It is a multivoltage unit that can be converted for use with 120V or 230V I'm in the U.S. so it's currently wired for 120V. It is second-hand. I have done a bunch of research and run some tests. The bottom line: I think the problem is electrical not mechanical; I don't believe it is due to a ground loop; I do believe it could involve left-right channel cross-talk in the 5500. My tests: 1. Connect the Adcom 5500 to power using a 3-prong power cord to a new Furman power conditioner. Connect a pair of speakers. Nothing connected to input jacks yet. 3. Power On: I get silence. Yay. 4. Connect a pair of patch cords high quality or low, doesn't seem to matter between the input jacks on the Adcom and a pre-amp again - high quality or low, doesn't seem to matter, I've tried several. Pre-amp has no other connections it doesn't even have to be plugged in to AC power - same result if it is or isn't. 5. Power On the Adcom: buzz! It's an audible buzz that can be heard from several feet away, even as far as listening position. It might be accompanied by a 100 - 120Hz hum, I'm not sure. 6. Disconnect either patch cord left or right, doesn't matter and the buzz goes away. Connect patch cord again, buzz comes back. 7. Again, the only connections are: Adcom Furman conditioner AC outlet; Adcom speakers; Adcom pre-amp. Pre-amp is otherwise unconnected to anything. If I take the Furman out of the equation and plug the Adcom directly into the wall, no difference in results. I have tried this in two different geographic locations, one an apartment complex, the other a single-family home - no difference. I don't have any experience testing electronics, but I do have a multimeter, and the patience and determination to use it. ; One other point: I said this was a multivoltage unit. I found a diagram online that shows how the toroidal transformer is supposed to be wired up to the power switch posts for 120V vs. 230V. The weird thing is, the way my amp is wired up doesn't match either of the diagrams. I know the amp was serviced by the previous owner at an electronics shop in Philly I have the shop's contact info and a service/job no., so it's feasible someone didn't wire it correctly - on the other hand I'm a newbie and this thing packs enough punch to kill me, so I figure it's prudent not to mess with the main wiring. : Any help, questions, or experience will be greatly appreciated! Thanks very much, Tim H.
Things to check:
1. With your multi meter, measure from left ground to right ground. What do you get? Measure from each input ground to chassis. What do you get there.
2. Get or make a pair of shorting plugs. Plug those into the inputs. Power on. Hum there or gone?
3. Turn the pre-amp on. Connect. Level of hum change with preamp on?
4 additional information. How efficient are your speakers? This is a big amp, high-efficiency speakers may show some noise, while low-efficiency speakers won't.
A trip back to the shop may be indicated, too.
Testimonial: "Thanks, Jay! 1. Assuming I am using my multimeter correctly, readings are 0.0 mV all around (left ground - right ground, left ground - chassis, right ground - chassis). Same readings whether patch cords are connected or not. 2. Any recommendations on a quick way to get my hands on shorting plugs? They don't seem to be as easy to find as they used to be – I followed several links online but they were all stale or broken (the links not the plugs :) ). 3. No change with pre-amp on or off. 4. I get the buzz with every speaker I've tried; right now I have Yamaha NS-777s connected; their sensitivity rating is 89 dB. Any further thoughts? Thanks!"
I think I found out why my readings are all 0.0 – there is what I think is an extra wire connecting the channels (see attached pictures – the one without arrows I found online; the one with arrows pointing to the extra wire is mine).
Ever seen this? Any idea why it is there, or whether it was likely installed during a service?
other picture
First, reading 0mv means you're not measuring resistance, but voltage... Yes, you are using your meter incorrectly for the task...
I don't know enough about your specific amp to be able to tell you if your extra wires are the issue... Please do go back and measure resistance between the points I suggested. That may tell us much.
I don't know if I am doing this correctly but here goes. I am using an ETEKCITY MSR-P600 multimeter –http://www.amazon.com/Etekcity-MSR-P600-...
I unplugged the amp from A/C power, my speakers connected, and nothing connected to the inputs. By ground I think you mean the metal outer portion of the RCA input jack. Readings were a little tough to get because they fluctuated – I had a hard time getting them to settle down. Tell me if these readings make any sense:
R ground to L ground: 0.2-0.3 ohms
R ground to chassis: 3.8-3.9 ohms
L ground to chassis: 3.7-3.8 ohms
The speakers are 6 ohms.
Thanks,
Tim
Yes, you've done what I requested, and your audio grounds are reasonable. Have you tested your amp with the inputs shorted? Essentially, when you connect up input wires, the wires are acting as an antenna, picking up the strongest signal around, 60 Hz hum.... Shorting the inputs should eliminate that.
Measuring that the "audio ground" and "chassis ground" are connected is worth doing, but I suspect your results may indicate that there is something "in the way", not allowing the 60Hz you're picking up to drain off. You might want to _try_ running a wire from one of the audio grounds to the chassis and see if that makes any difference.
For more expert (free) help, you may want to join the diyaudio.com forum. We're worldwide, and some are very knowledgeable. Many will help.
Hi Jay,
Thank you for your advice thus far.
