The hum from hell

Moofy

I am running a Dodd tube preamp and Dodd Hypex amp. I have a loud hum emanating from my Polk SDAs. Symptoms are:

The hum is not affected by the volume control on the preamp.

The hum stops when I mute the preamp.

The hum stops when I mute the amp.

The hum is independent of the input selector on the preamp.

Hum with or without SDA cable connected.

Hum whether Polks or other speakers are connected.

Hum with no inputs connected to preamp.

Hum with Hafler amp (a spare) or Dodd amp (although significantly louder with Dodd)

Hum will go away but soon return following what seems like any change: new outlet (tried upgrading to a hospital grade outlet), different amp (Dodd/Hafler, as mentioned above), disconnect/reconnect inputs, etc.

This has been especially frustrating as I can, for example, disconnect/reconnect ICs from inputs, enjoy no hum for up to 45 minutes with music playing, go to sleep, and wake up in the morning with hum having returned.

I don't think it's a ground loop problem.

I'm ready to call an exorcist!

Any advice is greatly appreciated. Thank you.

Seth

F1nut

Seems like the common denominator is the Dodd pre amp. Can you borrow another pre amp?

A few possibilites;
A bad tube
Cold solder joint
Loose RCA jack

Speedskater

For troubleshooting, some good starting conditions are:

a] Everything powered from the same AC wall outlet or power strip.
b] Eliminate power conditioners, UPS's and the like.
c] Use common (ordinary) cables for AC power, interconnects and speakers.
d] Start simple and add one component at a time.
e] Only then can you connect cables from the internet or cable TV or other systems.

treitz3

I would start by calling Gary about the Dodd.

Tom

erniejade

Any florecent lighting in that room or a calbe box?

drumminman

Try this: connect a wire (any old insulated wire) from the chassis of the pre to the chassis of the power amp. I had this exact issue in my two channel system with different equipment than yours and this worked for me. Nothing else did: different IC's, plugging components in one at a time, lifting ground with cheater plugs, plugging components into different outlets on different circuits, disconnecting the home theater including the cable from the cable box.

I was on the verge of returning it to the manufacturer when I decided to do the above.

treitz3

Hello, erniejade. The florescent lighting is an excellent possible culprit for the cause [if present on the circuit] as well as any SS dimmer type product. As far as the cable box goes, Moofy had mentioned that, "Hum with no inputs connected to preamp" so that can not be the cause. A cable box could definitely add a hum though.

Also, please allow me to clarify that [as previously mentioned by F1] I was talking about the Dodd pre as a likely culprit.

Tom

Moofy

Thanks for all of the advice. I did try swapping out tubes...no change (except, as I mentioned, the issue seemed to resolve initially, then came back). No fluorescent bulbs in the house. I will try some of your other ideas later today. Cheers.

Seth

ViperZ

As F1 mentioned, it seems like the preamp is what is causing the hum. Have you tried hooking up your system in a different room - i.e. on a different circuit / different phase in panel?

Moofy

I was able to plug the preamp and amp into another circuit today, and despite my positive thinking, got negative results - same hum. I guess I'll have to wait until I hear from Mr. Dodd. I've called and emailed, but no response yet (but what with the holidays, yadda, yadda). Hope springs eternal for a hum-free 2014. Happy New Year all.

Seth

dromunds

Gary just came home not that long ago after major heart surgery so he's probably still recovering. Be patient with him.

Moofy

Thanks for the update. I hope he's doing well.

Moofy

Well, the plot thickens and my frustration level increases. Our contractor came by today and installed a dedicated 20 amp circuit for the audio equipment. I plugged everything in - dead silence. Nirvana! I put on Dark Side of the Moon. By the time the album was over, the hum was back. I'm starting think the Dodd preamp is fubar, but Jake ran it at his house for several days with no hum. WTF can this be? The hum comes and goes go mysteriously that it seems to evade diagnosis. Does anyone have any new ideas? Thanks!

Moofy

Thanks for the idea. I built and experimented with a couple of ground loop eliminators like this http://www.youtube.com/watch?v=CuvDMjxhE68 several weeks ago. No joy. After reading your post I tried one again: still no joy. Is the circuit in the Ebtech materially different from what I'm using? Thanks.

F1nut

You don't have a ground loop problem. The problem is IN the Dodd, so have a tech look it over.