Quad 33/303 baffling hum

[PaulR]

I have recently bought the above and carried out the Dada upgrades to both of them. Everything works and sounds good, however there is an odd low level buzz/hum from the left channel.

It occurs when some things are connected, specifically a tuner (any tuner) and the TV set. The CD player doesn't seem to make it hum. Earthing the tuner, or not, doesn't stop the hum. It happens with a Stereofetic tuner and a much more modern Cambridge one.

It happens whether or not the item connected is switched on or even plugged in.

It is independent of the volume control but is somewhat affected by the balance control.

It is independent of which source button is selected

It remains on the left channel if the circuit boards on the pre amp are swapped but stops if the left board is removed.

It stops if the left channel is disconnected from the tuner. But not the other way round.

As a matter of interest it hums quite badly on both channels if the earth to the record deck is disconnected whether or not "Disk" is selected.

Help!!

[PJC58-Hythe]

Have you tried swapping the left and right leads over to see if it moves to the right channel.

[PaulR]

Yes, whatever I do it remains on the left channel. It also occurs on both Radio inputs and using different 5 pin DIN adapters

[frsimen]

Hello Paul,

I've had a similar problem with a Quad 33/404 combination. Connecting the shield from the lead connected to the television to the earthing terminal on the 33 made a big improvement in my case.

I also had a lot of trouble with breakthrough from the television when any other input was selected, but only if something was plugged in on that input! Adding an attenuator on the output from the television helped.

Paula

[PaulR]

Thanks Paula, I will have a poke around with an earth lead. I feel it has to be something to do with earthing. What is so odd is that it hums even if the tuner is unplugged.

The inputs are quite sensitive and part of the Dada update is to replace some resistors to reduce the sensitivity. This reduces it for all inputs including the Disk one so I have also relied on an external attenuator. I wondered if the hum was something to do with that but it remains whether or not the attenuator is used.

[fetteler]

Did it hum at all before you did the updates?

Steve.

[PaulR]

I can''t say. I briefly tested it with my phone to make sure it worked but I didn't really examine it.

[fetteler]

Ahh, ok.
I was wondering if somehow the hum was a result of the updates. It might still be worth assuming that it is and double triple super checking that the work done is 100% correct and all components are in the right place/right way round, all joints good and no little bits of solder bridging tracks etc.

Steve.

[PJC58-Hythe]

I agree with Paula it suggests an earth loop, although I would have expected it to be on both channels. Have checked the continuity of the earth (shield) connection on the left channel?

[Craig Sawyers]

I had an irritating problem with intermittent channel loss, and odd noises from my unmodified 33.

Someone on the list said - resolder all the circuit board connections. Which I did.

Problem solved - it has not missed a beat since.

I'm not saying that is the problem the OP is reporting - but it was so in my case.

Craig

[PaulR]

Right - thanks all.

Regarding the modifications, they are done to the two circuit boards, the disk pre-amp board and the power supply. Swapping the boards makes no difference and I doubt that the disk or power supply ones would cause a fault on one channel only. Having said that I will go over the soldering.

I think I agree with the hum loop/earthing theory.

[PaulR]

As Paula suggested earthing the tuner to the earth terminal of the 33 has stopped the hum. There is still a slight noise which sounds as though it is picking up some SMPS interference but it is much better now.

I have just noticed a nice Rifa cap across the mains supply. Odd that Dada didn't send a replacement for that. Anyway I will replace it with a class X cap

[PaulR]

Swapping the Stereofetic tuner back in stopped the SMPS noise. Totally different eras!

I'm not sure why connecting the tuner to the earth connection stopped the hum. The screens of the connecting leads are also connected to it. Strange things, hums!

[frsimen]

That's a good result.

I think the hum problem's source is from low level mains leakage currents in the tuner finding a path to earth via the DIN socket. I get the impression that the earth path there isn't the best and some of the hum gets picked up by the nearby signal wiring. Wiring the shield directly to the earth terminal provides a lower impedance path to earth and shunts the leakage current. This is more of a problem when connecting to equipment that has class Y capacitors connected to a non earthed chassis.

Paula

[frsimen]

The Rifa capacitor is a snubber, with a resistor built in. The capacitor part is a class X but ordinary class X capacitors don't have the built in resistor.

Don't be tempted to leave the snubber out if you value your loudspeakers and hearing. Without it, turning the Quad 33 on or off sounds like gunfire!

Paula

[PaulR]

Right, thanks Paula. I will bear that in mind

[PaulR]

I wasn't totally happy with the solution to the hum problem with the 33 as grounding the radio (now an FM3) to the 33 helped but didn't completely remove it. It was also there when the output from the TV was connected. This was whether or not either the TV or radio was plugged in, or whether its output was selected.

I reasoned that the only connection both had and the CD player (which does not hum) hasn't is an aerial. Unplugging the aerial from each stopped the hum completely for both the TV and radio. I don't know what the process is for the hum to be introduced. The TV aerial comes from another room via a signal amplifier but that is not earthed and unplugging it doesn't stop the hum.

Can anyone help please?

[Cruisin Marine]

Does it go with the TV unplugged (not just switched off)?
This applies to anything else in the vicinity of the Quad too.

[PaulR]

Having googled this it seems like quite a common problem.

Would something like this stop it?

https://www.amazon.de/-/en/Wentronic...6G52S%3Fth%3D1

Or this?

https://www.amazon.co.uk/Isolator-El...096VLG1RN?th=1

[PaulR]

Sorry Crusin Marine I didn't see your response.

Yes it happens even if the TV or radio is unplugged

The CD player doesn't cause a hum though