Multiple Altec 9440A amplifiers ground loops

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.

Are you running jumpers or transformers in the input sockets?

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.

Quote:

---

Originally Posted by rghinton

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

---

Sure they are... just not;

Quote:

---

Originally Posted by rghinton

...(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...:D

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:

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.

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.

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!

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.

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.

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.

hey reed...
great to hear about your theatre again! im running anniversary series amps in mine.. and while i use a surround prossecor, i also run through a mixer, a splitter mixer , and active crossovers. tho i dont get hum, i do get a static crackle that seems to build up after hours of play.. a slight pop or crackle sound that only occurs very seldom. i attribute this to grounding issues as well. in my setup, i convert to balanced right out of the mixer.
can you give directions to the 12 page article on grounding?
and please repost the pics of your build on those 820's!..please

It was an 8 page article from Rane. They have several good articles about hum and grounding.
The article that I was referring to is at:
http://www.rane.com/pdf/ranenotes/So...connection.pdf

Quote:

---

Originally Posted by rghinton

Back to the starting point... I purchased the EBTech 8 channel Hum Eliminator plugged it between my surround sound decoder and amplifiers and the hum went away...

---

That's good news R.

Looks like something the 9440s stock xfmr isolation could have cured too, but that EBTech specs nicer. Looks like they nicely fill the consumer/pro conversion niche.

Now that you're in business, would it be too much to ask for a review? Plenty of us have enjoyed sharing your system developement, I'll bet I'm not alone among the curious...:coffeedrinker:

Quote:

---

Originally Posted by rghinton

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.

---

Sounds like one Awesome system ! :2thumbsup:

I have had many amplifiers over the years but the 9440A's are the best sounding amplifier I have ever heard. I don't know what words to use but every sound whether it be gentle music or full blown action movies is as clean and pure as if the performance or action were live. I have also replaces my preamp. I was using a preamp with all of the bells and whistles that tended to alter the sounds from natural. I am now using the Technics SH-AC500D. The unit is probably 10 to 15 years old. I found one that had never been used. It is a "Digital Surround Processor". It doesn't do anything except decode the surround (Dolby, Dolby Digital and DTS). It doesn't equalize, no tone controls, no special hall settings but it allows the setting of large or small speakers and individual channel level adjustments. The only minor problem with the unit is that it takes a fraction of a second to determine the source and clips a fraction of a second off the first part of a song from a CD or DVD. This is not a problem with a DVD once the movie starts. I can live with this little problem because the sound is so pure. The processor also sends L,C,R,LS,RS sound from a music CD to the proper amplifier or you can select stereo mode for just L & R channels. Music DTS CD's are awesome. The combination of this processor and the 9440A's create a sound that is all around me. Every channel is important. I have never heard such separation and blend at the same time. All of the VU meters work and look really cool but I removed the meter lamps because too much light leaks in to the theater. With the Altec speakers that I am using, I don't feel the need to alter the sound with an equalizer, tone control or effects. The sound is great unaltered. I am a happy customer. I am just a "poor boy" and feel that if someone would recreate the 9440A's that I would not be able to afford the new amplifier. Even a used 9440A including the purchase price, parts and the time to get them in working state is not cheap. My amplifiers are mounted in a relay rack just outside of my home theater. There is some mechanical hum from the power transformers in the 4 amplifiers but in the theater with even low level source material the mechanical hum is inaudible. The hum out of the speakers is 99% gone. I have been learning as I go and I know that I can get rid of the last 1% of the hum by moving the signal cables inside the amplifiers. I know this because I got all of the hum out of my surrounds by moving the cables from the input jacks inside the amplifier. It is a rascal to pull the amplifiers out of the rack to do anything inside the amplifier. My wife has been very patient with amplifiers and amplifier parts everywhere (dining room, kitchen, garage, bedroom and halls in the house) I am taking a little break but someday I will work on getting rid of the last 1% of the hum. I just watched "The River Wild" on this system. I love white water rafting and the sounds are so realistic.

Thank you!

Yeah, for $999 in '75 money, you'd pay $3939.24 in today's $. Mr. Rumbaugh really built a winner there.

This helped with me in getting rid of the 60hz hum I was hearing with two of my 9440A's.
One I used as my sub amp running a Contra Bass sub,
The other running my second set of mains, JBL 4435's
The worst was the hum coming from the sub amp.
I found that if I made a run of quad shield RG6 with RCA fittings on the cable from the source to the amps and used an adapter to the 1/4 inch plug on the 9440A's, it went away. Occasionally I found it would return after I moved the amps and ended up bending or breaking the cable ends when I push the amp too far back pushing and forcing the cable to bend too much.
I never got around to going from the RCA signal out to XLR in.
I fixed a nightmare problematic sub hum one day years ago after an install I did for a health center. The sub would hum here and there for no apparent reason and at times the hum would go on and off several times a minute.
Then one day during a quiet time, the sub started to hum a little then a lot and then it blew up. The lady described it sounded like a body hitting the roof. After I got done asking her how did she know what that sound was, I figured it out.
Somebody near her place, she was out in the boon docks, was using some sort of high powered CB or similar that was in a car. The people there said the car had several long antenna mounted all over the car. This time the car drove, down her road and the people inside the center heard the car. Then the speaker started to hum and blew.
So all I had with me at that time was RG6. I made a run of it and no hum.
Been using it since.
All kinds of ends and adapters are available for the cable too.
I had a pair of RCA shielded cable that I could hear a hum. Replaced them with the RG6 and I couldn't hear the hum.
I love my 9440A's also.
The Altec Dealer on Maui once told me that it was basically a bullet proof amp and could run 200 watts through a screw driver without damage. Sounded like a story of experience.
I have sent most of my 9440A amps to Iratec for a check up and cleaning.
He knows them inside and out.
Ira Tec Pro Audio's Homepage

Give the RG6 a try running your signal lines from the processor to the amp. Who knows, it was a lot easier and cheaper than having me send my amps in to be checked out or chasing ghosts.

hope it helps
Steve

I forgot to add in my previouse post that I found before I did the RG6 cable switch was having all the amps, processor, etc. be on one circuit if at all possible. Even using another outlet, even though on same circuit and about 8 feet away, induced hum.
I also found that the cable tv coming in to the house was a major culprit also.

Steve

I thought the 9440A was in the price range of $2,900 to $3,200 in 1975.
I know of an Altec dealer who still has a NOS 9440A on his shelf with the original price tag, showing $3,000 and some change. At least last time I saw it about 3+ years ago.
He didn't want to sell it either.
A lot of hotels used the amps back then also.

Steve

$999 was the '75 listed price for a GSA contract;

http://www.hostboard.com/forums/hbmc...2010/10/67.jpg