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Old 2nd November 2008, 12:46 AM   #1
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Default Quad 405 hum

I recently posted a thread about a humming Avondale modified Quad 405 power amp - plenty of readers but no answers.

Has anyone else had a 405 modded by Avondale that hums ?

The main ps caps are about 10 years old as are the Avondale mods.

It get's quite warm too particularly on the right channel.

Any simple but effective component change suggestions
out there to make it quiet ?

Thanks

Andrew
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Old 2nd November 2008, 01:21 AM   #2
MadMutt is offline MadMutt  Australia
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I believe the standard fix for a humming 405 is to replace all electrolytics.
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Old 2nd November 2008, 08:29 AM   #3
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Default Quad 405 hum

Mad Mut Hi

Thats what I wanted !!

Great thanks

And
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Old 2nd November 2008, 08:30 AM   #4
Mooly is offline Mooly  United Kingdom
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Did it hum before it was modded ? What were the mods ?
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Old 2nd November 2008, 09:32 AM   #5
latala is offline latala  United Kingdom
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I can remember working on a 495 with an intermittent hum ! The problem was traced to a poor connection on one of the psu capacitors merley touching the connector could stop it humming
regards Trev
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Old 2nd November 2008, 11:08 AM   #6
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Replace C5!
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Old 2nd November 2008, 12:23 PM   #7
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Andrew,

why not give Les at Avondale a ring? He has been known for the good support of his creations...

Best,
Oliver
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Old 2nd November 2008, 08:11 PM   #8
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Default Quad 405 Hum

Thanks for the replies !

To Mooly - I got it pre modded and therefore have no experience or knowledge of how it used to be.
The boards look like no other I've viewed - completely different.
It does use MJ 15003 x 4 successfully and when playing music
it sounds OK when the volume is above the hum level.
It's also quite a bit warmer on one side of the heatsink than the other.

To Latala - The connections to the PS supply caps are all good.
I did attempt to agitate them ( gently ) whilst the amp was at idle
and there was no change. - thanks for the suggestion though.

To Roger Gustavvson - I'll do C5 as soon as possible and report back - many thanks

To Core Dump - I think your suggestion is probably the way to go
ultimately as I'm not completely comfortable with ' tech ' stuff.
I've built a 3886 based Gainclone ( with good instructions and diagrams ! ) and built a couple of nice passive attentuators but that doesn't qualify me at all when things get complicated.
I'll try Roger's C5 suggestion and then make the call to Les !!

Many thanks to all for doing some thinking for me !!

Regards

Andrew
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Old 3rd November 2008, 07:29 AM   #9
Mooly is offline Mooly  United Kingdom
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Does it hum with no inputs connected or more specifically with the inputs shorted. Are you actually sure it is the power amp. Worth asking.
If one side is warmer than the other that suggests the outputs on that side are drawing more current for some reason, unless there is some other component ( regulator been added or something ) that is causing it. The original 405 runs at fixed and what is really zero bias, so should run cold.
It's an unknown quantity I'm afraid.
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Old 3rd November 2008, 08:37 AM   #10
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Default Quad 405 Hum

Mooly thanks.

Absolutely no hum with nothing connected to the inputs
and speakers connected.

What's odd is that I'm running it with a passive stepped attenuator PRE that's completely quiet with another power amp.

Only other active component in the chain is the CD player.

Regards

Andrew
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