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Old 5th June 2004, 01:20 PM   #1
garbage is offline garbage  Singapore
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Default buzzing line drive preamp by the mod squad

hi all

i have a Line Drive by The Mod Squad that looks like the one below.
Click the image to open in full size.

it is a passive preamp, but it is causing a buzzing sound on my setup.

the internals are just a couple of selectors, 2 nobel pots, one for balance and the other for volume and lots of wires.

the whole thing is star-grounded... mostly.

the buzzing comes and goes and is quite random. but when the buzzing is around, it gets worse when i put my finger on the wires that lead from the volume pot to the output RCAs.

i think the buzzing could be due to the nobel pot on the volume control... or it could be due to the resistors on the volume pot.
there are 2 resistors on each channel of the pot for a total of 4.

don't think the wires are buzzing...

any ideas from you guys? i would like to isolate the part so that i can go get a replacement for it.

thanks in advance
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Old 25th August 2006, 08:09 AM   #2
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You didn't say whether this is a new installation.....

The first thing to remember is that, because this unit is passive (and, therefore, has a high output impedance), it is VERY sensitive to stray magnetic fields causing hum. [I have a Creek passive preamp with the same type issues.] If you are replacing an active preamp with this one you may have to rethink where your wires are run and where components are located.

The unit itself will be sensitive to where it is sitting (especially bad would be on top of the power amp - which has a big trasnformer). Because the output impedance is high, the interconnects running from the preamp output to the power amp will be VERY sensitive to hum. Running them anywhere near a power cord or a unit with a big transformer could be a problem. The fact that it hums when you put your finger on the output wires is normal for high impedance circuitry - and having the cover open will make it much more sensitive to hum as well. Fancy, super-shielded, wires may help this - or they may not.

Use as short interconnects to the power amp as possible - and keep them away from everything. If you run long interconnects - like to a subwoofer - they will probably be a problem. Make sure your interconnects stay as far from power cables as possible - especially NEVER run interconnects alongside power cables. Make sure that all of your interconnects are tight and making good contact. You could try running a chassis ground wire from the preamp (a case screw) to the power amp case - or to your wall outlet GROUND screw (this will either make things worse or better).

If this is a new problem - and the unit has been in use for a while then I might suspect an internal ground problem - if you have just replaced an active unit with this passive one then I would suspect you are just seeing the increased sensitivity of the passive unit. You may have to rethink where components are located and where the interconnects lay.
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