Comments:
- I am familiar with a 60Hz hum, and that is not what I am hearing. It is a buzz (sounds like an insect), possibly accompanied by a faint hum at a higher pitch, maybe in the 100Hz-to-120Hz range I've read about; but it's the buzz, not the hum, that is bothersome.
- I hear the buzz only when I connect leads to both channels between the amp and pre. Anything short (excuse the pun) of a complete "interconnects path" between amp and pre does not produce a buzz.
- I tried connecting a wire between the chassis and one of the channel grounds – there was no change in the buzz. Should I do this with the other channel as well, or is that question asked and answered?
- When you say "short your inputs" are you referring to using the shorting plugs you mention in your first reply on this thread? If I put shorting plugs in the inputs of my amp, how do I tell if they are doing anything? (See previous point – I won't be able to connect anything to the pre, which is what produces the buzz.)
- Finally (here I risk alienating you by fully revealing the extent of my electronics ignorance): My pet theory, based on observations so far, is that there is something about the completion of a signal path (not ground path) between the channels of my amp (I base this on the fact that connecting leads from both channels of the amp to the pre is required to produce a buzz – even when the pre has nothing else connected to it, not even the AC power cord). My question is: What would happen if I connected a single interconnect between the left and right inputs on my amp? That would meet the condition of creating a signal path between the amp's channels. If I don't blow everything up, will it tell us anything about the problem?
Again, thank you. As you suggested I have joined DIYaudio (as thefferon) and am looking for advice there as well.
Best,
Tim
The shorting plugs are instead of the connection to your preamp, and is an attempt to further isolate the problem. The question in the back of my head, "Is the buzz/hum a product of amplifier instability caused by the capacitance of the input wire, RFI (Radio Frequency Interference), or something else?"
If it's RFI, the shorting plugs should make the amp silent, and we can talk about a "pi filter" to reduce it at the input to your amp.
So far the only condition under which I have heard the buzz is when the amp was connected to the pre. If shorting plugs are occupying my amp's only pair of input jacks, how do I know whether the buzz is gone, since I can't connect the amp to the pre to listen for it?
In other words, how can I know if the amp is silent if I can't reproduce the conditions under which I would normally hear it?
Tim, the shorting plugs can help to reduce the uncertainty for where the buzz is coming from. Even turned off, your pre-amp could be picking up RFI and sending it down the wires. Shorting plugs should be just as silent as the amp with nothing. If it isn't, then the amp has a problem. If it continues silent, you may have a pre-amp problem to solve.
Do you have another pre-amp? Worth testing, too.
I am making shorting plugs, but all I have at hand are 2000-ohm resistors to solder between signal and ground on the rca jack. Is it worth using them, or should I seek out something in the 0.5-k range?
Use a piece of wire not a resistor. Shorting plug is zero ohms.
:D
With shorting plugs in, amp is beautifully silent.
I have a set of low-capacitance interconnects arriving in the mail Monday or Tuesday:http://www.bluejeanscable.com/articles/h...
The pre I have been using is very nice so I'd be surprised if it is the source of the buzz. Here's hoping the Adcom is simply in a league that requires me to use better cables than I have been using.
In the meantime, what's a "pi filter," and is it now indicated?
Thank you again.
First, I don't think the Adcom is the problem at present... It's nice and quiet itself.
A "pi filter" is a filter based on two capacitors and a resitor, often used to reduce RFI.https://en.wikipedia.org/wiki/Capacitor-...
The trick is to use the correct values....
I use quite small values for the capacitors, and say, 5k instead of a coil/inductor for L1. Use, maybe 100 pF caps. You can mount these right on the input sockets inside the Adcom.
Hi Jay, low-cap cables arrived early, and as you tacitly suggested, they make no difference :( . Before I hook up my pi filters I want to make sure I understand you. Please let me know if the attached diagram looks correct. Does wattage rating of the resistor matter? The ones I have are 1/4W.
Thanks, Tim
1/4 watt is fine, essentially no current will flow. circuit inputs are on one side of resistor, output on the other side.
"circuit inputs are on one side of resistor, output on the other side"
I want to make sure I hook up my pi filters correctly. As seen from the inside of the chassis, there is currently hooked up:
- to signal (center pin): 1 wire, other end terminates in a plug at channel-specific circuit board mounted on side of chassis
- to ground (3 wires, soldered together at ground node): 1 wire, sheathed with signal wire mentioned above, going to same plug in channel-specific circuit board; 1 wire going to a plug at circuit board mounted in center of amp (board where caps are all mounted); 1 wire going directly to ground of other channel's input socket (may be after-market wire)
So: Exactly where do I solder the two ends of my pi filter?
– Thanks very much
Tim, remove the connection from the center of the plug. Connect one end of the resistor there, the other end of the resistor to the wire you just removed. Connect the capacitors from both ends of the resistor to the socket ring (ground).
I again suggest joining diyaudio.com for better advice than I can offer. I dabble, there are serious builders and fixers there that will help you.
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Jay,
I entered a response to your Answer as a Testimonial, so I don't know if you saw it – please check.
I also added two photos with two red arrows pointing to where a single wire connects the left and right channel grounds. I'll include them again here.
Thanks
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