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Understanbly so

Posted by Victor Khomenko on January 5, 2001 at 05:19:58:
***i am confused by your explaination below regarding "differential" phono stages. i have an aesthetix io phono stage which i run with "balanced" phono cable and interconnects. your statement below says that a 6db gain in noise performance is derived from a single-ended input; am i missing something? my understanding is that a "balanced/differential" input would yeild a 6db gain in noise reduction.
A typical confusion. Often people say that balanced line has twice the signal, sort of implying that there is an inherent 6dB S/N advantage, but this is simply not so. S/N has to do with with the max signal, and the noise level of the input circuit. So yes, if you CAN boost your signal, then you DO gain S/N, but his doesn't apply here, because the cartridge comes with fixed output - it is whatever it is designed to produce, say - .2mV full scale. No matter how you connect it, you still get that .2mV max signal.
Given that the source voltage is fixed, you now need to reduce the circuit noise in order to improve your S/N - that's the only way. Unless you are willing to buy a new cart that gives you perhaps 1mV output.
Now, let's pretend we have a single tube, a dual triode, allocated for the first stage duty. Now you have the design decision to make.
First, you can configure it as a differential amplifier, a long tailed pair, for instance.
What happens in that case, is that each triode has the internal input noise source that is now connected in series with each input. You have two uncorrelated noise sources in series with your .2mV, and they add up so there is 3dB more noise than there would be in case of a SINGLE triode.
Another possibility, as long as you have the same dual triode to play with, is to connect the two triodes in parallel. Now you only have a single noise source in series with your signal, plus this noise source is now reduced by 3dB - that's how two uncorrelated noise sources add up if connected in paralles.
Net result: two triodes connected in parallel will produce 6dB less noise than the same tube conected as a differential input stage. Provided, again, that your source amplitude is fixed and can not be increased.
Of course, this is just one design consideration, there are many others, so your final answer might be different, but noise-wise this is how it works.