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RE: Volume Control

Very often, the O/P impedance of tube equipment is too high for a passive control center to be satisfactory. :>(

Let's say that a 50 Kohm load is needed by unit X. That's easy enough to do with a 50 Kohm control. The problem is downstream from the pot., where a 500 Kohm I/P impedance is in order. To avoid high freq. roll off, with that sort of I/P impedance, a pentode or cascode I/P stage (low Miller capacitance) is indicated. Lots of power amps have 100 Kohm impedance triode I/P stages and, therefore, mate badly with a 50 Kohm passive control setup. Keep the 1:10 impedance rule firmly in mind.

The 1st order of business is working out the amount, if any, of line stage gain needed. Then, sensible advice about line stage configuration can be provided.

BTW, the "classic" RCA passive EQ phono circuit requires a minimum of a 220 Kohm impedance downstream. The 6SL7s, 6SC7s, and 12AX7s found in early mono mag. phono preamps are all wimps that can't drive loads.


Eli D.


Edits: 06/18/15
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