Quad 520F

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08-Mar-2019 11:50 PM

rupert

rupert

Posts: 11

Just received my "new" Quad 520F from an auction. It is in very good condition. Boards are 12828-7 versions and the serial number is 5602.

The amp inputs are wired as follows: 1 NC, 2 Hot, 3 Cold.

I am getting quite a loud hum from this setup with input connectors in. When there out the hum disappears.

If I connect pin 3 (cold) directly to the chassis the hum is almost totally muted

I checked the wiring of the XLR's (inside the amp) and they are wired as above but with the shield on the connector body (which also links it to pin 3 (cold))

Logically, this wiring should be the same as connecting pin 3 directly to the chassis but it seems it isn't.

Does anyone know where to look ?

Thanks All.

Greetings


photos are here (copy/paste, ignore subscription)

https://www.dropbox.com/sh/bg2s5r9jz3mfz8k/AAAWVJx0yHSp6_3Z90W48Z2qa?dl=0

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09-Mar-2019 12:16 AM

rupert

rupert

Posts: 11

Correction. 

 Pin 3 (cold) and shield are not linked as I thought. 

But if the are bridged hum disappears as well. This would be an easy fix. Is it ok to do so ?

Thanks


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09-Mar-2019 12:00 PM

Margus

Margus

Posts: 9

hi,

found some schematics for 520F from internet

i assume you have "optional floating input module" installed in your amp. In that case if using unbalanced signal you must connect pin3 to ground, otherwise it will not work. Module has transformer on input, leaving "cold" floating doesn't allow transformer to do it's work. But i believe you will need to raise signal level going into amp, balanced signal's normal level is usually higher.

 Alternatively, you can use unbalanced to balanced converter, like Sonifex 

Sonifex CM-ULX1

wb, margus


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10-Mar-2019 01:02 PM

rupert

rupert

Posts: 11

Hi, and thank you very much for your reply. 

I connected pin 3 (cold) chassis ground. The noise level has gone down significantly but there is still some residual hum when nothing is playing. 

In the second pic you can see the XLR connector with pin 3 soldered to the same lug as the shielding. But I'm still not quite sure this is the best way because there doesn't seem to be any "optional" components in this amp (see pic 1) which would suggest an alteration to the recommended wiring which is marked on the chassis as Pin 1 NC / Pin 2 Hot / Pin 3 Cold (pic 3)

Greetings


amp board top

https://www.dropbox.com/s/oxkhyr0tz70p3i5/IMG_9420.jpg?dl=0

XLR connector

https://www.dropbox.com/s/v2bd210ygyh4wot/IMG_9432.jpg?dl=0

Chassis markings

https://www.dropbox.com/s/4ll59csccs4ht93/IMG_9430.jpg?dl=0

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10-Mar-2019 03:14 PM

Margus

Margus

Posts: 9

oookey..

seems like this is not the same version as on schematics i found. 

First, if you disconnect pin3 from ground and disconnect input cable from xlr, is hum still there? If so, i would suspect problem with amp power supply.

Second, can you track down if input blue wire is connected directly to ground on amp board? More probably it is connected directly or through resistor to this oval black thing slightly above input wires, it looks like input transformer. 

If so, then it is possible that Quad has used a bit unorthodox way for connection. Pin1 is marked as NC, usually this meansnot connected. If you have a piece of microfone cable in hand, you can try to make a cable where on xlr side is connected pin1>shield, pin2>red wire, pin3>blue wire (or whatever color code cable has). On RCA side connect red to signal pin, blue to ground pin, leave shield open. I have heard of such practice, never seen it in use. But theoretically it gives additional protection against external noises.

wb, margus

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10-Mar-2019 11:59 PM

rupert

rupert

Posts: 11

The good news is without input cables connected, there is hardly any hum. So it seems the PSU is fine.

I connected XLR pin 1 to the shield (amp side). But of course, it made no difference since pin 1 is not connected to anything. So, I bridged pin 1 to pin 3 (blue, cold) which cancelled out the noise immediately (see photo of plug) with the connectors in place.

But, as soon as I connect a volume attenuator and / or a digital source (DAC) and turn the amp volume knobs up, the noise starts again.

Greetings



XLR with pin 1, 2 and shield connected together

https://www.dropbox.com/s/syqdm3c0o9atviu/IMG-9433.jpg?dl=0


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13-Mar-2019 12:25 AM

rupert

rupert

Posts: 11

I have reset the input wiring (Pin 1 NC, Pin 2 hot, pin 3 cold) and connected pin 3 to the chassis. This seems to be the best configuration to diminish the noise level. 

I hooked up a Quad 34 preamp to the 520F.

There is absolutely no hum from the amp with only the Quad 34 connected but it starts as soon as a source is attached to the preamp. Same results with a relay attenuator.

Thanks any help would be greatly appreciated.


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15-Mar-2019 11:16 AM

rupert

rupert

Posts: 11

On my research into this issue, I came across a post which mentioned that sometimes the ground screw could come loose and thus cause ground noise. 

On my amp it was securely tightened but just to be sure, I sprayed contact spray onto all the screws making contact to the chassis... and see... this has reduced the hum to an almost inaudible level.

:-)))

Greetings.

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