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Old 14th May 2010, 07:40 PM   #1
steevo is offline steevo  United Kingdom
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Default Quad 909 buzz problem

Just bought a Quad 909 from ebay 6 months old with 6 months waranty left on it. Excellent condition but one problem, a small amout of buzz coming from both speakers when plugged into my preamp. When not connected one channel does not have any buzz at all and is perfect the other has a very faint amout. when I connect each channel to the preamp individually, the bad channel with the very faint buzz increase in magnitude, the other channel has absolutley no buzz at all when connected on its own to the preamp. Connecting both channels causes a small amount of buzz to appear at their outputs. This is obviously destryoing the distortion and signal to noise ratio performance of amplifier and is annyoying me. I would expect more from quad. I Suspect there is an internal gronuding problem within the power amp or grounding system is wrong. Incidental there is no earth connection at the mian suupply. The mains supply is a standard kettle plug but the earth pin has been removed from the socket on the amp.

Anyone any ideas how to fix the buzz problem.

Cheers
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Old 4th July 2011, 05:20 PM   #2
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Default 909 buzzing

Hi, this seems to be a bit of a common problem. Fidele Audio have a good fix but at the moment this is only avaialble with an upgrade. Quad have a fix but it seems there might be some sound quality degredation with there fix.
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Old 5th July 2011, 12:15 AM   #3
Tajzmaj is offline Tajzmaj  Slovenia
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It looks like there is something wrong with your grounding system....?
606,707,909 and smaller 306,520...amplifiers has no center tap at the psu transformer but has floating ground which is fixed by help of 2 small transistors.....and because you remove ground from mains plug all other components connected to the 909 grounds them to this floating ground via 10E resistors on each board. My advice is to remove all the inputs and than observe this buzz. If there is nothing than amp is ok. If there is still some buzz than amp is defective....so send it back ......
Best, Taj
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Old 26th October 2011, 04:44 PM   #4
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after two unsuccessful attempts by Quad to cure my buzz problem and somehow them muting the musically of the amp to boot in their attempts i gave up bit the bullit surrendered nearly four years Quad extended warranty and sent my 909 to Fidele Audio. they fixed it, carried out upgrades for me and removed the Quadlink. Result .........absolutely superb. regards gw
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Old 5th January 2014, 10:01 PM   #5
impuls is offline impuls  Yugoslavia
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I have a quad 606 that has a problem with bad sound that the oscilloscope shows the oscillation a little below of the zero crossing. Oscillation is very small but you can hear as a very bad sound. I went through all the components and all are correct. I put the new capacitors but it did not fix the problem. The only thing that solves the problem is to reduce the supply voltage. When the voltage drops to 200V AC Quad works great. How to raise the mains voltage and increases noise. At higher forces, the noise disappears.
Did anyone have this problem and found that the circuit works on the border of stability?
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Old 6th January 2014, 06:24 AM   #6
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In the amp mentioned we replaced the 3 diodes in the Class A output (in the 606 D4,5,6) with fast diodes (not important) with lower ON voltage (important). However this has not fixed all amps with this problem. Might be worth a try. Also try lowering the value of R8 in stages down to a minimum of 47R. Hope this helps. Peter. Fidele Audio retired.
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Old 6th January 2014, 09:09 AM   #7
impuls is offline impuls  Yugoslavia
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I have already change this diodes with fast and with schottky but not fix the problem. I try with R from 44 to 56 ohm and all the same. Today I'm going to try to heat the components to see what changes the situation.

edit: sorry, i have changed R29 and R30 not R8. I`ll try decrease R8

Last edited by impuls; 6th January 2014 at 09:21 AM.
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Old 6th January 2014, 04:47 PM   #8
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I don't know if you've noted this but the buzz is actually induced from EMF from the transformer. If you move the module away from the transformer the buzz reduces and even goes away. It might help to use some sort of shielding, e.g. copper sheet. If you ever see inside the Quad Elite (the latest version of the 909) they have added a shield to each module. Although I managed to fix a few of these I never really understood the root cause. Thx, Peter.
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Old 7th January 2014, 07:51 AM   #9
impuls is offline impuls  Yugoslavia
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Peter, thanks a lot.
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