Let's play Find the Ground Loop

[dlwearl]

OK....I've had my system hooked up for a while now, with no relief in sight. Here's what it looks like:

I have a Pioneer receiver-as-preamp (RCA, unbalanced) connected to an Ashly XR4001 crossover network (XLR, balanced).

From there, the Ashly goes to four amps: Crown D-75 (XLR, balanced; 2405h slots), Yamaha PC2002M (XLR, balanced; 2446h compression), Yamaha P-2200 (XLR, unbalanced; 2123h) and Crown Com-tech 800 (XLR, balanced; 2241g). All cables are home-made from Mogami Neglex 2534 quad mic cable.

Everything is mounted in an equipment rack and the Pioneer is grounded from its ground post on the back to the rack. When I did this, the atrocious hum I was receiving throughout the entire system almost disappeared....almost.

The only amp giving me any real problem right now is the Com-tech. When it's gain controls are at, say, 50% I can hear 60hz hum. I hooked up an oscilloscope and measured it at 62 hz or so, confirming that it is indeed hum. None of the other amps exhibit this behavior. The P-2200 when the gain is 100% exhibits ever-so-slight hum, while the other two just exhibit hiss, not something I"m too surprised about considering the 2002M powers compression drivers.

I'm near my wit's end with this. I've tried isolating the crossover, cutting the shield to one cable going to the com-tech, increasing the gain on the crossover, all to no avail. The power and signal cables are on opposite sides of the rack and everything is hooked up to one outlet. I'd really rather not have to purchase isolation transformers or power conditioners, so if anyone has some thoughts, I'd be grateful.

Thanks so much for all the help this forum has given me. I've tried others where I just can't get the same kind of assistance.
David :)

[just4kinks]

What is in front of the Pioneer? Is there a cable (TV) hookup anywhere in the system?

[Baron030]

Hi dlwearl

Your system is not all that much different from mine.
I too am running an unbalanced feed from a preamp into Ashly XR4001 crossover.
And after the XR4001, I am running balanced lines into 5 crowns, a K2, K1, and 3 - D75a amplifiers.
Just4kicks is spot on about asking if there a TV hook up anywhere.
Without an isolation transformer, my system would hum like hell.
So, if you have any TV hooks at all, then order the following:

http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=180-075 (http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=180-075)

Once, I hooked the isolation transformer between my Comcast cable and the TV, the hum disappeared like magic.

Baron030 :)

[dlwearl]

Hi guys,

There's no cable hookup going to the receiver. It - for the foreseeable future - is a music-only system. I just tend to listen to a lot of classical music and I find that during the quiet passages the hiss and hum and get a little annoying.

Thanks,
David

[boputnam]

I have a Pioneer receiver-as-preamp (RCA, unbalanced) connected to an Ashly XR4001 crossover network (XLR, balanced).I would try disconnecting the Pioneer from the rest of the system. With the signal path "starting" at the XR4001, with nothing into the inputs, is the rest of the system OK?

I suspect the "Com-tech" is merely showing the symptom, but is not itself the problem. The ground you tried on the Pioner helped, but cannot IMO remove the problem of itself not being "properly" grounded.

If you insist on using the Pioneer, you will need the handy-dandy Radio Shack Ground Loop Isolator (http://www.radioshack.com/product/index.jsp?productId=2062214&cp=&sr=1&origkw=isolation+transformer&kw=isolation+transformer&parentPage=search), recommended here many times with many happy users.

You should consider moving-up to a balanced pre-amp. It will improve your S/N dramatically.


If not, you need to start at the bottom and work backward, one amp at a time back to the Ashly. This is done by disconnecting everything.

1. Listen to the amp1 by itself. No GL? Move on...
2. Connect the amp to the Ashly, and listen. No GL? Move on...
3. Connect these to the Pioneer and listen. No GL?

Do-over with amp2, ONLY. No GL? Move on...
Do-over with amp3, ONLY. No GL? Move on...
Do-over with amp4, ONLY. No GL? Move on...

Do amp1 and amp2 together, only. No GL? Move on...
Do amp 2 and amp3 together, only. No GL? Move on...
Do amp3 and amp4 together, only. No GL? Move on...
Do amp1 and amp4 together, only. No GL? Move on...
Do amp 2 and amp4 together, only. No GL? Move on...

