Axiom Home Page Forums General Discussion Home Theater More amp woes

I got my repaired A1400-8 back from Axiom yesterday. It was nice to see it come back. Unfortunately, the loud humming it was exhibiting prior to failure of one of the channel pairs is still there. It's distractingly loud. Sometimes it lessens briefly, but it always increases again. I'm assuming it's an amp issue. Taking it out of the chain makes it disappear, and it persisted before when I switched from my pre-pro with balanced connections to an A/V receiver with unbalanced connections. It really is a loud hum. I'm going to be disconnecting the amp again, even though I'd really like to be using it. If any of you have any ideas, though, I'm all ears.

Because of the issues, it's getting harder to justify external amplification. I still want it, but the new Onkyo receiver has actually been doing a pretty good job. I'd still like extra power potential for my mains and center(s), but that's all. I may stick with receivers instead of pre-pros from now on. The receiver can power the remaining surround/wide/height speakers. I'm hoping Axiom lets me "upgrade" from this 8-channel amp to two of the 2-channel amps whenever they (hopefully) start offering amps again. I think that will be the way to go. If not Axiom, then maybe when the Outlaw monoblocks are on special sometime I'll pick up some of those, unless something shinier catches my eye.

Anyway, I just wanted to vent a little and possibly get some suggestions. Thanks for listening!

What if you unplug everything else in your system, except the pre and amp, no other equipment?

All my humming issues were solved by one or the combination of 3 things, in no particular order;

* Ground loop isolator on the cable line.

* Removing the ground screw from the back of my EP500

* Making sure all the plugs for my A/V system were on the the same circuit or at least the hot wire was on the same leg in the breaker box (i.e. every other breaker in a normal US house, or every third breaker for those fortunate enough to have 3 phase power at home)

Just thought of a 4th hum, recently solved. When connecting video output from my laptop, I had to eliminate the ground leg on the power converter (3 to 2 adapter plug)

I have not reas through all your amp woes, but I would guess that you've already covered those bases...

Sorry to hear about your amp woes, Charles. I hope Axiom makes things right for you.

Charles, I don't really have any new suggestions, since if the speakers hum with the A1400-8 powering them, but not with the Onkyo receiver, there's no ground loop question, but as you suspect, an amplifier problem. The power supply section of amplifiers, in particular the rectifier and filter capacitors, is supposed to provide clean DC free of audible hum when operating correctly. When you sent it in for repair of the dead output channels did you also describe the hum problem?

Looking back through my correspondence, it doesn't look like I did mention the hum. It must be because I hadn't really nailed down what was causing the hum. JC says that the unit would have been fully tested, and they should have caught the hum. In any case, they're aware now, and he's asking around for other things to try.

Okay, there's no hum if I have nothing hooked up to the amp, but with the amp still hooked up to the speakers. Does that mean anything? I'm thinking I need to borrow back the pre-pro I gave away and make sure that having it hooked up with the balanced connections still creates the hum.

Did you try a hooking up a battery powered, portable music player to the amp? Start with the volume all the way down to be safe.

I'll try that out.

With my portable player, the hum is still there, but not as strong. Hooking the receiver back up, the hum is definitely louder, but it's not as loud as it has been. Right now I would still consider it too loud, but at least now it could be drowned out at decent listening levels.

Since it hums with a portable player that's not connected to a ground, it is definitely not a ground loop.

Annnd... the hum can't decide how loud it wants to be. Thanks for giving me things to try.

My theory about that involves impedance differences between the output circuits of the various devices.

Is there a hum when nothing is connected to the amp other than the speakers? And does the hum vary with volume?

No hum when only speakers are connected. The hum doesn't seem to vary with volume, but the hum changes how loud it is on its own. Right now it's being really annoying, but it was somewhat quieter just a minute ago.

Reminds me of someone I know.

Oh it changes over time while the same piece of gear is connected? Then it's not my theory from above.

Well we are past the Skynet goes sentient date as someone recently pointed out. Perhaps your amp is simply trying to hum along to the music?

I haven't used a ground isolator since we don't have cable TV. I messed with all of the ground screws on my subs when I was getting hum from them. I still get a little hum from them, but not like before.

As for keeping everything on the same circuit, I know that's not happening right now, but I can't say for sure how many circuits are represented since I don't know how it's all wired in the walls. I really wish everything had been wired in the most logical way possible when we had our electrical re-done. So why do multiple circuits cause problems? I'm just worried for when I have multiple dedicated circuits for my home theater. All of my gear has a lot of potential power suckage, so I want to be prepared. Between the multiple amps I'll be running for my speakers, the amp in each EP800, my computer, the ButtKicker amps (I say amps since I'll probably run two to shake the riser I build), display, and whatever else I happen to have running, it could potentially want quite a bit of power. How would you (you as in anyone reading this) do this? I want the power to be there, but I also want my system to be as dead-silent as possible except for the signal.

It's not that multiple paths to ground are a bad thing, it just can be a bad thing.

The problem arises when one circuit's ground isn't as good a path as going through an interconnect, and then to ground through another component. As long as you have good, clean ground connections on each different circuit they will be preferred over the inter-component path.

Ahh, thanks for the explanation. Hopefully my dad can have an answer for me on that one.

My system is actually on 3 different circuits right now. Most everything is on one, but the left sub is on it's own, and the ButtKicker on another. No hum or ill effects at all.

