passive preamp design

[aragorn723]

Hi, so I have been looking at some schematics for a bunch of preamps and small amplifiers lately, and noticed that many have a resistor bridged across the positive and negative signal inputs (to set input impedance) that many people install across the 2 terminals on an rca input jack.  Is that necessary to have a good sounding preamp?  I built a passive preamp a while ago with just a dpdt switch to switch between 2 inputs and also a 100K alps potentiometer.  Would adding resistors across the positive and negative signals help the sound a lot, and in what way?  The passive sounds clean, just a little bit like a boom box at times :-p  It is clear, but somewhat congested/bland sounding.  Thanks!

[Paul Birkeland]

The resistor across the inputs is redundant if you have a volume control that follows.  On any of the Bottlehead power amps, you will see a resistor there to bleed off grid leak current from the first stage of the amplifier, and this also sets the input impedance.

Sometimes on a tube preamp you will see a resistor of very high value after the volume control, and this is just a failsafe in case the wiper lifts. 

The good news about this is that you can experiment to your heart's content with zero risk!  With a 100K pot turned up a reasonable amount, your output impedance is going to get kind of high, so you may find that a 20-50K resistor across the pot helps a bit with that.

-PB