Saturday, 1 January 2022

Guitar Amp Experimenters Bench PA — 12 Watt Popcorn PA


I sought a low power bench audio PA to use for guitar preamplifier development. This brick measures ~ 18 x 11 x 8 cm and delivers 12 clean Watts power into an  8 Ω load. On the front panels 3 RCA jacks provide regulated +/- 17 VDC plus a signal ground connection. Additionally, the signal input jack lies on the front panel. The rear panel sports a speaker jack, fuse and the AC mains input.

Above — A PA is built around a power supply transformer. This tall, heavy & old Hammond transformer has sat in my lab for decades: Although the label shows 25v CT, the AC voltages on each side measured 13.8 VAC under no load. Rectified and filtered this transformer provided +/- 18.89 VDC under a heavy load with no sag until pushed to the extreme. Thus it will function as a 10 -12 W guitar amp power supply — good for my purposes.

Above — The +/- 17 volt rail regulator/filter circuits. My rectifier included a full wave bridge and the main reservoir capacitors = 4700 µF.  I re-purposed The main Hammond chassis from some old project and this entire project falls under the low cost category since most of the parts were ancient specimens from my collection.

Above — Popcorn PA schematic. This is a slightly tweaked version of this Polytone-inspired PA. I recommend this PA over my original version since it offers way less distortion.

The small signal transistors = 2N4401/2N4403 - a pretty decent general purpose BJT I use from DC to HF. The input pair feature some emitter degeneration via a 100 Ω trimmer potentiometer. If you lack a trimmer pot, place a 49.9 to 68 Ω resistor on each BJT emitter instead.  

I ran out of BD140s plus sought a VAS with an fT of > 150  MHz, a low Cbe and a hfe as high as possible. VAS transistor choice seems to shrink every decade. Gone are the luscious  high voltage, high current, high beta offerings from companies such as Toshiba or Sanyo. For example, the KSA1381/KSC3503 or 2SC2911/2SA1209. Yes, these are still available from online auction sellers, but they seem very expensive and the whole bootleg part worry looms heavy. The BD139-140 seem the only low cost, readily available choice for a low budget PA like mine. The BD139/140 pair specs also widely varies - and some are just garbage. I now get mine from Digi-Key and test a couple to confirm they are OK.

In the end, I placed two 2N4403 BJTs in parallel with 10 Ω current sharing resistors for the VAS. This worked OK. Guitar amps are generally not hifi amplifiers giving ultra low distortion. Some might classify as hifi, but that is the exception. I avoided current sources, driving the VAS with a emitter follower and other distortion lowering techniques. 

The virtue of this PA = easy to build, easy to debug and sounds very good for the parts count. If you scratch build a complicated HiFi PA, you suffer a high probability of failure. Especially when current limiting circuitry and 2 current sources that work together with negative feedback go into your circuit. Often enough, your first sign something is wrong comes in the form of smoke; and by the time you figure out why, you may have destroyed some parts.

Above — FFT of the Popcorn PA. All harmonics are down at least 70 dBc. I tweaked the input pair 100 Ω trimmer to crush the second harmonic. Some of this spurious output comes from IMD in the TIP142/147 pair. In a PA, distortion can arise from many points along the signal path. I'm quite happy how this particular PA turned out.

Across the base inputs of the Darlington final pairs, I measured 2.064 VDC. Three rectifier diodes seemed to eliminate any detectable crossover distortion ( a dominant source of distortion in many PAs).

If you listen to a  guitar amp that lacks enough forward PA base bias and suffers crossover distortion, you'll hear a faint, fuzzy sideband sound along with your main guitar sound. You really hear this when playing single, sustained note phrases. This popcorn PA sounds clean & punchy with no hum except with single coil pickups.

Above — I built this version of my PA first. The BD-139/140 emitters get 120 Ω resistors so they don't need a heat sink and hopefully won't burn up if I made a mistake. I measured DC voltages, calculated current by voltage drops across selected resistors — and also tested it with a signal generator and dummy load. Only 2 diodes drops are needed to properly bias the BD139/140 pair.

With the amp working well, I then pulled the BD139/140 off the main board and wired up the chassis mounted TIP142/147 after adding 1 more diode to set the correct PA idling base bias. Finally, I added the current feedback loop. I chose the 7K5 Ω resistor during listening tests.

Above — FFT of the circuit shown in the schematic above (with the BD139/140). The distortion is about as low as I can measure with my DSO. All that is needed = a set of complimentary finals and a good PA might arise. A decent LoFi popcorn PA for guitar amps.

Above — The "brick". The Popcorn PA lies on 2 boards with carved islands for the positive, negative and 0 volt rails - and to anchor the rectifier diodes. After taking this photo, I screwed on the cover plate to seal it for safety against the pile of wires and other mess on my bench. This amp is not connected to AC mains unless I am using it and does not have an on/off switch since all my audio test equipment goes on a dedicated switched AC mains power bar.

Above — Rear view of the Popcorn PA. I'll start work on my next preamplifier circuit tomorrow and audio test it using this PA. I really like the small bench footprint this brick offers. I place the preamp circuit just in front of the brick and have small cables built for the 17 volt rails and input.

Click here for my guitar related index.

Saturday, 11 December 2021

Gibson GA-50 Inspired Guitar Preamplifier Tribulations


Like many, I enjoy the recorded sounds of — and feel inspired by the Gibson GA-50. That warm tone, full mid range and brown suitcase look epitomizes an American jazz guitar amp classic.
What do I like about the GA-50?  Its simplicity, the non-mega-scooped mid range, and of course, the warm, thick low and lower middle tones. Feeling inspired by these attributes, I embarked on a design journey to make a GA-50 inspired preamplifier to go with my simple experimenters PA stage.

