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PostPosted: 22 Jun 2017, 04:03 
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Joined: 22 Jun 2017, 03:59
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Cheers,

well I've heard a few kind of buzz / hums in amps, but this one is a strange case for me.

It's not a DIY amp, but there will be som DIY fixings, so I turn to you guys - bcs who knows how to fix amps better, than the guys who build them? :thumbsup:

I've bought this beautiful pre-owned CA Azur 640A amplifier , but there's a problem - nasty hum / buzz. The more volume you set, the more you hear it.
Sounds like this: https://www.dropbox.com/s/dxk9agjdi5qz9 ... 6.WMA?dl=0

Any ideas what could be the problem and how to diy fix it?

Thank you all for your advice!


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PostPosted: 22 Jun 2017, 18:05 
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Joined: 13 Feb 2013, 11:02
Posts: 38
Location: Canada
Check the input jacks, and board traces around the jacks. I've even seen these types of jacks break internally and you can't see the break.

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DTS Audio Electronics
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PostPosted: 23 Jun 2017, 01:46 
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Joined: 22 Jun 2017, 03:59
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blackdog wrote:
Check the input jacks, and board traces around the jacks. I've even seen these types of jacks break internally and you can't see the break.

Hi, thanks for the advice, I will check it for sure. Would this be valid also for this case, when also with no input connected the hum is there?


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PostPosted: 23 Jun 2017, 01:49 
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Joined: 22 Jun 2017, 03:59
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Oh and one correction for "The more volume you set, the more you hear it."
Not true, after further testing, it goes like this:
- you can hear the hum on minimum sound level
- adding volume = decreasing hum, until I find a "sweet spot" where it is only slightly humming...
- after that it goes up again


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PostPosted: 23 Jun 2017, 09:16 
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Hi, I ran into that problem a long while back with some S-5 K series amps. The case of the volume control was not grounded and was picking up emi and crud. Grounding it solved the problem. I would as others said look for a ground connection that is not good or disconnected. Also verify that the unit is not picking up something else from the power line or a nearby appliance. Some lamp dimmers, TVs, displays and florescent lamps put out stuff like that.

Good listening
Bruce

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PostPosted: 23 Jun 2017, 09:47 
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In rank order:

a) Short the inputs. See if the hum goes away. It should go away absolutely to dead-silence!
b) Check *ALL*, repeat **ALL** the capacitors, with special attention to the filter caps. As this is happening on both channels, that would be my first suspect after over-sensitive inputs as above. A filter cap just starting to leak will often cause a bee-like buzz that is NOT a hum!!
c) Look for broken traces, cold-solders and similar mechanical issues.
d) Look for local interference: Dimmer switches, cheap LED lamps, cheap CFL lamps, ceiling fans with bad caps, noisy switching power-supplies, ignition transformers, and similar.

That is the easy stuff.

The last time I had a SS amp with this symptom that a) through d) did not 'fix', I had a (1) leaky driver transistor in each channel. I replaced the pair in each channel as they were in a PNP/NPN configuration with a pair of 'equal' units. I hesitate to use the term "matched pair" when in PNP/NPN. I also upgraded considerably in ruggedness. However, it is very hard to discern this level of problem without a scope or unless you are very familiar with the amp circuitry.

Note: the above amp had worked for years and developed the buzz pretty much overnight. It also had a quiescent-state constant low-voltage power-supply, so I suspect the failure was caused by a spike.

If in only one channel - a different set of issues applies.


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PostPosted: 03 Jul 2017, 02:52 
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Joined: 22 Jun 2017, 03:59
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Hi Peter, thank you for your thorough answer... tried the step a) - nothing happened. so now i guess I'll try the rest of them.... but I keep wondering, why nothing happened when i shorted the inputs...


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