Ground hum driving me nuts!

[Bufo Bill]

Hi all, I wonder if you could give me some advice on a very little problem I have. When I use my Quad II and QCII, there is a little niggle. At volume on 2, where I listen to my Quad FM1, I hear nothing in the background that shouldn't be there. At Volume 5, where I listen to my Garrard turntable, I hear a slight buzz between tracks or during very quiet passages in play. If I turn it up to 7, it is higher than I need but I can hear it clearly and it resolves itself into an obvious grounding hum.
I have tried every component that I can get at in the preamp to see if there is a bad joint, but to no avail. I will have a look at the amp next.
Now, is this just par for the course on something this old and I just need to be more realistic in my expectations?
Or is this something I can fix? Any tips would be appreciated.
I don't expect a modern audiophile listening experience, I don't have upgrade-itis but I would like my equipment to work to its optimum, whatever that may be.
Cheers from Bill.

[RojDW48]

Do you get this with no input or just when playing a record? Could be a bit of rumble from your TT?

[Bufo Bill]

Hi, this happens with no record playing. I will check the empty inputs later, it's a good thought.
Cheers from Bill.

[GrimJosef]

Originally Posted by Bufo Bill

... I have tried every component that I can get at in the preamp to see if there is a bad joint, but to no avail. I will have a look at the amp next ...

If it's affected by the volume control then I don't think the problem can be in the power amp, which comes after the volume control.

Good luck tracking it down. Hum can be caused by many, many things. Does it get better or worse if you unplug your deck from the mains ?

Cheers,

GJ

[Bufo Bill]

Hi GJ, thanks for responding. It's worse if you unplug the deck.
If there's all sorts of potential problems then I may leave it be.

[Radio Wrangler]

These things are harder to describe than to do.

Define: "If you unplug the deck"

Do you mean mains plug, or the signal lines from deck to amp? The ground wire from deck to amp?

For a test, leave the deck plugged into the mains and running. Unplug the signal leads at the amp end. Now, can you make connection with the grounds of the signal leads only... leaving the signal paths not connected. Try with and without the deck/amp ground wire on.

I'm looking for a hum loop round the turntable and the preamp and the power amp.

David

[yestertech]

A possible cause is an unintentional link between the pickup ground connections and the deck mains earth. Thus is often caused by the way the tagstrip under the deck is mounted/wired.
check this by unplugging all the cables from the deck and measuring with an ohmeter between the mains earth pin and the signal lead grounds - there should be no connection.
If there is the deck needs modifying.
The metalwork on the deck should be connected to mains earth, the signal grounds remain separate and are grounded when the leads are connected to the QCII.
If the tuner and other inputs are fine, then the QCII and power amp shouldn't be the cause.

Many years ago I had an annoying buzz at high volumes on a Garrard SP25 fitted with a magnetic cartridge. The eventual cure was a small metal screen formed around the tagstrip under the deck. Unusual, but it worked.

Incidentally, knowing the usual QUAD arrnagements, How is your main system earthed ?


Andy

[woodchips]

Doesn't, by any chance, use phono connectors does it?

I was very kindly given a cassette deck off the forum, and it hums. It is driving me mad. Mostly fixed by squeezing both the centre and outside connections on the phono plugs, but still there. It also manifested itself by the signal level jumping up and down whilst playing.

At the moment I have got the Philips amp with the covers off looking for dry joints.

Other discovery is that after decades of not being used the pins, all tin plated, have corrosion on them which stops a solid connection.

Good luck.

[ajgriff]

I'm guessing that this is Bill's Garrard 401 with the SME 3009 S2 Imp. If so it's important to ensure that the arm's earth is kept separate from the signal grounds as already mentioned. Also if the 401 is earthed via its mains plug rather than the amplifier the arm can either be earthed via the turntable or the amp but not both. If the 401 is earthed via the amp the arm can be earthed via the same route. The key is to make sure that neither the turntable nor the arm has more than one route to earth in order to avoid a loop. Hope that makes sense.

Alan

[Bufo Bill]

Hi guys, thanks so much for the help! I will go through the tests you discussed and get back to you.
Cheers from Bill.

