Ground loop hum between preamp and amp. Help!

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Member
Joined 2020
Paid Member
McIntosh MA7900 (via unbalanced RCA outs...no balanced so unbalanced my only choice) to MC275 tube amp.

If things are hooked up with my 89 dB. speakers...can only hear it if in a DEAD quiet room and my ear is right up against the tweeter, there is the faintest noise I can barely hear. However, on my sensitive, no-crossover, full-range driver speakers (which are brutally revealing), I get some annoying hum. The hum is the 60 cycle ground hum (as validated by some YouTube clips).

--If the speakers are connected to the SS amp (MA7900)...nothing
--If the speakers are connected to the MC275 and I switch off the MA7900, goes away 100%
--It is not volume, input sensitive (same level from speakers no matter what, even if I mute the MA7900 or shut off the output to the amp).
--If I power off and/or unplug the MA7900, the hum goes away, and instantly comes back when I switch it in (even during its ~10 second warm up where it is not outputting any signal)
--Both power cables are connected to the same bank on a Panamax 5300 power conditioner
--If I unplug the RCA to one channel, the hum goes away 100% and remains on the other channel
--I tried plugging both directly into the wall outlet and it didn't change (meaning there was nothing else within 10' plugged in to power, including anything like phone, coax, etc.)

If it were not for the sensitive speakers, I would never have noticed. Even with the sensitive speakers, it is just *barely* noticeable from listening distance, but feels like it should not be there at all.

However, I now have that 'itch' to try and fix it and curious where to start. Based on the above, can you help me isolate where to start, what to try? If the answer is get a real preamp with balanced connections...well that would work!

Thanks!!!
 
Last edited:
Several questions. Do all units use 3 wire power plugs? IF is is a ground loop issue you do not have to go
balanced line at both ends. Balanced on the receive side only will get rid of your problem. You can try a cheapy
audio transformer between the units. This is only a test. If the transformer fixes the hum you can add a balanced
line receiver to the amp. This can be a laser trimmed amp from THAT corp or Analog Devices OR Burr-Brown
(T.I.) OR you could build your own using matched resistors and an opamp which might work almost as well as the
first 3 options. BTW there are commercial products using 'balanced' inputs with an unbalanced source.

You can TEMPORARILY use a ground lifter under a 3 wire plug for testing. If that solves the problem the pseudo
balanced input will fix the problem permanently. I personally try to avoid audio transformers because of response,
distortion, size, cost and magnetic sensitivity.

 
Member
Joined 2020
Paid Member
Thanks for the reply. Yes, all of these devices use standard 3-prong power cords (including the power conditioner, and modern house with modern wiring...no retrofit or old wiring).

Yikes, while I appreciate that solution, would love to find a fix with what is there (not having to add parts). I guess asking another way, WHY would I have this ground loop at all? What is the root cause? Can I target and fix the root cause?

I have a few other things to try, among them hooking up a Rotel integrated amp (that has preamp RCA outs) to see if that changes anything.
 
Member
Joined 2020
Paid Member
Correct. I am using the second set of variable RCA outputs to drive the MC275. This is explicitly the purpose of them (to bi-amp, run another amp, another amp for another pair of speakers, etc.).

I disconnected the other devices that are connected. Turntable (using the internal phono pre...ground wire is connected) and a few digital sources. I unplugged these and it didn't make any difference. My troubleshooting was to isolate the MA7900 and MC275 from everything else in the system.
 
Member
Joined 2020
Paid Member
Try only connecting only the single pair of RCA cables, and the speaker wires.
Now plug the two AC cords into the same output power strip. Is the hum still there?

Yes, I did the ol 'remove things until the problem goes away' method, and unfortunately I was left with no change. I do stand corrected, I did not plug both into the wall outlet and bypass the power conditioner completely (I did plug the MC275 into the other outlet on the wall below where the power conditioner is connected...with the MA7900 still connected to the conditioner).

Sucks with a tube amp I have to power cycle to try different power plug options - which is why I am trying to see if there is something else I can focus in on.

Is there any chance this is caused externally (issue with the outlet or further away from the system), or does that have to be localized?

Thanks!
 
Member
Joined 2020
Paid Member
Cycling tubes, especially power tubes, on and off isn't doing them any favors in terms of longevity. You can hear them clicking as they heat up powering on, then as they cool after you switch off the power. I'm in the 'on for the day, off before bed' school of thought for tubes vs. turning them off for an hour or two, then back on again. If someone has a different point of view I am very much open minded to hear this.

Instead to dumbly trying things, if there is likely one root cause, I'd like to start there and give the tubes a break.
 
Member
Joined 2020
Paid Member
Sure, here is an image (from their site). I cannot comment on what has changed (or not) since the original.
 

Attachments

  • MC275B Right.jpg
    MC275B Right.jpg
    126.1 KB · Views: 95
I have a model MA7000 in my workshop. Signal ground is connected to
chassis. It will be the same with your MA7900.

The same connection signal ground to chassis and so safety ground is in
the new MC275, visible on p. 7 of the service manual. This alone is reason
enough for a ground loop between both amps.

It is bad practice to connect SG to chassis directly.

The connection should be replaced by a resistor of 10 .. 30 ohms, 2 ..5 watts.
It is sufficient to do so in one amp, and easier in the 275.

This should be done by a competent electrician or repairman only.
 
Member
Joined 2020
Paid Member
Thanks! Great info.

OK can I ask this...would using balanced interconnects (from a properly-designed preamp, like anything McIntosh builds) solve this issue?

Anything else that would help fix this, or at least improve it?

Again, it is not loud but obviously reducing it would be nice.
 
Member
Joined 2020
Paid Member
As in, the hum is inherent to the electronic design vs. some flaw elsewhere...and you won't speculate on a miracle cure (like putting one of those $300 noise stones in top of it!)?

What resistors are you referring to, something different than your suggested fix I assume?
 
I have a model MA7000 in my workshop. Signal ground is connected to
chassis. It will be the same with your MA7900.

The same connection signal ground to chassis and so safety ground is in
the new MC275, visible on p. 7 of the service manual. This alone is reason
enough for a ground loop between both amps.

It is bad practice to connect SG to chassis directly.

The connection should be replaced by a resistor of 10 .. 30 ohms, 2 ..5 watts.
It is sufficient to do so in one amp, and easier in the 275.

This should be done by a competent electrician or repairman only.
Perhaps you should elaborate more, You used "SG", but did not indicate if you meant signal ground or safety ground. If safety ground is via a 10 to 30 ohm resistor, it will defeat the safety.

Jn
 
Member
Joined 2020
Paid Member
Confirming 100% if I isolate things as much as possible, still get the hum.

I have a 'outlet splitter' (like what you buy for $5, plug into one grounded outlet, get 3 grounded outlets, no surge, nothing, just a simple block). That, with the MA7900 and MC275 connected, issue remains.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.