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franklinh

Well-known member
Joined
Aug 18, 2004
Messages
89
Location
Troy, NY USA
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
 
Yes, RF will be received by loops in the chassis.  If the holes in the chassis, like vent holes, are the right size they can pick stuff up just like an antenna.

The old joke goes, When is ground, not ground?  When it's an RF conductor!

Ok so I made that up but RF can travel on grounds almost as easily as it can regular conductors.  It's a hard beast to kill if you have a true RF ingress problem.  Even the ground has to be filtered to keep RF out.

If you do wrap foil around connectors or holes in the chassis, make sure it's well grounded to the chassis itself.  Simply touching won't be enough.

Add some decoupling from the signal to ground, try 10-100pf first maybe keep going up until the hash is gone.

Maybe add ferrite in series with the inputs or between stages.

there are lots of things that could be going on here.
 
Tried 91pf from signal to ground - no change, it seemed more prone to react to where my hand was over the chassis.

Speaking of the chassis - the lid is painted inside and out - I tried scratching off some paint near a screw hole, when I apply pressure there the hash gets louder - improving the loop maybe?

With the lid off, if I hold a flat hand over the output stage the noise diminishes significantly.

When I touch a lead connected to the chassis to the can of the main filter cap, I get less noise in the channel nearest the can.

The internal signal wiring was also replaced with solid core - unshielded.  I don't know if the original stuff was shielded to begin with - I do have some shielded single conductor I could play with... hmmm.

What's a good source for ferrites?

 
RF pickup in audio equipment is sometimes a tough nut to crack.

First, the PAS 2 preamp has a high input impedance as well as a high output impedance. Both of these attributes cause trouble with stray pickup of RF signals. The fact that you can turn the chassis and reduce interference tells me the RF is getting picked up primarily inside the box. The audio leads may contribute to the problem, but if you can place your hand inside and affect the RF pickup, that's where it's getting in.

PLEASE NOTE:: I'm not sure how experienced you are with electronics, so anything I may recommend you do at your own risk!! I'm not responsible for any injury to yourself or your equipment caused by following what I may recommend.

Anyway, one possible place the RF may be getting in is through the tubes themselves. Tube rf and tuner circuits use metal tube shields grounded to the chassis to prevent and rf getting in. Unfortunately, the tube sockets in the PAS 2 are mounted over phenolic boards so you can't readily do this. However, you mentioned that the top of the case is painted. This would prevent a connection between the top and chassis, inhibiting the top from shielding the circuitry.

You can try removing the paint on ALL surfaces that contact the chassis; screw holes, sides, along the faceplate, and along the sides and back. Sandpaper would probably work best. Next, I would get some "Scotch Brite" pads, and shine up all points of contact between the top, chassis, front panel, back panel, and other individual chassis parts that touch. Wipe with a clean cloth. Reassemble everything, make sure screws are tight, and try now.

Another area that sounds like trouble: you connected a lead from the chassis to the outisde of the can capacitor and the interference changed. This tells me the ground connection between the can cap an chassis is not very good. This will be more difficult to fix, and is beyond the scope of this post.

Another thing you can test is taking a test lead; one end connected to the chassis of the PAS 2, the other, the center screw on the power outlet it's connected to. This will directly ground the chassis. BE CAREFUL! Check and see if there's any change in RF pickup.

Try removing and rotating the PAS 2 power cord 180 degrees and putting it back in the power socket.

Keep all connections between equipment as short as possible. Try these suggestions one at a time, and write notes about any changes. Report back your results.

Good hunting!

jD
 

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