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Old 6th January 2008, 08:51 AM   #11
GeeVee is offline GeeVee  Australia
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Originally posted by Mooly
Hi George, Earlier on I asked was it a hum or a buzz(Harsh sounding), as this is a big clue as to it's origin.Your circuit shows a +/- 15 volt supply, (the 7815/7915 regulators.Do these feed just the phono amp ? Anyway first off you need to be sure that all rails are clean.You have to use the correct grounds when making measurements like this, also if your 'scope is earthed (mains earth), as well as the amp this can cause false readings.Is the amp earthed through the P.S.U. ? To check the rails connect probe ground to main signal ground on the phono P.C.B. and measure ripple on +15 -15, & +35 -35 .
Hi Karl.

Thank you for your response.

Your comments have certainly given me some more ideas to try out later tonight.

The noise is definately more of a low level buzz rather than a hum.

The buzz is present when the preamp is connected into the system, when the phono is selected.

The only supply to the phono board is the +/- 35V. the 15V rails are for the high level stage.

I have not yet tried the shorting of the phono plugs, as I have been concentrating on the PSU measurements.

Tonight I will re examine the earthing, and try to find a "clean" earth and re-examine the results on the scope.

PS those electros on the phono board are a secondary filtering of the 35V rails.
The original specification called for 1000uF capacitors, but I used 2200uF, thinking that more would be better.

Thank you for taking the time to assist me.

I will post my results later on tonight.


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Old 6th January 2008, 09:52 AM   #12
Mooly is offline Mooly  United Kingdom
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Do you have circuit for signal stages, if phono stage runs on +-35volts would have thought there would be more regulation for the phono section.
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Old 6th January 2008, 12:28 PM   #13
Gordy is offline Gordy  United Kingdom
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Originally posted by Mooly

Let us know how you get on and remember EARTHING,EARTHING EARTHING.
... and Gordy chips in with his standard answer...

George, you may have seen these references already, but if not I recommend that you spend an hour having a read.

Ground loops



Balancing and grounding

Grounding and shielding from Rane

Interconnection from Rane

Grounding in ADCs

Supply and ground in digital systems

Good luck.
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Old 6th January 2008, 01:21 PM   #14
SY is offline SY  United States
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GV, the picture didn't help as much as I would have liked- too much hidden. And it's possible that the ground layout on the circuit boards is poor, which will not be trivial to fix.

The sort of ground loops that are caused by earth grounding aren't the issue here- this noise appears to be from power supply grounding and the strong possibility that grounds from other parts of the circuit are connected in different spots within the high-current loop between the rectifiers and smoothing caps. I'd pull the PS board and examine how the grounds are applied.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 6th January 2008, 01:25 PM   #15
AndrewT is offline AndrewT  Scotland
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I agree with Sy, thinking this is a PSU problem.
It could be caused by bad wiring or bad PCB or bad components.

Powering up the PSU alone and testing/measuring how well it works with various loads on it may be enlightening.
Then check that the installation matches the bench results.
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 7th January 2008, 02:07 PM   #16
GeeVee is offline GeeVee  Australia
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Hi Karl, Sy, Andrew, Moody and all others.

Once again, thank you very much for your assistance.

Last night, I tried to locate an alternative earth / ground location for some CRO measurements.

This did not bear any fruit.

I also tried alternative wiring / grounding arrangements, and this did not alleveiate the problem.

AS you gentlemen have suggested, I am now more than ever convinced that the problem is within the PSU.

Tomorrow night I will strip the preamp down, and test the PSU in isolation, with some resistive loads. I will record the measurements, and photograph critical CRO screen shots.

As i mentioned at the start of the thread, this preamp is a complete rebuild, which includes the fabrication of a new PCB which contains the PSU, high level and muting.

I did not make any major alterations to the track layout, especially the grounding and main filter cap area. The only area where i did a substantial track redesign was for the 4 voltage regulators / pass transistors.

Atatched is a copy of the original board layout, as published by AEM.

I will post a copy of my layout very soon

Thank you


Attached Images
File Type: jpg psu.jpg (88.2 KB, 134 views)
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Old 10th January 2008, 12:05 PM   #17
GeeVee is offline GeeVee  Australia
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Location: Adelaide, South Australia
Default Progress Report.

After three nights of investigation work, I am now more convinced that ever that the problem lies within the power supply.

Unfortunately, despite my best efforts, I am still unable to resolve the problem.

As mentioned in one of my previous responses, I did alter the circuit layout, but only the pass transistors / regulator area. The 0V layout remained essentially the same.

This is what I tried:

1. Looked at different sampling points for the probes.

2.Disconnected the phono stage and loaded up the power supply with a resistive load.

3.Tried different earthing techniques

4. Replaced the main filter caps.

5. Changed the regulators and pass transistors

After all of this, I did manage to get a reduction in the charge peaks, but that is about it. The problem still persists.

I suspect that the problem is in my modified board layout. I have not given up just yet. This weekend I will be building a new external power supply, and conducting further tests.

My sincere thanks to all that have offered advice and assistance. It is greatly appreciated.

I will post my next round of results as soon as possible.


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