It's just that if you get hum, that's one of the first things to check.

If your buttkicker gets a ground loop hum, does it just tickle a bit?

I've actually thought about that.

Charles - As a test to determine if you have a ground loop, or some other grounding issue, try using a ground isolating adapter. I don't know the proper name for it, but it's an adapter than eliminates the ground wire and allows you to plug a three pronged cord into a receptacle that only uses line and neutral wires. Every one of my external amps has created a 'hum'. I've had to chase down the source, which can be quite frustrating. The adapter will at least let you know if you can eliminate one possibility.

Charles, I still don't have any new suggestions, but I'm puzzled by all the discussion relating to ground loops. Unless I misunderstood your first post, replacing the A1400-8 with the Onkyo receiver as the power source resulted in no hum. If so, there's no ground loop issue, but an amplifier issue, as I previously commented.

I'll have to find one of those cheater plugs. I think my dad has one or two somewhere. Otherwise, I'll pick one up. That's what you're talking about, right? So I should just hook the amp up to the wall with one of those to see if the hum goes away?

And yes, no hum with the receiver powering the speakers. No appreciable hum, anyway. If I turn the volume way up, I can hear a tiny bit, as you'd expect. The hum on the amp is unrelated to volume level.

I'm also waiting to hear more from JC who's been talking to Andrew. I really appreciate all of the help, guys.

If you have a 65 year old house like I've got, those 3 to 2 prong plugs are laying around all over the place.

The house is old, but the electrical was completely redone a couple of years ago, so at least there's that.

John, my dad did have a cheater plug, and the hum remains when I use it.

Sure, Charles; the use of a cheater plug is meaningless when a ground loop isn't the problem, and nothing that you reported indicated that it might be.

Have you called Axiom Charles since you got the amp back?

I've been trading emails with JC.

Charles,

How many active inputs do you have connected into the A1400-8? Try to have an even number of inputs (2, 4, 6, or 8) active even though that input module may not be connected to a speaker. Also, is the hum coming from all output channels of the amp?

Right now I'm using three channels. I disconnected one so I was only powering the mains, but no change to the hum. And yes, the hum is coming through on all speakers. How's your A1400 treating you?

Did balanced vs unbalanced inputs make a difference?

Interesting that with no input signal, there is no hum.

My two A1400-8s produce a slight hum with single ended inputs but no hum with balanced inputs or, oddly, with RCA to XLR adaptors.

What did JC suggest to try?

I haven't been able to get my pre-pro back to verify that it's still the case, but if this hum is the same as it was before I sent it in for repair, which I assume it is, then no, the balanced connections didn't make a difference. I know when I first moved to balanced connections, noise did seem to be reduced.

That's interesting about the RCA-to-XLR adapters. JC was saying that if balanced connections defeated the hum before, then it would be possible for those adapters to do the same thing. I didn't see how. This would confirm that balanced connections wouldn't help, though, since I'm using the adapters now, and the hum is as strong as ever.

Actually, the only thing JC suggested I try was the balanced connections, if possible, and I explained how the hum had been present when I was still using them, and it persists with the RCA-to-XLR adapters. I think the things you guys have had me try has helped me head off some of the suggestions he would have made.

He'll be forwarding the stuff I've tried to Andrew, and he'll try to help me pinpoint the issue. Based on the hum being absent without any inputs hooked up, and the amount of hum changing based on whether it was the receiver sending the signal or the portable player, Andrew said they could likely rule out the amp as being the issue. His theory was a ground issue of some sort. JC still has more details to forward, so we'll see if that changes.

If you cheated the ground using a 3-2 prong adapter on your equipment and the hum is present, its impossible that a ground loop is the problem. There is something else going on here. My guess is the unit will need servicing.

The cheater plug does not eliminate the slight hum this amp is known to have with RCA/single-ended inputs.

Charles, the supplied RCA-to-TRS adapters don't work to eliminate the slight hum, so I am wondering which adapters you used. In my stereo setup, the preamp does not have balanced outputs, so I am using the Hosa GXM-133 adapters. These eliminate the slight hum completely. In the HT setup, my pre-pro does have balanced outputs so I just use them.

Those Hosa adapters don't even attempt to create a balanced connection. "Pin #2 Hot. Pins #1 & #3 tied together," that just ties the pin that should be carrying the inverted signal to ground (which is a shared chassis/signal ground on RCA cables).

That results in a half amplitude signal entering the gain stage of the amp. Don't know why it would reduce hum. Unless the circuit integrating the differential signal is the issue. It would require you to run the input volume at a higher level to get the same output.

Using Axiom's supplied adapter actually triggers a switch inside the jack and puts the amp's input in single-ended mode.

The Hosas are what I ended up with, just to try.

I just turned everything on today, and in addition to the loud hum, there is now a scratchy sound happening quite frequently. Makes me think something wants to fizzle out and die.

It's certainly looking that way. Thanks!

Bummer. Your amp is bad. Could be something real simple, like a loose connector. Might be worth a few minutes of your time to pull the lid and look around a bit. But I suspect there's something more going on.

John - the hum can come from noise on the neutral, not just your typical ground loop. 110v > 24v transformers being powered from the same power source can cause this hum too. I don't fully understand why or how this chatter / hum is picked up with an amp. I can demonstrate this to you on two of my systems that use an outboard amp though. No hum with the AVR powering the speakers, hum when adding the amp(s).