Isn't that the truth! I wish I could make money as well as I can find projects
Southwest Technical Products Corp. (SWTPC) Headphone Amp 202-C
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Isn't that the truth! I wish I could make money as well as I can find projects
Tone Control
New Member
I built one of these when I was in high school in 1973. Noticeably fantastic sounding. Got real warm on idle. I'm still using the 198/A preamp.
Do you happen to have the manual?
As I previously wrote, I'm fairly certain the headphone amplifier is a Plastic Tiger re-biased for Class A. So that would explain the heat at idle. Always burning power.
I still haven't done anything with mine.
I, too, have the 198/A preamplifier. Would benefit from replacing the opamps with lower-noise units (taking care to limit bandwidth to avoid oscillation), and the noisy voltage-regulators replaced with modern units
As I previously wrote, I'm fairly certain the headphone amplifier is a Plastic Tiger re-biased for Class A. So that would explain the heat at idle. Always burning power.
I still haven't done anything with mine.
I, too, have the 198/A preamplifier. Would benefit from replacing the opamps with lower-noise units (taking care to limit bandwidth to avoid oscillation), and the noisy voltage-regulators replaced with modern units
You should see my place... lol
I believe I have a full set of schematics and manuals on the SWTPC "Tiger" amps. I still have the amp boards somewhere. This won't help you, but you triggered a memory.
Thanks for the offer, but I have the amplifier info for the traditional Tiger line and am well familiar with the myriad of postings as to its peculiarities and limitations. All of which give me pause.
As I wrote, I am reasonably convinced that the headphone amplifier is the Plastic Tiger biased into Class A.
Which is highly concerning, again, is that this design is similarly plagued by the twin problems of runaway oscillation — well known to destroy the amplifier and speakers by dumping DC into the voice coil — and exceeding of the SOA for the devices, leading to pure DC dumped into the cans.
PeteB has done exemplary work in analyzing and simulating the Tiger circuit, and he nicely details and explores its defects and deficiencies. See, for example:
His analysis of the Plastic Tiger design and implementation is neither encouraging nor confidence inducing. See, for example (emphasis added):
I would, in fact, label his extensive commentary (easily found here and on diyAudio) as inducing me to put down the 202C and carefully back away lest it lash out and destroy my vintage cans. The solution, of course, is to upgrade the devices, add anti-oscillation features as per the modifications to the standard Tiger, and add a DC-protection circuit to switch off the output in the event DC appears. That's a level of re-design I'm not yet prepared to make, as the payoff appears to be minimal.
I certainly would never use the Tiger, even though this was considered to be The Amplifier when I was in high school, because of its combination of high power and low cost. Students in high school, of course, are generally ill-prepared to evaluate designs, and the classic design triangle — price, reliability, performance, pick two out of three — is an alien concept. All that matters is highest wattage at the lowest cost. The upper-right corner of the risk-management matrix, of course, is an alien concept at that age, which is why the leading cause of death among those under 35 is Death by Misadventure. I would consider any SWTPC amplifier product to fall into that category.
As I wrote, I am reasonably convinced that the headphone amplifier is the Plastic Tiger biased into Class A.
Which is highly concerning, again, is that this design is similarly plagued by the twin problems of runaway oscillation — well known to destroy the amplifier and speakers by dumping DC into the voice coil — and exceeding of the SOA for the devices, leading to pure DC dumped into the cans.
PeteB has done exemplary work in analyzing and simulating the Tiger circuit, and he nicely details and explores its defects and deficiencies. See, for example:
His analysis of the Plastic Tiger design and implementation is neither encouraging nor confidence inducing. See, for example (emphasis added):
https://www.diyaudio.com/community/threads/1971-plastic-tiger.92036/post-1110396
A friend gave me a Plastic Tiger amp with at least one channel blown. This reminds me of how, this particular amp made us think back in the day, that these designs were seriously lacking in design margin. One could even say that the design was completely flawed. The output devices are MJE3055/2955 which have a Vceo of 60V, yet the design runs on +/- 40V rails. This means that a full output signal with no load will certainly exceed the Vceo specification of the output devices. Also, they'll only handle 1A at 40V. These amps should be run at a maximum supply voltage of +/- 30V when using these cheap output devices. Really, for the small increase in price, there is no point in using these cheap devices. There are modern plastic devices with much better SOA and power handling.
