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Effects Loop Ground Problem (I think)

Discussion in 'Amps and Cabs [BG]' started by Chris Fitzgerald, Feb 27, 2003.


  1. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    Hope this thread is in the right forum, and that it isn't a duplicate. That said....

    Tonight I just racked up my new EA iamp800 into a 3 space SKB rack. I have also purchased what is by all accounts a high quality (slightly used) 31 band graphic EQ to use with it for DB - an Ashly MQX 1310, which I placed in the space above the iamp. I'm using a Raven Labs PMB-1 as a mic/pickup blender on my DB. My plan was to go into the blender, have the graphic wired into the effects loop of the blender, then go out into the iamp.

    When I turned everything on, there was a hum/buzz coming out of the speaker, so I checked the signal chain, pulling out one thing at a time, until I discovered it was the EQ (when I unplugged the EQ from the effects loop, the hum went away). What's weird is that when the EQ was wired into the effects loop, the hum was there whether the EQ was "in" or bypassed, and it was still there even when the EQ was turned completely off! Pardon the expression, but ***??? Aside from the hum, the unit doesn't seem to be noisy at all, and I've heard nothing but great things about this brand of EQ. I'm stumped.

    I'm also the first to admit that I've never used an effects loop on a bass amp before, so I might be doing something really obviously wrong. If anyone could check my signal chain and make any suggestions that might help, I'd be very grateful.

    THE CHAIN: Stereo out on DB - - -Stereo in on Blender - - -Effects "send" on blender - - - Balanced "in" on EQ - - -balanced "out" on EQ - - -Effects "return" on Blender - - - Line Out on Blender - - - Input of iamp.

    The VARIABLES:
    1)The Raven Labs blender has an "Effects Blend" pot on it. The hum is worse when the knob is turned full left or full right, and better (but still present) when it is at dead center;

    2) The ins and outs on the EQ are balanced. The unit has both XLR and 1/4" ins and outs. I was using the 1/4" connections. I tried both balanced (TRS) and unbalanced cables. (I should mention that the only balanced cables I had were part of an 8 cable recording snake, but they are dead quiet for my recording gear).

    3) Thinking the problem might be the effects loop on the RL blender, I tried using the effects loop on the iamp, but got the same results, even when the Ashly EQ was turned off.

    I'm stumped. If anybody can help clear this up, I'll owe you big time. Thanks.
     
  2. Bob Gollihur

    Bob Gollihur GollihurMusic.com

    Mar 22, 2000
    New Joisey Shore
    Big Cheese Emeritus: Gollihur Music
    Are both the EQ and amp installed in the rack? If so, remove the EQ unit so its chassis is not grounded to the rack rail; it could be a ground loop. If that solves it, there are stand-offs to isolate devices from one another.
     
  3. It sounds like a ground loop. You may be able to make up some special cables to alleviate the problem. Worst case you'll also have to get some isolation mounts that keep the chassis of the EQ from making metal-metal contact with the rack. It just depends on how it is constructed.

    For more info than you ever wanted to know, read this Rane Note on cables and stuff:

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

    And here's the main page with a plethora of info:
    http://www.rane.com/library.html

    Chris
     
  4. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY

    Thanks for the links, but HOLY CRAP! :eek: I printed the first one and read it about three times, but I'm afraid it's way over my head. I'm sure the concept of chassis ground is simple enough, but I'm not getting it yet.What I'm walking away with at the moment is the notion that I should have probably bought an EQ with unbalanced connections? But since I didn't, I'm not sure what to do....

    I tried taking the EQ out of the rack as Bob suggested, but that didn't change the hum at all. So now, based on what little I understood from the Rane document, I'm supposed to modify my connecting cables to make sure that I have acheived "chassis ground", whatever that might be. The Ashly unit has three different types of available connections on the back: XLR, 1/4" TRS, and also three screws for both "in" and "out". The ones on the INPUT side are labeled Input Ground, (+), and (-), while the ones on the OUTPUT side are marked Chassis Ground, (+), and (-). I'm getting the dim notion that in order to fix this problem, I'm going to have to either:

    a) Connect some part of my incoming/outgoing cables to the grounding screws; or

    b) Fashion cables in which one end terminates in a 1/4" mono male connection, and the other terminates in wires which should be connected to these screws in some configuration which I can't even begin to fathom yet.

    Am I even in the ballpark? My first reaction to this kind of stuff is to run screaming from the room, but I'd like to stick with it and try to get it sorted out even if (as it has so far) trying to do so makes me feel like a complete moron. Can anyone give me a clue about what to try next?

    Wow - I think I'm beginning to figure out what my students must feel like when they find themselves running into their own personal "Music Theory" walls while I'm trying to explain something that seems simple to me. Paybacks are hell, eh? :D
     
  5. I'll have to study your layout to see if I can condense those Rane Notes down to the correct answer for you. I was thinking that you may need to make some custom cables. It'll be easy to try different configs with those screw terminals.

    If I can figure anything out, I'll post it in a little while.

    Chris
     
  6. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Here's something to try: run a short, shielded cable from the effects out to the effects return. That way you can check out the effects loop by itself.

    If you still have hum, you'll know that it's within the preamp. If the hum is gone, then the problem is with the EQ.

    The fact that the hum is predominantly at the maximum or minimum settings of the effects blend pot makes me think that the preamp will prove to be at fault.
     
