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In Reply to: RE: Please help me troubleshoot my new SET posted by rebbi on January 27, 2015 at 12:41:56
I don't think signal level from the digital source is the problem.... (But I could be wrong.)
"Subterranean" bass takes up gobs of power..... It could be a simple case of the deep bass clipping an 8-watt 300B amp driving 92 dB efficient speakers. Especially where the bass response of the speakers isn't extended. (If I recall, the Reference 3A De Capo is a smallish speaker.)
Since you did build the amp, the only other thing I can think of is maybe the bias is off..... That will cause an SET amp to clip (saturate or cut off) at lower power levels.... And deep bass will bring out the problem.
Follow Ups:
Todd,
Thanks a lot for the post. As I mentioned earlier, Brian doesn't think it's the DAC output, but some problem with the build; perhaps I mis-wired something or I have a bad tube on the driver board. We're working together to systematically troubleshoot this and I'll report back when we find the problem. What's odd is that bass-heavy material creates no distortion on the analog side. It sounds fantastic, in fact! But the fact that the DAC is putting out 2.2 V and the phono preamp only 550mV may mean something here. As a last resort, I may take the amp to a local and very good stereo repair shop (the guy specializes in vintage equipment and knows tubes) and enlist his troubleshooting help. Brian has assured me that he's committed to making me totally happy, which means something for sure.
The digital and analog output levels are not the same measurement. The analog voltage is derived from the cartridge output, rated in mV at 5cm/sec. That is the nominal VU peak (short-term average). Actual instantaneous peaks can be much higher, with good carts able to handle as much as 50cm/sec; the old-school working assumption was that peaks are 14dB above VU peaks.
Digital is always rated by full-scale peak, and corresponds to the analog level plus 14dB (5 times the voltage).
Note that these days, when extravagantly powerful compressors are widely used, that assumption of 14dB headroom is ... optimistic? ... yeah, that's a nicer-sounding word.
Paul,
Thanks for the post.
So, in layman's terms, are you suggesting that an 8.5 watt 300B SET amp simply can't handle certain kinds of music in the digital realm?
If that's not what you mean, can you clarify your remark in non-technical terms?
Thanks again.
"So, in layman's terms, are you suggesting that an 8.5 watt 300B SET amp simply can't handle certain kinds of music in the digital realm?"
I'm not sure if an 8-watt 300B SET amp will have enough power to drive 92-dB efficient speakers to satisfying volumes with music containing lot of deep bass energy. (The same amp might be just fine w deep bass with > 98 dB efficient speakers.)
By the way, I think Paul's suggestion of using a high-pass filter at the input is a good idea..... If the speaker doesn't have extended bass response, you won't miss much down there.
Sorry for the lack of clarity. I was only addressing the concern about the apparent difference in levels - with well-recorded material it should be small in practice.
I don't see enough information in this thread yet to know what the problem is with the digital material. A number of good possibilities have been suggested, and I think the "overloaded first stage" theory is unlikely if that stage is operating correctly - which you are already engaged in checking, I think.
If it is very deep bass overloading, either deliberately encoded in the digital or caused by an noise/instability/error in the digital side, then a simple high-pass filter at the amp input would be a possible test.
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