Popcorn QRP Audio PA for Receivers and Projects

Greetings!  For 2-3 years, I’ve received emails from readers seeking a simple “popcorn” discrete transistor PA to substitute for the LM386 part in their DYI projects. Readers wanted 3-4 transistors maximum & no differential amplifiers with current sources — and hopefully low distortion up to 1 watt with a ~12 VDC single supply.  

That seemed a tall order, but I did it (more or less). I’ll define ‘popcorn’ to mean that at maximum clean signal power, all harmonics are down to -50 to -55 dBc. This amp behaves well until driven to about 1.3 Watts.  I made a video that lies in the last section.

Above — The final Popcorn QRP PA.  4 transistors. Voltage gain = 28. Quiescent current = 73.5 mA.
This is a power amp designed to cleanly drive a speaker even at loud volumes.  To reduce distortion + boost stability, I applied ample local + global feedback which lowered gain. I suggest readers consider building up their AF signal voltage with low noise, low distortion, feedback-containing voltage amplifiers -- and not rely on their PA stage to make all the voltage + power gain.  Getting most of your voltage gain in your PA adds too much noise into your AF chain.

Above — An FFT of the Popcorn AF PA driven to exactly 1 Watt output power. The load = a 7.9 Ω “resistor” consisting of 3 two watt resistors in parallel. The second harmonic lies ~53 dB down.

Above — LM386 driven to 808 mW. This is the only LM386 scope trace I had where the voltage gain = 40 plus I had applied a good negative feedback network. Therefore, this practice seems a reasonable head-to-head test against the most venerable LM386. The Popcorn PA makes less distortion at 1 Watt, than the LM386 does at 0.81 W.  At 1W power, the LM386 begins compressing into a square wave.

I promote bench experiments – and developed this amp on my bench. I began with a lower power version using 2N4401/2N4403 complimentary emitter followers to drive the speakers. Push- pull drive as opposed to a single-ended PA driver seems the best way to go for decent output power. You might substitute any number of small signal BJTS such as the 2N3904 for the 2N4401 (or the PNP equivalent) in this project.

Let’s start where I began. I’ll show the development of the Popcorn AF PA and give ideas to consider in your own experiments.

TABLE OF CONTENTS

[ SECTION  1 ]   LOW POWER DEVELOPMENT VERSION
[ SECTION 2  ]  OUTPUT STAGE BIAS
[ SECTION 3 ]   FULL POWER VERSION
[ SECTION 4 ]   VIDEO

----   [ SECTION  1 ]   LOW POWER DEVELOPMENT VERSION   ----

Above — The schematic of the initial & fledgling Popcorn PA using paired 2N4401/2N4403 as the complimentary emitter followers.  In 1956 while working for RCA, H.C. Lin developed the first transistor power amp that didn’t use an output transformer. By around 1968, output transformers in solid state AF power amps had all but disappeared in professional designs.

Audio transformers suffer from non-linearity and in the case of the tiny transformers employed in cheap transistor radios of lore — these gave distortion, poor bass response -- plus very low output unless run in push-pull fashion. I suggest there are < 2 coherent reasons to use AF output transformers for solid state designs in 2023.
 
Input Stage

Without a differential pair as the input stage, I chose a PNP for the Q1 input amp with global negative feedback coming from the output rail going back to the Q1 emitter. The Q1 emitter also gets local feedback -- AC degeneration through the 330 Ω resistor. Because of all the feedback on Q1, Q2 provides most of the voltage gain and gets around double the collector current.

