I recently acquired a Mcintosh MC 7104 from my father in-law. It is in mint condition, and works perfect except for one annoying issue. There is a physical/mechanical hum eminating directly from the transformer. It is not loud, but is audible from a few feet away when the room is completely quiet. It is sometimes noticeable on quiet passages from my chair. He did not noticed it as it was installed in an AV closet. I haven't had this problem with any other audio equipment with the same outlet, (Classe, B&K, Yamaha amps) but feel (hope) it might be a power issue. Also, my house and wiring are 61 years old. I have been led to believe the cause is due to:

1. laminating issues with the transformer
2. bad transformer
3. DC power from outlet.

I am not too experienced with this; but, using a friends multimeter set to measure DC volts at the 20 range I get a .34 - .67 reading, 0 when removed. Is this "excessive" DC voltage? How can I know if it is a defective transfomer, normal condition for this amp, or DC induced noise a DC filter component will eliminate? If I just ignore it will it cause any further harm to the amp?

System:
B&K 4090 pre
Sony S9000ES DVD/CD/SACD
Mc 7104 (bridged to 2 ch) replaced B&K 120x2 that produced no hum
Yamaha MX830 (for sub)
B&W 705
Peerless XXLS subwoofer 12DVC (DIY)
Dahlquist active crossover
audioquest and vampire interconnects

My MA6500 is less than a year old and also makes that kind of xfo noise, but I didn't investigate any further because now it is in a AV cabinet so it doesn't annoy me anymore. It may be "normal"... I'm eager to hear what other have to say on this...

BTW (off subject), how do you like your 705's ??

For the size, IMO it is the 705's are the best speakers you can buy. I had the opportunity to compare them to the $10k Krell LAT2's at home and wouldn't trade. (except to sell the LAT2's and keep the extra cash after I bought 705's again!)

My MAC-1900 also has some hum from the transformer. It is quiet, but certainly there!

It is used in the back bedroom so I don't think about it to often...

Hum level measured- db meter says 29db@about 8in. If this is normal, would putting a sound dampening material like Dynamat on the transformers housing hurt anything? when I put my hand on the housing, the hum is drastically reduced, so I am figuring dampening might help. (before anyone mentions it, I am sure the the hum is coming from inside the housing and is not the direct cause)

First, I assume that the tf in this unit isn't potted as I've never seen a 7104. Obviously, the first things to check are the 4 bolts holding it together and then the 4 that secure it to the chassis. Make sure they're snug.

Then, try loosening one or two of the bolts that hold it to the chassis. Of course, use common sense when doing this. This is a long shot but at least it's cheap and doesn't take much time. :smoke:

I have a MAC-1700 with a singing tf and it's not as loud with the transformer a little loose.

Murray

Seems DC was the problem as it went away at a friends house and when I put a DC filter on at my house. Thanks for all the great suggestions!!

What DC filter did you use?? I think that I have the same problem.

I was loaned an AH! DC filter by my local store. I plan on buying it. There are others made by PS Audio. They cost more, but I think the AH! at $170 is just as good.

let me know if DC was the problem