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.

 Prelude

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.
 

STAGE 1


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.

STAGE 2

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

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.

STAGE 3

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.

Wednesday, 20 November 2019

Clean Jazz Guitar Amp Builder Notes — Part 2: Power Amplifier




This is part 2 of  a series about a complete prototype guitar amp.

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


 Power Amp Stage Notes:


Section A   The Final Amp | 21 Watt power amplifier for clean jazz guitar.

Above — Schematic of the final power amplifier developed for this series. Much of the design keeps the VAS + VBE multiplier, and ultimately the final output pair from instant destruction when things go wrong. For example, a speaker disconnection, or accidental bench experiment electrical short.  Of course,  we likely can't provide fail proof protection to sustained adverse events.

This amp includes an abundance of medium power transistors: BD139 | BD140 , plus current limiting in the VAS + the final drivers.

Most resistors outside of the input differential pair circuitry are 1/2 watt rated.  Most caps are 50 VDC rated.  In the schematic above, I've got the 1000 µF filter capacitors going to signal ground for clarity. Actually, they return to their own special ground @ the power supply as discussed in Part 1.

Above — 12-bit FFT of the maximum clean signal output | 21 Watts. With a beefier power supply , ( such as +/- 30 to 40 VDC on the rails),  better heat sinks on the power follower, and slight adjustment of the 2 current source emitter resistors, this amp could work OK to <=50 Watts or so.

On the bench, I hooked it to my dummy load.  I applied a 1 KHz tone and drove it to the brink of distortion as described in here on my Dummy Load page. That's what you see in this FFT.




Above — PA board installed in the amp.


Above 2 photos — In chassis for testing.  I'm using some of the installed rear panel jacks as temporary paths for input into the PA. This allows PA testing and real-world tests of the preamp + tone shaping stages.  2 speaker output jacks wired in parallel provide output possibilities.

I'll go through some notes:

I build circuits from DC to ~ 1.5 GHz on FR4 board using Ugly Construction. Evidently, this frightens some of my readers.
 

Board Construction & Heat Sinking

I built the PA on 2-sided , 1 ounce copper clad board. Circuit paths, or islands were carved with a motorized tool while outdoors. I also isolated each  board mounting hole so the board does not get connected to chassis ground when bolted in. I seek star grounding for each circuit board.

The PA board gets lifted off the chassis by spacers that prevent shorting the power BJT mounting bolts to the chassis [ recall that they're connected to each collector terminal]. This also allows air flow under the PA board for cooling.


Above —Heat sinking the power followers. I bent ~ 20 gauge aluminum with a brake to fashion crude heat sinks. Further, under each heat sink lies a small piece of 24 gauge copper sheeting and under that lies more 24 gauge aluminum stock.

The copper sheeting + aluminum heat sink is secured to the main board with the main power transistor mounting bolt plus an additional 6-32 bolt below each transistor. The additional bolt presses the heat sink & sheets  firmly onto the main board  — and lowered BJT temperature 3-4 degrees C more than without the extra bolt during tests.

For my 21 W amp, this crude heat sinking worked OK for this, my first solid-state guitar amp.  In a test setup with a dummy load, I applied a 1 KHz signal to give 20 Watts output in the DSO & left it that way for 24 minutes ( a torture test ). The big NPN measured 47-49 degrees, while its PNP sister measured 54- 56 degrees C.  My heat sink resistors measured 82 degrees C . When playing guitar at a comfortably loud bedroom volume, the transistors run around 23-24 degrees C.

Transistors

For the small signal transistors, I call for the MPSA06 and MPSA56 BJTs, however, by this time, I'd run out of them and applied 2N4401/2N4403.  My experiments showed that the MPSA series transistors  are better matched from BJT to BJT and likely prove a better choice for the differential input pair at least.

While I chose the very husky MLJ21194 +  MLJ21193 as power followers, a 2SA1302  2SC3281 pair, or even the TIP 35C | 36C could work well.  All of my parts were brand name & purchased from reputable American dealers. I saw a couple of cheap bootleg transistors let their smoke out during my several PA builds under heavier current.

In previous versions of this amp,I employed T0-92 encased transistors for all but the finals and the BD139-140 BJTs in earlier versions of this amplifier. This proved a big mistake. With mishaps + during torture tests,  the VAS transistor quickly or progressively failed  and the VBE multiplier died instantly. This led to a current surge and caused the fuses on my AC transformer secondary side to blow.

While T0-92 transistors provided a lovely high Beta, they led to a lot of frustration. In audio amps, and in particular guitar amps, "overbuilding" seems important. Thus, I used a lot of medium power transistors — even for the current sources.

Current Sources

For my build, I ran out of BD140s, so I bolted two TIP42A's onto my circuit for the current sources transistors. I isolated their mounting points and also sought them for mechanical rigidity — a needed thing when you build using Ugly Construction. The current sources exhibited nearly 0 temperature drift once they ran for ~ 1 minute.

