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Old 23rd August 2008, 08:21 PM   #41
jaycee is offline jaycee  United Kingdom
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Twosheds, you are correct, the presets on the board are how each channels bias is adjusted.

To adjust, first turn both presets to minimum. Connect a voltmeter on the mV range across one channel's resistor - for example, across R41. You are best doing this with some small clips, or tack solder some wires to each end. Make sure you've not got any shorts, and switch on with no speakers or input devices connected.

Adjust the channel's preset slowly until you read 7mV. As the amplifier warms up, this reading will increase - adjust the preset lower to keep 7mV. This will probably take about 30 minutes.

Then repeat with the other channel, using eg R141 to measure. You don't have to get it spot on - roughly 7mV will do. A bit less is fine, especially as this amp is very skimpy on heatsinks!

The Alpha was a very nice amp in it's time, but these days it's a little long in the tooth. It's certainly fine for your son's intended use - better than any PC speakers that's for sure!
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Old 26th August 2008, 09:06 PM   #42
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Thanks again guys. Followed your instructions.

The presets are rather sensitive aren't they? I managed to get both readings to be floating around 7mV. They were quite higher when I started.

The amp is running much cooler now it appears.

Although using a PC for input, the amp is driving Wharfedale Diamond 4's, which sound quite nice in this combo.

Thanks again,

twosheds.
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Old 27th August 2008, 06:31 AM   #43
Mooly is offline Mooly  United Kingdom
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That's good
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