In all PA versions, Q1 bias gets set by a potentiometer (20K here). The pot proves necessary since all of us use a slightly different DC power supply voltages. The potentiometer allows you to optimize the Q1 bias for the lowest possible distortion with whatever DC power supply you use. When satisfied, you may remove the pot, measure it, and replace it with 1 -- or 2 series or parallel resistors to try to get as close as possible to the measured pot value. Alternately, you hard wire in a 20 – 25K trimmer potentiometer.  
In the final Popcorn PA version, I show a fixed Q1 bias resistor and a procedure how to set this value

The Q2 “stack” includes Q2 & all the parts connected to the Q2 collector going straight up to the positive DC power supply rail. Q2 serves as the main voltage amplifier. I placed a 10 Ω emitter resistor as local negative feedback to stabilize the stack against HF during development. I have not found any HF instability in the Popcorn PA with or without that 10 Ω resistor.

With the 2K Q2 collector resistor, the stack draws ~ 2.5 mA. Let’s look at some DSO outputs:

Above — DSO time domain output. The first draft PA driven to 2.01 volts peak-peak. Lovely sine wave.  Power = 64 mW.

Above — The FFT of the PA driven to 2.01 Vpp or 64 mW into a 7.9 Ω load.

Above — Left PA driven to 4 Vpk-pk [ 253 mW ] and 5 Vpk-pk [ 396 mW ]. Only the fundamental 2nd,3rd and 4th harmonics shown.  The 3rd harmonic tone starts to rise as the amp is driven to 4 Vpp. You can see the limitations of a single pair of TO-92 transistors such as the 2N4401/2N4403.

We’ve already exceeded the harmonic distortion goal for a popcorn PA amplifier. That is --- all harmonics must be down 50-55 dB at the maximum clean power

Above — FFT with PA driven to 6 Vpk-pk or 570 mW.  The 3rd harmonic is only 27-28 dB down. These TO-92 transistors are getting hot and starting to stink. Some of this distortion might be Beta droop from the high collector current plus heat.

Regardless, this seems like unacceptable distortion. You could easily hit power level this high on a strong Morse code (CW) station.
At this point, the 2N4401/4403 emitter followers seem only good enough for headphone level listening.

What can we do to try boost their linearity?
 

Technique One — Bootstrapping

Above — Boot strapping Q2.

Q2’s 2K collector resistor gets split to make a tap for a 330 µF bootstrap capacitor that provides positive phase feedback from the output rail to the collector. This raises collector impedance and reduces the loading effects of the Q2 collector resistance on the input of our 2 complimentary emitter followers. The positive feedback lowers Q2 signal drop.

Above — The FFT of the PA driven to 2.0 Vpp or 63 mW into a 7.9 Ω load. If anything, the 3rd harmonic is about the same while the rest are a bit worse. Bootstrapping is not helping here.

Above — The FFT of the PA driven to 4 Vpk-pk or 253 mW . The third harmonic is about the same without bootstrapping, while the other tones look a bit worse.

Above — FFT of the PA driven to 6.03 Vpp or 753 mW.  In this case, the harmonic distortion has improved. For example the 3rd harmonic improved by about 7 dB.  But overall, the net distortion exceeds our harmonic distortion goal.

Theoretically, bootstrapping may help and often works as well as driving the Q2 stack with a current source.  However, it doesn’t seem to work in this simple amplifier with a 2N4401/2N4403 pair.

Above — A fun FFT of what happens when you submit the 2N4401/2N4403 pair to 1 Watt power. Lots of compression, square waves & those emitter followers are smoking hot + stinking up the room.

Technique Two — Current Source

Above — I biased a single PNP to function as a current source. I set the output current as close as possible to that of the Q2 stack with the 2K collector resistor (limited by standard value resistors). The current source provides high impedance drive to the emitter follower pair. I won’t show any tracings because the current source, like the positive feedback, didn’t reduce distortion --- and in for some tones, worsened it.   I went back using a collector resistor.

Technique Three — Reducing the 2K collector resistor to 1K Ω

With the 2K collector resistor, the stack current measured ~ 2.5 mA. I measured the Q2 stack current at 4.83 mA when reducing the 2K Ω resistor in half. The results seemed unimpressive.

