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In Reply to: RE: First, disconnect the tuner. posted by Al Sekela on October 27, 2010 at 15:27:26
Moved the tuner and receiver from the audio shelf into the next room. Still hear the same AM station very faintly.
Disconnecting the pre-amp from the power amp. That solves the 'problem. However, that would not be a solution.
Reconnected the pre to power amp, dis-connected all sources from pre, still have the AM station coming in faintly.
There is no Radio tower close by.
Follow Ups:
Repeat your experiment with the preamp connected to the power amp and nothing connected to the preamp, but with shorting plugs on the input jacks.
If you still hear the AM station, then the preamp needs work.
If you don't, then experiment with shielded cables from the source devices to the preamp.
Well, I only had one pair of shorting plugs and I only could find one.
So I unplugged the source selected and plugged one shorting plug in and the AM station went away. So I did some more 'playing' around. Turns out that the radio station come and leaves sometimes when I touch the RCA input jack, also after leaving the AM station came back when I switched sources.
Looks to me that the AM station comes in with a ground loop problem, though I don't hear a ground loop problem.
Just a 'light' come on.
If it's ground loop based, than a cheater plug may solve the issue.
Well, found a cheater plug, inserted it and and so fare the AM station has not returned.
Let see what happens the next days.
On which piece, preamp or power amp, did you defeat the ground?
Put the cheater plug on the power cord of the pre-amp. Haven't tried at the power amp yet.
The fact that you can make out the AM station's program suggests there is a detecting junction somewhere in your preamp or the AC wiring that supplies it. The ground loop acts as an antenna, but something is converting the radio frequency energy into the audio band. This could be something as simple as dirty wire contacts to the AC outlet. The problem also could be inside the preamp, say, a dirty selector switch. It could also be a consequence of the design (how the audio ground is related to the chassis ground).
I would do a thorough cleaning of all the contacts and check for cold solder joints.
The designer of the pre-amp thinks it the ultra-wide bandwith of his pre-amp that makes it sometimes susceptible to these kind of interference. Also, the selector switch used is made for 5 inputs, but we only have 3 inputs soldered to it. Maybe I need to solder 75 ohm resistors across the unused pins on the selector switch.
Also, install 75-ohm shorting plugs on any unused input jacks.
Depending on how far you want to go with modifying the preamp, you could use the old Hewlett-Packard trick of diode offset for the safety-earth connection. Together with some low-value, high-current inductors, this should provide an RF barrier while maintaining a semblance of third-wire protection.
how do you do the diode offset 'thingy'?
Connect two silicon junction rectifier diodes in anti-parallel and wire the result in series with the safety-earth connection. The diodes should be big and beefy to permit fault current to flow long enough to trip the circuit breaker. Because the diodes are pointing in opposite directions, there is always one that permits current to flow with no more than a single diode drop across it, no matter which polarity the AC voltage has at the instant of the fault.
This is a safety issue, as some would argue that the theoretical maximum fault current is so large that any reasonable size rectifier diodes would explode and interrupt the connection to safety-earth before the breaker trips. However, in most cases, the AC branch circuit impedance is high enough to prevent this. It is important to NOT use fancy rectifiers in this application, as they usually have less tolerance for overloads.
An alternative is to use a potted rectifier bridge with the + and - terminals wired together, and the AC input terminals wired together. This puts two diodes in series in each direction. The bigger ones come with metal jackets and are easy to mount on a heat sink. I use 35-ampere versions.
It is also possible to wire this into a power cord to try the concept before making permanent changes in your equipment.
The reason this works is that the diodes have very high impedance for small noise voltages. Since junction rectifiers have capacitance, adding some heavy inductors with about 1 microhenry of inductance in series will make the RF impedance even higher. The end result is close to an open circuit for noise voltages, but a low impedance path for fault current.
Try putting shorting plugs on the preamp inputs. If the AM goes away, you could try putting small capacitors from hot to ground on the RCA jacks.
All 3 inputs and 1 output are occupied. Need to contact the manufacturer and ask him about placing a cap between the hot and ground.
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