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-   -   Pre-Amp distortion (http://www.diyaudio.com/forums/solid-state/221461-pre-amp-distortion.html)

Jolida 13th October 2012 06:10 AM

Pre-Amp distortion
 
I am getting a strange distortion from both channels of my pre-amp (Harman Kardon Signature 2.0). I had not been using it since more than a year now. When I recently started using it, I started getting a strange kind of distortion, as if the speaker cone is torn. The vocals sound torn especially with recordings of a high input level. I tried eliminating every piece of equipment & interconnect in the line-up & the problem lies with the preamp. Now it's getting worse, with the balance shifting a lot towards the left channel. The left channel sounds louder & brighter than the right. Please Suggest :(

MagicBox 13th October 2012 06:32 AM

Sounds like dirty potentiometers but there's also a chance capacitors in the signal path or powersupply have worn. It would be worth looking into these first.

Jolida 13th October 2012 06:38 AM

It's a digital volume control. Will this still have a potentiometer inside? There is a big volume
Knob on the amp but it changes the volume digitally.

MagicBox 13th October 2012 06:54 AM

It's not the knobs then, no mechanical volume / balance control :) What remains then would be a check-up on the important capacitors inside the amp. Are you familiar with working a bit on electronic circuits?

jitter 13th October 2012 06:56 AM

According to page 14 of the service manual of that amp, the volume control is electronic, i.e. the knob doesn't operate a conventional potentiometer.

I take it you tried every input on the preamp and get the same result on each input?
Since input selection and volume control seem all electronic in this device, I'd start with measuring if all voltages in the PSU are present and correct.

Jolida 13th October 2012 06:59 AM

Yes I'm quite ok at electronic stuff but not anywhere close like u guys :)
If u could please guide me as to what has to be looked at, I would really appreciate.

Jolida 13th October 2012 07:03 AM

Yes I have tried all inputs on the amp & the problem still remains

MagicBox 13th October 2012 07:08 AM

I just peeked at the service manual as well and you'll have quite the number of components to check! Like jitter told, using the manual, knowing a bit about the used ICs, you could check up on all the voltages to see if they are proper; measure the supply output and if that is ok, measure the supply pins of various ICs throughout and if that looks okay, try to find any capacitors that may look like they're puffed up or leaking or otherwise show anomalies.

It's going to be quite the hunt.

Edit: Oh and forgot to mention another 'easy' checkup: Internal connectors gone bad (bad contact) so reseating those may also help.

Jolida 13th October 2012 07:16 AM

Great. I also have an Oscilloscope but have never learnt how to use it to debug such issues. Or a multimeter is just enough ?

MagicBox 13th October 2012 08:09 AM

To measure supply voltages a multimeter is just fine. The scope starts to become useful when you also have a device to generate a test signal, often a sinus. Then, you just probe the signal path starting at the input all the way towards the output and see where the signal goes weird.


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