QRP HomeBuilder - QRPHB -: Suzu 12 — All Discrete Component Guitar Amplifier for 2023

In January & February 2023, I built 4 smaller size versions of the GAA -12 Practice Guitar Amp that we call Suzu. My design goals included fresh & unique circuitry, all discrete components, all split supply amplifiers plus a clean & simple signal path. I'll show my 4th and best version. Serving as my upstairs guitar practice amp, I specifically designed it for the T-style or Fender Telecaster ™ guitar and a 10 inch speaker.

The overall tone flavor of this amp harkens the Gibson GA-50. I avoided a mid range tone control and deep middle frequency scooping. If you boost the bass and treble controls, you do create some mid scooping but it's low Q and quite subtle compared to old black panel Fender guitar amps of lore.

Note this was originally published as an update on Dec 4, 2022. I added much new content and then re-published it on Feb 20, 2023.

— C O N T E N T S —

1. Preamplifer 1

2. Preamplifier 2 + design spreadsheet to download

3. Power supply

4. Power Amp - PA -

5. Speaker

6. Miscellaneous Photos

Click here for my Guitar-Related Index

1. Preamplifer 1

Above — First preamplifier schematic. Preamp 1 and 2 connect directly to the main DC power supply with no voltage regulation to get the maximum possible rail to rail AC guitar signal. To subdue power supply ripple and to isolate the preamplifier from the PA supply, a ripple filter feeds the preamp stages DC. I employed further RC low pass filtration on each stage to enhance ripple & noise rejection in this single coil pickup purposed guitar amplifier.

The input 12K stopper resistor and capacitor form a low-pass filter to prevent AM radio detection. Eleven volt zener diodes clamp excessive signal amplitude from popping the input. This cold/dry Winter [coldest temperatures every recorded here in 2022] caused a lot of electrostatic buildup and discharge. Shocking. Sadly, empirically, I learned that static discharge can easily blow up front end circuitry & that all guitar amps need input protection.

A low-noise JFET with 1 megohm gate resistance provides a high input Z to the guitar pickup(s) and drives an emitter follower so the following stage tone circuit sees a low output impedance. The JFET voltage gain is set to about 3.3 with the 2K7 gain setting, source degeneration resistor. I normally set my maximal input stage voltage gain between 3 and 5. The JFET source current = 1.3 mA. The emitter follower collector current = 2.4 mA. When AC coupled to a 1K resistor load, the JFET + emitter follower can pass a 1 KHz signal with a magnitude of ~8.6 Vpp before it starts to clip. Lovely.

I prefer to bias each preamp block with a signal generator and DSO running and temporary resistor load AC connected. I strove to run the lowest possible current for each stage along the signal path. I chose the FET drain resistor value by temporarily substituting in a 10K potentiometer while adjusting it to get the highest clean signal swing at my bias point and then swapped in the nearest standard 1% metal film resistor. Almost every resistor is a 1% metal film and I happily grew my metal film resistor collection this Winter.

2. Preamplifer 2

Preamplifer 2 functions as the heart of my amplifier. I spent a month on this stage alone. Most of my discrete circuit designs resembled op-amps: For example, differential input, a voltage amp, plus a low Z output, however, but I found it wasn't necessary since I was not pursuing a ultra-linear preamp design. Some guitar amps built with op-amps and careful local + global feedback are said to sound sterile or too HiFi. Perhaps this rings true?

I did not get hung up on an ultra-linear signal path, rather tried my best while avoiding the emitter-coupled pairs found in op-amps plus many other analog ICs. It's fun to bias discrete transistors, calculate & measure things like input impedance, or the feedback values needed to get a particular gain and so forth. I miss this stuff. Old school electronics for analog dinosaurs like me.

Above — Second preamplifier schematic. The 22 µF input capacitor gets driven by emitter follower Q4 from Preamp 1. Preamp 2 voltage gain = 17.7

The Baxandall tone circuitry time constants reflect that T-style guitars generally sound bright. For the classic 100 Hz / 10 KHz Bass + Treble 3 dB turnover tone section, you might wish to run 100 nF and 15 nF for the capacitors respectively. The 50K bass potentiometer works well since I tend to 'pump the bass' & this prevents the impedance from getting too low at the extreme wiper setting seen when when boosting hard.
The treble and bass are fairly independent and the boost / cut is just over 10 dB. Clearly op-amp tone controls boost and cut with more amplitude, but this work OK and proved very simple. The emitter of Q6 provided a convenient node for negative feedback into the tone circuit.

The two 100 µF coupling capacitors help boost the low end for bright T-style guitars.