Above — For reference, With the 2K collector resistor driven to 3 Vpp.  [142 mW power]

Above — With 1K Q2 collector resistor driven to 3 Vpp. The 2nd harmonic improved by ~ 5 dB and the 3rd by about 4 dB.   At higher power like 500-600 mW, , the distortion was still too high for my liking. Further, the increase in amplifier quiescent current for the net reduction in harmonic content wasn’t worth it.

I’ve gone as far as I can with the simple 2N4401/2N4403 emitter followers. I’ve got to add some current gain and get some proper power followers. 

Before, we go to Section 3, the high power version of the Popcorn QRP PA -- Section 2 quickly covers output stage biasing:

----   [ SECTION 2  ]  OUTPUT STAGE BIAS   ----

2 diodes produce a voltage drop of around 1.3 volts providing sufficient bias for the 2N4401/2N4403 output emitter followers. From reading & my own experiments, the output bias may affect PA output distortion. The most obvious way is by giving crossover distortion.

Above — DSO screen capture of the low-power Popcorn PA with only 1 bias diode across the emitter follower bases. You may easily see (and hear) crossover distortion.

Above — An FFT of the 1 diode output bias with only the amp driven to 36 mW output power. The distortion dominates with odd order harmonics.

Above — FFT after adding back the 2nd output bias diode. This reduced the amplifier distortion shown above. Crossover plus output follower switching distortion pose factors we must live with. How far the output pair are biased from Class B towards Class A may also affect amplifier distortion.

However, using 2 diodes, we don’t have much control over that. You may place a small value resistor in series with 1 diode instead of using 2 diodes -- or in series with 2 (or more) diodes to change the output bias. An alternate way is to remove the diodes and replace them with a transistor.

Above — Schematic with the 2 diode bias replaced with an NPN referred to as an amplified diode or Vbe multiplier bias generator. Normally, this BJT has a trimmer resistor as R1 in the schematic for tweaking the voltage divider bias. The trimmer gets adjusted while watching the output in a test circuit to find the sweet spot of bias -– the setting that offers the lowest distortion in the output. 

I normally temporarily make R1 a trimmer pot, set the bias and then remove and measure the trimmer pot. Then I replace that with a fixed resistor such as the 1K8 Ω shown.

Since this is the popcorn PA stage, we’ll stick to plain diode biasing of the output followers.

[ SECTION 3 ]    FULL POWER VERSION

Above — Device under test. The best part about bench building is getting to use your test equipment. Glory and fun on the bench. Since I usually make 22 – 50 watt PAs, my electrolytic capacitor collection are all rated 50 volt to 100 volts. They look quite large in the Popcorn PA.

Above — Popcorn PA with DC voltages. Q1 shows fixed bias. I’ll give the bias procedure soon. The 10 Ω Q2 emitter resistor got dropped since this adds 1-2 dB of lower tone harmonic distortion under heavier drive.

I didn’t bother with the standard Zobel network in parallel with the speaker as seen in most AF PAs. This series cap + 10 Ω resistor across the speaker serves to lower the Q of the resonant peak of the speaker’s peak impedance at somewhere between 80 and 130 Hz.  While important for crossover design + function, I’ve left it out. You may need it with some speakers perhaps.

Power Followers
 

 

Above — I swapped out the TO-92 finals for some big boots.  In many lower power amps, to get current gain you’ll keep the TO-92 followers and then drive another set of power followers such as the BD139/BD140 pair. This works well and is recommended, however; we’re going full on popcorn on this project.
Thus, we’re keeping the emitter follower driving an emitter follower theme, but combining both in a packaged Darlington pair. This keep the parts count down -- plus provide the high Beta and current we seek to drive our speaker with room filling, low distortion loudness.

The TIP122/127 pair are only 1 example of packaged Darlington current amplifiers. I’ve got 3-4 other in my parts bins such as the BD94C/93C or TIP142/147 pairs – but again, usually I build higher power amps.

