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Cambridge Audio 540a hum on low and high
Cambridge Audio 540a hum on low and high
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Old 3rd December 2018, 11:01 PM   #1
MattDD is offline MattDD  United Kingdom
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Default Cambridge Audio 540a hum on low and high

A newbie with a 540a question:


I noticed the other day that my Cambridge Audio 540a has a 50Hz hum at low volumes and at high volumes, but in the middle the hum fades to inaudible (around 10 oclock on the volume control).


The hum is there with and without inputs (input leads even disconnected). The sounds varies a little depending on which input is selected, but not much.


I removed the connection between input board and power amplifier which stopped the hum (not sure if something may shut down the power amp if no input connected at all - but didn't seem to trigger the protection circuit).


It seems to point to something in the input stage - but why would the hum fade out at a certain volume and be there on high and low - is there some kind of resonance in the power supply to the input stage at a very specific current draw?



I could just leave it in the 'quiet' position which is about right for normal listening, and adjust the input volume (Hifiberry+ Raspberry Pi), but I am a fellow who likes to mess with things........and it's not exactly a priceless piece of audio equipment.


Do the experts on here have any ideas as to the likely cause?



My search before posting did throw up this question from a long time back that sounded like the same symptoms (for all I know this is the very same amp!) - but no replies then.
Cambridge Audio Azur 540A amp noisy (50Hz?)
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Old 4th December 2018, 04:23 PM   #2
Drakula38 is offline Drakula38  Ireland
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It seems to me that you having problems in grounding, you need to check the board for loss contacts, checks signal path, or you might need to change the potentiometer
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Old 5th December 2018, 06:59 PM   #3
sgrossklass is offline sgrossklass  Germany
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That's some sort of ground loop issue, and unfortunately those aren't usually an easy fix - otherwise the amp designer would have caught the problem.

What I would try first is disconnecting the Pi and using a floating (battery-operated) source instead. Any difference? The offending current may be traveling down the audio ground path from the Pi's power supply EMI capacitor to PE, seeing that the 540A appears to sport a 3-prong IEC power connector.

In light of this, I would further check which of
* RCA / audio ground
* case
and
* IEC mains earth
are connected, and in which way (are the RCA jacks insulated? and maybe follow the earth wire for a bit).

If this turns out to be an IEC Class I device, that would explain a lot. (Not the most practical design for a consumer integrated amp, which is why your average Japanese design is Class II - double insulated. Maybe the toroidal doesn't have a shield winding.) If so, using a line isolator and giving power ground on the Pi an earth connection ought to fix the issue.
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Old 6th December 2018, 10:16 AM   #4
MattDD is offline MattDD  United Kingdom
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Thanks for replies so far..

Yes, it's an IEC Class 1 device - earth terminal on the 3 pin plug is wired to the chassis, and a separate wire leads from chassis to the middle of the main power amp board.



I can't see an obvious way that the earth is carried to subsidiary boards (input switching, tone/volume control etc) but it seems the power connectors include a 0v line, which sounds like it might be the thing.


I've looked at the circuit diagram and measured resistance between various places which are supposed to be earth/ground - and so far it's coming out at <1Ohm which is probably the limitation on my cheap digital multimeter and sounds about right. I'm a bit restricted on test points as I cant get to the back of the boards without removing them which will be a major piece of work.



It is now apparent that somebody has had this apart before - there are a couple of jumper wires that look as if they have been resoldered and the board has been taken out and put back. So I suspect there may have been something damaged in the past and repaired.


The ground loop is definitely nothing to do with the inputs - with no inputs or leads connected at all, there is still a hum that varies with volume setting - so it's being generated 'inside' the amp somewhere.


I will do a more extensive test at the weekend - plug in to a separate mains socket, well away from all other potential EMF interference sources.


Is a simple resistance test and a bit of wiggling of wires (with no 240VAC power connected!) likely to show any earth faults or dry connections or do I need to find an oscilloscope?

Thanks again.
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Old 6th December 2018, 10:28 AM   #5
Mooly is offline Mooly  United Kingdom
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Cambridge Audio 540a hum on low and high
It does sound a little bit like some kind of internal issue... and I've encountered this effect before on diy amps.

It may be very difficult to pin down what is happening but in the first instance you need to be sure that the problem is still present with nothing else connected and that the line level input you select also has shorting plugs fitted for the purpose of the test.

I assume you mean the hum is a pure deep tone (50Hz sine) with no hint of buzz or harshness (which could be 100Hz ripple/rectifier related).
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Old 6th December 2018, 11:01 AM   #6
jaycee is offline jaycee  United Kingdom
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Are R113/C86 stuffed on the board ? They are near the "PREAMP OUT" phono jacks
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Old 6th December 2018, 11:07 AM   #7
jaycee is offline jaycee  United Kingdom
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Never mind. I see that the amp hums on its own.

Does it hum if you disconnect the EARTH pin connection ? my guess is it still does.

Can you take pictures of the bits that look as if they've been fiddled with ?
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Old 7th December 2018, 11:20 AM   #8
MattDD is offline MattDD  United Kingdom
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jaycee - as suggested I've tried disconnecting the earth in the 3 pin plug to the wall - no impact on hum - earth now reconnected.


Hum measured on smartphone audio analyser suggests 200Hz actually - I can't help thinking the variability with volume position is odd - suggests something is acting like a tuned filter.....



Attached a couple of photos of the inside - the speaker board has some jumper leads where the soldering looks a bit different from the others in neatness. Also in the middle the two boards have been split apart and not put back quite in line.


Perhaps these are both original state from the factory - without seeing the back of the boards its hard to see what has been replaced. It looks as if removing the boards to see the back is non-trivial and likely to require some desoldering/resoldering of jumpers which may cause more issues.
Attached Images
File Type: jpg small board 2.jpg (33.1 KB, 55 views)
File Type: jpg Small board.jpg (47.7 KB, 57 views)
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Old 7th December 2018, 03:09 PM   #9
jaycee is offline jaycee  United Kingdom
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200Hz sounds unlikely, but 100Hz is... and would point to bad power supply capacitors.

I would say the issue is in the preamp. The capacitors Cambridge use are not great quality to start with, so I would probably try replacing C32 and C33 to see if that cures the problem.
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Old 7th December 2018, 03:35 PM   #10
diyralf is offline diyralf
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Before you headless change any components, first measure the ripple.
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