Above — A DSO trace of the Q6-Q7 feedback amp probed at the 22K load resistor. I measured 26 Vpp output clean signal voltage — at 26.1 Vpp, the lower half started to clip. This image shows a virtue of split DC supply for making amplifiers: better headroom. Not nearly as good as an op-amp, but pretty good headroom all the same.

Feedback Amp Notes

Above — This is my favourite AF feedback amp in single DC supply. In Suzu version 4, I employed this particular feedback amp for Preamp 2 with a split DC supply. Simplicity, wide bandwidth, stability — and medium to higher voltage gain make this a favourite amp for me. It goes well after a follower since the input impedance is relatively high and won't load down a source or emitter follower. I use a VCC from 3 to 28 volts DC in my single supply design work and whatever I can muster from my power supply in my split DC supplies. Of course, you have to watch the transistor collector to emitter breakdown voltage. I stock (hoard) high voltage BJTs knowing they are getting scarce and more expensive.

In late 2021, retired EE Ken Kuhn suggested that I learn to make every discrete amplifier in split DC supply. (Paraphrasing) Ken wrote ... "any circuit can be biased to operate on single or split supplies and split supplies do not have to be symmetrical (i.e. +5, -12). All that matters is the total supply voltage."

To that end, I learned to make all the common configurations such as common emitter, emitter/source followers and differential amplifiers with both BJTs and JFETs at various total supply voltages. I struggled with some feedback amps as the calculations seemed tricky and I had no example circuits to inform my own designs. I sent Ken the above 19 volt single DC supply feedback amp requesting help to convert it to split DC supply.

To my delight, Ken made a spreadsheet that did all the calculations and allowed the user to change supply voltages with the ability to adjust the gain to a desired value (combination of RE1 and RF).
Big thanks Ken! You may change parameters like VBE -- it might be best to measure VBE and input that value, however, if not, the spreadsheet gets you close and offers a great learning tool.

Spreadsheet taken down for re-location to another server.

Above — A screen capture from the spreadsheet manipulated to fit this image file. This shows an example of using the tool to run the calculations for my single DC supply amp shown earlier. Note that the feedback resistor idealized value = 510 Ω, not 560. I adjusted RF using standard resistor values so that the 2 values VC2 center and VC2 actual were as close together as possible -- in this case 0.16 volts.

Above —My actual single DC supply amp with RF = 560 Ω. The difference between VC2 center and VC2 actual is only 0.6 volts, so well within the +/- 2V specified by Ken's spreadsheet. Notice the unloaded voltage gain rose by .91 . In reality my measured voltage gain was 11.7 -- the spreadsheet gets you close. You can manipulate RF and RE1 within reason to target more or less gain. The spreadsheet has a split DC supply example design defaulted into it. Between that example and my single supply examples here, the spreadsheet should prove easy to use if you ever build this feedback amp.

Within Suzu, RB1 can be made from parallel and/or series values, although my collection of resistors over 100K seems quite limited. To provide the Baxandall tone circuit with a higher input Z, I increased RB2 to 10K and made RB1 from two parallel 120K 5% resistors placed in series with a 150K 1% metal film resistor. I measured 208K from this resistor block -- it worked perfectly.

You may also stick a temporary pot for RB1 [ I used a 250K potentiometer] to find the exact center for the Q1 bias on the test bench. With a 1 KHz signal generator and DSO probe on the 22K resistor, I drove the amp just into soft clipping and tweaked the pot to find the sweet spot for a perfect bias voltage. I removed the pot and measured just over 208K. Do not leave a regular potentiometer or trimmer pot in the actual circuit as it may add noise and potential for oscillations.

The feedback amp also provides a soft start and silent power off for the guitar amp.

Output Filter

Preamp 2 contains a crude RC low-pass filter on the output. Some of my 10 inch speakers sound shrill -- and this switchable low-pass filter tames that down. Further, the added stopper resistor(s) changes the dynamics of the power amp. I like the 2nd or middle position switch a lot, as it seems to make the guitar sound more “woody”.

I did make some active low-pass filter using FETs and BJTs and found they did not better,my tone. In the end, I preferred the RC filters since the added stopper resistors, plus the shunt caps provide me 2 additional practice tones to enjoy.

3. Power Supply

Above — A basic power supply. The different green and orange LED resistors try to equalize their relative brightness on the front panel. 1 LED for each DC rail.

Above — For the first time ever, I'm using a commercial grade bridge rectifier and will also apply this part in my high powered amps. You may heat sink the GBUE2560 for high power amplifiers.

