Potentiometer wiring in preamp

[Andrew]

I am making a small passive preamp for my hi-fi with an ALPS Blue Velvet pot.

Standard wiring is much like a guitar where you progressively allow the signal to ground as the volume is reduced. But I am worried that this will damage the output stage of the CD player as you are effectively shorting out the line-level signal at very low volume.

Is it okay to do this? Can you allow the signal to ground without damaging the output stage?

[triodecr]

So the signal path is CD player ==> Preamp ==> Hi Fi amp? If so, connect the outer lugs of the pot so the CD player always has the fixed value of the pot for a load and the wiper controls the level to your hi fi.

Really crude diagram:

CD OUT + ---------

\

/

\<-------------------------------- wiper to HI fi input

/

CD out ground -------------\---------------------------------- hi fi input ground

[Armitage]

... how do you make a passive preamp?

[HAMERMAN]

I doubt it will hurt the CD player but to be safe you can add a resistor in series to each output of the CD player. That way even when the volume is at zero the CD player is still seeing the series resistor.

Modified really crude diagram:

CD OUT + --RESISTOR-----

\

/

\<-------------------------------- wiper to HI fi input

/

CD out ground -----------------\---------------------------------- hi fi input ground

[fractal]

Based on the diagram, the CD will always see the resistance of the pot, since the + and ground are on the outside lugs. The wiper may be grounded, but never the CD out.

Someone correct me...

I doubt it will hurt the CD player but to be safe you can add a resistor in series to each output of the CD player. That way even when the volume is at zero the CD player is still seeing the series resistor.

Modified really crude diagram:

CD OUT + --RESISTOR-----

\

/

\<-------------------------------- wiper to HI fi input

/

CD out ground -----------------\---------------------------------- hi fi input ground

[HAMERMAN]

Based on the diagram, the CD will always see the resistance of the pot, since the + and ground are on the outside lugs. The wiper may be grounded, but never the CD out.

Someone correct me...

You sir are correct. That's what I get for answering a post when I really should have gone to bed a few hours earlier! :-)

[Andrew]

A passive pre-amp is a just a potentiometer or attenuator in a box with an input and output.

I cannot believe how brilliant a simple passive pre-amp sounds. Mine cost about £40 ($60) to make with an ALPS Blue Velvet, but I am already so convinced I will be upgrading to solid silver wire. How do Hifi companies get away with selling expensive pres?

Anyone with hifi separates with poweramps - ditch the active pre-amp and get a passive. All CD players have the output to run poweramps at high volume, so you just need to trim the output a little for normal listening. A 10K pot is usually enough.

[Armitage]

...then it's not a preamp... it doesn't amplify anything. It attenuates only.

[Andrew]

...then it's not a preamp... it doesn't amplify anything. It attenuates only.

Absolutely! It's just shorthand to call it a passive pre-amp (an oxymoron, I know). No gain, no unwanted colouration.