I used JFETs & op-amps instead of octal tubes. Perhaps, now, even op-amps are getting outdated as digital algorithms simulating some old amp of lore push the bleeding edge of design. I’m not a fan of using tubes in jazz guitar amplifiers – give me solid state any day for clean signals. Perhaps 1 day, I'll possess the skills for digital amp design. 
I’ve studied hundreds of solid-state, analog designs and it seems that many just copy someone else's solid-state guitar amp design. Most of the ingenuity in solid-state design seems to go into the distortion circuitry.  I found a patent by someone who made a so called 'tube sounding' clipping circuit by putting zener diodes in the feedback loop of an op-amp. Really?!?  They were doing this in the late 1960s albeit for other reasons such as op-amp voltage limiters or zero crossing detectors.

It's more than the 'lack of tubes' that make some solid-state amplifiers sound poor. Design elements such as putting electrolytic caps in the signal path, along with brash, tinny-toned, picofarad coupling capacitors. Add in toxic sounding distortion — and not to mention the grinding noise from mega-high, tube-era, resistor values, and sometimes poor gain distribution that often compromises the noise performance and/or headroom.
The classic, passive Fender, Marshall et al. mid scooping bass/middle/treble tone stacks looms prevalent in solid state guitar amps. You will find them in countless guitar amps from as many different companies. While they do sound good in many designs, they depart from the desired GA-50 tonality and boy they exhibit loss.

My whole adventure focused on the study and testing of tone circuitry. I will blog more about that in future posts, however, as much as I wanted to keep the passive tone circuit of the original tube GA-50, it performed with lackluster results in my solid-state versions. Yes, these simple tone controls work, but such passive tone circuits seemingly lack versatility for getting a consistently warm tone with different guitars, speakers and speaker boxes.

That left active tone control circuitry and about 3 things to play with: [1] shelving bass or treble circuitry (basically these are variable low-pass or high-pass tone controls respectively) [2] peaking/resonant circuits [3] choosing a Q for my tone control circuits that gives the natural sound you hear in amps like the GA-50.

I spent ~6 weeks on the bench & computer working on my tone circuits along with the basic gain stages. I also studied non-linear design including distortion circuitry, switching, line-out circuits and speaker box emulation circuits for DI purposes. I filled 2 notebooks and learned much.

I burnt thru 3 soldering iron tips and tons of parts on my tone quest. I’ll start by showing you my latest design as of Dec 11, 2021.  I strove to keep my signal path resistor values down in value to reduce Johnson or thermal noise arising from these resistors.

Above — The complete preamplifier schematic. Click on the image for a better view since it's 1121 X 821 pixels with lossless compression. This is actually version 7 & sounds sweeter, plus uses less parts than the previous 6 versions. This design was built around the glorious 5532 op-amp. I measured no instability.

Buffers. Isolation buffers lurk everywhere !

In your lab, you're likely not making a cost-conscious design for the production line. Therefore, use as many op-amp buffers as you like to enhance stage isolation and to prevent the loading of your tone circuit at extreme settings of the potentiometer wipers. I love buffers and isolation — almost to a fault. Go ahead and remove some of these voltage follower buffers if you wish.

The input 10K resistor adds noise at a low level spot in the preamp. However, a series input resistor follows normal practices. My listening experiments yielded my personal preference for this resistor to lie between 10 and 12K ohms. That resistor and the 150 pF shunt cap are wired right on the input jack provide RF filtration in a critical spot.

The 47 nF input cap comes right from the GA-50 which uses a 50 nF cap. This capacitor value works perfectly as a high pass filter pole to attenuate the low frequency rumbling noises emitted from an arch top guitar. Further, it simplifies the design by allows readily available for a reasonable price 1 µF signal chain caps for DC blocking in a clean signal preamplifier design. My 1 µF caps = Panasonic metalized film polypropylene jobs — chosen for their low distortion.

Active volume and master volume controls help with noise and headroom management. In many solid state guitar amps you'll see a stage of massive voltage gain immediately followed by passive attenuation via a 100K or so volume potentiometer that's shunt to ground. That voltage gain stage runs at its maximum gain (and noise) all the time. Why boost — then right away attenuate the signal if you don't have to ? I tried to copy the GA-50 and just provide 1 volume control, however, found too much compromise in headroom and noise performance. Distributing the gain stages and adding an active master volume control improves the preamplifier and 1 extra volume pot on the front panel does not seem too onerous.

Tone Circuitry

I tried many tone control circuits for bass, treble, lower mid and high mid range. I made variable Q parametric stages; EQ style stages with an op-amp gyrator to simulate the inductor at Qs ranging from 0.7 to 5.1; Baxandall circuits with shelving and/or peaking; and also Wien bridge type treble and mid range circuits. Some of these Wien circuits were fixed while others went variable frequency. I also explored some passive circuitry like those found in Fender, Marshall, Hiwatt, Pignose .....etc. amplifiers as well as circuits from old hi-fi amps. I even copied an old Hughes and Ketner design that plys passive middle and treble controls, then an active, op-amp bass stage that worked pretty well.

After listening to these circuits over many weeks I came to a few conclusions in the context of a GA-50 inspired amplifier:

  • I disliked EQ style tone controls. If the Q was 0.7 to 1, they seemed a bit more tolerable
  • In general, circuits with a Q > than about 1.5 tend to sound unnatural and may trigger listener fatigue
  • I'm not a big fan of deeply scooped midrange
  • Shelving bass and treble sounded better than peaking bass and treble, however, variable frequency shelving seems quite desirable for versatility
  • Combining shelving low and high; plus peaking or resonant type mid-range controls is common in mixer boards and other professional gear and sounds OK. Again, when the Q is <= 1.5, the peaking/resonant mid range tone circuit seems more natural sounding to me

Based on my conclusions above, you now know why I went with the variable frequency shelving low and high frequency controls that forms the heart of my preamplifier. I struggled with potentiometer interdependence in early shelving circuits. When running both a high and low shelf, the low boost/cut pot may affect the high shelf and visa versa. Further, when the shelf frequency pot is rotated to lower resistance to get a higher frequency, distortion, weird behaviour, and/or noises might creep in a some settings of the boost/cut pots. Hence voltage follower isolation amps help make all that pain go away — and provides reasonable potentiometer separation. My chosen circuit still needs work.

