Southwest Technical Products Corp. (SWTPC) Headphone Amp 202-C

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I have acquired a SWTPC 202-C headphone amp and need a schematic or, better yet, the assembly manual for it. It needs a recap at a minimum, and likely other work.

I've searched the Interwebs, including SWTPC.com, but didn't find any documentation.

So any documentation or leads would be greatly appreciated.
 
Confirmed SWTPC builder and user, and I have never heard of this item Jim
 
Not one of the most popular SWTPC audio items. Which is why I've been having trouble finding a manual for it.

You'll find it in the catalogs, though.
 
While I have received a few PMs from SWTPC fans about never even seeing one of these, I'm still looking.

I've puzzled out some of the circuit, which seems to be similar to a Class A Plastic Tiger, a manual would be very helpful.
 
This could have been a popular electronics project, many SWTPC products were basically kits produced for magazine articles. Have you checked the archive at American radio history?
 
I did a bit of searching, and came to the conclusion that you own an orphan product, which is very interesting, because it operates in class A.

Since it's only a headphone amp, how about some reverse engineering? I bet if I push the button on a stop watch, you could have a schematic and bill of materials drawn up in pencil in about three hours. My guess is it's probably a scaled down version of the JLH 15W class A amp of the late sixties, except with TO-220 output devices. It probably has no more than five transistors per channel.

The catalog says it has two PC boards inside.. if there's one for each channel, all the easier to reverse engineer. If you need some help on this front, just take a whole bunch of photos and post them, I'd be happy to give you a hand.
 
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As I noted above, I believe this to be a Plastic Tiger slightly re-biased for Class A.

Yes, I can trace out the circuit, but I wanted the actual manual.
 
As I noted above, I believe this to be a Plastic Tiger slightly re-biased for Class A.

Yes, I can trace out the circuit, but I wanted the actual manual.

Understood, but after three years I figured you'd be interested to hear it.. mind you if it's a known unstable design which has been modified, and then DC coupled to headphones, I'm not sure how enthusiastic I'd be to plug in my expensive Sennheisers. Is it DC coupled like the plastic tiger?
 
That's exactly why I haven't done anything with it.

DC protection measures would work, of course, just as it does for speakers.
 
That's exactly why I haven't done anything with it.

DC protection measures would work, of course, just as it does for speakers.

Right, but the headphones could still be damaged by high frequency oscillation, which is what the SWTPC amplifiers were famous for. I'm starting to see why you've had it sitting for years. If it is as described, it looks like it is most likely a regular complementary symmetry amplifier which has had the bias messed with. I wonder if the differential pair is even matched well. Maybe this one should stay in the 1970s? I got a kick out of the price list of ICs in the back of the SWTPC catalog... no wonder tubes were still viable!
 
Oh, I'm very familiar with the SWTPC issues and the complementary-differential issues. I've passed on the Tigers for this reason many times.

The headphone amplifier is sitting here in case I decide to fix the oscillation and DC output issues. I must build some DC protection for my speakers, so building a few more boards is easy enough as is adding Zobels to prevent HF oscillation. I just need to simulate it to make sure some other issue doesn't exist.

Or, better yet, use the box and drop in a more stable Class A topology.
 
Oh, I'm very familiar with the SWTPC issues and the complementary-differential issues. I've passed on the Tigers for this reason many times.

The headphone amplifier is sitting here in case I decide to fix the oscillation and DC output issues. I must build some DC protection for my speakers, so building a few more boards is easy enough as is adding Zobels to prevent HF oscillation. I just need to simulate it to make sure some other issue doesn't exist.

Or, better yet, use the box and drop in a more stable Class A topology.

I think that the benefit of 45 years of progress in semiconductors and also in circuit design, would certainly be beneficial. The other obvious option of course, is to change the capacitors, and use it as-is with some cheap headphones.
 
Better semiconductors generally make oscillation worse, not better, because of the increased bandwidth. This is why the "improving" opamp equipment with new devices usually results in oscillation as the slew rate is 10x the original. A new transistor is good to 50 MHz, while the old ones struggled to hit 1 MHz frantically flapping their wings. Old and slow is the order of the day.

I suspect the big issue with the SWTPC stability is Muntzing plus a lack of simulation plus a lack of zeitgeist to diagnose the issues. A Zobel on the output devices, or perhaps some resistors to lower the Q for > 20 kHz, should stop the oscillation, and the feedback loop may have poor stability which can be improved. I haven't simulated it, but I was going to use Pete's spice model for the Tiger, suitably updated for the Plastic Tiger's characteristics and the Class A bias.

I've seen similar Class A headphone amplifiers which are rock stable. So the design needs some study.

It's a back-burner project, but I'm periodically reminded to bump the thread in case a new member says, hey, I have that manual!

Your caution, however, while known to me is nonetheless appreciated as if I didn't know about either the oscillation issue or B+ on the rails could/would result in a blown pair of cans. Always worth pointing that out. Many headphone amplifiers have marginal designs. I have the Ramsey portable headphone amplifier which I must rebuild to remove oscillation, and improve linearity and transient response. It's destined for my shoulder bag to be used with an inexpensive pair of cans; if it gets stolen or fails I'll not be out too much.
 
Better semiconductors generally make oscillation worse, not better, because of the increased bandwidth. This is why the "improving" opamp equipment with new devices usually results in oscillation as the slew rate is 10x the original. A new transistor is good to 50 MHz, while the old ones struggled to hit 1 MHz frantically flapping their wings. Old and slow is the order of the day.

I suspect the big issue with the SWTPC stability is Muntzing plus a lack of simulation plus a lack of zeitgeist to diagnose the issues. A Zobel on the output devices, or perhaps some resistors to lower the Q for > 20 kHz, should stop the oscillation, and the feedback loop may have poor stability which can be improved. I haven't simulated it, but I was going to use Pete's spice model for the Tiger, suitably updated for the Plastic Tiger's characteristics and the Class A bias.

I've seen similar Class A headphone amplifiers which are rock stable. So the design needs some study.

It's a back-burner project, but I'm periodically reminded to bump the thread in case a new member says, hey, I have that manual!

Your caution, however, while known to me is nonetheless appreciated as if I didn't know about either the oscillation issue or B+ on the rails could/would result in a blown pair of cans. Always worth pointing that out. Many headphone amplifiers have marginal designs. I have the Ramsey portable headphone amplifier which I must rebuild to remove oscillation, and improve linearity and transient response. It's destined for my shoulder bag to be used with an inexpensive pair of cans; if it gets stolen or fails I'll not be out too much.

Ah, I understand. I end up searching high and low for manuals too, until eventually I can turn them up. It's always the hardest with cottage industry stuff like this amplifier, which was never on the bench of service techs, and never had a Sams manual produced. Of course the main point of curiosity here for me is the fact that it's a commercially made Class A amplifier. I wonder if the heatsink is too small, given other SWTPC products, I wouldn't be surprised at all if they pushed the limit with temperature rise of the heatsink.

I enjoy reading about projects like this, because I can only take on so many myself, and it's interesting to learn about what others are doing.
 
The double-winged TO-220 heatsink is likely of an acceptable size. So I don't think it is thermal runaway.

Given the Tiger issues, this seems more a fundamental issue of oscillation arising from Miller tanks or feedback phase shift.

The problem with projects is that more keep showing up on the doorstep despite the giant neon sign flashing "No Vacancy".
 
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