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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Input pot question

kacernator

Member
2008-01-09 10:38 pm
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

Member
2006-12-30 11:55 pm
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

Member
2008-04-01 1:34 am
input pot

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

Member
2006-12-30 11:55 pm
Re: input pot

sneetch57 said:
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.