Another Look at the LM386

On a whim, I took another look at the LM386-4 one cold Sunday afternoon this November. My focus was to drive a loud speaker and not headphones. I won’t personally use the LM386 for a headphone amp as we enjoy so many better options. For example, an op-amp driving a pair of TO-92 followers, or perhaps placing 2 NE5532 op amps in parallel as the headphone PA stage.

Since the mid 1970’s the LM386 has enjoyed popularity amongst hobbyists for low to flea power AF power amplification. The NE612 mixer IC plus the LM386 have literally formed the basic building blocks of innumerable radio receivers amongst Hams and hobbyists for decades.

Although, imperfect like all other linear ICs, the LM386 design team delivered a simple, flexible, low power AF amp with reasonably low distortion.

This part is noisy though. The input noise density = ~ 50 nV/√(Hz)  — about  10X that of an NE5532 op amp. So. if you use this part in high gain mode [with a gain of 100 to 200] and drive it with a low-level audio signal, you’ll really hear the noise (hiss) in your speaker.

Others online have provided detailed analysis about each stage of the LM386, so I won’t bother. However, I will comment about why it might be noisy. Normally, in modern AF power amps. the differential input pair emitters get 50 - 100 ohms of degeneration to boost linearity at the expense of noise. Other than that, in the IC only current sources connect to the emitters (and usually active loads to the collectors -- i.e. no resistors), However, the LM386 input pair get multiple large value resistors connected to their emitters. This translates into lots of Johnson noise from thermal agitation within conductors, plus related high-level input current noise that all gets amplified by the NPN voltage amp and delivered to the output stage.

Above — My basic test setup superimposed on the internal schematic of the LM386. I RC low-pass filtered the 12.24 VDC and ran two 1 Watt resistors in parallel as my resistive load. The measured load resistance for my power calculations = 8.4 ohms. I AC coupled a 1 KHz, ultra low distortion signal generator with a gain control to the input and watched the output in a DSO containing a stalwart FFT with 12-bit sampling. 

Above — An early photo during my initial bread boarding for these experiments. The non-inverting input resistor was changed to 10K for my experiments.

Index of this blog post

1. Gain = 20 Mode
2. Bass Boost and More
3. Gain = 50 Mode
4. Gain = 200 Mode -- plus lifting and AC bypassing Pin 2

I'm avoiding math in 2022, as data shows that my blog readers don't care for it.

Above —For comparison and contrast, here is the FFT of a discrete guitar PA pushing 3.4 Watts in this photo. The 2nd harmonic lies ~ 64 dB down.

1. Gain = 20 Mode

Above — The data sheet suggested amplifier with Gain = 20, using minimal parts. 

Above — The FFT of the above schematic with the fundamental plus 4 harmonic tones showing. I'll show many traces with a "standardized" output voltage of 5 Vpp or 372 mW into my 8.4 Ω load. This allows comparisons of various circuits. Note the 2nd harmonic is only 46 dB down  [ -46 dBc ].

Above —FFT with only 1 change -- I bypassed pin 7 with a 10 µF capacitor. The 2nd harmonic decreased by ~ 9 dB. Always bypass Pin 7.

 

 

Above — Apart from the 2nd harmonic, this FFT shows very low distortion at low signal levels (20 mW)  The clever 1/2 positive DC supply biasing scheme within the LM386 isn't perfect and the input pair are not perfectly balanced due to both variable AC and DC factors and this effects 2nd harmonic suppression. We also see some crossover distortion in the output. Don't get me wrong — I feel amazed by this design. Like you, I kind of like the humble LM386; plus have used it a lot over time.

 

Above — This is the cleanest sine wave I can drive before clipping using my eyeball. This is 725 mW output power.

Above —  I kept advancing the gain knob until clipping appeared on the top half.

Above —  Switch to FFT mode, although you can still see the yellow coloured sine wave. At this point, the tones are almost level and 45 - 47 dB down. You may easily hear distortion at this level.

Above — I kept advancing the signal generator gain knob until the tones look really strong. You can see the sine wave is now both bottom and top clipped. These FFT screen shots and my notes serve as the basis of what follows. I learned a lot doing these experiments and hope you like them.

2. Bass Boost and More

 Above — The data sheet suggested schematic for Amplifier with Bass Boost.