A friend gave me a Plastic Tiger amp with at least one channel blown. This reminds me of how, this particular amp made us think back in the day, that these designs were seriously lacking in design margin. One could even say that the design was completely flawed. The output devices are MJE3055/2955 which have a Vceo of 60V, yet the design runs on +/- 40V rails. This means that a full output signal with no load will certainly exceed the Vceo specification of the output devices. Also, they'll only handle 1A at 40V. These amps should be run at a maximum supply voltage of +/- 30V when using these cheap output devices. Really, for the small increase in price, there is no point in using these cheap devices. There are modern plastic devices with much better SOA and power handling.
I would, in fact, label his extensive commentary (easily found here and on diyAudio) as inducing me to put down the 202C and carefully back away lest it lash out and destroy my vintage cans. The solution, of course, is to upgrade the devices, add anti-oscillation features as per the modifications to the standard Tiger, and add a DC-protection circuit to switch off the output in the event DC appears. That's a level of re-design I'm not yet prepared to make, as the payoff appears to be minimal.
I certainly would never use the Tiger, even though this was considered to be The Amplifier when I was in high school, because of its combination of high power and low cost. Students in high school, of course, are generally ill-prepared to evaluate designs, and the classic design triangle — price, reliability, performance, pick two out of three — is an alien concept. All that matters is highest wattage at the lowest cost. The upper-right corner of the risk-management matrix, of course, is an alien concept at that age, which is why the leading cause of death among those under 35 is Death by Misadventure. I would consider any SWTPC amplifier product to fall into that category.
I
Rebuilt the Tiger amps and they would eventually oscillate and dumped DC into the speaker, burning up the woofer voice coil. I even rebuilt them twice to try to beat the oscillations with no luck. Sounded pretty good when working.
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As I understand it, the cause is ringing at crossover as the devices fail to fully turn off. Circuit changes are required. I won't use those amplifiers. I thought the headphone amplifier could be fixed, but, as I above wrote, I remain dubious about using it without DC protection circuitry.
For me, at this point the headphone amplifier is turning into a collectible which cannot be safely used.
Smart move!
When I built things in high school, price and performance were always prioritized. I was well aware of the triangle but having no money and big expectations, something had to give! To the extent of leaving out any sort of fuses, and operating power transformers with 120V on the 100V winding, to get more power. Never mind it got hot and buzzed. It's a wonder I never caught anything on fire, although I always did have great sound!
I never had any electronics courses in High School... I had to go to college to get the education....most of the schools I went to were quite small, and didn't have many extra programs.
I never had any electronics courses in High School... I had to go to college to get the education....most of the schools I went to were quite small, and didn't have many extra programs.
Me either, but between the library, magazines, and frequent trips to Radio Shack I hacked my way though it until education was an option. That's why threads like this one about old magazine projects are so fun and nostalgic to read about.
I agree... Radio Shock was very useful, and I miss the chain. I have about 6 years worth of the excellent "Audio Amatuer" and "Speaker Builder" magazines still. Some incredible knowledge in them!
I agree... Radio Shock was very useful, and I miss the chain. I have about 6 years worth of the excellent "Audio Amatuer" and "Speaker Builder" magazines still. Some incredible knowledge in them!
Those were excellent magazines! I've got popular electronics, radio electronics, electronics the Maplin Magazine, wireless world, stereo review, audio, hifi world, and on and on. Print media was easier to learn from than the internet, because there was more information and less opinion. I can't imagine trying to learn about amplifiers now, and wading straight into conversations about brands of capacitors, or something like this.
I find it much easier to read and study printed material, by far. It may be an age thing? lol I'm annoyed because I've forgotten so much theory, but at the same time I've learned a lot from 43 years in electronics service. Everything from Medical equipment to Industrial lasers to toasters! LOL
I was just searching for the SWTPC headphone amp and saw this post - I built mine from a kit new around 1980 or so. I still have the schematic somewhere -
I bought the kit new and built one and I still have it. I have the build instructions somewhere - I run across them now and then - but I have one right here that I built about 45 years ago. What info do you need?
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Mine always ran hot enough to fry an egg on but it never failed - I still have it but I haven't turned it on in at least 30 years:
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The Plastic Tiger was the first amp I built - I don't remember the circuits in detail - I didn't know a lot of electronics back in high school - but if I remember correctly the schematics didn't look anything alike, except both amps used the same output transistors. After I built the Plastic Tiger I built the Headphone Amp and then I built 15 Tigersaurus' amps and started a sound company in the late 70's. I still have most of the amps.
