Axiom Home Page Forums General Discussion Stereo LFR1100 Actives hum

I don't remember who first mentioned the significant hum coming from their LFR1100 Actives, but that's been the case for me as well. It doesn't help that I don't have all of my equipment on the same circuit, and there's no easy way to remedy that. I read at one point it can help to have all relevant circuit breakers on the same side of the electrical panel, and I don't even have that. I still need to run an experiment at some point in which I run extension cords to some equipment so they're all at least on the same side to see if it actually makes a difference.

In the meantime, I heard someone had luck eliminating their hum with this product:

Emotiva CMX-2 Precision Common Mode AC Line Filter With DC Offset Eliminator

Seemed worth a shot for an easy solution. It arrived today, and I think I shot myself in the foot. I tried it in various configurations. Since it's just two outlets, first I did both DSPs, then both amps, then just the amp and DSP on one side. Each one made the hum worse. As one last test, I connected it upstream and plugged the AV receiver into it. Same thing. The already significant hum was significantly worse. Deciding it wasn't going to be my miracle, I took it out of the chain completely and reverted to the way I had it. Aaaand the hum has remained at the higher volume. Whereas before the hum could be drowned out with content playing, now I'm hearing it even when I'm playing stuff. Can't live like this!

I emailed Axiom for ideas, but I figured I'd post here as well to see if any of you have suggestions.

CV, by chance do you have an amplifier in line for cable tv?
That was the culprit in my case. When the cable tech installed it he didn’t bother grounding it.
Once I had hooked up a grounding wire to the earth ground my hum was gone! Dead quiet.
What a relief.
Jeff

I have cable internet, but not cable TV. And the hum is new with the Actives. Oh, I did have some hum before, which went away after fiddling with the grounding screws on the EP800s, but after that I had no noticeable hum until I got the Actives.

Charles, if you unplug at the DSP the amp to DSP cable, is the buzz eliminated or reduced? Leave the other end of the cable connected to the amp.

Yes, the hum goes way down when disconnecting those.

And if you have all connected up normally and power everything on, but then turn just the DSP off, the hum is still there, correct?

Yes, that's correct.

So just think about that, Charles. Draw yourself a picture either on paper or in your mind's eye:

1. Amp to DSP cable disconnected at the DSP only = (less or) no hum

2. Amp to DSP cable connected at the DSP and amp but DSP off = hum

What component (amp or DSP) therefore is the conduit of the hum? What is the source of the hum?

I'll admit it's tricky. Even for electrical engineers.

I would say the amp. For clarity's sake, AVR, since that's what I've been assuming with the tests. While I think something has happened to it with the introduction of the line filter, the original increase in hum came with the Actives, which is what makes it hard for me to work out. What would you say made that hum at that point versus the gear I already had with the passive LFRs? Going with what I have now, by process of elimination the AVR certainly seems to be the big culprit. If I have all of the gear on with a source active (as in on, not necessarily playing content), the hum is loud through all connected speakers. If I turn just the AVR off, the hum is way quieter, though still there.

While I plan to replace my AVR, I'm not quite there, and I don't want to buy something new only to have the same behavior with the potential of it worsening.

Isn't it strange, that with your AVR and DSP off, and the ADA to DSP cable connected, the hum is still there? But when you remove the ADA to DSP cable, the hum subsides.

I have a theory and the explanation is technical but I will try to explain as simple as possible. This is just a theory which has not been tested. There is a 60Hz noise source in the ADA. It is between a signal input line and ground. In technical jargon, this is called a common mode noise source. The source of this noise within the ADA is not known but it is parasitic and it is related to the 60Hz power fed into the ADA.

Why do I suspect this? When you disconnect the ADA to DSP cable at the DSP, the only thing "across" the cable circuit is the capacitance of the ADA to DSP cable. This capacitance is tiny - on the order of 150pF. This small capacitance cannot pass the 60Hz current from the noise source or passes very, very little. Therefore the voltage across the cable capacitance is tiny. This voltage is right across the ADA signal input. Therefore you hear very little buzz if any at all. I don't hear anything BTW when this cable is disconnected.

When the cable is connected to the DSP, the DSP has a finite output impedance. That impedance is right across the cable capacitance. It allows 60Hz current from the noise source to pass. Hence, you hear the buzz.

The theory therefore is a parasitic, common mode noise source in the ADA that arises from the ADA's input power and is somehow impressed on one or both of the ADA's input signal lines.

If this theory is proven true, there is nothing you personally can do about it.