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OTL Asylum OTL, Output Transformerless Amplifier User Group. |
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OTL Asylum OTL, Output Transformerless Amplifier User Group. |
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In Reply to: Anyone have experience with the RCA OTL design? posted by tomcat on June 27, 2001 at 14:45:23:
I built a very similar OTL in the mid-seventies. It was an in-house design from Packard Bell Space systems. Reviewing the schematic, it's hard to believe how similar it was to the RCA version. I could swear the two designers were in league.I had several problems with this design, but I did eventually get it to work well. It was never the sonic equal of the Atma-Sphere design, but still far better than any other transformer coupled amp I've ever heard.
The fact this amp has no power line isolation is it's greatest problem. Avoiding expensive transformers is a great trick, but it's so very dangerous and this practice makes it difficult to interface with other equipment. I used two isolation transformers per channel for the output supplies and a larger 6.3 VAC filament transformer, mil-surplus, of course. I also used a separate transformer(s) for the driver stage too.
I'll admit my construction technique wasn't so good in the seventies, but I still had way too many problems with stability. RF oscillation was a real problem. It could blow away the local TV very well. It was very sensitive to the coupling caps between the driver and the output stage. The only thing I ever found that would not cause it oscillate were the big oil caps.
It used to to take out output tubes very well too, since it is operated fixed bias. This was before the availability of the more rugged Russian 6AS7Gs. This design also suffered from balance problems and lack of deep bass. These problems were solved by Bruce, over at Trancendent. His output stage topology is very similar.
In short, I'd never do it again. Get in touch with the Atma-Sphere factory. Buy an M-60 assembly manual for about $20. It has a schematic included. It is vastly superior.
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Follow Ups
- Re: Anyone have experience with the RCA OTL design? - mark 15:46:04 06/27/01 (1)
- Re: Anyone have experience with the RCA OTL design? - tomcat 13:47:54 06/28/01 (0)
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