I bought the TIP122/127 pair for $2.30 Canadian dollars & they look husky and tough. You don’t even need to heat sink them for 12 VDC power.  If you need to heat sink them, then it's easy to do. Some readers emailed me to say they had smoked countless 2N3904/2N3906 pairs in their PA building adventures. Some soldered several in parallel to make a "power follower", etc..  

While purists may dislike a packaged Darlington pair – they seem perfect for popcorn PA stages and practice guitar amps alike. We have to add 2 more diodes to properly bias both Darlington transistors.

I added the Q2 boot strap back in. For this version, it significantly helped boost linearity from low to high power.

I kept the 1 Ω emitter resistors of the low power version. In pro audio, these are usually 0.1 or 0.22 Ω but of course, those amps are making some serious power.
In the past, I placed two or three quarter watt 1 Ω resistors of 1% tolerance in parallel to get the maximum possible output power. I left the popcorn emitter resistors at 1 Ω to ensure this project is stable for anyone choosing to experiment with it.

Play with every resistor value on the test bench. You’ll probably make a better PA than I did.

Let’s go through some FFT’s of the Popcorn PA at various drive levels:

Above — FFT at 3 Vpp. This proved the lowest 2nd harmonic tone measured @ -56 dBc. You could further experiment with the output bias, add a current source, or perhaps make other tweaks, if chasing a lower 2nd harmonic tone is your goal.

Above — FFT with the Popcorn PA driven to 6 Vpp or 570 mW.  Looks about the same as with 3 Vpp.

Above — FFT at 7.5 Vpp. Again it look similar to the Vpp = 3 or 6 FFTs.

Above — Cranking up the drive! FFT while driven to output 8.39 or 1.11 Watts. Still meets our popcorn goal of all tones down 50-55 dB at maximum clean power.

Above — FFT while driving the PA to 9.18 Vpp.  The harmonic tones are starting to rise!

Above — FFT while driven to 1.34 Watts. Things are falling apart.  Ok, let’s finish up.

Above — Set up schematic.  If your power supply is close to 12 VDC, then consider just building the fixed Q1 bias version shown earlier. However, bigger is better in PA stages. If you’ve got 13.8 or 14 VDC, then your maximum clean output power will go up. You may choose to optimize Q1 bias for a different DC supply.

In big power PA’s the DC rails are often unregulated. Fortunately, most of our ~ 12 VDC single power supplies are regulated which makes setting up the Popcorn AF PA a snap.

Terminate the output with a 10 ohm or lowish value resistor – or your bench 8 Ω load. Do not connect anything to the input.  Preset to maximum resistance, connect a 10 - 25K pot from the DC power supply rail to the Q1 base. Clip your positive voltmeter probe to the output rail and tweak the pot until your measured DC = your DC supply voltage divided by 1.82. Remove the pot and measure. Substitute the nearest standard value or place 2 in series or parallel to get close to this voltage.

If the output rail voltage lies between VCC/1.82 & VCC/2 you’ll be fine. Of course, you may experiment to find the optimal Q1 bias for your particular build -- that serves as the best way to optimize linearity.

[  SECTION 4 ]   VIDEO

I made a short video so you can hear the Popcorn PA in action. I connected it to a CD player plus my 8 inch guitar speaker and cranked it loudly to show its linearity under heavy audio drive.

I sampled at 44.1 KHz into mono using a Lewitt LCT 440 large-diaphragm condenser mic --- the same mic I use for my voice overs. I like the LCT 440 since it offers a flat bandwidth + very low added noise at a reasonably low cost.

Above — It seems better to watch this video on YouTube directly.

Addendum:

To clarify, I think the LM386 is an awfully good part. Imagine if your design team made a linear IC that went into hundreds of thousands of projects or products?  I'm a fan of the LM386 and the designers left us IC pins to add negative feedback with.

I cover this in the following blog post:

Link to my LM386 Experiments from November 2022