1. Preamplifer 1

 
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.

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.

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 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 — 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.

6. Miscellaneous Photos

Best regards!   Click here for my Guitar-Related Index

Greetings!  This Fall, I built the first of 2 planned practice amps. Inspired by simple 1950’s tube guitar amps I too kept it simple. In those Golden-era amplifiers, you plug the guitar in 1 jack, the speaker in the other and hit the switch. Modern solid state guitar amplifiers with effect loops, frequency compensating gain control stages and features galore may just complicate things in the guitar - amp - player interface. While perhaps cool and fancy,  these added stages may carry high-value resistors that boost op-amp input current noise and also increase resistor-related Johnson noise too.

My goal = make a low noise jazz / clean guitar amp as opposed to a low distortion, high-fidelity practice amplifier. I remember having to turn the volume pot on my Stratocaster to 0 between songs in my Marshall 50 - 100 Watt amp days of lore. The amp sounded great, but was super noisy unless the rest of the band was playing loudly to drown the amp noise out. At my age, a quiet amp seems desirable.

Note, I completely redesigned the preamp on November 15th after first posting this amplifier on November 6, 2022. Two things changed to trigger that : [ 1 ] I moved to 10 inch speakers [ 2 ], I moved to playing Fender Telecaster guitars 95% of the time instead of an arch top. With my back and wrist pain, the Telecaster proves much easier to play --- and also it's Leo Fender's gift to humanity. Such a joy to play. Thus, I re-designed my practice amp around playing a Telecaster through a 10 inch speaker. The result is a basic preamp with few AC coupling capacitors in the signal path.

 

Project Index

1. Preamplifier and Tone stages
2. Power Amp

3. Power Supply

4. Miscellaneous Bench Notes

5. Video Links  (only 1, but more coming later)

 1. Preamplifier and Tone Stages

Above — Input stage also showing ground loop reduction techniques to eliminate 60 cycle hum.

In tube amps, we employ our quietest 12AX7 or alternate preamp tube in V1 -- or the first preamp position, since all arising noise gets boosted down the signal chain. Same for solid state design. We seek to input the guitar signal, filter off radio frequency interference, plus control & boost signal amplitude while adding minimal noise and hum.

I prefer a 12 K Ω input resistor for Telecaster guitars and I didn't have any in metal film, so placed two 22 K Ω resistors in parallel got get the 11K Ω shown. For picofarad level caps, I use MLCC types with C0G temp compensation in all of my projects from AF to microwave. Both the positive and negative op-amp DC voltage pins get a 100 nF capacitor shunt to ground as close to the op-amp package as possible. It's OK if the temperature compensation of those particular 100 nF MLCC caps is X7R from my experiments.

An active gain control keeps the noise down. Like in tube amps, many solid state guitar amp input systems maximally boost the signal in the input stage(s) and then immediately attenuate it using a volume pot. This functionally works OK, but when a stage is operating at maximal gain, it’s also making maximum noise and today we may choose to apply noise - reducing active gain circuits with our op-amp & transistor design work.

I chose a warm, jazz guitar amp voicing inspired by the lovely Gibson amps of the 1950’s.

 

The above schematic also shows 1 ground loop prevention strategy to consider. Each stage including the power supply and PA are electrically isolated from the chassis by carving away copper around the mounting bolts + nuts. A single, insulated ground wire from each isolated board goes to the master star ground node located on the power supply board. Classic star grounding.

 

At the guitar input jack, the chassis becomes connected to the input jack bolt ground lug when you tighten the bolt. An insulated wire from the input jack ground lug runs to the EARTH ground on the AC receptacle. The non-grounded input jack lug coax centre runs to the op-amp input, however, the braid of the coax at this end goes to the star ground system as shown. The chassis gets connected to the star ground system only through the coaxial braid at the guitar input end of the coax. The result is no hum. I use RG-174, but any coax or shielded wire may work OK. No other coaxial cable are used in this guitar amplifier.

 

 
Above  — The entire preamp went onto this board. This photo shows an earlier iteration. I place some local DC filter capacitors on each board in my projects. On this board, 100 µF and 100 nF were placed. The blue and white wires move DC to the op amps positive and negative terminals. A guitar signal flows down copper wires along with its DC supply.

I employed genuine Texas Instruments brand NE5532s with a typical input noise density of  5nV/√Hz for the 2 op-amps that make the preamp. I enjoy this lovely, quiet part.

Above — The tone stack, buffer, plus final preamp stage with master volume control. This board uses a hybrid approach to tone control — a passive 1960's tone stack for bass, middle and treble -- plus active bass with a Baxandall circuit. A regular Fender/Marshall style passive tone stack cuts too much bass for my needs. The active bass control turnover frequency is 80 Hertz and offers ~ 15 dB cut or boost.
I kept the impedance higher to allow hard boosting with no distortion or noise. This amp with a 10 inch speaker gives more bottom end than many solid state guitar amps with a 12 inch speaker.

 

The scaled to nearest standard value capacitor, classic Fender tone stack RC network use relatively low value potentiometers plus higher capacitance to reduce noise. With the active bass, this tone circuitry offers a wide variation in tone control. Fender / Marshall et al. tone stacks work best driving a high impedance, thus it drives a nJFET follower with 1.7 mA source current. This, in turn, drives the Baxandall circuit with a preferable low impedance.The FET drain is RC low-pass filtered and connected to the regulated, positive op-amp supply rail.

