cmoy volume pot question

[alane]

Hi all,
I built my cmoy over the weekend and as far as i can tell it is working as designed... (thanks tangent

) however due to an errant mouse click on my part i got the wrong pot in my order and it was single rather than dual .. so no good ..and nobody local has one

so i havent installed any pot but i boxed it up into a nice enclosure with the intention of adding the pot later.. when i turn the amp on i get a hiss (even with no source connected) that i presume is the fact that the amp is running like i had the volume all the way up .. so i just turn it up at the source and this covers the hiss..
my question is if i add a volume pot on the input will that hiss not always be there (masked by the input signal to be sure but it is still audible during quiet passages in the music and when i turn the volume down) and would it not be better ( i am asuming as my electronics knowledge is only basic ) to put the pot across the output ... therefore dropping the noise floor effectivley.
Also I experimented with putting some resistors into the input path .. but they seemed to make no difference until i used a 100K and that made the signal distort at medium volume ( i am only using spare resistors from the ones that came in my order for the amp )

Any ideas would be appreciated

Alan
Ireland/Dublin

 

[MonkeyMan]

You should always use the very best pot you can get your hands on

Modify your consciousness first, and all else will fall into place as it should...

 

[till]

Quote:

Originally Posted by alane when i turn the amp on i get a hiss (even with no source connected) that i presume is the fact that the amp is running like i had the volume all the way up .. so i just turn it up at the source and this covers the hiss.. my question is if i add a volume pot on the input will that hiss not always be there (masked by the input signal to be sure but it is still audible during quiet passages in the music and when i turn the volume down) and would it not be better ( i am asuming as my electronics knowledge is only basic ) to put the pot across the output ... therefore dropping the noise floor effectivley.

You could try to reduce the gain, it will lower the noise floor. Did you use the standard gain of 11? I use 5 or 6 and I do not have problems with hiss. A metal enclosure could also help, if you don't already use one; maybe the amp is picking up RF.
I'm not sure if it is a good idea to put the pot after the amp because the output resistance is crucial, you would drastically raise it. But I'm still a beginner, too. Sure someone with more experience can clarify.

But you could add a load resistor, as suggested in CMoy's original article.
I quote:
"R5 is an optional load resistor, which reduces residual hiss when the amp is driving low impedance headphones such as the Grados (32 ohms). Because the voltage drop across R5 reduces the maximum output of the amplifier, I recommend trying a 50 ohm resistor first (or as low as 30 ohms). If there is still residual hiss with low impedance phones, then increase the resistor value to 100 ohms"

Quote:

Originally Posted by alane Also I experimented with putting some resistors into the input path .. but they seemed to make no difference until i used a 100K and that made the signal distort at medium volume ( i am only using spare resistors from the ones that came in my order for the amp )

If the hiss comes from the amp, putting a resistor before the opamps won't help, I think. Also, resistors act as voltage dividers, so they cannot be used to do anything if they are in the only way the signal can take. Apart maybe from heating them up

HTH

 

[alane]

Thanks till ..
yes i am using the gain of 11 setup .. i might reduce that as to be honest i will never turn it up loud enough to use that .. unless i am going to use it for some sort of torture device... i have the R5 in there .. i have to find a pot somewhere local .. i can't order just one online!

Cheers again

Alan
Ireland/Dublin

 

[till]

Alan, if you don't find one locally, you could contact guzzler - I remember he has some Panasonic EVJs for sale (which aren't available in Europe except through Digikey) and he's in Scotland, so shipping costs might be bearable. They are said to be a good value for the money.

 

[alane]

Cheers Till I shall have a gander

Alan
Ireland/Dublin