Having 2 low and high frequency pots allows you to find a sweet spot for each respective shelf with respect to the room, guitar and whatever speaker your running at the moment.

Ultimately, I opted to not put in a mid-range peaking/resonant tone control for my GA-50 inspired amp. That just seems wrong. On the other hand, the 2 shelving frequency controls allows some alteration of the middle tones which boost versatility.  Shelving networks exhibit a Q of less than 1 and subtly, gently alter your guitar's tone. I'll blog more about my tone controls circuit experiments in the future — I've got lots of interesting material to show you.

Choosing Time Constants

In many low + high frequency shelving circuits, the designer chooses the same capacitor value but up a decade for the low frequency network. For example, .0082 + .082 µF. This allows a bit of overlap between the 2 shelves. I opted to run my high frequency network a little higher and get some calculated response slightly above the 10 KHz zone with the 20K high-pass potentiometer set to 0  Ω.

The standard calculation applies. Frequency = 1 / (2 pi *  R * C ). So, for the low-pass network with the 20K pot set to maximum resistance: Freq = 1/ (6.28 * 22200 Ω * .082 E-6) = 1/.011432 = 87.4 Hertz.  If you seek mega bass, try swapping in a 100 nF capacitor instead.

Above — An early version of my preamplifier using nJFETs as amplifiers. I found IMD and clipping on the output of the Q3 source follower. I later replaced it with an op-amp buffer, and perhaps another after the volume control before abandoning JFETs altogether. The low frequency shelf circuit is by Douglas Self in his book Small Signal Audio Design. I've got 4 of his lovely books and Mr. Self is my favourite audio design book author. I noticed that in his latest version of Small Signal Audio Design, he's added a chapter on guitar amplifiers. Yay!

This schematic includes my favourite simple passive treble network. The 20K pot is used to variably shunt high frequency to ground. I also added a "fatten" switch to roll off less low frequency for use in 10 inch and 8 inch speaker applications. I would normally switch this capacitor in or out using a front panel switch that remotely controls some DC voltage to a JFET switch using the J111 or PF5102.

Above — A whimsical idea that I tested and discarded when I tried to keep the original passive GA-50 tone circuit topology in my preamplifier. When I don't like the result of a circuit I write FAIL on it so that I never make it again.

Above —A schematic excerpt from the solid-state Gibson G-35 guitar amp. This amplifier kept the basic topology of the GA-50 tone circuit. By that time, most of their solid state amps had abandoned this style of tone control circuit and were running a passive Baxandall tone stack instead.


At first, I built my preamps on small boards that I lifted in and out of my experimental chassis to work on. I did not bolt them in — for they were just held in place by the DC voltage, ground and wires going to and from the various potentiometers. This worked OK, but soldering and unsoldering all those wires became tedious. Later, I moved to modules that sat in front of the chassis that contained integral pots. I only had to solder or unsolder just 3 wires: B+, B- and the ground wire. Much easier.

Above — 2 identical modules that contain all the circuitry from input to U1b pin 7 of the inverting amp in my Dec 11 schematic. The circuits were tested with a signal generator + DSO. Then I began installing the low frequency shelving network boost/cut potentiometer as shown in the closer or proximal module. 

Above — The completed Dec 11, 2021 module with the DC voltage and ground wires attached. An alligator clipped lead connects the output to the PA input. A Boss digital reverb permanently lives on my bench for testing circuits with reverb. I'm addicted to plate reverb. The speaker cable runs just to the left of the reverb pedal. I've currently got five 8 Ω speakers in my lab for amp testing.

Above — A closer photo of the final module. The master volume pot lies on the extreme right hand side. I chose a 2.2 µF output cap since it lies in series with a 1 µF capacitor that's soldered to the PA input. In my final, proper build, I would just likely use a single 1 µF capacitor. My 2.2 µF polypropylene caps are huge 630 volt jobs. I've got a few 100 volt caps @ 2.2 µF, but save these for final builds. Brand name capacitors are not cheap.


No doubt some of the sonic signature tones from the GA-50 come from the Alnico magnet 12- and 8-inch speaker pair. I’ve learned that speakers and perhaps even more importantly, the box they’re mounted in prove crucial to realizing your desired guitar tone.  I’ve tried 2 speaker pairings: 12 + 8 inch, 12 + 10 inch and 8 + 8 inch. The wide sound field somehow disturbed me. The only combination I seemed to tolerate was the 8-inch pair, however, they need to go in a box designed to boost the bass response for my taste. Thus, I just use one 8-ohm speaker with this preamp + my PA.

With my preamplifier, if you drive a speaker mounted in a cabinet designed to give bass extension, the available low-end response might amaze you. This is definitely a warm, low-end focused amp – lacking the brash high-mid & raunchy treble response associated with some “transistor amps”. However, the high end is still there. Sometimes it’s pleasant to put a bit of shimmering > 8 KHz treble into your tone.

Above — An early breadboard using Ugly Construction. The power supply is a simple interface to my bench +/-15 volts regulated, 5 Watt power supply.  Islands are carved out in the circuit board for temporary connection to the power supply when developing circuits prior to putting them in the guitar amp. I measured DC voltages, DC current, AC peak-peak voltages and usually hook up a signal generator and look at the FFT also. I use the grounding techniques shown on this web page so despite all this wire, I hear no hum unless I use a single coil pickup.

