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RF interference, phono stage hiss.

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Posted on May 26, 2003 at 12:12:26
Zack P
Audiophile

Posts: 1
Location: Virginia
Joined: May 26, 2003
 I have a PAS 3 which sounds good, except for the very audible reception of a nearby radio signal. This is present in both channels equally, but varies a little in intensity from day to day. It is also present using all inputs. Has anyone had similar trouble with this preamp? What steps have worked best to control this?

There is also a fairly loud hissing or blowing sound in both phono stage channels, that was not present when I first got the preamp. There is lots written about 60 cycle hum in these amps, but very little about this problem.

 

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Re: RF interference, phono stage hiss., posted on May 27, 2003 at 12:55:22
Joe Curcio
Manufacturer

Posts: 1261
Location: Northern CA
Joined: October 10, 2001
 RF interference can be difficult to eliminate especially if you're near a transmitting tower (or even a local ham operator). Note that there are several places where the interference can get into your preamp - any of the input cables or the power cord. I would first disconnect all of the inputs and using shorting plugs on the selcted input, see if you still have the problem. Chances are that if you still have the interference the RFI is coupling into the line cord and you may need a power line filter (inside the preamp). If the interference disappears, either more robust interconnects are needed or possibly ferrite beads or small input caps may be needed. Either way, your first step is to identify the source.

The noise in the phono stage is _probably_ not the power supply (usually a hum problem stems from the power supply). It could be the tubes or cathode resistors (although since the symptom is appearing in both channels it's difficult to tie to the tubes or any component). It could also be an oscillation set into motion by the EMI cited earlier. In your above experiment, you should also place the input shorting plugs into the mag input and note if the hiss disappears - if so the EMI is probably responsible for the hiss as well.

 

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