Above — Rectifier and 2 gorgeous reservoir caps for the DC power supply.

Above — The power supply transformer just sitting in the chassis prior to wire shortening and mounting.The Hammond 166L25 gives 12 watts out, while the166L20 gives about 8 watts clean output power. Further, if you regulate the op-amp DC supply with the 166L20, this means running +/-12 volts split as the unregulated DC voltage sags downs to less than 14 VDC on each rail when driven hard.

I also tested a larger transformer with 29 VDC unregulated on each rail & for awhile, Suzu was running at 27 Watts output power. The Hammond 166L25 and 166L20 have identical dimensions. In the end, I opted with the 166L25, since its higher output DC voltage allows running the preamplifier rails at 17-18 volts DC unregulated to get maximum headroom.

Above — The power supply section mounted and tested.

Above — My downstairs Telecaster ™ with a Seymour Duncan Phat Cat single coil pickup in the neck slot and his Alnico 2 Pro™ in the bridge position. I added my newly designed, switchable treble bleed circuit in February 2023.

4. Power Amplifier — P A —

Above — PA schematic. I chose different transistors for the input emitter coupled pair and also for the finals compared to the original GAA -12 Practice Guitar Amp. Further, I sank a little more current in the emitter coupled pair and the VAS/driver stack. At this point, I only plan to run voltage feedback in the global feedback loop, although, I can easily add current feedback if desired.

I measured a β of 540 for BC546B matched pair. The whole BC54-X- series seem an incredible BJT collection offering low noise figure plus high β and, of course, is long obsolete. I've got 30 pieces of the über low NF BC549 in my parts bins for future 12 volt single-supply, discrete, low-noise AF amplifiers.

Above — Notice from Mouser. The day after I installed the power Darlington complimentary finals, I got this notice by email. Obsolescence might be the central story of my electronic hobbyist career ? Happily, I've got enough genuine power follower pairs -- both standard and Darlington style to last me for a long time.

Above —The finals mounted in their heat sinks. Once again a hack saw helped fashion DYI heat sinks.

Above — The finals and PA mounted in the "cake pan". The power transformer sat unmounted in this photo. Suzu with it smaller chassis and will go upstairs in our living room to serve as my main practice amp. The downstairs GAA -12 amp serves as my main transcription amplifier. I spend time downstairs transcribing horn solos. I rarely listen to guitarists other than if a guitar happened to be on the song of the horn player whom I'm transcribing.

Above — Suzu's PA offers low distortion. I'm very happy with this PA stage. The matched input pair have obliterated the 2nd harmonic and I believe what's left are crossover + some intermodulation products from interactions with my outboard circuit, test leads, clips and probes.

5. Speaker

I chose the Eminence Legend 1058 speaker for my upstairs practice amp.

Fortunately, many kind YouTube posters have uploaded head-to-head trials with various 10 inch guitar speakers for comparison. I tend to favour Alnico magnet 10 inch speakers, however, dislike their cost. My "non Alnico" preference seemed to the the Legend 1058 in several videos. So I bought one and found it well suited my purposes. — and the added bonus, it's not expensive.

Above — The large dust cap makes the speaker look bigger than 10 inches in diameter. This speaker is a gem. Ferrite magnet and weighs 2 Kg.

Above — Transfer function of the Legend 1058 from Eminence. It directly connects to what I hear with actual playing tests. In a cube shaped cabinet with my preamp circuit, the bass is OK while lower middle response sounds a little scooped. There is 1 "sharp" peak at ~2700 Hz, but the treble response starts to fall down a cliff at around 5 KHz. Perhaps a good fit for a Fender Telecaster ™ through a 10 inch speaker? I prefer scooped lower mids for rhythm, but stronger lower mids for lead playing. There is no 'ideal' speaker for me it seems.

Above — My wife designed & built a prototype cabinet from a plank of 12 inch wide, 3/4 inch thick pine. The final specs are 12 inches depth x 12 inches height x 14 inches width [ or 30.48 cm deep x 30.48 cm height X 35.56 cm width ]. I stuffed some fibreglass pink insulation in the cavity to dampen any reflecting waves. The back is partially open with a 2 inch gap across the top end. This keeps out cats (protects the speaker), keeps in the insulation and gives punchy bass tones with some room audio fill through he back of the speaker cabinet.

Above — I've got a Jensen Mod 10-35 in another identical cabinet at the moment. I like the strong mids for neck pickup solos better when compared to the 1058, however, it sound quite bright. It's best to listen to a speaker for many months before you write in in or off.