  7. OK, I think the problem is that the effect loop send and return are unbalanced, and the EQ Out and In are balanced. Correct me if I'm wrong on that.

    If that is the case, about the only thing I know to do is use a couple of isolation Lo-Z to Hi-Z transformers to couple those inputs. You can buy a couple at radio shack that will plug into the XLR jacks and have a 1/4" unbalanced jack on the other end, and even if the fidelity isn't exactly hi-end, at least it will tell you if throwing in those Lo-Z/Hi-z transformers will fix your problem.

    I'll keep thinking about it.

    Chris
     
  8. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    Thanks for the tip. based on this test, I discovered that the problem is DEFINITELY with the EQ. Whether it is powered up or not, bypassed or not, the moment I run a cable from the "send" on the preamp to the "input" of the EQ, the hum begins. When I ran the short cable from the send to the return on the EQ, I could detect a tiny[/b] difference in sound as I rotated the effects knob, but I had to stretch quite a bit to hear it. The hum from the EQ, on the other hand, is readily apparent the moment the "send" cable touches the "input" jack.
     
  9. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Good, I think we're getting it nailed down. Since the Ashly EQ otherwise works fine, it would seem to have what's called a pin 1 problem. That's where the input connector grounds are not immediately connected to the device chassis, but instead to signal ground, power supply ground, or whatever. Proper grounding methods weren't always understood by or even made known to design engineers until Neil Muncy really started beating the drum for it back in the mid 90s.

    To read more about pin 1 problems, see:
    http://svconline.com/ar/avinstall_equipment_wiring_internal/index.htm
    http://www.mmproductions.co.uk/ground6.html
    http://www.engineeringharmonics.com/papers/s_vc.htm
    http://www.rane.com/par-p.html

    Isolation transformers (www.jensen-transformers.com; www.sescom.com) will help a pin 1 problem, although they won't correct the offending device, and they are yet something else to keep track of in your bass rig. Modifying the unit with the pin 1 problem is a more robust and reliable solution, but it involves opening the unit and working on it, and it's best left to a qualified technician.

    Contact Jim Schwenzer (jschwenz@ashly.com) at Ashly and tell him about your situation, tell him your EQ has a possible pin 1 problem, and ask him to suggest some solutions.
     
  10. Bob,

    Isn't the root cause of this the fact that the balanced input of the EQ is being connected to an unbalanced output of the Effects Loop? That means that the XLR (-) connection is getting connected to signal ground at the preamp.

    I think the pin-1 problem type stuff is secondary to the Unbalanced-Balanced problem.

    Correct me if I'm wrong about that (which I very well could be).

    Thanks,
    Chris
     
  11. http://www.ashly.com/photos/equalizers/mqx/rear panel/mqx1310-b.jpg

    And there's the back of the thing if that helps. Note that the outputs can be run unbalanced, but the inputs have got to have a balanced signal coming in.

    I vote for a Lo-Z to Hi-Z transformer on the input of the EQ. Beyond that, there may still be a grounding issue.

    Chris
     
  12. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Unbalanced lines are inherently less immune to induced noise than balanced lines are. If you connect a balanced input to an unbalanced output, you lose the noise-cancelling benefits of a balanced line, but that doesn't make it worse than a plain unbalanced-to-unbalanced connection. The straight-through test of the effects loop demonstrated that an unbalanced line in his rack isn't in too hostile an environment.

    I wouldn't go with hi-z/lo-z transformers; if I opted for a transformer solution, I'd go with straight 1:1 isolation trannies, balanced on the balanced side and unbalanced on the unbalanced side.

    My preference would be to fix any pin 1 problems in any device in the system, but then I'm a geek who likes to open things up. :cool: Isolation transformers would be my second (but still perfectly acceptable) choice.
     
  13. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Hey, Chris, thanks very much for the link to the photo. That's helpful and it gives me some more to think about.

    Chris F: Is the metal jumper from input ground to chassis ground in place on the screw terminals? Having the jumper in place should eliminate a pin 1 problem.

    Here's something to try: Get a short balanced cable, TRS to TRS or TRS to XLR male. Mark a TRS end as the preamp end. Open up the TRS plug on this end and solder a jumper between the ring and sleeve terminals, then put the plug back together.

    On the other end of the cable, open the connector (TRS or male XLR) and cut the shield connection (pin 1 or the sleeve), then reassemble it.

    This makes what's called a quasi-balanced cable. Use it for the effects send to the EQ input. Use a regular unbalanced TS to TS cable for the return to the preamp. See if that helps.
     
  14. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    Like I said, I'll wait for one of the people who knows more than I do about this stuff...but thank you both (Bob and Chris) for spending so much time trying to help me get this figured out....it's been driving me crazy, and it's good to know that somebody's got my back. Thanks. :cool:


    P.S. - oooh, forgot to mention that I also tried the high Z - Low Z transformer...no go. I suppose that should also fill in a piece of the puzzle, but I'm not sure what.
     
  15. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    No, I said the sleeve, not the tip. That's if you use a TRS plug. If you use an XLR connector, it would be pin 1.
     
  16. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    Ah. See, I told you you were dealing with a tech dummy! :D I'll try that when I get back home. I also got a nice email back from Jim @ Ashly, who suggested that I try removing the jumper between the input and output grounds on the back of the unit, or if that doesn't cut it, to try "ungrounding" the unit from earth entirely. I'll report back in when I've had a chance to get my hands back on the unit. Thanks again!