noisy pre

T

T-bone Sanchez

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Theres something Im missing here and my brain wont click.

The MF pre is finished for now and Im quite happy. However, last night I decided to put my system back together but as a result I got terrible noise from one of the inputs. So I went back in, de-soldered all the inputs, the pot and all grounding. Cleaned the pcb, scrapped back some of the tracks etc etc. Everything read right on the meter, I couldnt find a thing. Hooked it all up again and its still there.

When selecting the cd input the pre is amazingly quiet, at full volume right with your ear right next to the speakers theres nothing there. It seems it doesnt like an open input, Ive never had this problem before and whilst it doesnt effect those inputs with something attached I would like to sort it. Any ideads? Shorties perhaps?
 
Because, if the tuner is connected to an antenna and the antenna has an amplifier (as it will in many cases if you do not have your very own roof-mounted aerial), this is a classic suspect for an earth loop in your system. Solution: a small capacitor in the antenna cable.
 
While I have no idea what preamp you are talking about - are you saying 'it makes a noise when there is nothing connected, but works OK when connected'.......?

You need resistors from input to ground to bias the input stage - are there any there? Unless its a virtual earth input.
 
The pre started out as a Musical Fidelity X-Pre (the two tubed variety) but has since been quite heavily re-worked. I took the tuner out out its totally gone. Its silent with the 3 remaining inputs open so it looks like your right.

There are resistors to ground and all grounding reads spot-on on the dvm. Its time to try the cap across the antenna, what size do you recommend?
 

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