I closely measured them in a variety of situations and placed some of this data in Table 1 on the schematic.  The input pair get sourced with a lovely high impedance with a current that varies from 6.25 mA to 5.7 under test conditions. I sought around 10 mA for the VAS supply , and measured a range from 10.5 to 8.61 mA with the amp driven to a constant 3W, 5W , 10W and 20 W output. 

This shows what happens to my DC power supply under different amplifier power loads.  I placed in DC measures in Table 2 below:


Since I'm only going with  1 pair of finals, the VAS current needs to be high enough to fully drive the top half of the AC signal swing @ high output to a potentially low output impedance . For example, with two 8 Ω  speakers in parallel which doubles the current needed to drive a single 8 Ω speaker.  The emitter follower located before the VAS helps that cause immensely.

Ultimately, For lower current, distortion, heat sinking requirements and  DC ripple, I plan to stick to an 8 Ω load, however, i plan to experiment with different speakers and all of mine are currently  8 Ω speakers. If  I find 2 speakers I like better together than just a single 8 Ω unit, then I'll order these 2 speakers in 16 Ω for a final future amp.

Amplifier Topology

A basic 3 stage power amp that borrows heavily from the recommendations of Douglas Self. The classic differential transconductance pair serves as the input stage to subtract negative feedback from the input and drive an error voltage to the voltage-amplifier stage or VAS.

In professional power amps, input differential stage emitter + collector resistors get replaced with high Z current sources + mirrors.  Discrete transistor current mirrors improve the balance of the input pair across a wide frequency spectrum to cancel the 2nd harmonic. You'll see a variety of current mirrors used extensively within op-amps to boost linearity & reduce HF distortion.

Some designers employ boot-strapped resistors to supply the VAS current and this evidently can also work very well. I went with a current source.

The voltage-amplifier stage [ VAS ]  provides all the voltage gain in a solid state power amp. I applied Douglas Self's recommendation of driving the VAS with an emitter follower to reduce distortion and garner more current drive for the followers. 

The VAS circuit also contains 2 small COG or NP0 ceramic RF caps to stop HF oscillations. The current limiting transistor helps protects the VAS in event of a catastrophe.

When I first built the amp,  I did not connect the final power transistor collector nor base node wires. Every other part was wired in however. Thus I had a small functional amp to test and measure with a signal generator, DMM,  DSO or oscilloscope, plus a dummy load.


Above — Output of the amplifier with just the BD139-140 drivers in-situ with a +/-15 volt DC supply.  I first test any PA circuit with my +/- 15 VDC bench supply before going to higher DC voltages.

You won't get much power with the 100 Ω degenerative emitter resistors, but can fully test the amp without a lot of current flowing.  By tweaking the 1K pot on the VBE multiplier, I also set the bias across points a and b at 1.2 VDC with no input AC signal applied.  Thus the bias is at a good starting point for when you connect the power followers.

Finally, connect the power followers and adjust the bias while looking at the output in a scope or DSO plus FFT.  The quiescent current of the entire PA ran ~ 22.4 mA.  At full clean signal power it will consume around 714 mA if you connect an ammeter between the supply and 1 of the DC rails.

Of negative feedback, folklore and tribalism

This design applies mixed mode negative feedback: voltage and current negative feedback. Both provide series feedback to provide the same function --  to boost PA linearity.

Negative feedback also affects an amps gain, bandwidth and frequency response, plus its input and output impedance (Z). Negative current or voltage feedback affect linearity + gain similarly, but with respect to output impedance, current feedback increases output Z by the size of the feedback factor, while voltage feedback decreases output Z by the feedback factor. To calculate the feedback factor for each type, a different equation gets applied and I won't go into the math.

A classic HiFi solid state amp with just voltage feedback exhibits a very low output Z that is just fractions of an ohm.  Delivered power decreases as the speaker Z rises.

For decades, solid state guitar power amp designers have applied current plus voltage negative feedback. This leads some to suggest that the solid state amp is now behaving more like a transformer-coupled tube amp that offers a higher output Z  while exhibiting a lower dampening factor. 
Some engineers who design audio amplifiers hotly debate current versus voltage source drive. See this brief EDN article by Esa Meriläinen who argues for driving speakers with current rather than a voltage source. I've got papers by respected authors who state the opposite.  I'll stay out of this debate.

In my PA,  output current is sensed by measuring the voltage drop across a 0.22 Ω resistor in series with the output voice coil. This resistor senses a fraction of the output signal and feedbacks a small voltage to the input stage that is phase inverted and thus "negative feedback".  The voltage drop across the 0.22 Ω resistor goes up or down proportionally to the output current. This variable voltage is fed to the 510 Ω emitter resistor in the 2nd half of the input differential pair, As the fed back voltage increases, amplifier gain decreases. E.g. increased voice coil current increases the voltage drop across the current sense resistor and this bucks the amplifiers input signal.

The 10K resistor connecting the power follower collectors to 1/2 of the differential pair's base node provides the voltage feedback.

Output Impedance and Dampening Factor

Going back to your electronics training, for example -- studying linear power supplies.  In current feedback, the circuit attempts to keep the output current constant meaning that the output impedance has to be high.  For output voltage to be constant the output impedance has to be low. Regardless, whether constant voltage, or constant current, the power delivered is still a function of speaker impedance versus frequency.

