Multiple Altec 9440A amplifiers ground loops

[rghinton]

In my home theater I have 4 Altec 9440A amplifiers mounted in a metal relay rack along with my surround sound processor and DVD/CD player. This is a new arrangement and now I am starting to fight ground loops. If I connect the left and right channels out of my processor to one of the amplifiers, there is no hum. If I add another amplifier from my processor for the center channel, there is quite a bit of hum. Any amplifier by itself is quite, it is just when I try to add the second amplifier that the hum starts. When I have time, I am going to take the 4 amplifiers out of the metal rack along with the processor and see if the hum is still there when I add multiple amplifiers. The 9440A is the most musical amplifier that I have every heard. I like it so well that if I cannot solve the ground loop problem I will have a home theater with 2 channels.
Note that I am trying all of this with unbalanced cables.

[bfish]

Are you running jumpers or transformers in the input sockets?

[rghinton]

I am using the unbalanced 1/4" input sockets that override the Balanced High Z Inputs. Since the Balanced High Z Inputs are overridden (per the Altec 9440A operating instructions), the transformer sockets are out of the circuit. I do not have the transformers installed and I have no jumpers (jumpers are not an option with the 9440A). The first chance that I get, I will remove everything from the rack that ties the chassis of all units together and see if the hum is still there with none of the units connected to the common metal rack. Thanks for your interest bfish.

[bfish]

... (jumpers are not an option with the 9440A)...

Sure they are... just not;

...(per the Altec 9440A operating instructions)...

Gives you essentially the same thing you're doing, only with the XLR cable.

You can run more than the listed input xfmrs too, see Paul R's excellent answer to that Q. (I know, loss of sens/BW). You've got a lot of holes there to fill though...

Probably unrelated, but a prime suspect with HT ground loops is the cable/dish ground. Usually a separate ground rod, and not hooked the house AC ground. Easy enough to trace, just unhook the cable.

I'm picturing you swapping rack hardware on those beasts. :doh:

[rghinton]

I have the amplifiers and processor out of the rack. I have not had time to check for hum.
I have the 4 amplifiers, a 5 channel surround sound processor and a dvd player. I do not have cable/dish in this system.

[rghinton]

I plugged everything in with all of the components (2-amplifiers, surround sound processor, DVD player) sitting on a table. I used only 2 of the amplifiers for this test. One amplifier for the left and right channels and the other amplifier for the center channel. The same hum was still there. I then put the little connector (69 cents at Home Depot) that opens the 120V A.C. ground on one of the amplifier cords. The hum was slightly reduced. I moved the little connector to the other amplifier. Same, the hum was slightly reduced. I then put one of the little connectors on each of the two amplifiers. 99% of the hum is gone. I believe that the very slight remaining hum may be because I probably should replace the 9800uf filter capacitors. I will do this when I can afford them. I have adjusted the Null and Bias in all of my amplifiers.

Next problem. Some of the volume controls are weird. With the volume control all of the way down there is no hum. As I start increasing the volume I start to hear hum which increases until control is about mid position then the hum starts to decrease and is gone when the control reaches maximum. I haven't had time to really check which amplifier or which channels have this problem. I know that it is only channel 1 or channel 2 on a couple of the amplifiers. Could this just be old pots that need replacing. I know they are 20K Audio pots but I don't know the manufacture or model number of the pot.

[bfish]

I've never had a flaky 9440 volume pot, at least one that cleaning wouldn't cure, but there's always something new.I doubt that's the real culprit especially if more than one has the issue.

I've run up to 3 9440s in a common rack system with none of those issues.

Common ground loop wisdom says it's usually multiple ground paths, but my experience has mainly been from ground paths that had some resistance (usually from poor contact).

Since you eliminated the chassis ground and still have issues, look to the signal ground. Resistance there can have the same effect. Might be as simple as a flaky interconnect or loose/dirty phono jacks.

Good luck!

[rghinton]

You say that you have run up to 3 9440A's in a common rack with none of these issues. Have you used a surround sound processor with a common signal ground to feed 5 channels of the 9440A's with unbalanced cables?

You may have misunderstood what I have found. By removing 2 of the amplifiers, the surround sound processor and DVD player from the rack about 20% of the hum has been eliminated. By lifting the ground from the power cord of both amplifiers 99% of the hum has been eliminated. I have no ground issues operating like this.

In this configuration I now have left, center and right channels with 99% of the hum gone. This sound is very good and clean. Like I said, I think the 1% of hum that is left is probably from the old filter capacitors which I will replace someday.

I do not want to continue to run the amplifiers without the power ground wire but do not know what to do to eliminate this ground loop. Both amplifiers are fed from the same 20A outlet.

[rghinton]

I tried removing the ground shield from one end of each of my signal cables and then running a good ground from the chassis of the processor to the chassis of the amplifier. The hum increased.

My next step is to replace the filter capacitors (9800uf) in one of the amplifiers and also replace the volume controls. Then I will get back to my hum problem. I want to hear one of the amplifiers with 100% of the hum gone and no volume control issues. I have the capacitors ordered and also have the volume controls ordered.

This is going to be a tough problem to fix but I know it will be worth it in the end because I really like the sound of the 9440A amplifiers.

[rghinton]

Back to the starting point.

I replaced the two 9800uF electrolytics in 2 of the amplifiers. I moved the signal cables away from the A.C. cables inside the amplifier. I mounted the amplifiers in a metal relay rack and plugged them in without a ground cheater. A.C. and grounds are connected. I patched cables from my unbalanced surround sound decoder, which has common grounds, to the unbalanced input of the 2amplifiers. Hum was still really, really bad. It was too bad to listen to anything. I purchased the EBTech 8 channel Hum Eliminator plugged it between my surround sound decoder and amplifiers and the hum went away. I now have a very clean sounding left, center, right, sub-woofer system. When I get a chance I will change the electrolytics in another amplifier and mount it in the relay rack for my left and right rear Altecs.

I read a 12 page article about ground loops. Near the end of the article the author said "May the force be with you". Ha! Ha!

I have spent hours, days, weeks and now months working on this problem.

My three front channels - Altec 817 cabinets, 515E woofers, 805 horns, and 288K drivers. My left and right rear - two 820A cabinets, 515E woofers 511E horns and 288K drivers.
My four sub-woofers 8182 cabinets with Altec 18" speakers with new accordion cones.

It was worth repairing the amplifiers and getting the hum out of the system.