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Old 20th April 2007, 02:24 PM   #1
clm811 is offline clm811  United States
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Default Need help combating power amp hum

I got a vintage Forte Model 5 power amp used. It is a nice sounding amp, but is plagued with a constant, low level HUMMM which is just audible from my listening chair when the room is very quiet. I understand hum modulates/distorts the music which is undesirable. I've checked all cable/cord grounds.

My speakers are an MTM design with 90dB sensitivity (the other owner used Magnapans and didn't notice the hum).

The amp uses a toroidal power transformer and input signal wires are shielded & routed carefully to the rear, away from the power supply components (see pic.)

The hum occurs even when the inputs are shorted/disconnected and when the power amp is the ONLY component used (i.e. everything else except speakers was unplugged) it appears to be within the power amp itself. Unfortunately there is no balanced option in my system, as all components use single-ended RCA.

Any ideas/suggestions to combat this hum?
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Old 20th April 2007, 07:30 PM   #2
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If it is both channels--start looking at the power supply filter caps.If its in one channel or the other you have a small bypass cap causing the humm.

Dave
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Old 20th April 2007, 08:20 PM   #3
clm811 is offline clm811  United States
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It's in both channels; however, the filter capacitors were replaced recently and they tested good.

Any other ideas (could it be the transformer and if so, what can be done short of replacement)?
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Old 20th April 2007, 08:56 PM   #4
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Could you post a closer detailed pix of the powersupply rectifiers or bridged rectifier....I'm thinking possibly a noisy rectifier.
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Old 20th April 2007, 09:28 PM   #5
clm811 is offline clm811  United States
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Rectifier bridge was also replaced- still hums.
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Old 20th April 2007, 09:40 PM   #6
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Here's a closeup of inputs and rectifier.
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Old 20th April 2007, 10:02 PM   #7
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Quote:
In addition, capacitors (typically 100nF) are often used across the bridge rectifier, effectively in parallel with each diode. These may help reduce noise
http://www.sound.westhost.com/psu-wiring.html

I dont think your transformer is the problem.Have a read of this article.It may help.

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Old 20th April 2007, 10:03 PM   #8
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I think you connected ground via this metal tab wrong way.
You created a closed loop of chassis/transformer center screw/ ground tab so that it forms an additional secondary winding and adds hum to ground.
My advice is connect reservoir capacitors to each other different way, not by metal tab connected to transformer screw, use just a piece of wire instead.
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Old 20th April 2007, 10:12 PM   #9
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Quote:
the filter capacitors were replaced recently and they tested good.
Were they replace for the hum problem.Does the transformer get warm?
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Old 20th April 2007, 10:59 PM   #10
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Quote:
additional secondary winding and adds hum to ground.
Transformer shorted turn causes transformer to heatup and also induces noise-good eye darkfenriz--

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