Gibson GA-50 Inspired Guitar Preamplifier Tribulations

 Introduction

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 and its offspring such as this PA.

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 Perhaps, now, even op-amps are getting outdated as digital algorithms simulating some old amp of lore push the bleeding edge of design.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.

Modules

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.

Speakers

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.
 

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