 

The master volume active gain stage uses the topology from first preamp stage — the additional resistor ( 1K here ) causes the 10K pot to change gain in a more linear fashion. As you age, your near vision worsens --- and also when playing, room light is often poor, so you might just adjust volume knobs “by ear”. This amp sounds very loud for 12.4 Watts and when cranked up, vibrates the walls in my den at low frequencies with a 10 inch speaker.

 

Above —The complete preamp board with some test wires and a temporary 1/4 inch input jack for bench testing.

2. Power Amp

Above — The PA schematic. 

 

Since this is a low DC voltage amp with plus/minus ~ 20.85VDC (unloaded) on the rails, common, low-voltage transistors such as the 2N4401 emitter coupled pair shown will work in the transconductance amp. All resistors = 1% metal film types as possible. Most are rated at ¼ watt. The emitter couple pair get degenerated with 49.9 Ω resistors to boost linearity. Some designers leave them off, however, PA distortion will increase dramatically. I think 49.9 Ω is a reasonable value for guitar PA transconductance amplifiers.

The pair get sunk by a current source biased for 1.48 mA. Even a simple current source design like I used greatly surpasses old-school long tail resistor biasing since the high collector resistance helps boost differential balance to reduce noise and distortion. For my current sources, I opted to use cheap BD139 transistors instead of small signal TO-92 types.

This PA lacks any protection circuitry for when something goes wrong. Thus, I overbuild to keep it running when something does go wrong. Guitar amps may suffer lots of punishment including when you are building and testing them. I feel that the current sensing and limiting protection circuits found in many commercial PA circuits move away from the spirit of the 1950’s style amplifiers where simplicity proved a key feature.  After all, if my PA fries a transistor or 2, I can fix it.

The voltage amp or VAS = a  genuine NXP brand BD-140 PNP job. I tried 5 PNP BJT’s in this slot: the venerable high voltage classic KSA-1381, a BF-423, a BD-238, the BD-140 -- and a suspect bootleg MJE-350. The MJE-350 gave poor gain and went in the garbage. The KSA-1381 offered the most gain but seemed a bit overkill -- and the others provided similar gain and PA clean signal power with bench testing.
In the end, the BD-140 seemed the logical choice for a practice amp.  Since I wanted this PA to offer good gain, I only degenerated the VAS emitter by 10 ohms which may lead to instability in some designs. You’ll commonly see resistor values of 33-47 ohms used in some commercial designs. Increasing emitter degeneration boosts stability plus noise while the lowering the PA gain. The measured PA voltage gain = 56.

The most sensitive part of the entire PA is the collector of the VAS transistor. I found a strange phenomenon. When I put the 'standard' 68 to 120 pF cap between its collector and base, HF oscillations occurred and I saw distortion of the PA output when looking in my PA in a DSO with low levels of 1 KHz signal generator input. I actually remembered to save these image files as an FFT and a sine wave:

 

Above — Distortion caused by the VAS feedback capacitor that went away when I cranked the input signal up above 10 Vpp. When I removed the cap, no distortion appeared at low levels, but re-emerged at high levels of input drive. I left out the 120 pF feedback cap and instead installed a 10 Ω resistor plus 56 pF shunt capacitors on each arm driving the Darlington complementary power followers.  This eradicated all instability at all input signal levels.

Sadly, when I built the master volume gain control circuit and connected that up to the PA and then my signal generator to its input, the distortion problem re-emerged! Thus, I added back the 120 pF feedback capacitor and the PA stabilized at all signal levels. Likely, increasing the VAS emitter degeneration would have helped, but I can live with 3 small capacitors stabilizing my PA. I find it best to add stabilizing capacitors after you build and look at your PA with a dummy load, signal generator and DSO (‘scope). Then decide what capacitors you need add to remove HF oscillations.

Continuing on ... a similar current source biased for 1.77 mA sinks the VAS  & output driver stack. I stopped using diodes for biasing the output followers in my PA stages – rather, I prefer to run a single BD-139 with fixed bias for simplicity. The 1K5 resistor going from collector to base gets soldered in. Then, I temporarily solder in a 5K pot between the base and emitter nodes and tweak the pot until the crossover distortion disappears. The pot is then removed and measured with an ohmmeter.
A nearest standard value resistor ( in this case 2K7) gets substituted for the pot and then a final check is done with a DSO with or without an FFT as you can see easily crossover distortion in a sine wave on your 'scope. You might even further check this by ear into a speaker while playing single notes on the thicker guitar strings. The voltage across the power follower biasing NPN transistor is just over 2.1 VDC.

For guitar amp PAs, I now prefer using complimentary Darlington style transistors like the TIP142/147 pair. It simplifies design and works well. Other transistors I may evaluate in the future include the BDX33C/BDX34X, BDW93C/BDW94C and the TIP 127/TIP 122.

Above — The maximum clean signal power into a dummy load with all harmonics < 60 dBC. Very happy.

3. Power Supply

LED apparent brightness is adjusted by the current limiting resistor to each. 1 LED monitors each rail in my designs. Orange = positive is my personal standard. If your PA is self-oscillating, you might even see an LED flicker.

Above — Twins! I purchased these 2 light, low power transformers for my 2 practice amps. The 25 VAC RMS centre-tapped  transformer [1L6625] went into the GAA-12 amp. The 20 VAC RMS transformer will go in an even smaller practice amp for our living room. It will hide it on a bookshelf and drive a 10 inch speaker.

 

 

Above — Parts for the op-amp voltage regulators. Again, I overbuild. These hulky, slow transistors will last longer than I will -- and provide extra stiff voltage regulation.

 

 

4. Miscellaneous Bench Notes

 

Above — A bench test jig that contains a TIP 142 and TIP 147 pair that I use for PA board development.