Future Experiments

Above — Future things to add to my GA-50 inspired amplifier. Thanks to the books of Douglass Self and from studying many schematics, I've got confidence to work out ways to record my amp without speakers + a microphone and/or add software-based digital effects. The features found in many newer amps such as integral DI patching, speaker cabinet emulation and ground loop noise isolation circuitry will help take the GA-50 inspiration into new realms. I feel that if Gibson's Seth Lover and Ted McCarty were alive today they would embrace the latest technology including digital signal processing and of course, op-amps. 

I'll continue to work on my basic GA-50 inspired guitar amp from time to time. I've got parts coming to make a small, complete low-power amp Dec 11 version for practice. I've already tweaked the improved Polytone inspired PA from my last posting to lower its noise floor & distortion a little.

I only wish I had more time to spend on the bench.

Click here for my guitar-related index.

Wednesday, 3 November 2021

Clean Jazz Guitar Amp Experimenters Platform — Power Supply and PA


Greetings! I purchased a chassis to hold a power supply, a power amp and future preamplifiers. I drilled a bunch of holes for potentiometers, switches, plus 2 inputs and 1 output.  I'll use this basic experimenters platform in my lab and blog about my results over time.


My goal is to explore solid stage guitar amp preamp and tone shaping designs for clean, jazz guitar purposes. My aluminum chassis is strong & large to avoids the pains of working in a cramped box.  

I've cut enough holes for 2 separate preamplifer stages to allow A - B comparisons of 2 different preamp or tone shaping circuits. Today, I'll blog about the power supply, power amp and 1 simple preamp stage — this will set the table for future, specific, component-level experiments.

Power Supply

Above — The simple split power supply.

I had 4 usable transformers in my collection and the first 1 applied was fried. I replaced that with a lighter +/-18 VAC output  transformer purchased on eBay last year. To my delight, the no load DC output measured +/- 26.7 VDC.  A zener diode regulated portion sits on each rail for the preamplifier op-amps and JFET/BJT amps. Choosing +/-17 VDC garners maximum headroom and dynamic range from op-amps.

I went into great detail about guitar amp DC power supplies here and won't repeat myself.

This platform features no power switch since it gets connected to an AC power bar with an integral switch — this guitar amp only sees AC mains power when in use.

Above — 2 photographs of the power supply mounted in the chassis. The 2 rear panel mount fuses are not currently in use. The panel mount IEC320 C14 receptacle features a built in 20 mm fuse holder. I've got a 2 Amp slow blow fuse inside.

Each rail is "monitored" by a separate front panel LED. This proves handy. If you shunt or short a rail, the corresponding LED will dim or turn off respectively.  If your amp goes into oscillation, 1 or both LEDs will flicker at the oscillation frequency.

Power Amplifier

Only a 10 Watt or so PA is needed for an experimenters platform. This nicely keeps the weight  (from the transformer) down. Additionally, 10 Watts seems about perfect for a practice amp. I know enough guitarists with hearing impairment to feel some concern about this topic.

I built 3 PA's but eventually settled on a simple design that boasts only 5 transistors. I took the PA from the Polytone model 100, 101, 102 and perhaps others and modified it so it makes less harmonic distortion. I love this PA for its simplicity:

 Above — Power Amp stage.

With my current supply, this PA provides 12 Watts maximum clean power. Maximal power rating mostly depends on your power supply transformer, and at full power, my DC rails measure around +/-19.7 VDC.

The first change included replacing the original Sziklai pair with a standard Darlington pair in the form of the TIP 142 and 147 complimentary power followers. These husky 10A TIP transistors prove tough and stable in my low power guitar amp. Mine have SOT-23 cases. 

To drive these transistors the BD 140 voltage amp or VAS runs 9.8 mA quiescent emitter current. Rather than going with an adjustable bias for the power followers, a simpler 2 diode scheme works fine and crossover distortion is acceptably low. The DC voltage difference across the TIP 142/147 bases = 1.44 V.  Adding a 47 µF capacitor across the 2 biasing diodes did not reduce crossover distortion.

I replaced the original 470 Ω collector on the input BJT differential input pair with a 1K potentiometer and tweaked the pot for the lowest possible distortion while viewing an FFT in my digital oscilloscope. After locating the sweet spot, I removed the pot and measured 699 Ω. This resistance was nearly achieved with a 4K7 in parallel with an 820 R. I stuck that into the circuit. With that amp set to maximum clean power into a dummy load changing resistor from 470 to 699 Ω lowered the amps distortion profile significantly.

The 100 pF capacitors arrangement in the Polytone was supplanted by that shown. A 100 pF collector cap shunt to ground (instead of to the 0 volt rail) plus another from collector to base stabilized this amp.

Although 1N4007 rectifier catch diodes were added to clamp any back EMF spikes caused by changes from the inductive load of a speaker, I added no PA current limiting circuitry.  For a 12 Watt experimenters amp, this seems OK.  With all power amps, overbuild them with parts that can stand some current; the only TO-92 transistors in the whole PA = the differential pair.

Above — The original Polytone PA that served as the basis for my power amp.

I liked the switchable "dry or wet" current feedback selector switch on old 1970's Polytones.
During listening tests, another version arose: a series 712 or 156 Ω which varies the midrange response heard in the speaker.
For my forthcoming experiments, I'm using 2 Jensen 8 Ω speakers:  a C12N or a C10R mounted in the same open back cabinet (using either 1 speaker or the other). I tested my switched feedback resistor values listening to these particular speakers while playing guitar and recommend doing this to anyone making a guitar amp. Find your current feedback resistor mojo to suit your guitar(s) and particular speaker(s).