Above — Voltage gain versus frequency graph showing the 6 dB peak at ~ 90 Hertz. In the LM386, both AC and DC feedback runs back from the output to the non-inverting input emitter. Pins 1 and 5 allow us nodes on either side of this 15K resistor so we may provide additional AC feedback in parallel with it. The bass boost example provides us insight into possible feedback strategies: a 10K resistor + 33 nF cap form a network to boost the bass and also low-pass filters the frequencies out to ~ 5 kilohertz.

If you listen to this amplifier with music through a speaker, it sounds muffled and somewhat lacks the important mid frequencies for both voice and music. The hiss is definitely attenuated though. Further, the voltage gain goes from 20 without feedback down to around 8 with the 10K + .033 µF network added. There are other potential side effects to with such heavy feedback which I'll show soon.

Above — FFT with fundamental + 8 tones of the bass boost circuit driven to output 5 Vpp. The feedback has suppressed the 2nd plus all other harmonics effectively. Feedback certainly holds promise in reducing harmonic distortion in the LM386.

Above — The bass boost circuit may produce weird distortion when driven hard in some circuits. The bottom half of the sine wave clips initially.

Above — The FFT of the above DSO tracing when pushed a little harder. This looks and sounds terrible.I do not recommend people use the bass boost circuit, or if you do, please check for instability.

Let's adjust the feedback network and perhaps find something that works better.

In a classic PA with a differential transconductance input pair, 1 BJT base serves as the input stage while the other base receives the negative voltage feedback. The transconductance pair subtracts that negative feedback from the input and passes the difference voltage onto the voltage amplifier stage that follows. This doesn't happen in the LM386 -- negative feedback goes to the non-inverting BJT emitter which is often also the input side of the input emitter coupled pair.

Negative feedback also affects an amps gain, bandwidth, frequency response, plus its input and output impedance (although the output impedance is just fractions of an ohm). When we add AC feedback between pins 1 and 5, our network is in parallel with the 15K resistor and may be affected by other amplifier parameters including the gain and input impedance.

From the data sheet, In low gain mode, we should strive to keep the amplifier's closed loop gain 10 or greater which happens with the 10K resistor in our R C network. It's quite easy to turn your LM386 into an oscillator with too much feedback. I've noticed that feedback networks that look OK in SPICE simulations may actually oscillate in real life bench work -- especially with higher gain and/or drive.

I took the bass boost circuit example, kept the 10K resistor, and tried different capacitor values. If you lower the 10K resistor, you'll have to watch for oscillations at input drive levels high enough to cause distortion. This also may also reduce the LM386 voltage gain considerably.

Above — Our base schematic to evaluate different values of C1 and view the resultant FFT and voltage gain.

Above — FFT at 5 Vpp where C1 = 0.01µF. Outstanding results! This turned out to be the best feedback capacitor of the few I tried. The LM386 voltage gain dropped to just under 11 with that particular capacitor value for C1.

Above — Driving it as little harder to give 623 mW output power. Still fairly clean compared to other tracings.

Above — FFT with the top just starting to clip. 3rd harmonic re-emerging. 735 mW output power.

Above — Pushed a little harder to 761 mW. Things are getting ugly. C1 still = 10 nF. Let's decrease the C1 value by a decade to 1 nF:

Above — FFT at 5 Vpp where C1 = 0.001µF. While not as impressive as when C1 = 10 nF, it's still quite good and the LM386 voltage gain is around 19.

Above — FFT at 7.14 Vpp or 759 mW output power, The second harmonic is somewhat better than the case where C1 = 0.01uF.

Above — FFT at 5 Vpp where C1 = 470 pF.  Another favourable reduction of harmonic distortion when the LM386 amp is running at reasonably high, unclipped power levels. I measured no loss in voltage gain with a 470 pF cap + 10K resistor.

I also tried a 220 pF cap - it worked somewhat, but the harmonic suppression started to fall off at this point. The overall best unclipped harmonic suppression occurred where C1 = 0.01 µF in my experiments, albeit with 45% voltage gain loss.
To decide on a C1 value, it's important to listen to it to with actual audio to ensure that any frequency peaks, or more importantly, the low pass effects caused by the network doesn't wreck the audio you listen to.

