Eliminating Possible Ground-Loop Hum

As per my signature, my current stereo system has a B&K Reference 50 Pre/Pro feeding two amplifiers that I built myself. When no source is connected to the amplifiers, there is no discernible noise through the speakers (maybe a tiny bit on the tweeters, if your ear is just about touching them, but not enough for concern). However, when connected (by RCA interconnects) to the B&K, there is a low-frequency hum that is audible from a few feet away. For this reason, I suspect it is a ground loop issue that I didn't address while building them rather than some kind of interference generated within the chassis.

Furthermore, when sourced by my laptop fed through a USB-powered DAC (i.e. no secondary grounding aside from the amplifier's own), there is no perceptible hum - there is just a bit of high-frequency "hiss" that could probably be attributed to the quality of the Nuforce uDAC feeding into a high-gain amplifier directly.

What I'm asking is, is there any simple way to eliminate or reduce this humming? I can't even really think of where to start, aside from putting a resistor in series with the ground path inside the amplifier to cut down on the ground loop current. I just bought a Yaqin CD3 tube buffer today which has an output impedance lower than that of the B&K, so maybe that will help as well.

If anybody could help, it'd be much appreciated. I very much enjoy the amps, I'd just like to eliminate the hum.

Thanks!

For troubleshooting plug the pre/pro and the 2 amps into the same AC power strip.
Use ordinary co-ax RCA interconnects. One with very heavy braided shields are best.

When all else fails, do what Audio Precision does.

Attachment 81557

for me I normally try a Ground Loop Isolator
https://www.google.com/shopping/prod...d=0CI0BEPICMAU

and or a Ground Lift Plug
http://en.wikipedia.org/wiki/Ground_lift_plug

OR a Panamax 4300 power line conditioner.
https://www.google.com/search?q=pana...w=1307&bih=666

Hope thus helps, it normally fixes the issue for me on jobs.

They actually are plugged into the same power line conditioner (Rotel RLC-900), which is why I've been surprised that there was such a hum.

Without having to interfere with the AC ground by using a cheater plug, is it possible to do a ground lift with single-ended interconnects without interrupting the continuity of the signal chain? The Wikipedia article primarily discussed balanced interconnects.

I put together two albums of pictures: the innards of one of the amplifiers and the innards of the RLC900

Amplifier boards are these with some resistors changed to facilitate running at a lower voltage. Power supply boards are also from eBay, here.

Some things that I noted:
-The input jack and wiring are physically close to the transformer. While this could induce noise, it probably can't be blamed for this particular problem.
-All grounding wires (speaker return and the two from the power supply board) are grounded directly at the IEC inlet. The RCA jack is connected to the +/- input terminals on the amplifier board, not directly to ground.
-The RLC900 has an internal shared ground wire for six of the plugs, including the unswitched one (used for triggering purposes), the two amplifiers, and three "accessories".
-The final three plugs, for "digital equipment", have their own ground wire. The two ground wires are joined with the one from the board via star ground near the inlet.

I can't recall if the Reference 50 was plugged into accessories or digital equipment. If it was on accessories, it might explain the ground loop. If not, then I'll have to keep looking around.

You might take the insulators off the RCA jacks.
Where does the Safety Ground wire from the IEC connector go? It should go straight to the chassis not over to the circuit ground point.

I do also need to correct that part of the amplifiers. Grounding is not currently a star configuration on the chassis, so there is not currently a safety ground wire from IEC inlet to chassis, and the signal and power grounds are both at the IEC inlet.

Could that cause issues aside from the obvious safety hazard it presents?

The problem wasn't solved by plugging the Reference 50 into one of the digital equipment plugs. It also persists when the B&K is completely removed from AC power but the amps are still powered on and connected to it. It stops when the interconnects are unplugged.

Perhaps that could mean it's a ground loop issue between the amplifiers (via RCA grounding inside the B&K), and not between the amplifiers and pre/pro?

Reading through the DIYaudio thread on the amp board I saw a picture actually illustrating a sample wiring diagram:
http://i.imgur.com/dfGUls0.jpg

The power supply there uses a center-tapped transformer, whereas mine is dual-secondary, but it's a pretty moot point.

The actual wiring for the ground reference is very different from what I currently have (i.e. entirely on the DC side of the supply rather than referencing earth on the AC side):
http://i.imgur.com/2IUhjZm.png

With that said, I'm about to rearrange my wiring to be in line with what is shown there. However, I'll still prefer to leave the amps out of commission until I can remedy the safety/chassis ground problem. I'll have to find something that can drill through a steel chassis.

For diagnostic purposes you can leave the chassis safety ground connected on both amps buttemporarily disconnect circuit ground from chassis ground.If that solves the issue then there are a couple of easy permanent fixes.Also be sure your RCA con's are insulated from the chassis.

Disconnect all the Green wires!
Connect the AC power IEC connector Ground to the chassis near the connector.
Run a wire from the power supply common to the amp common.
Connect either the amp input (-) or the amp supply common to the chassis near that connection.

Be sure that your RCA jack's are connected directly to the chassis.

Do you have a link to the audio circuit schematic?

Quote:

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Originally Posted by FTGV

For diagnostic purposes you can leave the chassis safety ground connected on both amps buttemporarily disconnect circuit ground from chassis ground.If that solves the issue then there are a couple of easy permanent fixes.Also be sure your RCA con's are insulated from the chassis.

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I tested signal continuity in all kinds of places. RCA jacks are definitely insulated from the chassis.

I've eliminated the ground loop hum by switching the wiring per the "recommended" wiring diagram above. The only noise that persists is a small amount of high-frequency static, not audible from more than a foot or two away from the speaker in a quiet room. It's a relatively minor concern compared to the hum.

Quote:

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Originally Posted by Speedskater

Disconnect all the Green wires!
Connect the AC power IEC connector Ground to the chassis near the connector.
Run a wire from the power supply common to the amp common.
Connect either the amp input (-) or the amp supply common to the chassis near that connection.

Be sure that your RCA jack's are connected directly to the chassis.

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You and FTGV are giving conflicting information on whether the RCA jacks should be insulated from the chassis or not. They are currently insulated from the chassis.

Here is a more accurate diagram of how the wiring looks now:
http://i.imgur.com/oBQ2DS3.png

The black speaker terminal runs to one ground terminal on the power supply board; the ground terminal on the amp board runs by a separate wire to the other ground connection on the power supply board.

There isn't currently a wire between the ground of the power supply board ground and the chassis. Should there be one?

There is a short wire in place between the earth terminal on the IEC inlet and the chassis.

Quote:

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Originally Posted by Speedskater

Do you have a link to the audio circuit schematic?

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The manufacturer of the boards doesn't supply a schematic, but it is nearly identical to the reference circuit for the IRAUDAMP7, found here.

Quote:

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Originally Posted by blairfrischx

...conflicting information on whether the RCA jacks should be insulated from the chassis or not. They are currently insulated from the chassis.

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Good,not isolating them could create a ground loop.

Quote:

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There isn't currently a wire between the ground of the power supply board ground and the chassis. Should there be one?

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This is what I was suggesting to try to eliminate the hum.You can connect circuit ground to chassis ground through a 10 ohm 5 watt resistor or use a bridge rectifier such as N Pass and some others do.

Quote:

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There is a short wire in place between the earth terminal on the IEC inlet and the chassis.

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Thats as it should be to provide a safety ground.