If/when you find a GL, the unique device in that configuration is the culprit.

[edgewound]

Maybe this will help:

http://www.rane.com/note110.html

[boputnam]

Maybe this will help:

http://www.rane.com/note110.htmlYea, but the OP seems pretty knowledgable. I suspect the Pioneer...

[Don Mascali]

Part of it may be the Pioneer being grounded causing the loop. (unbal/bal).

The crossover is filtering everything out of the other amps, as the Com Tech is the only one handling 60hz.

As for the hiss, read here;
http://www.rane.com/note135.html

There is a tendency to amplify the noise floor if the gains are set too high. I had a huge problem until I set the gain structure properly.
It isn't noticed as much in a Pro Rig but at home it is in your face.

Good luck.

[dlwearl]

Well guys, here's what I've tried:

I hooked up the bass speakers to the amp. Without any xlr cables plugged into it - so just the wires going to the speakers (which are only about a foot long), no problems - no hiss, no hum at any gain setting. As soon as I plug one or both XLR connectors from the amp to the crossover and turn up the gain, the hum starts, on both channels.

I wonder if the ground connection on the crossover and the amp at the wall is causing a ground loop? Although no way am I removing the ground pin on the plug....

Maybe I could cut the shield on the other cable? I doubt that will fix anything.

David

[boputnam]

It isn't noticed as much in a Pro Rig but at home it is in your face.Good post, Don.

wrt to "pro" rigs, actually, WADR, that is a misconception - all these issues are in-fact much larger. The noise floor is very evident at the SPL's we run. And, any evidence of GL cannot be tolerated - they are given a maddening chase.

Using headphones ("cans"), we solo every input and output, tracing the culprit. This problem arose five times last Sat - three combined occurrences with three seperate acts. Solutions involved DI's with Jensen transformers, mic'ing instead of any direct connection, and in one instance convincing the keyboard player his rig needed more maintenance service than we had time for and we jettisoned that POS off the stage taking him direct through wedges and the PA - it worked genius.



I hooked up the bass speakers to the amp. Without any xlr cables plugged into it - so just the wires going to the speakers (which are only about a foot long), no problems - no hiss, no hum at any gain setting. As soon as I plug one or both XLR connectors from the amp to the crossover and turn up the gain, the hum starts, on both channels.
Cool, we are getting somewhere...

Is the crossover disconnected from the Pioneer?

[boputnam]

One additional question - I forgot to ask... :o:

Is the pre-amp on the same AC circuit as the amps and ALL the peripherals?

It must be. It really does matter, you know... :yes:

[edgewound]

Have you tried moving the crossover away from the amps?

Could be the power supply is picking up EMF from the amp's transformers/power supplies. Since the crossover is a preamp it can be susceptible to extraneous noise from magnetic fields.

I'd also lift the AC ground as you're not lipping up to a live mic. Also as Scotty said is pin 1 tied to pin 3?

[boputnam]

Two things:


How do you know when you have the proper pitch on a banjo?

When it flies into the dumpster without hitting the sides!:rotfl:That is almost correct. It goes like this...

"Perfect pitch" is when you throw a banjo into the dumpster and it lands on an accordion. Now THAT's funny...

[Baron030]

Hi David

Are you mounting your XR4001 and amplifiers on the same rack?

There could be a ground loop running along the rails of your rack.

In my own system, the XR4001 is not mounted on a rack.

So, it is electrically isolated with only the xlr cables and its power cords running in common with all the other amps.

And I don’t have the ground loop problem that you are describing.

Baron030

[edgewound]

I'm sorry if this sounds insulting, but...and I'm the first to admit guilt on this one...

Did you double-double check your cables to make sure all the conductors are going to the correct pins, and the solder joints are clean and shiny?

Just a thought.

I bet that Pioneer doesn't have a grounded plug either.

[Baron030]

Quoted by Edgewound

I bet that Pioneer doesn't have a grounded plug either.


The preamp not having a grounded plug may not be an issue.

In my own system, I have a Carver preamp that does not have a grounded plug.
And I don’t have any ground loop issues.

Now, I do have a heavy 16 gauge grounding wire running from the preamp to the wall outlet cover’s mounting screw, which provides a good solid ground to the preamp.

