I am talking specifically about adding a passive attenuator between source and amp, where there is no active circuitry following the attenuator. In that case, the output impedance seen by the amp IS the impedance of the passive attenuator. True, a 10K passive into a 100K input impedance amp is not bad, but with the rule of thumb ideally being 1:100, not 1:10, you get the idea.
I speak from both experience and engineering knowledge. Before I fully understood why, I used a number of very high quality passive attenuators and the Goldpoint is a great one! It is a great volume control, period! I am just saying that if you use even the BEST passive attenuator between source and amp, there will not be a good impedance match.
My personal experience many years ago was that, while the best passive, always sounded very clean and quiet, it lacked body and weight. I then added an active preamp and found the body and weight that was missing. After this, I always used an active preamp. It has to be a GOOD active preamp however!
I have since learned that it is really a matter of impedance! The use of a good buffer AFTER the Goldpoint or similar excellent volume control/attenuator WILL give you the best of both worlds! The active circuitry must be good, quiet, well designed and not harm the signal of course. I have long since determined that adding circuitry to the signal path is NOT always a bad thing! The same can be said of signal path transformers.
Many of use believe that less is more, always! I don't agree with that 100%. I am not a fan of tone controls and I am not crazy about DSP, because it all involves manipulation of the audio signal, in ways that often creates more problems than it solves. I don't like my analog digitized and I find that when any signal is digitized, manipulated and decoded again, there is too much lost. DSP room correction for low frequency issues IS a good thing, but I would NOT like to have the entire signal treated with DSP.
Now, we are not talking about digital or DSP, but it illustrates a point of adding circuitry for a net positive gain...or not!
Many people feel that adding signal transformers will always limit fidelity. I believe this comes from the fact that tube amp output transformers, especially SET transformers, must be very well designed, in order to not limit bandwidth and add distortion due to core saturation. I will argue in this case, that well designed SET transformers can and will produce the MOST BEAUTIFUL music playback when done well!
Push Pull tube amp output transformers are MUCH less impacted by the issues that SET transformers are because there is very little net DC voltage seen by the core. As long as global feedback is not used, P-P tube circuits can be AMAZING sounding and with very good BW! Our P-P 300B Integrated is flat to 50Khz!
Non-gapped, audio transformers (do not see DC voltage) are an entirely different matter. I use Lundahl line-level non-gapped transformers extensively in our best products! Jeff Rowland has done the same for years, also with great results. Yes, you are adding a copper coil and magnetic core to the signal path. However, you are also getting galvanic decoupling, inductive reactance and out of bandwidth filtering (say > 60Khz rolloff) which is ideal for digital circuits. Transformers can also attenuate signal level and allow for impedance matching. They are amazingly useful and musical reactive devices! I would much rather create a fully differential balanced output from a single ended circuit with transformers than use op-amps (feedback based) or other active circuitry to achieve the same!
So, to sum all of this up...
The addition of a tube buffer falls into the same camp of addition to the signal path WITH net benefits, not sonic degradation. For the design with gain, we will use transformer coupling and likely the same for balanced input and outputs. The tube circuit offers very low distortion, very low noise and bandwidth flat from 20Hz-100Khz.
Those using Goldpoint or other high quality passive volume controls will find that the addition of our Analog Bridge after the attenuator and between the amp will offer VERY welcome sonic improvements!
Thanks,
Dan
