franklinh
Well-known member
Hello there everyone!
I'm having an interesting time trying to eliminate some annoying pickup happening in my listening system - I have a dynaco PAS-2 preamp ( http://www.curcioaudio.com/pas_mnl.pdf ) that is really great at demodulating cell phone hash - that sort of machine gun white noise type. It wasn't always this way, it became an issue when we moved into a new place with more windows on the 3rd floor with amazing cell phone reception.
The rest of the setup is a SS power amp that is on at all times (and exhibits no pickup by itself, connected to the preamp or not). Everything is quiet with the preamp off. The preamp has had some mods neatly done to it by the previous owner - the EQ and loudness switch are jumpered out - although stuff may still be hanging. Caps and resistors replaced with modern parts.
What I've tried - if I rotate the preamp physically it's possible to find somewhat of a null, but not a complete one, and now the knobs are facing sideways. Aluminum foil (I know... aliens) wrapped around the output jacks to the amp - seems to help, especially when I close my hand around it. I'm familiar with the shielding properties of human flesh, so I have deduced so far that this could be a matter of shielding, termination, or both. I've gone so far as to short the grids of the output stage to ground, with marginal results.
I remember reading somewhere in here in a discussion about where to ground chassis and shields, that RF propagates near holes in the chassis, or protrusions like connectors - is that right? Short of putting it inside a bigger metal box (just haven't found the right one yet) are there things I could try inside the preamp?
The ground in the apartment is certainly suspect, there's not a ton I can do about that being on the 3rd floor. With the right combination of putting my hand on the foil and putting the preamp in the "null" place things get almost enjoyable.
Thoughts?
Frank
I'm having an interesting time trying to eliminate some annoying pickup happening in my listening system - I have a dynaco PAS-2 preamp ( http://www.curcioaudio.com/pas_mnl.pdf ) that is really great at demodulating cell phone hash - that sort of machine gun white noise type. It wasn't always this way, it became an issue when we moved into a new place with more windows on the 3rd floor with amazing cell phone reception.
The rest of the setup is a SS power amp that is on at all times (and exhibits no pickup by itself, connected to the preamp or not). Everything is quiet with the preamp off. The preamp has had some mods neatly done to it by the previous owner - the EQ and loudness switch are jumpered out - although stuff may still be hanging. Caps and resistors replaced with modern parts.
What I've tried - if I rotate the preamp physically it's possible to find somewhat of a null, but not a complete one, and now the knobs are facing sideways. Aluminum foil (I know... aliens) wrapped around the output jacks to the amp - seems to help, especially when I close my hand around it. I'm familiar with the shielding properties of human flesh, so I have deduced so far that this could be a matter of shielding, termination, or both. I've gone so far as to short the grids of the output stage to ground, with marginal results.
I remember reading somewhere in here in a discussion about where to ground chassis and shields, that RF propagates near holes in the chassis, or protrusions like connectors - is that right? Short of putting it inside a bigger metal box (just haven't found the right one yet) are there things I could try inside the preamp?
The ground in the apartment is certainly suspect, there's not a ton I can do about that being on the 3rd floor. With the right combination of putting my hand on the foil and putting the preamp in the "null" place things get almost enjoyable.
Thoughts?
Frank