Now, if David’s Pioneer preamp does not have separate grounding wire, then I would recommend trying that next.

Baron030 :)

[dlwearl]

Hi Guys,

When I tested the amp with the crossover, nothing else was plugged into the crossover. No receiver, no other amps. The power cords are connected to a power strip that is plugged into one wall outlet; all of the amps and receiver come out of that power strip when everything is plugged in. Neither of them were in the rack, the cables have been double-checked after I soldered them, and the two were some 6 feet apart (far enough, I hope, to avoid induced currents). The hum is also independent of cable: I have some XLR cables I had purchased some time ago and I get just as much hum using them as I do using the ones I soldered.

I tested the crossover using a multimeter and did find something I believe to be odd (or, at least, I'm not sure why it's designed like this). With a cable plugged into the crossover, the resistance between the ground on the plug and pin 3 is 100 ohms, consistent across the entire unit. There is a 50000 ohm resistance between pin 2 and cord ground, consistent across the entire unit. Pins 1 and 3 measure 100 ohms when connected together, also consistent across the unit. From plug ground to pin 1, all measure less than 0.5 ohms or so.

The Pioneer receiver does not have a three-prong plug, but I think that is irrelevant because the ground loop comes when the receiver is not turn on or connected to the crossover. I will admit that I don't have it grounded with the heaviest gauge wire to the rack, but grounding it to the rack relieved my system of almost all its loop problems.

Thanks,
David

[boputnam]

...the ground loop comes when the receiver is not turn on or connected to the crossover...That is very relevant info. Let me ponder that, and re-read the rest of the post.


With a cable plugged into the crossover, the resistance between the ground on the plug and pin 3 is 100 ohms, consistent across the entire unit. Since it's consistent, I don't think it relevant - it may have to do with the Ashly approach to "balanced". I have an XR1001 in the shop and mine gives the exact same measurements.

Have you tried swapping the duties of the amps? Meaning, is the Com-tech ALWAYS the culprit? And, normally you have the amps and crossover in a rack? I'm wondering if there is something just not right with the Com-tech...

[edgewound]

In SR, say a guitar player floats the ground on his amp - whenever his lips graze the mic, he becomes a potential path to ground (through the PA system, down Pin1) for that amp. He could get really whacked.



Been there....done that. 120 volts to my vocal mic. I coulda swore that the lights dimmed, but apparently it was my pupils fluttering. The band had to stop the song to see if I was o.k.

I don't recall if our power amp had a problem or the nightclub's electrical system, but our amp had 120 volts to the chassis.....which found my lips.

Luckily...I saved my Les Paul Custom from a crash landing.:p:D:applaud::blink:

That was about 25 years ago....them good 'ol days.

[Akira]

the ground loop comes when the receiver is not turn on or connected to the crossover.
This statement I don't understand. Are you saying the hum is caused by the receiver merely plugged in and not turned on or physically connected to the cross over? Does this mean the hum goes away when the receiver is powered on and connected to the rest of the system?

From what I understand: A hum exists between the connection of the cross over and the Comtech which both use balanced XLR's. The hum only appears when the receiver is plugged into the power source but not turned on, or connected to the rest of the system.

Removing pin 1 ground out of the XLR connecting the Comtech and cross over should eliminate the ground loop. If you do this, ground the chassis of both units together.

[dlwearl]

Well, here's what I did do (not sure why I didn't think of this before). My dad has an M552 active crossover, so I brought it up, connected one xlr to it and the com-tech amp and the hum decreased dramatically.

This told me it was the Ashly crossover, so I took it apart (took the back panel off) and tried to clean up some of the grounding points on the chassis. I scraped away some of the paint on the contact points and added some star washers where the back screws bolt the chassis together. Unfortunately, when I reassembled everything, I found that I had damaged a capacitor (I thought the hum was bad before..dear Christ I'm glad I didn't have the gain turned up), so I've ordered replacements which I'll have to solder.

Upon opening it up, I noticed that there were several points where the paint had already been scraped away (I can only assume by the previous owner), but the inside front panel where all the xlr potentiometers contact the chassis looks like it has been sanded with an air tool sanding disk or metal brush. I imagine it came like that from the factory, but it could be that previous owners have had their own problems grounding it. There's not much I'll be able to do for the next few days, but I"ll keep you all posted.