Above — Listening to monaural jazz from my CD player into the LM386 and then into my 8 inch lab speaker. Although C1 = 0.01 µF gives the best reduction in harmonic energy, it rolls off too much high frequency audio for my tastes. I usually start at 0.001 µF and work up in capacitance. To my tastes, a 0.0018 or 1.8 nF cap sounded best. We've entered subjective territory. Variables may include personal taste, your hearing + age, your AF signal source overall tone, speaker size -- and perhaps whether the speaker is mounted in a cabinet, etc.. You might consider choosing a feedback cap between 0.01 µF and 470 pF according to your needs and wants.

Above — Listening through my 6 inch lab speaker. My C1 preference = .0039 µF for this speaker.

3. Gain = 50 Mode

Above — Datasheet example: Amplifier with gain = 50

Above — FFT at 5 Vpp with a 1K2 and 10 µF cap between Pins 1 and 8. Although we see the 2nd harmonic at about 52 dB down, it's still OK for the LM386. Signal noise will appear louder compared to the "Bass Boost" feedback variants with the same output power.

Above — I pushed it hard to 7.39 Vpp or 813 mW output power. This is the nastiness you'll hear on loud signal peaks.

4. Gain = 200 Mode -- plus lifting and AC bypassing Pin 2

Last section. Here's the famous schematic used by millions to make simple DYI audio projects:

Connecting Pins 1 and 8 AC bypass a 1.35K emitter resistor in the input pair -- and unleash the hounds. We get a full menu of gain, noise, and potentially harsh sounding distortion.

Above —In this separate experiment to showcase the worst-case scenario, I've manipulated & then pushed this particular amp into raucous distortion. Note the strong 3rd and 5th harmonics relative to the 2 even harmonics. This is worst case fuzz box stuff. While this might sound bad with your ears, it's great fun to see it on a DSO.

Above — Back to the main experiments using the schematic shown above... The FFT at 5 Vpp or 372 mW output power. The 2nd harmonic lies at -46 dBc.The various tones do not go down much at lower input signal levels.

Above — Increasing the signal generator output to push the amp into clipping. The 3rd harmonic looks ready to break open.

Above — A slight increase from 7.01 to 7.09 Vpp. The 3rd harmonic is about 41 dB down.When in full gain mode, the LM386 tends to offers more odd harmonics

Above — Top and bottom sine wave clipping translates into wretched distortion.

My question -- will feedback similar to what we used in the Bass Boost variants lower the harmonic distortion?

Above — At our standard of 5 Vpp, the affects of feedback leap out at us. [ 10K plus 0.01 µF cap ] Feedback is our friend?  However, the gain dropped to around 100 and you will hear lots of high frequency roll off.

Above — Vpp = 5 with a feedback network consisting of a 10K + 0.001 µF cap. The response is lack luster compared to 10K + 0.01 µF capacitor. The drop in voltage gain was only 2%. The 2nd harmonic is maybe 1-2 dB better than without the feedback network?

Above — Vpp = 5 with a feedback network consisting of a 10K + 220 pF capacitor. Interesting FFT ! The 2nd harmonic is now -50 dBc where without the network it measured -46 dBc.. Some of the tones dropped around 4 or 5 dB too. No change in voltage gain by adding this network.

Above — There's an old trick left to try. Normally, most builders will ground Pin 2 like I did throughout this blog post. What if we AC couple Pin 2 to ground through capacitor C2?. This may help to better DC balance the input pair bases ( may reduce DC offset ) and perhaps even bypass some portion of the distortion to ground.

Does this work?

Above — The FFT tracing shown above is with C2 in place showing that C2 does decrease distortion in certain cases. I could not superimpose 2 FFTs, so I made a red line above each of the 4 harmonic tones.

The bottom of the red line is the exact peak of the each tone with Pin 2 shunted to ground. Above the red line is the measured improvement for that particular tone caused by C2. I installed a switch across the C2 capacitor to make comparisons. . C2 = 0.01 µF in this particular experiment.

In my experiments with a gain of >=150 and no feedback network, when the LM386 is pushed into harmonic distortion, C2 lowered the harmonic tones by 4 to 8 dB. I tried C2 values of 0.01 to 0.27 µF
and changing the value of C2 within that range seemed to make no significant difference. Replacing C2 with a resistor of any value did not work to lower distortion.

