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Old 9th February 2013, 08:38 PM   #1
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Default Krell KSA250 large channel imbalance

I measured the right channel of my Krell KSA250 as having a gain of 16 dB when it should be 26. The left channel measures fine. A 1 KHz sine wave going through it seems to look fine. I suspect I should check the feedback resistor but I have no idea which one it is. Anyone know or has any suggestions? I recently performed a recap on it but the problem was there before.

Thanks,
Will
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Old 10th February 2013, 01:47 AM   #2
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pls post schematics

Perhaps it is close to the KSA50? Some indications in another thread I see.

I have no component reference other than a resistor in the area about 12KOhms, and a 470Ohm in series with this ending in a capacitornetwork with two electrolytics in series turned polarities, and a filmcap parallell to theese.

Theese five components should be the feedback.

Values of theese may differ.

But I have no schematics to back it up.
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Last edited by TANDBERGEREN; 10th February 2013 at 01:54 AM.
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Old 10th February 2013, 02:25 AM   #3
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are you using a balanced or unbalanced input signal?? if balanced are you sure both input phases are working? sounds like half of something in the input stage is not working correctly.
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Old 11th February 2013, 10:45 AM   #4
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Half of something in the input stage not working seems plausible. I am using a single ended input for testing purposes and noticed if I remove the inverting pin (3) to ground jumper the input relay repeatedly clicks on and off. This does not happen on the left channel.
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Old 11th February 2013, 11:22 AM   #5
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yup you have an input stage problem! the protection circuit is probably seeing a dc shift when the input pin is removed. something is drifting and when a large enough DC offset is present the relay kicks in.

My KSA-250 has a faint crackle in one channel as it is warming up from cold so I am probably headed towards the same problem!
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Old 26th February 2013, 10:17 AM   #6
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The problem was a transistor in the pre-driver stage (WPS42). I replaced all the transistors in this stage including the WPS92s. All is well now.

As the amp warms up I could see some fluctuation in the DC offset on both channels where it would jump momentarily by perhaps 40 mV positive or negative. Both channels exhibit this behavior though not in a synchronized manner. Sometimes one channel will fluctuate, then the other, and sometimes both at the same time. I wonder if anyone else with a version 3 KSA-250 can study the offset over time to see if your amp behaves the same.
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Old 26th February 2013, 11:11 AM   #7
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I sure wish we could get schematics for these monsters!
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