Dampening factor is another term that may polarize some guitar amp lovers; especially by those who adore distortion. Dampening factor (aka dampening) = the ratio of loaded Z ( typically 16, 8, 4 Ω ) to the amplifier's output impedance. In guitar world, higher dampening is often considered a bad thing by the cognoscenti. A very low output Z solid state amp dampens the speaker more than a higher impedance transformer-coupled amplifier. This is true according to the arithmetic.

Raising the amplifiers output Z reduces dampening, however, the most significant factor affecting dampening is the series resistance of the speaker voice coil. I once significantly changed the dampening on a friend's solid state amplifier with no soldering — I put in a different speaker with a much lower voice coil DC resistance. He loved the result.

How I chose the 820 Ω feedback resistor.      Color you PA Tone

The other board bolted in the amp chassis thus far contains a split-supply voltage regulator for the op-amps used in the preamp and tone shaping circuits, plus this board also houses the first preamp & it's connected to a 1K gain pot.

I connected my guitar to the amp input and routed the output of the first preamp into the power amp input. It's interesting to listen to a guitar amp with no tone control circuit -- and just 1 volume control.

I connected an 8 Ω 12 inch Jensen speaker to the amp and played through it. Instead of the 820 ohm resistor, I place a 1K 2 Watt potentiometer in the feedback loop and tweaked the pot as I played.

I seemed to hear some changes in the bass & treble frequencies at different rotations of the pot. This aurally supports the claim that current feedback tends to boost lows and highs and thus provides a mid frequency scoop. Further, I seemed to prefer 1 particular setting on the pot.  I removed the pot and measured it with an ohm meter.

Continuing on, I tried 4 other 8 Ω speakers that I had on hand at the moment. I also tried different speakers in parallel.  In most cases, the setting on the pot that I seemed to prefer was different.  I also tried longer speaker wires  ( 16 foot, 8 foot versus 3 foot  ) -- this exhibited a small effect on 2 of the speakers but this in turn felt dependent on the pot setting.  Speaker cable resistance can potentially exhibit an effect on dampening in a very low output Z power amp.  Normally, I use a 3  foot ( 91 cm) speaker cable

The 1 pot setting that worked as a compromise setting was then determined: 811 Ω, I soldered in a 820 Ω 1 Watt resistor in for now. When I finally complete the amp and decide which speaker(s) to use, I'll repeat my test with a potentiometer.

The current feedback resistor allows for a potential change in the output impedance that can allow for some equalization of the speaker - power amp system in concert with the speakers frequency response + resonant frequency [ speaker resonance often lies somewhere between 75 - 120 Hz and varies from model to model ].  I wonder if adjusting the pot allows you to enhance the resonant frequency of the speaker?

This function paled in comparison to actually changing the speaker. Using speaker choice to dial in whatever desired guitar sound lurks in your head seems critical. Then you can try to tweak it with a current feedback resistor choice.




Section  B   Faux Amp


Above —  Schematic for a faux amplifier I built for plus and minus 15 volts all on 1 PC board to investigate PA stages, plus different preamplifer circuits.  Clean signal output = 9.5 Watts, as my bench split supply has a small transformer and normally gets used for developing small signal op-amp circuitry. This was my first split supply power amp and I learned a ton and gained some confidence.


Above — The finals were mounted directly on 2-side copper clad board. At under 10 W power, they did not get above 30C even during maximum power torture tests for 15-20 minute stints.


Above —  RCA inputs used for connecting the +/- 15 volt split DC supply to this board. This is OK for a low power experimenter board, or for op-amp circuitry, but not recommended for real power amplifiers.


 Above —  Entire board. In the forefront lies an RCA jack connected to the PA input.

Above —  FFT with maximum clean signal. Above this drive my power supply voltage sagged excessively and distortion increased exponentially.


Above — Time domain. Compared to above,  increasing the drive by 1 Vpp produces harsh clipping.



Above — I've connected a preamplifer module to the power amp. I'll cover several preamp schematics plus discussion in Part 3 of this series.


Above — Side view. You can see the power supply rails. Generally you want to keep them closer together, but this was my first go and gave me lots of building space.


Above — Output section. The emitters connect to their resistors through small holes. I wound the coil and held it together with tape. This OK for a temporary project. I connected the output jack to either my dummy load or speaker(s).

I enjoyed this little amp and it sounded great. Surprisingly, a 9.5W amp cranked up proves quite loud in an electronic lab.

Miscellany


Above — This power amp featured parallel power followers to reduce current-induced Beta drop when driving 4 Ω loads into 60 W. I learned from this board that the most important stage in your entire amp = the power supply.

To get desired a output power, your DC supply needs to provide the needed current without sagging too much under full load. Using transistors that are specified to have lower Beta drop, plus putting them in parallel does help the Beta drop issue. The BJTs = MLJ21194 & MLJ 21193. Heat sinking was provided by copper plating on the back side to the board.