C2 seemed to have less of an effect when the LM386 gain was lower than 150. At Gain = 20 with no feedback, I observed a maximum 2-3 dB maximal improvement in any 1 tone. With feedback, the effect diminished a little further, however, results were inconsistent. C2 does not appear to lower the harmonic distortion when the audio signal is unclipped -- rather, it seems to reduce distortion due to clipping when it happens. 

I performed other experiments such as bringing the feedback to Pin 2 with Pin 2 connected to ground via a resistor or resistor + capacitor (like what you do with an op-amp or discrete AF amplifier). I also tried lowering the feedback 10K resistor value at various gain levels. Often enough, the result was that the LM386 would go into a writhing spasm when pushed into distortion. See below.

Above —Fancy feedback experiments often resulted in the above tracing. It seemed better to explore  simpler ways to lower distortion.

Conclusion

Wow, this was a lot of work, but proved fun. I encourage you to perform your own experiments with the LM386. While no panacea, and a little long in the tooth, the LM386 reflects a simpler, mostly analog time for many of us home builders.

I suggest you consider using the LM386 with lower gain and build up your audio signal voltage with a low noise preamp using an op-amp like the NE5532. 

Further, consider adding feedback [ 10K plus some value of C1 ] from Pins 5 to 1 and also AC coupling Pin 2 to ground. I did both of these tricks in my 2 photographed bench CD player listening tests shown in Section 2. 

とてもいい 

GAA-12 Practice Guitar Amp

 

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.

Above — Heat sinks fashioned for the TIP 142/147 power follower pair. I ran the amp in test mode with signal generator + dummy load at 10 Watts for 30 minutes and the PA temperature measured ~29 degrees C. Hulky 10 amp transistors on big heat sinks in a  low power guitar amp should last a long time.
 

3. Power Supply

Above — The split DC voltage power supply. I employ no switch as my amps AC plug into a certified, high-grade, commercial power bar that is turned off and then unplugged when the amp is not in use. Power supplies involve voltage + current that may cause injury, death or fire. Only work on power supplies and/or amplifiers if you are a certified to do so. You incur all liability arising from all electrical equipment problems, accidents, or mistakes. Safety. Safety. Safety.

 

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 — The genuine Nichicon brand capacitors that went on the power supply board to filter. We now have to worry about bootleg transistors, capacitors, power resistors, linear ICs and more. Such as pain! Caveat emptor.

 

Finally, the op-amp voltage regulators went on their own small PC board:

 

 

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.

Continuing on... a couple of the latest amp chassis photos I call the passive tone stack 'bass', the fat control:

 

 

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.

 

Above — The reverse view of the PA test jig seen from the opposite angle. It contains a pair of 0.22 Ω resistors, a Zobel network and an isolated speaker jack. The yellow wire passes through to the 0 volt centre rail. 

Above — The GAA -12 PA board under test with 1 or 2 temporary parts attached. Since the PA transistors are normally mounted in heat sinks in your amp chassis & connected with wire or PC board paths to the rest of the PA circuitry, this test jig mimics them well. You can instant tell if you made a mistake or parts are broken etc.. 

When I mounted the PA board in the amp chassis and the PNP transistor did not work, I knew it was not the PA board at fault. It turns out that the 0.22 Ω power resistor connected to the PNP follower inside the amp chassis was open circuit. I didn't have any more, so, then changed both to 0.1 Ω emitter resistors as I had several of these in stock. From now on, I'm sticking with Vishay brand wire wound resistors as bootleg power resistors have sadly made their way into our parts bins. I prefer 0.22 Ω emitter resistors to boost stability in the power follower pair.

 

 

Above 2 pictures — The transformer and power supply, input, master volume/ DC regulator & PA boards -- plus the heat sinks all mounted in the chassis. The guitar amp input is as far away from the power supply as possible.

Above — Reverse view of the the entire preamp module under development. This likens a blank canvas with 2 op amps plus all the potentiometers installed. It's now up to you to install the right combination of resistors + capacitors to make a nice guitar amp. It's really that simple in 1 aspect. This entire module goes into the amp chassis and the pots line up with the holes drilled in the metal chassis.

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

My videos look better on YouTube proper

 

The only video so far is this short 1 already posted on Oct 22, 2022

 

 

 

 

My YouTube Page :  QRPHB YouTube

My Guitar-related Index :  Click

 

Ciao!