[kmcdonou4eq5]

I hooked up my Monarch Audio SM-70 Pros, run as monoblocks, and have some severe hum through my speakers.  I realized I had a ground loop and temporarily installed a cheater plug. This allowed me to keep the amps on (the hum/buzz without the cheater plug was so loud I was afraid my speakers would blow) with only a mild hum. 

However, about twice in the first hour while listening to music I had a static surge sound through the speakers.  This happened about 4 times before I realized it was coming out of the speakers and not my baseboard heating.

At this point I disconnected the Squeezebox (serving as preamp) from the amps.  With only the amps connected to the speakers I can hear a hum from across the room.  If you didn't realize there are speakers in the room, you might have a hard time noticing the hum or realizing where it comes from.  Yet, if you get closer to the speakers it is quite noticeable.

Since these amps are running on a dedicated line (I have a separate line for digital equipment) and no other equipment is connected, what could be causing the hum? It can't be a ground loop can it, with no other equipment connected? I noticed the hum did not lessen if I attached cheater plugs while in this configuration, unlike when the SB is connected.

I was thinking about getting a Jensen transformer for the ics, but if I am experiencing a loud hum without the preamp connected, I feel I must have another problem as well as a potential ground loop. Also, I have no idea where the static surge came from either.  Maybe it has something to do with my unshielded interconnects.

I'd appreciate any help.

[mtodde]

It sounds like your output transformers could be damaged.  What you describe can be a symptom/result.

Have you had a qualified technician check out your amps?

It also is possible that the electrician didn't do as good of a job as you thought.  I saw some crazy stuff back when I worked on satcom & tube gear in the Navy that resulted from poor electrical work.

[Gordy]

Have you tried running them one at a time in stereo mode?  Just thinking it could have something to do with them being run in bridged??? 

[kmcdonou4eq5]

I hope my transformers aren't damaged.  I just got the amps new a couple of months ago. 

I talked to my headphone amp maker just because he is pretty knowledgeable about these things.  He said I need to hook up my preamp to the amp to see if the hum goes away (I forgot why he said I should do this).  Well, I put a cheater plug on the preamp (Squeezebox), none on the amps, and turned it on.  The hum was gone; only a small hiss from the speakers when you were really close, which I suspect is normal.

So, why is there a hum without the preamp hooked up?  What is causing this?

EDIT:  Just after I submitted this message, thinking everything is good, I started reflecting again on that random pop/static sound that I heard when playing music (about a couple times per hour).  Lo and behold, it just happened again.  Sort of like a pop mixed with static.  Very quick, lasted about a second.

What the heck is up with that?  Is this a possible an ic issue?

[bluewax]

I had something vaguely similar occur, though the issue seems to have resolved itself. For what it's worth, I thought there might be an issue either on the Slimserver end or on the Squeezebox end in terms of decoding. To eliminate both (as potential culprits), save your playlists, reinstall Slimserver and reset your Squeezebox. By the time you're done that, and if the problem persists, I'm pretty certain folks here will have a litany of good technical and electrical suggestions and insights. Cheers, bw   

[OTL]

Coupla things....the obvious first though. I suggest that you give Monarchy a call.

It's never a good idea power up an amplifier with open inputs and/or outputs or make any connection/disconnection with power applied to the preamp or the amp.  Trying to save 30 seconds can (and eventually will) cost $$$.  There, I had to say that.  Now....off the soapbox.

Assuming you're using RCA connections, have you double checked to insure that:

- The input cable is on the "A" input of both amps.
- Speaker cables are connected to Speaker A + (RED) and Speaker B + (RED).
- The phase switch is in the "UP" position.  (Switch s/b "DOWN" if using an XLR balanced input cable)

Using the SR-70 amps as mono-blocks provides (liberal rounding in use) ~5db increase in gain vs. the stereo setup. (40W stereo vs. 140W mono)  So any noise present on the input will be ~2.5 times louder with your mono setup as compared to stereo.  If you have a slightly noisy source and moderate to high efficiency speakers (88db/1W or better) with 140W on tap only requiring 1V at the input for full output, you have a recipe for HUMMMMM!  Unshielded cables in a noisy environment can be superb antenea.

Also, the input to the SR-70 (as memory serves) is quite sensitive to overload as it requires only ~1V P-P for full output.  I believe that the Squeezebox can output @3V or so.  (Squeezebox has a reputation for noise, sometimes at deafening levels.  The suggestion to reload the server and 'box is a good one.  This is well documented.) 

Lastly (whew!) most (all?) balanced inputs require a shorting plug when operated with RCA inputs.  Not sure about the SR-70's though.  Again, check w/Monarchy.

My guess is the Squeezebox and cable combination.  Too much noise at too high an output level.

Hope this helps.

Mike B.

[kmcdonou4eq5]

Assuming you're using RCA connections, have you double checked to insure that:

- The input cable is on the "A" input of both amps.
- Speaker cables are connected to Speaker A + (RED) and Speaker B + (RED).
- The phase switch is in the "UP" position.  (Switch s/b "DOWN" if using an XLR balanced input cable)

Check on all of these

Using the SR-70 amps as mono-blocks provides (liberal rounding in use) ~5db increase in gain vs. the stereo setup. (40W stereo vs. 140W mono)  So any noise present on the input will be ~2.5 times louder with your mono setup as compared to stereo.  If you have a slightly noisy source and moderate to high efficiency speakers (88db/1W or better) with 140W on tap only requiring 1V at the input for full output, you have a recipe for HUMMMMM!

I see what you are saying, but the hum is only noticeable when the SB is not hooked up.

Also, the input to the SR-70 (as memory serves) is quite sensitive to overload as it requires only ~1V P-P for full output.  I believe that the Squeezebox can output @3V or so.  (Squeezebox has a reputation for noise, sometimes at deafening levels.  The suggestion to reload the server and 'box is a good one.  This is well documented.)

My SB was modded by Bolder Cable.  This process reduced the output to 1.1 v or so.  I will try reloading the Slimserver software, although it is a fairly new install. 

Lastly (whew!) most (all?) balanced inputs require a shorting plug when operated with RCA inputs.  Not sure about the SR-70's though.  Again, check w/Monarchy.

When I was researching this isssue I read about needing to use a shorting plug.  There is nothing in the Monarch manual to suggest this is true, but I will send a message to CC Poon.

Thanks for the help.  As for the random static/popping, I am trying a different set of ics to see if that prevents it.