Re: Which tube expert can do custom works?

John Byrns wrote:
In article <fmdv03$b11$1@xxxxxxxxxxxxxxxxx>,
Ian Thompson-Bell <ruffrecords@xxxxxxxxxxx> wrote:

So the lowest possible level of noise from the output of our mic pre is -100dBu and at gains over 30dB it gets worse due to input noise.

OK, I think I follow you this far, for the case of the mic pre example you are discussing, although I don't think Henry Pasternack would care much for your use of the term "output noise".


From what I can find out about him on the net he seems to have very little to say about noise. In any pre-amplifier using several active devices (be they tubes or semiconductors)(and we will need several to achieve the gains we are talking about) the noise at high gains is predominantly determined by the noise contributed by the very first device. The other devices, including those at the output end, do of course produce noise but it is low enough to be masked by the noise generated by the first stage. At lower gains this is not true and the noise from the later and output stages significantly contributes to the overall noise appearing at the output. That is what I mean by output noise. It was a term in common use when I was at Neve in the 70s.

The next question is what level should we mix at? Since our noise level is always -100dBu or worse we want to mix at as high a level as possible; that is we want out signal to be as far above the noise as we can get it. Since a CD can give a theoretical 96dB S/N we want to our mix to achieve at least this S/N which implies a mixing level around 0dBu. Since we know we can design preamps that will deliver +20dBu, choosing a 0dBu mix level gives us 20dB headroom and a potential 120dB dynamic range.

You seem to have left some assumptions out of this part of the discussion and your logic here seems to be dependent on the particular mic pre you defined in the previous paragraph. Not that it isn't necessarily true, but you haven't presented evidence that it is necessary to mix at 0 dBu to achieve at least a 96 dB S/N. For example if we had a mic pre with 10 dB less gain, and 10 dB less "output noise", it isn't obvious to me that we couldn't mix at a level of -10 dBu and still achieve the desired 96 dB S/N. I guess I will have to try and flesh out the example myself, but this gets back to my earlier question about the noise contribution of the mixer network.


What you say is true. if your mic pre has an output noise of -110dBu then you could mix at -10dBu and still achieve 100dB S/N and you could make up the extra 10dB of gain later in the chain.

I used a -100dBu output noise value as it is a value readily achievable in a mic pre. -110dBu is somewhat harder to achieve.

In a professional mixer there are other considerations. One is the requirement to provide either channel outputs and/or insertion points. These are for connecting outboard gear which operates at line levels ~0dBu, so the mic pre then does need to incorporate all the required gain.

So since our output level also wants be be in the region of 0dBu we want want any additional gain after mixing (except to make up for mix losses) so in practice all our gain ends up being in the mic pre.

So mixing at -10 dBu as in my example would require an additional 10 dB gain following the mixing network, but since the output noise of the mic pre is also 10 dB lower, it all washes out as far as the noise contribution of the mic pre goes. However any noise contribution of the mixing network/circuit would be increased by 10 dB, this is the part I don't have a grip on yet, how the actual mixing network contributes to the noise at the output of the "mixer unit"?



Let's say we have 10 channels to mix together and we use voltage mixing as described earlier by Graham and for the sake of argument say we use 10K bus resistors. One channel is fed via a 10K resistor and the other 9 off 10K resistors load the end of this 10K resistor so the bus signal from this channel is 10 times lower than the mic amp output so the mix amp needs 20dB of gain to make up the mix bus loss. The mix amp sees at its input the 10 off 10K resistors in parallel (1K) in series with its own noise resistance (typically about 300 ohms) as its total noise source resistance plus the noise from any feedback resistor needed to achieve the 20dB gain. Ignoring the feedback noise for now, the 1300 ohms noise resistance seen by the mixer amp produces an input noise level of about -121dBu. If we mix at 0dBu our signal on the bus will be -20dB so we can just about maintain out 100dB S/N ratio. If we mix at -10dBu with this scheme, even if out mic pre produces only -110dBu, the noise from the mix amp will predominate and our S/N ratio will drop to 121 - 30 = 91dB.

Even if we now change our mix resistors to 1K, the noise at the mix amp input is still only -126dBu because now the 300 ohm noise resistance I have assumed for the mix amp dominates. Clearly the mix amp is needs to be as noise free as our perfect mic pre.

HTH

Cheers

ian
.

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