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trustchild 3rd December 2007 04:36 AM

Kenwood KA-9100 Amplifier Buzzing

I have a Kenwood KA-9100 amplifier.
It emits an incredibly loud buzzing sound when the power switch is turned on.
The buzz is coming from an electromagnetic switch on the same circuit board as the fuses.
The electromagnet doesn't seem to be generating enough force to hold the moving part of the actuator still. Instead it vibrates and buzzes loudly.

Can anyone help me fix it? Do I need to replace the actuator? Where would I find that part? Or is it repairable by cleaning it? How would I do that?

Thanks for any thoughts.


K-amps 3rd December 2007 05:01 AM

Check/ replace the diodes and caps on the relay circuit. You should be good to go. Both cause buzzing when failed/ aged.

EchoWars 3rd December 2007 10:53 AM

2 Attachment(s)
That would be a start, but the buzzing is the relay not getting enough voltage to turn on. Two common causes are a weak driver transistor for the relay, and a failing protection diode that is mounted (electrically) across the relay coils to protect the driver transistor from inductive spikes as the relay is turned off.

The diode is mounted at the rear near the relay, so that's not a problem. The driver transistor is mounted on one of the boards that the four large power supply caps are soldered to, and is a major operation to get to and replace. If you go to this much trouble, it would be pretty foolish not to replace the three small electrolytic caps on it.

One last thing I just remembered is that the speaker select switch is part of the current path to the relay (on a working 9100, you'll note that the relay turns on and off between speaker selections when moving the switch). It is possible that you might be losing a bit of voltage through this switch...clean it.

Here's the board the driver transistor and protection circuit is mounted on...

trustchild 4th December 2007 02:38 AM

Thank you both for your responses.

I fixed the buzz.

This 9100 is new to me, so I know nothing of it's history. After taking the moving part of the relay off I was able to slightly bend the four metal contacts. Just enough to let the electromagnet pull the moving part completely up and closed. I'm no electrical engineer so I felt pretty smug when it worked. Of course there was some cursing involved when the tiny white wires broke off of their soldered connection points. I'm a hack with a soldering iron, but I got it all put back together and now it turns on, doesn't buzz, and it turns off.

I'm still adjusting to the sound of the amp. This is the highest power amp that I've owned, and I'm quite unaccostomed to the intensity of the dynamic range and the transparent color to the sound. It's open and intense sounding. I'm noticing a lot more of the high frequency audio data. My ears are tuned to hear high stuff and I'm hearing a lot more of it with this amp. It's making me rethink my audio sources cause all of the compression artifacts in my mp3s are glaringly noticable.

Thanks also for the circuit schematic. I'm saving it for future reference.

K-amps 4th December 2007 02:49 AM

I am waiting for the day when Mp3's will sound nice on my main system.... ;)

jaycee 4th December 2007 10:01 AM

There are better codecs than MP3... Musepack for example, or if you want to be a purist you can use a lossless format such as FLAC or WavPack.

K-amps 4th December 2007 10:54 AM

True... but it helps me not when my bro comes with his stack of Mp3's and says I thought these would play well on your high end system.... I am speechless... and in no mood to explain why.

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