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Please help me troubleshoot my new SET

Posted by rebbi (A) on January 27, 2015 at 12:41:56

Hi, Folks,
Last week, I finally finished building my brand-new Audio Note Kit 1 300 B SET amplifier. I also blogged the build on my WordPress site.
The amp is still very new and settling in – I don't think it has more than about 10 hours on it. I should also note that it is using the stock tubes, except for having substituted a 274 B which Brian of Audio Note Kits sent me instead of the stock rectifier tube.
The amplifier is a beautiful, heavy beast that seems to be working fine in the sense that all the voltage checks were good, the tubes are all lighting up, and so forth.
The good news is that it is dead quiet – at least with my 92 dB efficient Reference 3A De Capo's I can crank the volume potentiometer as high as it will go and there is absolutely no noise, hum or tube rush coming from either the speakers. Music emerges out of the proverbial black background.
The analog side of my rig also sounds quite great, again with the caveat that the amp still has, according to Brian, at least a couple of weeks to burn in. I have a Bottlehead Seduction Phono Stage.
The trouble I'm having is on the digital side. I am running an old Music Hall CD player as a transport. I also play high-resolution files from my Mac laptop. My DAC is a Musical Fidelity M 1.
What I am finding is that when I play CD tracks that have significant bass energy – and here I am talking about either fat, subterranean synthesizer bass tones or prominent bass drum whacks, accompanying vocals will start to sound like the singer is gargling and the bass also gets a stuttering quality. I assume this means that the amplifier is clipping!
I consulted with Brian about this. He thinks that there is a mismatch between the DAC and the input stage of the Kit 1. He is suggesting that the DAC is putting out too high a voltage and that with the input stage amplifying the input signal 14 or 15 times I am getting a clipped signal amplified. He is suggesting trying a DAC or CD player with an adjustable output for the analog output. By the way, I also tried taking the DAC out of the equation and just running the CD player straight into the amplifier, but the results were not much different. Again, I am not seeing this kind of trouble playing LPs, except, *maybe*, in the opening bars of the LP version of Donald Fagen's "Morph The Cat," which again features fairly exaggerated bass. But that may be my imagination.
As you can imagine, I'm feeling very frustrated at this point, having put hours and hours of (admittedly very enjoyable) time into building this amplifier, not to mention that the kit is not cheap.
Please, I'm not looking for opinions about the wisdom of having selected this particular kit to begin with. What I'm hoping is that some of you can bring your technical expertise to this and help me figure out what's going on. Does this business of the DAC voltage being too high and saturating the input stage makes sense to you? What do you think it's something else?
Many thanks, in advance!

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