Bottlehead Forum
General Category => General Discussion => Topic started by: aragorn723 on May 02, 2013, 03:31:28 PM
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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!
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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