amp hum question

hyperba55 · #1 03-22-2010, 12:06 PM

Hi all -

I have an amp question that I thought I would reach out to the collective wisdom of TBL. I e-mailed Aguilar, but in case they don't get back to me ...

I have an Aguilar DB359 (all tube) amp head that is getting a severe hum. It sounds like a ground loop hum and it gets louder when I turn the amp up.

I checked all of the tubes and set the bias - all of the tubes are still good, and the bias adjust didn't help. After some troubleshooting, I found that the hum went away when I have an XLR cable plugged into the XLR out jack and the "ground/lift" is set to "ground" - when I set the switch to "lift," the hum returns. In other words, the only way to get rid of the hum is to plug an XLR cable into the XLR out jack and switch the "ground/lift" switch to "ground."

I looked through the amp and I didn't see any obvious loose connections or soldering issues. There's not enough slack on any of the internal cables to move them around much either.

Any help is appreciated.

Thanks!

billfitzmaurice · #2 03-22-2010, 12:12 PM

That's really odd, as connecting just a cable to the XLR out shouldn't have any effect unless connected to something at the other end. Even then that usually ends up causing hum, and you use the lift to remove it. Their reply will be interesting.

hyperba55 · #3 03-22-2010, 12:36 PM

Sorry, should have been more specific. If I "just" plug an XLR cable into the jack, nothing happens, but if I plug the XLR into my PA or another amp with an XLR input, the hum goes away.

BassmanPaul · #4 03-22-2010, 01:21 PM

Sounds like you have a failed ground connection in your power cord. Check that the ground pin on your plug is still there. If not replace the plug with a hospital grade plug.

Paul

billfitzmaurice · #5 03-22-2010, 01:59 PM

Quote:

Originally Posted by BassmanPaul

Sounds like you have a failed ground connection in your power cord. Check that the ground pin on your plug is still there. If not replace the plug with a hospital grade plug.

Paul

+1. It may be the cord, may be the connection to the chassis internally.

hyperba55 · #6 03-23-2010, 09:51 AM

Thanks for the advice. The cord looks fine and I couldn't find any loose connections in the amp itself. I am going to try to replace the cord and will also wait to hear from Aguilar.

Hopefully it's just the cord :-)

TaylorO' · #7 03-23-2010, 10:22 AM

i had the same issue. i have a mesa boogie 400, all tube. i went to Home Depot and got a Belkin 6 outlet power strip/surge protector, which also had some "noise reduction" capabilities as well. no more hum.. the room we practice in has terrble outlets. maybe its yer outlet.

Rick Auricchio · #8 03-23-2010, 10:40 AM

1. Use an outlet tester to verify that the outlet is in fact providing ground.

2. Use an ohmmeter to verify that the cord is good. Just looking isn't good enough.

hyperba55 · #9 03-23-2010, 11:01 AM

Thanks for all of the advice - I think I am getting somewhere.

The outlet is good. The cord is good. In fact, the connection to the ground on the chassis is good as well. I tested the ground wire in the amp - it tests good up to the first connection with a circuit board. After the first connection, though, I get no reading from the ohm meter.

Should I? I thought if the ground was connected properly, I should be able to get a reading anywhere in the amp that the green wire goes - or am I wrong on this? Amp is powered off, BTW.

billfitzmaurice · #10 03-23-2010, 03:41 PM

Quote:

Originally Posted by hyperba55

Thanks for all of the advice - I think I am getting somewhere.

The outlet is good. The cord is good. In fact, the connection to the ground on the chassis is good as well. I tested the ground wire in the amp - it tests good up to the first connection with a circuit board. After the first connection, though, I get no reading from the ohm meter.

Should I? I thought if the ground was connected properly, I should be able to get a reading anywhere in the amp that the green wire goes - or am I wrong on this? Amp is powered off, BTW.

You should get a zero reading between any of the ground planes on the boards and the chassis. For that matter you should get some reading from any traces that aren't capacitively isolated. Perhaps the ground connection from the XLR via the mixer is providing the ground on a circuit board that has a bad trace or wire to where it's normally supposed to be grounded.

hyperba55 · #11 03-24-2010, 12:44 PM

Thanks for all of your advice and help on this. based on this, and what Aguilar told me (I have been working with their tech support - fantastic, BTW) - I found the issue.

There is a small capacitor on the signal path of the ground wire that is blown - it's actually the first capacitor in the ground signal path, and it's physically blown. Luckily enough, it seems like a quick solder job to replace.

I wonder though - did it blow because of another issue with the amp, or did it just blow (either bad or freak occurance)?

Any thoughts? What would cause it to blow?

**bass4worship** · #12 03-24-2010, 02:31 PM

You have to wait until you replace the cap.

hyperba55 · #13 03-26-2010, 12:38 PM

I am actually using the wrong terminology - it's resistor, not a capacitor.

The resistor is located in the path of the ground line, which explains the hum from lack of ground.

Does anyone know how I can tell what kind of resistor I need to replace it with? Infortunately the outer coating is completely gone (burned off).

Thanks

johnk_10 · #14 03-26-2010, 12:46 PM

you'd need the schematic or at least the resistor's part number to determine its value. can't Aguilar help you with that?

hyperba55 · #15 03-26-2010, 02:08 PM

I just heard from Aguilar - they provided the spec.

It's a 10 ohm, 3 watt resistor. Does anyone know where I could perhaps pick up one locally? I see them on eBay in packs of 100, but I only need 1

I am going to try the local Radio Shack. Any other suggestions?

johnk_10 · #16 03-26-2010, 02:19 PM

i don't know if you have a frys electronics near you, but they would probably have something that would work.