Above — An FFT of my PA with a 1 KHz signal injected in the input and a dummy load serving as an output. Each vertical graticule is 10 dB. The third harmonic seems to be from crossover distortion and imbalances in the input pair — and also from the finals.

To get the 3rd harmonic down another perhaps 5-6 dB, I would have install an active current source for the input pair, add 68-100 Ω  degeneration to the input pair emitters, use separate emitter followers for the driver and final PA transistor, and change the 2 diode PA biasing to an adjustable "amplified diode" using a BJT plus a pot. Thus, the once simple Polytone amp now resembles a classic HiFi amplifier like I would build for a home audio PA.

Above — The nice looking sine wave in my DSO (time domain). I can not see any crossover distortion in this screen capture and while zooming in live on my DSO.

Still, though, the distortion in my original Model 102 version looked much worse. The modified version dropped the 2nd plus odd order harmonics down ~15 dB @ 12 Watts power compared to the original.  My current version proves a simple, easy-to-build, inexpensive, 5 transistor PA that sounds great during listening tests.

Beyond the tube versus transistor debate, you might argue that classic, beloved guitar amplifiers were highly non-linear from input to output.  The tendency of solid state amp builders to make their amplifiers ultra linear by using extensive local and wide area feedback circuitry and über perfect input stage balance perhaps makes them sound a bit sterile.  Warts and all, that's my PA.

Above — The final PA mounted in the chassis.

The board looks little messy from the previous 2 PAs built into it. On the right foreground, you may view the TIP42C BJT formerly used as a current source and now disconnected. I fashioned heat sinks from some bent aluminum sheet metal. The entire PA board is isolated from the chassis by cutting islands around the mounting bolts. At this point, the current feedback switch S1 (seen on the right panel) was not in use. Another panel 4 SPST multi-selector switch sits in the chassis in case that is needed in the experiments ahead.

Above — Front of the chassis with 2 input jacks and 4 switches mounted in situ. A third LED slot on the right is open in case an LED indicator is needed for future experiments with preamp and tone shaping circuitry.

Input Channel 1, Preamplifier 1

Above — I built this simple low noise preamp into channel 1.  The output was connected to the PA input with a piece of wire. This simple, low noise preamp allowed me to guitar test the PA and also set the current feedback resistors connected to S1 via listening tests. 

It's a great experience to listen to an amp with no tone controls.  I enjoyed playing through this amp for a few nights and even though preamp maximal gain ranks low, it's already a loud, small room practice amp.

Now I have a base platform for preamp and tone control circuitry experiments to blog about.

My guitar-related index is here

Monday, 15 March 2021


Clean Jazz Guitar Amp Builder Notes — Part 4: Pre Amplifier Notes

Part 1 lies here
Part 2
lies here

Part 3 lies here

Click for my Guitar-Related Index

Preamplifier notes from March 7, 2021 until present

Seeking my ultimate clean jazz guitar amplifier I continue on from my last posting with Part 4. Procrastination set in. I  did zero on the bench for several months; however, gathered new guitar, bass plus steel guitar amp schematics, plus thought about the next steps on my journey to find my ideal jazz tone machine .

Although I liked the bass and treble controls from my previous preamp circuit, after months of listening, dissatisfaction arose. A sort of cognitive-aural dissonance loomed. My amp wasn't making me happy anymore. When we hear soothing, ideal guitar tones , we transform — suddenly your playing feels inspired and we make up new phrases simply because your mind feels pleased from what your hearing roar from the speakers. It's difficult to describe this to non-musicians.

A good guitar or amp may inspire you. Sometimes, just something new gets you on track. Hence many players suffer from guitar (or amp) acquisition syndrome and the like.

Back to the bench. The mid range frequencies provide the sonic heart of a guitar amplifier. Nailing the mid range — with versatility & deft finesse (all while feeling sonic bliss) proves no small task. Do you prefer the scooped lower mid range Fender sound, or seek to hear thick tones with just enough sparkle to quell the mud monster that lurks only a few dB below?  Or perhaps you desire both?

My previous Polytone-like bass + treble stack provided a pleasant enough tone, but lacked the ability to blast thick midrange tones when I sought them. e.g. 'twas a 1-trick pony.  

So i built a 4 frequency Baxandall tone circuit and then kept experimenting with a separate preamplifer board that I'll work on over the next year.

Above — Preamplifer block diagram. I kept the early stage preamplifer shown in Part 3 and added a new tone stack and post tone circuit amplifier.

Above — The new section 2 & 3 circuit with the simple post tone stack amplifier attached to the tone stack for clarity. This version borrows from the work of Doug Self, who I've referenced in Parts 1 and 2.

This is a transition circuit. It got me away from the Polytonic sound of my previous tone circuit and into the territory where thick, but clear mids blast beautifully.  Still, though, this circuit does not produce sweet, musical scooped mid tones even though I can cut the low and high midrange controls.

I lined up multiple new circuits to try and will present them over the next few months. Stay tuned.

Above — the current tone circuit in my jazz guitar amp. I love thick mid-range tones, but not all the time. Let's work to grab some versatility. To the bench for experiments. Thank you!

Monday, 16 November 2020

Clean Jazz Guitar Amp Builder Notes — Part 3: Pre Amplifier

This is part 3 of a series about a complete prototype guitar amp.

Part 1 lies here
Part 2 lies here
Part 4 lies here

My guitar-related Index is here.

Part 3: Preamplifer from Nov 29, 2020 to March 7, 2021

Bench Notes:

I sought a low noise, clean amplifier. In the head of most jazz guitarists an ideal guitar tone sits —
it might be Jim Hall in 1965, or Tal Farlow in 1957, or Julian Lage in 2020.  I've discussed this topic previously and it essentially defines subjectivity. Tube versus solid state? 12 inch versus 10 inch speakers? Single coils versus humbuckers? The microphone!  Was EQ applied by the recording engineer? And so on. Let's glide past this dribble and assume the 'ideal guitar sound' in your head sounds as good as mine.