David

[dlwearl]

Just to clarify what I've been saying: I don't think the receiver has anything to do with this. It doesn't matter if it's plugged in or not, in or out of the rack, turned on or off. :)

I've isolated the components of interest, and that's just the com-tech amp and the ashly crossover. With those out of the rack and distant from each other, connected with one or both xlr cables to the amp I get hum. If I replace my Ashly unit with my dad's M552 with exactly the same setup, the hum disappears. So I'm pretty convinced that something in the Ashly unit isn't grounded properly. I"ll have to wait to test that though until some new caps arrive.

Thanks guys,
David

[boputnam]

Upon opening it up, I noticed that there were several points where the paint had already been scraped away (I can only assume by the previous owner), but the inside front panel where all the xlr potentiometers contact the chassis looks like it has been sanded with an air tool sanding disk or metal brush. That is just wrong. No-one unauthorized should have been inside the device. I'd send it back for a full maintenance service by Ashly. You have no idea whether the "sanding" actions resulted in metal particles being spread all around.


If I replace my Ashly unit with my dad's M552 with exactly the same setup, the hum disappears. So I'm pretty convinced that something in the Ashly unit isn't grounded properly. But you said...


My dad has an M552 active crossover, so I brought it up, connected one xlr to it and the com-tech amp and the hum decreased dramatically......but it was not gone.

I'm not convinced your conclusion follows the evidence. Yes, the GL behaves differently with the M552; however, the GL does not occur with the other amps, correct? Therefore, the unique condition, to me, is the Com-tech.


Removing pin 1 ground out of the XLR connecting the Comtech and crossover should eliminate the ground loop...:yes: Generally good practice to drop Pin1 at all load (input) points.

[edgewound]

Does the hum still happen with a different amp in the bottom end?

That might eliminate or implicate the Com-Tech.

[Baron030]

Hi David

I hate to have to say this. But, it sounds like you bought a lemon.
And all of the hum problems are internal to the Ashly XR4001.
And from what you are saying the previous owner had problems with it as well.
I sure hope you got this unit on the cheap, because are you are going to have to spend some real money on parts to bring this defective unit back to a state of good working order. If you need a schematic for the XR4001, just let me know and I will be happy to send you one. But, I think you would be better off having it repaired by an authorized Ashly service center.

Baron030 :)

[boputnam]

Does the hum still happen with a different amp in the bottom end?

That might eliminate or implicate the Com-Tech.Yea, that was my question, too.


Have you tried swapping the duties of the amps? Meaning, is the Com-tech ALWAYS the culprit? And, normally you have the amps and crossover in a rack? I'm wondering if there is something just not right with the Com-tech...The Com-tech seems to be the only device in-use which creates the symptom - it is the unique condition. :dont-know

[just4kinks]

...
I found that I had damaged a capacitor
...

Are you you sure that the capacitor wasn't already damaged? If there was a bad cap, then all bets are off. How many years old is the Ashly?

I would still take boputnam's / edgewound's advice and try another amp in place of the com-tech.

[subwoof]

Having a metal rack, chassis's earthed, different type interconnect cables and unbalanced lines WILL cause issues.

Remember that the system should only have one pathway to AC ground, IE one power strip.

Try running the system with NONE of the items mounted in the rack and physically isolated from each other ( cardboard, etc ) to see if the metal rack rails are the "second" pathway.

A hum from the 810 that stays the same without a change in volume is a loop with IT'S input / ac wiring. A hum that *changes* with the volume control is IN the signal being sent to it...

A VERY COMMON problem with the ashleys is that the solder connections on the rear panel jacks tend to crack when they are rack mounted and the connectors are left in ( basic road use ). resolder ALL of them and make sure that the small electyrolytics on that board are not broken off.

The ashley's "balanced" output is signal on pin 2 ( or tip ) and pin 3 is routed to ground by a 100 ohm resistor. While this makes the amp down the line "see" the same "impedance" on each leg, it is by NO MEANS a balanced line. It's just a way to save money in production.

The input on the crown 810 is dependent on WHICH card you have installed. Some cards are problematic because they unbalance the source and also might not route the singal you want to the input jacks ( the XOV is notorious for this )

what card is it?

[boputnam]

sub, dood - great post.

:applaud: