Input pot question

[kacernator]

Hi,

in my Aikido preamp I have this input pot:

http://satanda.aspweb.cz/schem/input.png

I noticed, that sometimes there is no 1M, just the pot and also seen different values of this pot - 50k, 100k.

When I disconnected 1M the sound was slightly more open, maybe clearer, but lost punch and bass so I connected 1M back.
When I lowered the pot value from 220k to 100k, the sound was little dirtier, lost dynamics and was all like under the blanket.

How do you determine the best value of input pot?

[jnb]

Were both pots identical except for their value? Make the pot the smallest value that doesn't stress the stage before and keep the 1M resistor in.

[kacernator]

I just added 220k acrosss the pot (input and ground).

What do you mean by stress the stage before, how do I know to not to stress?

[jnb]

There probably should be some experimentation and some consideration of your levels, but a rule of thumb is ten times the output impedance of your previous stage.

Often, 220k will be OK, regardless.

What is your previous stage?

[kacernator]

Previous stage is line out from audio card Gina20.

[jnb]

You could go much lower than 220k, even less than 50k but you probably need not go lower. This should be enough room to allow you to experiment.

[kacernator]

If I can go low as to 50k then why, when I bypassed the 220k pot with 220k resistor - thus making the 110k pot, the sound was very bad? Lifeless, with blanket, small dynamics?

[jnb]

I don't know, but try putting the extra resistor after the sound card but before the pot (just as a test). This should tell you whether amp loading is not the issue (then replace the resistor). You might also try a different brand or type of pot, the differences will be small if noticeable. The resistor only needs to be 1M, as it is just there to keep the grid pulled down in case the pot wipes over a dirty spot and drops out.

[sneetch57]

I'm a total newbie,so please don't laugh out loud at my input potentiometer question....why do amplifiers regulate their output(volume) by varying the strength of the input signal as opposed
to varying the output of the output transformers( I'm guessing that the answer has to do with high voltages at the end of the amplification chain).It seems as though you would want as strong a signal to enter the amplifier as opposed to attenuating it before the first valve?

[jnb]

Originally posted by sneetch57
varying the strength of the input signal as opposed
to varying the output of the output transformers

A potentiometer causes a power loss so it is best to do this at a low level. Furthermore, the amp will have more headroom at normal levels.

It seems as though you would want as strong a signal to enter the amplifier as opposed to attenuating it before the first valve?

This would ensure a good signal to noise ratio, and it is considered in amp design, but you don't need to go all the way to the outputs to get this one right. Naturally you wouldn't want attenuation too close to a phono cartridge, for this reason.