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Old 24th January 2005, 03:38 AM   #1
Bogie is offline Bogie  United States
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Join Date: Dec 2004
Location: St. Louis
Default Isolating a hum?

Okay... Last coupla weeks I installed some rack rails in my basement HT, and screwed everything in... Been without movies (except upstairs in the living room) for about three weeks now, and am jonesin' bad...

It looks like...

Kustom power conditioner
Behringer Feedback Destroyer
Denon 2803 sitting on rack shelf
Sony DVD/SACD sitting on rack shelf
Hafler DH-225*
Hafler DH-500 #1
Hafler DH-500 #2

Fired it up tonight after a little fun with a coupla hundred feet of 12 gauge, the sawzall, and some ceiling tiles.

Turned on the power strip, the BFD, then #1, waited, turned on #2, waited, turned on the Denon, and the DVD... I'm gonna be putting in a pair of 20 amp circuits Real Soon Now (probably next week - already bought the stuff), but for right now, I'm running the whole mess off the dedicated 15 amp "freezer" circuit (I don't own a freezer).

Anywho, listened for a coupla minutes, confirmed everything was working, and then hit the pause button.

HMMMMMM.....

Okay...

Comes from the four main speakers, and the center (still haven't hooked up the surround backs, which will require me to go outside to cut the brackets, and it's too dang cold), altho not so loud from the center.

Turned everything off. Pulled the amp cords, flipped 'em.

Turned stuff on.

Still a hum.

Tried plugging one of the amps right into the wall. Still a hum.

Got disgusted. Turned stuff off again. When I turn the Denon off, but the amps are still cookin', the hum goes away. The center channel, which is plugged into the Denon, has a hum, but it doesn't seem as apparent as with the other speakers.

Let's see... What else... The conditioner has a ground, and I think the BFD has a ground, but the Denon, and both Haflers, are just two prong plugs.

The plug _is_ a GFCI. Should I swap it out?

*The DH-225 is there, and am planning on using it for the center channel or the rear surrounds - haven't made up my mind yet. It's switchable to bridged operation, but I'm not quite sure how to plug in the speaker leads - says not to ground 'em, and to use only the left input, so I plug them both into the red (+) binding posts? How do I determine which one's really (+) in that case? The left one?
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Old 25th January 2005, 02:35 AM   #2
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You may have a little AC floating around between various amps and receiver. I keep a 3-4 foot long insulated wire with an alligator clip on each end. Check with your VOM to see if any large amount of AC differential exists between the chasses of various units. Then clip a 10 ohm resistor on one end of the cord and use it to connect two chasses. Eventually you may find the combination that gets rid of the hum. I believe your problem is from a ground loop.
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Old 25th January 2005, 02:57 AM   #3
Bogie is offline Bogie  United States
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Hi, Dick...

Pulled all the cables, left the center plugged into the back of the AV receiver. No noise. Plugged in the optical and fired up the DVD. No noise. Ran different cable to the DH-500 #1. Flipped da switch. No noise.

Ran different cable to the DH-500 #2.

Noise.

Tried a third cable.

Noise.

Switched the cables from #2 to the DH-225. No noise.

Something's freaky in #2. Ebayed it a few weeks ago from a place that sells mostly musician stuff. The RCAs had been replaced with quarter inchers, and it looks like they also bypassed the output fuses.

It just became a project. Gonna pop the top some rainy day, right next to #1 and figure out what innards to replace/mod to make them both look like #1. May do the caps and a toroid transplant, etc... Already got replacement caps for my DH-200, and am waiting for the proper rainy day.
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Old 25th January 2005, 06:25 AM   #4
Enzo is offline Enzo  United States
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I agree, you have the classic symptoms of a ground loop.

A ground loop does not mean anything is wrong with any of the equipment. It means there is a tiny difference in ground potential between two chassis. Once bolted into the rack, pieces of gear now have their chassis connected together. In pro audio, we often mount rack things with insulating washers to help prevent this. SOmetimes lifting the powerline ground on a piece helps.

SInce the loop is up the ground wire of one thng, through the ground/shield of some interconnection, and back down the other things ground wire, we can sometimes cure it by using cables without continuous shields. No ground path exists through it then.

If you can disconnect one piece in the rack and stop the hum, try unbolting it from the rack, slide it out a bit to clear the rails, put a magazine under it to keep it from sitting on its neighbor, and reconnect it. If the hum is gone sitting isolated like that, you are a candidate for insulating rack mounting washers under the screws.

Read up on ground loops. it is a constant problem in pro audio and in sound system installations.
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Old 25th January 2005, 06:41 PM   #5
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"The RCAs had been replaced with quarter inchers, and it looks like they also bypassed the output fuses. "

Bogie, are these 1/4" jacks grounded to the chassis or are they mounted with fiber washers that insulate them from the chassis? The schematic shows that these jacks should be grounded to the chassis. If they are not you could try soldering a wire to the ground connection between the two power supply caps and then attach the other end of this wire to the chassis. I usually just clamp it underneath one of the little bolts that hold the PS capacitor mounting rings.

Otherwise, you are in for a session of trial and error configurations to solve the ground loop.
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Old 26th January 2005, 12:00 AM   #6
Bogie is offline Bogie  United States
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Well, I've got a DH-500 assembly manual, so basically I'm going to start at Step 1, and go through it all. May learn something. Will probably replace the caps and swap out the transformer for a toroid or two as I go. I'm gonna fix the RCAs, fuses, etc., stuff first tho, just to see if it nukes the hum. I'm guessing that's what it is - probably wouldn't matter much in a pro audio/live sound reinforcement situation...
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Old 26th January 2005, 03:01 AM   #7
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Obviously no half way measures for you. All I suggested was to run one little wire from the ground spot between the two PS caps down to the chassis. Your star ground needs a single chassis reference point to help nuke hum. If the two 1/4 jacks are isolated from the chassis then you need the chassis grounded to the circuit at one spot, like the one I suffested.
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Old 26th January 2005, 03:42 AM   #8
Bogie is offline Bogie  United States
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Well, yeah, that's where I'll start... But I want RCAs, and hey, it can probably use new caps and an updated power supply... I figure I gotta learn somewhere, and (re)building a kit may be sorta interesting.

Just curious, but why retrofit one of these things with a three-wire polarized power connection? A while back I ran into some gnarsty hum problems with a sub amp until I eliminated the third prong, via one of the little orange adapters... Wouldn't that potentially cause more problems? The "working" DH-500 is a two wire, as is my DH-200 upstairs.
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Old 26th January 2005, 03:43 AM   #9
Bogie is offline Bogie  United States
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And I'll run the wire before super bowl weekend, okay?
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Old 26th January 2005, 12:50 PM   #10
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Default Ground Loop Hum

As a previous poster recommended you need to read up on what causes ground loop hum and what can be done about it.

The third wire in the electrical cord is attached to the chassis, to protect the user from a shock from the case. It has little effect on the circuit, except how it may or may not help cause a ground loop.

Professionals use interconnects that consist of an internal twisted pair for the signal and the RCAs are connected to this pair on each end. However, a shield (usually metallic braid) covers the internal pair and this shield is connected at only ONE end of the interconnect. Then, a regimen is followed to connect the shield of interconnects only at the signal source end.

This practice helps to pass the signal but not pass stray AC from one chassis to another. This is also why the question about whether the input jacks on your amp are grounded at the case is important. This is also why professional versions of many amps include a rear panel switch to "lift" the ground thus obviating the need to cut off the 3rd wire prong on the electrical plug.
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