Hafler 9130 Hum

[db5owat]

Just got a Hafler 9130 to use with my computer and audio set up. It replaces my DH 200 which will be repurposed.

It seems to have a significantly higher level of hum. It is somewhat annoying. Is this a characteristic of this amp or is there possibly a problem? In all other aspects it seems to function like all other hafler amps I have used. Just the hum is present.

Any suggestions are welcome.

The basic system is:

imac 1.8 ghz
Tascam US- 122
Hafler 9130 (placed far from other gear)
Alesis monitor one speakers.


Same system with DH200 ....No Hum.

Could it be as simple as inter-connect cables? They are RCA from US-122 to RCA on Hafler.

Thank you for the input.

[Mooly]

Hi,
What happens firstly with no inputs connected at all, and then with the inputs shorted ( use phono shorting plugs ) ? Any hum at all ?
Again with the amp ON and no inputs connected try just touching the ground ONLY of the interconnect ( all wired up as it would be in use ) to the outer ( ground ) of the phone input socket on the amp.
Always switch the amp off when actually plugging anything fully in to the inputs.

[bwaslo]

if it's anything like the DH200 design, there will be a resistor on one or both of the input connector's ground shield to the chassis ground. In mine, a lightning strike that knocked out other electronics in the house managed to blow those resistors open but otherwise left the amp undamaged (must have been a current blast around the system grounds?). The symptom from that was that the amplifier gave out a strong hum ONLY when an input cable to other components was connected (with no cables, or only cables to line isolated sources, it was completely quiet!). It took a while to figure out what was wrong, and putting new 2 ohm resistors between input shield to the chassis ground fixed it. So, if none of the obvious things fix it, you might want to remove the input cables to it and take an ohmeter from to input shield connections to the amp chassis to make sure there isn't more than a few ohms resistance there.

Of course another possible cause is that one of the big filter caps has gone bad (open) -- to test for that, you'd have to strap another one across it (minding polarity) and see if that fixes the hum. If you have an oscilloscope, you could instead look at the ripple waveform on the DC power lines to see if both positive and negative match. That's a problem that occurs with many power amps, particularly if left on all the time and allowed to stay hot.

[Dick West]

Follow Mooly's advice and determine if the hum is in the amp or is a result of its connections. You could be getting into a ground loop when connected to the sound card output of a computer. If your cables are all OK (no loose wires or grounding shield issues) you might need to connect a grounding cable between the computer and amp cases -- to remove any possible small voltage differentials present between them.

Are the computer and amp connected to the same electrical branch circuit? Have you tried reversing the plug at the AC outlet?

When is a ground not a ground?

The ground configuration of the DH-200 is not relevant. The 9130 is a very different type amp. I am using one connected to my Outlaw 990 pre-pro for Home Theater with no issues whatsoever. The 9130 is a great little amp.

[db5owat]

Guys thanks so much for jumping in and offering advice. Before I try Mooly's suggestions a couple of questions/observations.

First what exactly is a phono shorting plug?

Second Mooly says,

" Again with the amp ON and no inputs connected try just touching the ground ONLY of the interconnect ( all wired up as it would be in use ) to the outer ( ground ) of the phone input socket on the amp."

What part of the interconnect is ground? I take you to mean DO NOT plug it all the way in. Right? Just touch the ground (?) to the outer case of the RCA input jack?

Dick West says.
"Are the computer and amp connected to the same electrical branch circuit?"

Is this the same as a wall outlet? This leads me to wonder if my elaborate collection of plugs and power strips may be causing some of the hum? I have two large power strips into the same wall socket. Each strip has 4 or 5 plugged in devices. This is the same as it has always been with my DH 200 though.

I did notice that the amp seems to have less hum today. I have done nothing to it. It's just quieter today.

I guess it is painfully obvious that I am somewhat a new comer here. I have been involved with music as a teacher/performer (acoustic instruments) for years as well as a fairly avid stereo lover. I guess I have been lucky not having to deal with this hum issue prior to this.

Thanks again for your willingness to help.

[db5owat]

OK, I figured out the phono shorting plugs. I happened to have two in another integrated amp. I put them in and turned up the right and left gain on the amp to full. No Hum at all.

I guess I need answers to the previous questions to move to the next phase. This seems like a good sign javascript:smilie('')

[unclejed613]

one other thing that can cause hum is unmodulated RF on the inputs. things that can cause this are: induction heaters and semiconductor crystal ovens (anywhere from 1 Mhz to 30 Mhz) but these aren't usually active continuously. FM baby monitors and cordless (not cell) phonesand other devices in the 49Mhz band. an FM broadcast transmitter (88-108 Mhz) (these and the 49 Mhz devices put out a carrier that is not Amplitude Modulated). some amps are more susceptible than others to RF interference.

[Mooly]

Is it buzz-- a bit harsh sounding, plenty of harmonics, or is it a deep pure mains frequency hum--no harmonics. Ground loops cause the latter.

[db5owat]

Thanks Mooly!

Somewhat subjective, but I'd say it is "deep pure mains frequency hum--no harmonics."

Can you clarify for me? You said, " Again with the amp ON and no inputs connected try just touching the ground ONLY of the interconnect ( all wired up as it would be in use ) to the outer ( ground ) of the phone input socket on the amp."

What part of the interconnect is ground?

I take you to mean DO NOT plug it all the way in. Right? Just touch the ground (?) to the outer case of the RCA input jack?

[Mooly]

This sounds a bit like a true ground loop. The outer part of the phono -- you call them RCA I think -- plug and socket is ground.
Just touch the grounds together- outer parts only. See if there is any hum. Depending on what you find it's a case of working back through all the possible points that hum could be introduced.
With the amp plugged in and all coupled up, try first switching off various pieces of equipment, if that proves nothing try actually unplugging from the mains that same equipment. A piece of equipment doesn't have to be switched on to cause the problem, just the presence of the ground connection (loop) causes the effect.
Another possibility unrelated to ground loops is "induction" say from a mains transformer in close proximity to leads or a piece of equipment. If you stood a cassette deck on top of the amp it probably would have audible hum-- lift the deck away and it goes. The big transformer in the amp causes a magnetic field at 50/60 hz which is capable of inducing a 50/60 current in nearby conductors and components.
Try and pin it down a bit, starting with just the amp and connect just one thing at a time.