I chose to power 2 speakers bolted in an open back cabinet. I feel open back cabinetry sounds more natural and previously felt 100% confident that sealed, ported speaker cabinets were the only way to get my jazz tone. Bah non, t’as fait n’importe quoi. Thus my tone shaping lies orientated to driving an open-backed speaker cabinet while still getting some thumping bass response at low, practice-level volume if desired.

First comes the DC power supply for the op-amps. For maximum op-amp headroom, I ran the split supply DC as high as I could safely manage by choosing 17 volt zener diodes.

Above — I built the voltage regulators on the Stage 1 preamp board. These hulky transistors seem excessive, but in an earlier version, a BD139 failed and smoked up my lab. Don't get me going about how bootleg parts have polluted our parts drawers and reduce some of the joy. These 20 year old, Motorola TIP BJTs won't fail now or ever. My original design had diode current limiters, but I dispensed with them when I hurriedly rebuilt the voltage regulators after the NPN part failure. BP is ground; please refer to Part 1 for details on how I grounded the various stages to avoid hum and reduce noise.

Above — Block diagram of the preamplifer. I'll present each stage separately.


Above — First preamplifier stage.

The 10K and the 220 pF shunt capacitor form a low-pass filter at 72 kHz which attenuates high frequency signals such as AM radio stations that might sneak into the amplifier input.  You may go as high as 470pF if needed; or may raise the resistor value to boost HF filtration.  

Ideally your entire preamp should lie in a sealed metal box to reduce capacitive pickup, however, I designed as I built and fell short with shielding.  Any noise arising in the first preamp goes down the chain and Stage 1 sets your amp noise floor.

The 10K input resistor also serves  to protect the op-amp from large signal overload such as a static spike or a high amplitude signal. Further, 11 volt anti-parallel zener diodes clamp excessive amplitude signals. The diode value isn't critical, however, many engineers  would avoid zeners rated below ~ 6.2 volts as they may conduct prior to signals reaching the zener or knee voltage — and this might cause signal distortion.

How much gain and how to distribute it proves a vexing design consideration. Running lots of gain often means lots of noise.  Ideally, we want just enough preamp gain for the power amp to reach its maximum clean power, but not a drop more. Distributing some of the gain before and after volume controls is 1 way guitar amp engineers manage the noise. The main goal is to avoid amplifying noise at low volume control settings. I spend most of my practice time at low volume settings so I don't drive my family nuts with repetitive practice routines.

Imagine if you built a 15 dB gain preamp stage and then placed a passive volume control pot after it. Your preamplifier stage is always running at 15 dB gain and its noise and the noise of the potentiometer will go down the preamp chain. If you put another fixed 15 dB gain stage after this potentiometer, then even at low volume control settings, the system noise after the pot is still amplified by 15 dB.

Applying active volume controls  (an inverting op-amp stage with the volume pot in the feedback loop) is 1 way to keep noise down at lower volume control settings.  Your stage gain goes up & down with the series resistance of the volume control pot. You'll see this in countless guitar amplifiers as it avoids amplifying noise at low volumes while still affording decent headroom for the input.

Over several decades, many of the guitar amplifiers I've played were high gain "rock star specials" and quite noisy. By noise I mean Johnson thermal noise, shot noise, flicker noise, plus voltage and current noise from amplifier stages.  Crank your amp to maximum gain by turning all the pots to "ten"; unplug your guitar cord from the amp input, or turn the volume pot on your guitar to zero and listen in a quiet room. You'll hear the noise floor of your amp at it's worst. Advance the tone controls to boost and then cut. If 1 exists, rotate the reverb control from minimum to maximum  and see what happens to the noise you hear in the speaker. 

I liken this sound to frying electrons in a skillet. It reminds me of listening to galactic noise in an astronomy receiver. Pretty much, they are the same thing.

Many home brew and some commercial guitar amps are really noisy.  Reducing noise serves as a major design goal for me.  In this amp, I tried to keep noise down by using low value resistors (as possible) , quiet op-amps, active volume control and hopefully wise gain distribution.  Some noise inevitably arises in the tone circuitry and for me this is where most of my amp's noise gets generated.

Currently my home brew amp offers less noise and louder maximum room volume than my Polytone Megabrute and my blackface Fender Reverb. I still want it quieter in future versions however.

Referring to the stage 1 schematic, my first stage unity gain arrangement buffers the input from the 5K volume control pot. The buffer allows you to avoid the noisy 50 or 100K volume pot you often see in guitar amps.  The Baxandall active gain stage perfected by Doug Self gets employed.  I strongly recommend Doug Self's book: Small Signal Audio Design, now in 3rd edition. Self, an adroit author and wise clinician, plys measurement based advice that can't be found anywhere else.

The U1B buffer allows use of quieter low value resistors in the parallel U2 op-amp stages. This amplifier exhibits very low noise on lower volume settings and the only downside is the volume control isn't perfectly logarithmic across the rotation of the knob. You get used to it however.

I prefer to not go above a setting of "9" on the gain pot as the noise performance at "1-9" seems stellar.  

Another big concern is tone shaping the input. The capacitors C1 and C2 achieve this; especially C1. I placed a pair of alligator clips and tried capacitors in the C1 slot ranging from 1 nF to 10 nF. I settled on 4.7 nF as shown. C1 sets how much of your guitar's low frequency you want to high pass filter.

If you apply too low a value, your guitar may sound thin and tinny. To large a value for C1 might cause you guitar to lack highs. The effect maybe subtle, but still important. I agonized over choosing the correct C1 for 1 year  — pick something and stick to it ( for awhile anyway). You may help fatten a bright guitar with your C1 and C2 choice.

