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RE: Everyone said it could not be done???

Posted by Paul Joppa (P) on November 10, 2014 at 17:27:21

In Reply to: RE: Everyone said it could not be done??? posted by drsx on November 10, 2014 at 10:27:00:

The influence of the first stage output impedance is reduced by its proportion to the RIAA filter impedance. This is as true for pentodes as for triodes, and is of roughly the same magnitude.

First off, I am talking of passive RIAA networks between two stages, and driven by a low impedance, loaded by a high one.

Typically, let's assume a first stage of 10k plate impedance, an RIAA network of 100k impedance, and the second stage with a 1.0meg grid resistor. Drift in the 10k plate resistance is 10% of the 100k input impedance of the network, so its effect is reduced by a factor of 10. A typical pentode operates as a pentode might have a plate resistance of 1.0meg, which is 10 times that of the RIAA network, so its drift is again reduced by a factor of 10. Six of one, a half-dozen of the other.

I have used both approaches with success.

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