CPA-1, the Crap/Cheap Parts Amp (A Fun Challenge)

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Here's a little project idea that could still use some input from you guys.

Recently I happened to be thinking about whether you couldn't even make a lowly LM358 work in a headphone amplifier when it struck me - why not make a headphone amp using lots of parts that audiophiles would be just about 100% certain to scoff at? LM358s, Darlingtons, 3 MHz fT power transistors, ceramic disc caps, random 12VDC SMPS wall-warts, finest generic Chinesium electrolytic caps - hell yeah! :devilr: (Well, maybe not the generic Chinesium electrolytics after all. I like stuff that lasts a while.) The challenge, of course, is making all of this crap deliver actually good perfomance all-round.

I have a rough idea of what the circuit is going to look like, but first off - who is going to qualify? Are TL07x and 4558s still getting in or are those way too high-tech? Or are we better off doing this in two steps, first the easy way with parts that actually qualify as "audio" (CPA-1), later with the jumping through hoops that the more hardcore options necessitate (CPA-2)? Purely price wise, RC/NJM4558s are actually cheaper than LM358s, and LF353s aren't any more "spendy" either.

Topology wise, I was thinking roughly along these lines:
Balanced input - +6/0/-6/-12 dB or something, maybe buffered (I was thinking additional RCAs Bruno Putzeys style, but having to run another wire for a ground connection is also annoying, making an adapter cable seems more elegant)
10-50k pot
Fixed gain noninverting power stage w/ discrete front end, bootstrapped cascode, yadda yadda. Would prefer a single-stage buffer tbh, with either BD139/140(-16) or TIP31/32C or similar, but won't rule out a diamond buffer either. Output inductor / Zobel as needed.
Might consider an additional impedance-balanced output, not so much for headphone use but rather preamp use.
Some more features perhaps, like an input selector? Internal connector for a DAC or something?
Power supply wise, I was thinking two generic 12 V 1-1.5 A switch-mode wall warts, stacked. (It's not like I have that many of them floating around myself, but I encounter them quite often, so they have to be very common. Manufacturers seem to be split between 5.5/2.5 and 5.5/2.1 mm barrel plugs though.) Cleanup will no doubt be necessary. If that turns out to be a stupid idea (common-mode noise and all), using an old-fashioned split power supply plus 78/79M regs (maybe 05s propped up on zeners) would still be an option.

Any thoughts so far?
 
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Make it out of nothing but MPSA42s and MPSA92s. If you parallel enough of them, with balancing ("ballasting") resistors, you can get as many amperes as you wish. Since they're rated for 300 volts, you can run the amp off a 48VDC wall wart with no fear of overvoltage on the transistors. And they're cheap. Extremely cheap. See images below. Still in active production today, BTW.

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PRR

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I built a, IMHO, very fine monitoring headphone amp out of only 1990s Radio Shack parts. TL072 and PN3055/2955, un-fancy caps (I did use non-RS input caps from mil-surplus). You could still buy AC wallwarts so I made the +/-DC in the box. Zener to the chips, raw DC on the power devices. NFB gain control because the *main* criteria was a very wide range from hiss to clipping (so's I knew any hash was in my source, not the HP box).

FWIW, nobody ever complained about the LM386 HP amp I built for a synth studio (yeah, not real music).
 
Balanced input - +6/0/-6/-12 dB or something, maybe buffered

Anyway buffered, because by using 4-channel pot you will kill any CMRR achieved by amp design.

Fixed gain noninverting power stage w/ discrete front end

No, use cheap as dirt LM318 inverting with EF2 discrete OPS loaded to ~150-200 mA CCS. Add some resistors to negative supply as on fig. 37 from datasheet, or even CCS.

bootstrapped cascode

Yes, something like IRFZ24 could be used, but this will demand for additional supply headroom.

10-50k pot

10k linear pot with 2k inverting input resistance will provide you with nice log function, a way better than best potz.

random 12VDC SMPS wall-warts


No, use cheap as dirt sectioned El type ~10 Wt trafo, this will provide you with ~20-50 pF mains coupling. No regulation needed because of good PSRR of opamp, put simple CRC with ~7-10 Ohms R, 5 Wt rating.
 
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