For C2, I've got a 1 µF cap in place currently. Between 1 and 2.2 µF seems ideal for me. This helps prevent a flabby bass response when playing loudly. If you play distorted guitar, your C1 and C2 choices would likely be very different than mine. Down the chain I use 4.7 to 10 µF signal capacitors. My small, mostly donated  collection of signal capacitors are older, mostly metallized polyester film types rated at 200 volts plus, and thus are big and unwieldy. 1 day, I will order some smaller size caps.

Guitarists hotly debate whether plastic film capacitors insulated with polyester, polypropylene, polystyrene or some other exotic dielectric material "are the best". It's quite laughable. Doug Self tackles this myth with gusto in his Small Signal Audio Design book. I just prefer caps that don't distort the signal, leak DC, nor break my bank account.

Finally, I employed the lovely LM4562 op-amp in Stage 1. Without  the 100 nF COG/NP0 bypass caps on pins 8 and 4, I've seen this part oscillate at between 3.5 and 4 MHz.  The 5532 would also prove a solid op-amp choice.


Above — Second and third preamplifier stages.
I combined both Stage 2 and 3 on this schematic as they share the same op-amp; the Texas Instruments OPA2134 which potentially offers less noise than the TL072.

For ~ 1 year I had a 9 channel equalizer as the tone control control circuit in my amp. I grew to dislike it. Why?  It sounded unnatural, caused listener fatigue and lacked a 'musical character'. On Nov 29, 2020, feeling discontented and a little melancholy , I cut all the wires and unbolted the copper clad board. I was done with that circuit. ( Archived in Old Stage Two and Three Notes below).

What to replace it with ?  I felt a Baxandall tone circuit would work fine. The next question was 2, 3 or 4 frequency bands?  Remembering a jazz concert 35 + years ago triggered me to go with the simple bass + treble version shown in the Stage 2-3 schematic.

I remembered the guitar player playing a Gibson L5 through a Polytone 102 with a speaker extension cabinet attached.  His tone amazed me. I remember visually checking out his amp and saw it had tremolo,  reverb and just bass + treble tone controls. I also remember feeling surprised that it wasn't a tube amp. Polytone amps emit a spongy, warm, quite musical sound that comes from the Baxandall tone circuit, plus perhaps, a closed speaker cabinet stuffed with insulation.

Many Polytone amps seem to follow a interesting gain distribution pattern. The first preamp has a gain of 6 dB followed by a switch that throws it in a bass boost, bass cut, or normal. These circuit modify the op-amp feedback loop and by viewing Polytone schematics, it easy to see how they work.

After the first op-amp stage  is a Baxandall tone stack with either bass + treble or bass middle + treble controls A volume pot follows the tone stack — passive gain control.  From there, the signal may go to a reverb circuit, tremolo, or a distortion circuitry , however, all of these are summed with a final 20 dB maximal gain op-amp stage with active gain control.

Above — An excerpt from the schematic that came with my Polytone Megabrute. R18 is actually a 100K pot.

With the fond remembrances from decades ago,  I decided to put the 102's approximate bass + treble tone circuit plus the Polytone Megabrute's summing op-amp into my guitar amplifier. The Baxandall tone circuit was a standard HiFi circuit back in the day and perhaps even now. It seems almost antithetical to the Fender scooped lower mid range tone stack. However, if you turn up the bass and treble controls, you hear the scooped mid range response.  Just a bit more subtle.

I dispensed with the reverb, tremolo, and the infamous Polytone distortion circuit while lowering the resistor values as possible to reduce Johnson noise. The end result is what you see in the Stage2/3 schematic. My first op-amp offer much more available gain, and by far my amp has a very low noise floor.  My power amp also generates lower harmonic distortion and noise than Polytone amplifiers.

The 100 pF capacitor from U1a pin 7 to 6 is mandatory. If you turn the treble pot to boost treble, at some point, the tone stack will burst into high amplitude HF oscillation without this cap. 

I tried a few different values for the treble capacitor and settled on 3.9 nF.  I also boosted the low frequency by hiking the bass caps to 150 nF. With an open-back cabinet, this provides a little more available bass boost if required. For a closed back speaker cabinet  — for 10K pots: 100 nF seems a good choice for the bass caps.


Stage 3 = the simple  U1b active gain controlled inverting op-amp. 

Above — My front panel now has many unused pots and switches. I've stripped out my line-out, effects loop and other circuitry regressed to a very simple preamplifier circuit with no frills. Sometimes less = more.

Above — My amp on top of a home brew speaker cabinet. I'm no carpenter.  This cabinet holds a 10 inch + 12 inch speaker. I'll comment about speakers in Part 4 of this amplifier series. With cats, cloth speaker grills are out. I fashioned mine from an aluminum vent cover. 

I love playing through this amplifier. It has a Polytone-like warm, bouncy sound with crisp note definition.

Miscellaneous bench notes plus discarded circuitry

Above — The voltage regulators with current limitation. The zeners shown are 16 volt jobs. 1 diode compensates for the BE of the transistor, while the other diode limits the voltage across the emitter resistor to the diode ON voltage of around 0.6 volts.  The current limit runs a bit over 200 mA per rail.

Old Stage Two and Three Notes

Below lies the Stage 2 and 3 circuitry used prior to Nov 29, 2020.

I spent a lot of time studying and testing tone circuitry. I'm still uncertain what I'll eventually stick in my final amp version, however, here's my circuit prior to Nov 29, 2020:

Above — Second preamplifier stage.

As you can see, it's an equalizer. I've had this in place for over 1 year to learn what frequencies I seek to boost or cut. Upon reflection, it seems I mostly like to cut lower middle frequencies. I grew up listening to guitar amps with scooped lower mids and it's ingrained. 

