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Old 22nd September 2014, 04:08 PM   #1
mazur is offline mazur  United States
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Default Unusual buzzing situation

Hi.

I have a Soundcraftsmen A100 Pro amplifier. MOSFET outputs and input volume control. I have an audible buzz coming through the speakers as follows:

- with no input connected, buzz is gone when volume is all the way down and increases steadily and stays there with the volume all the way up.

- with an input connected, buzz is gone when volume is all the way down, then increases as volume is turned up, then it disappears when it's all the way up.

Same thing for both channels. Completely re-capped and pots cleaned. Ground is floating (not connected to the case). Couldn't find any ground loop.

It sounds very nice and with the volume all the way up it's dead quiet. I would like to use the volume control since this is driving my horns on a bi-amped system.

Any ideas?
Elias
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Old 22nd September 2014, 04:19 PM   #2
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Quote:
Originally Posted by mazur View Post
Hi.

I have a Soundcraftsmen A100 Pro amplifier. MOSFET outputs and input volume control. I have an audible buzz coming through the speakers as follows:

- with no input connected, buzz is gone when volume is all the way down and increases steadily and stays there with the volume all the way up.

- with an input connected, buzz is gone when volume is all the way down, then increases as volume is turned up, then it disappears when it's all the way up.

Same thing for both channels. Completely re-capped and pots cleaned. Ground is floating (not connected to the case). Couldn't find any ground loop.

It sounds very nice and with the volume all the way up it's dead quiet. I would like to use the volume control since this is driving my horns on a bi-amped system.

Any ideas?
Elias
Is it this model?
Sweet Soundcraftsmen Score
Where is the case (chassis) connected ?
It must be connected with GND at least about a resistor and/or (parallel switched) cap (go to this example at Mission Cyrus integrated amp
http://img84.imageshack.us/img84/8545/img7505b.jpg
here it is R121 near the volume control shaft at bottom, right site.
In case of bown resistor one have the same situation as you mentioned at post #1.

Last edited by tiefbassuebertr; 22nd September 2014 at 04:24 PM.
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Old 22nd September 2014, 05:07 PM   #3
mazur is offline mazur  United States
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Quote:
Originally Posted by tiefbassuebertr View Post
Is it this model?
Sweet Soundcraftsmen Score
Where is the case (chassis) connected ?
It must be connected with GND at least about a resistor and/or (parallel switched) cap (go to this example at Mission Cyrus integrated amp
http://img84.imageshack.us/img84/8545/img7505b.jpg
here it is R121 near the volume control shaft at bottom, right site.
In case of bown resistor one have the same situation as you mentioned at post #1.
Yes, it's the same model, but with volume control.

The ground is not connected to the chassis. It's floating. There is a common ground in the board that holds all capacitors. The volume pots are connected to the ground point. There are no additional resistors from the pot. When all the way down, it grounds the input signal. When all the way up it provides a 50k input resistor to ground and a direct path for the signal to the input cap.

Elias

Elias
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Old 22nd September 2014, 06:55 PM   #4
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Quote:
Originally Posted by mazur View Post
Yes, it's the same model, but with volume control.

The ground is not connected to the chassis. It's floating.

There is a common ground in the board that holds all capacitors. The volume pots are connected to the ground point. There are no additional resistors from the pot. When all the way down, it grounds the input signal. When all the way up it provides a 50k input resistor to ground and a direct path for the signal to the input cap.

Elias
This is the reason for the unwanted buzzing noise. Connect the chassis to the GND connector of the volume control or the input female connectors and observe, what happens in the max. position without input signal and without connected preamplifier resp. source device (i. e. open input plugs).
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