[Bufo Bill]

David, following your instructions, with the earth lead from the deck removed, the signal earths connected result in an increase in hum. With the earth lead connected there is no difference when connecting the signal earths. This is with the deck plugged in and motor running and a record playing. At no time does the hum disappear.
Even with no connections from the deck attached, there is still a hum.
Andy and Woodchips, your theories are next to be tested.
Cheers guys, Bill.

[Bufo Bill]

I should also note that the hum is present on the empty inputs too.

[G6Tanuki]

When you say "empty" inputs, are they just left free-floating or are they fitted with shorting-plugs to tie the 'signal input' pins to ground?

Unless an input - even an unused one - is properly terminated with a 'load' that matches what the input is designed to see - all bets are off.

[Bufo Bill]

I had never heard of shorting plugs until now, but an Internet search provides me with some info . . . I may knock some up and see if they help.
Cheers from Bill.

[Synchrodyne]

An outside chance, but is there a “backdoor” hum loop via the tuner? If the FM aerial system is earthed, deliberately or inadvertently, then so could be the outer (braid) of the coaxial feeder. As the outer of the coaxial input socket of the Quad FM is connected direct to chassis, then the possibility of a hum loop exists.

My recollection, admittedly from circa 1970, is that if the tuner is earthed, as well as the control unit, then the hum from the resultant loop is inaudible on the radio inputs, but is audible on the higher gain disc input. This was determined when I was playing around with the earthing arrangements for the Quad AMII tuner, but I think it would be applicable to the FM.

Completely disconnecting the tuner (both HT/LT and AF) would quickly check (and probably eliminate) this possibility.

Regarding aerial systems, I understand that it is considered to be good practice to earth both the structure and coaxial braid, and somewhere I have seen that BREMA recommended doing same. In some territories such earthing is mandated by the applicable codes, as well as the requirement that either the main earth spike for the dwelling be used, or if not, that any separate earth is bonded back to the main earth. This creates potential earth loop problems with equipment where the aerial input is not fully isolated from chassis earth. The customary solution was to use in-line “braid-breaker” transformers.

With proper earthing and absence of hum loops, this type of Quad equipment should be essentially hum-free. Again from the distant past, I recall using a Quad 22 with Shure V15 Type II (not improved) cartridge. Output of the latter was really lower than the maximum sensitivity (4 mV) disc input of the 22 expected to see, and so rather high volume control settings were required. This resulted in audible hiss between LP tracks, but no audible hum. The QCII behaved similarly.

This indicates that it is worth pursuing the hum problem through to elimination, and that it is not something “to be lived with”.


Cheers,

[Bufo Bill]

Hi, I have checked that the deck mains earth is separate from the arm earth, as Andy and Alan suggested, a reassuring open circuit reading was obtained. I checked Woodchips phono idea, again no joy. I disconnected the FM1 from the system, we can discount that too.
Someone asked about the main system grounding, which is all done via the preamp ground tag connected to the mains ground. I will check the mains plug again but it was checked not long ago. Synchrodyne, thanks especially for the encouragement to continue.
Cheers from Bill.

[Radio Wrangler]

It begins to look like an earth loop.

Did someone say you were using a 301 and 3009?

If so, I would make sure the arm and support is fully insulated from the turntable metalwork. Then ground the arm from the amplifier... that's the arm alone.

So the arm becomes an outpost (oops bad pun) of the amplifier while the turntable itself is fully independant. That's one ground loop broken!

David

[Bufo Bill]

David, it's actually a 401 in a solid plywood plinth. But yes, the arm is a 3009 improved S2. It is grounded via the preamp.

[GrimJosef]

If you unplug just one of the signal leads into the pre-amp, say the R one, does the hum in the other channel (L, obviously) get less ?

The reason I ask is that if the signal cable screens are connected to one another both at the deck end and at the pre-amp end then the screens themselves will form a ground loop.

Cheers,

GJ

[lazythread]

As it's an SME3009 improved S2, it's possible the headshell or socket pins might need cleaning, or the springs behind are binding. Unlikely with an SME but it's something I have come across.