I actually built a partial 1/3 octave equalizer  (50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1K, etc.) but stopped at 6.3 KHz to experimentally see what frequencies I like to adjust in a clean guitar amp. The frequencies I seemed to care about are those I picked for the Stage 2 equalizer.

Above 2 photos — My temporary outboard equalizer with 12 possible pots ( 2 were missing at the time of the top photo). On my guitar amp, I placed a line out after the first preamp — and a line in going to the PA stage with a bypass switch (if needed) so I could test various external tone circuits. On the outboard EQ, I tested up to 12 frequencies at once.

I'll discuss my final Stage 2 EQ, going from left to right; or low frequency to high starting with the bass boost.  

My speaker tests get covered in Part 4, however, occasionally, I practice with 6 or 8 inch speaker mounted in open-back cabinets.  A switchable, low Q, 75-80 Hertz bass boost makes a huge difference for these 2 speakers.  It's also good for low volume playing when you really wish a super fat bottom end for a change in pace.  A front panel switch enables or kills this filter.

I really like the main bass control to be wide (low Q) and centered at 125 Hertz. A center frequency of 150-160 Hz is too high for me.  A slightly lower bass center frequency sounds OK, but since I have the bass boost at ~75 Hz , a 150 Hz bass control worked well.

Low mid range frequencies

For many, the lower mid range tone shaping defines the guitar amplifier.  Some of you have studied the transfer function graphs of classic Fender, Marshall and other famous amplifiers to see the importance of lower mid range shaping to set the signature tone of an amplifier.

From making the partial 1/3 octave, EQ, I learned I strongly dislike 250 and 315 Hz. These along with 400 and 500 Hz (respectively to a lesser degree)  make up the so called mud  (or mud range) frequencies to my ears. Thus I fixed cut 250 and 315 Hz. I found that 400Hz should be adjustable (to vary the cut)  and that 500 Hz is OK, but should never get boosted from my experiences.

800 Hz ranked important in my listening tests. Generally a little bit of cut helps my tone at this frequency.

I can't handle 1 KHz at all.  For me, its a nasally, festering, fingers down a chalkboard frequency. For some reason, 1.2 KHz seems a  bit more palatable.  Depending on the room and guitar, I may slightly boost, set neutral, or slightly cut this frequency. Since I favor the 1960's Jim Hall sound, I added 2K and 3K2 Hertz but nothing higher in frequency. The Hair control is more just a general treble boost or cut — very subtle.  Although I omitted 5 KHz, it might belong on some guitar amps.

For the lower mids and highs, a Q of 5.1 sounded better than lower Q versions. I think I tested a Q of somewhere around 1.7 and then 0.85 for some of these frequencies.

Since there is DC on the pots, I stuck to the format employed by countless amp designers to avoid scratchy pot adjustments: I built with the TL072 op-amp ( 6 of them ). I just ordered some OPA4134 for future exploration.

Other tone circuits

In jazz amplifiers, it seems that the Baxandall  tone circuit reigns supreme. A few older Fender solid state amps ran their classic R-C passive tone circuit ( the Fender scooper ) plus later in the preamplifier stage, a Baxandall type tone control circuit to keep the classic Fender midrange but still allow some boost/cut of the bass and treble etc..

I first built the old bass and treble control employed in the early Polytone amps such as the Mini Brute or Model 104.

I reworked it with 10K pots to lower Johnson noise plus op-amp current noise in keeping with the solid advice and influences from Doug Self's book:

Above — The basic bass & treble tone circuit that reasonably keeps the time constants of the early Polytone tone circuits.  This stage works well for all its simplicity.  I've put a version of this circuit in radio receivers, and a code practice oscillator. I prefer this circuit over the bass, middle, treble Baxandall tone circuit employed by Polytone in later amps including the Mega-Brutes.

You will find countless versions of 3 Baxandall frequency tone circuits in guitar amplifier service manuals. I built a few and found a 4 frequency tone control suits me better than a 3 frequency circuit.

Above — My favorite 4 frequency tone circuit. 20K pots prevent the input impedance going too low when boosting heavily, You find interaction between some of the peaking filters, but this is pretty sweet for 1 op-amp. There are also scores of 4 frequency variants in guitar amplifier service manuals that cut/ boost up to 20 dB.

Above — The board I put the 4 frequency tone circuit in. I added gain controls and 5532 op amp to turn this into a simple, complete jazz guitar preamplifer. It sounded pleasant.

Further, from his aforementioned book — I built Doug Self's low noise, variable frequency, variable Q, state variable,  mid range parametric equalizer. Of the 3 parametric EQs I built, his worked the best in terms of noise performance and function.  I just never feel parametrics work the mid range the way I seek for a jazz guitar amplifier.

I also tried some high-pass filter circuits. 1 stood out.

Above — A high-pass filter for guitar input. This filter works well and seems to enhance bass frequency tightness. We have no use for frequencies below 60 Hz. I built my guitar input stage from stage 1 right on the first unity gain buffer.

Stage 2 Conclusion

After using the Stage 2 equalizer for ~ 1 year, I learned that in a future circuit, I will probably only place a quad FET op-amp and make a 4-band equalizer for  150, 250, 315 and 400 Hz with 250 and 315 Hz in a fixed cut.  

Perhaps I will add a separate 1 op-amp stage bass boost as well. The high mids and highs will likely get handled with a single op-amp with a switchable peaking/ shelving EQ circuit.  Who knows? Only time and experiments will tell.


Above — Third preamplifier stage. This is similar to the stage 1 input amplifier section. The last buffer is made from the left-over TL072 op-amp stage on the equalizer.
The master volume pot dramatically  lowers the noise when gain is reduced.

Miscellaneous Photos


Above and below — The amp chassis with equalizer during testing. No hum!

The index for this project is on my guitar-related page. Click here.