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DBX 160A Compressor hum and noise

Discussion in 'Amps and Cabs [BG]' started by Josue Cabalceta, Sep 11, 2019.

  1. Josue Cabalceta

    Josue Cabalceta

    Sep 11, 2019
    I just bought a DBX 160A Compressor, and I'm getting this strange hum or noise when I set the gain knob above 0.

    The compressor is connected to the effects loop of a Gallien Krueger 800RB amp. The send of the amp is connected to the input of the DBX, then the output of the DBX is connected to the return of the amp. That's all... They all are connected with TS cables (Tip/Sleeve).

    When the gain of the DBX is set below 0 everything works just fine, but when the gain is set in 0 or above, there's this really intense hum...

    I tried connecting it BEFORE the amp (not in the effects loop) and there was no hum at all, but the idea is to use it through the effects loop of the RB.

    Is it the compressor, is it the amp, are they the cables? What should I do?

    Here is a video of what the hum sounds like (use headphones, the hum is intense but very low): https://photos.app.goo.gl/91A8Swt6Xdg6Y6H19
    Last edited: Sep 12, 2019 at 2:47 PM
  2. Jeff Scott

    Jeff Scott Rickenbacker guru.......... Supporting Member

    Apr 11, 2006
    If the compressor sounds fine going into the front end of them, my guess is the amp is the culprit.
    Josue Cabalceta likes this.
  3. alembicguy

    alembicguy I operate the worlds largest heavey equipment Supporting Member

    Jan 28, 2007
    I've run my DBX 160A into the effects send and return the exact way the OP did on my Mesa 400+ and have zero noise.
  4. BogeyBass


    Sep 14, 2010
    yeah. the idea of a effects loop is nice.

    in reality i find them annoying 80% of the time.

    first you have input gain that brings levels way way way up. then a effects loop that brings levels way way way down.
    then another stage that brings everything way way way up again.

    in english that means a buttload of noise. more notible with a compressor or distortion effect.

    oh yeah 90% of effects are pedal based. most effects loops dont work well with them.....wooo genious

    thing is the compressor coud still have a hum. its just more noticeable in the loop. or the loop is noisy. neither would surprise me.

    if the compressor has a magical tube in it. aside from the usual noise a compressor makes, the tube will just help that much more
  5. Josue Cabalceta

    Josue Cabalceta

    Sep 11, 2019
    Thank you! I've read in other forums that many people have the same hum-problem with their DBX 160A... This should mean that it is something with the Compressor, but I haven't find an answer
  6. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    The DBX-160A should be used in the effects loop, it's a +4dBu line level device. There were different versions made over the years, some of the early ones have a +4/-10 or -20dB switch(s) on the back, they should be in the +4dB position.

    In an effects loop, generally you will want the output level or trim control set to 0dB, above this there is additional gain added (called make-up gain, sometimes used with heavy compression to bring the average level back up to line level knowing that the input will be compressed accordingly).

    With 20dB of make-up gain available, there will also be 20dB of additional noise, so any flaws in your bass or signal path will be much more obvious.

    I spent many years with this product (at FOH) and have used this specific model when testing line level loops on bass amps. It works and works well (provided it's not damaged)
  7. spigmu


    Mar 25, 2009
    Brooklyn, NY
    The 160A manual says this about the ins and outs:


    The Tip/Ring/Sleeve phone jack and XLR-type jack are wired in parallel; either INPUT will accept an audio signal for processing by the 160A. The phone jack accepts a standard TRS 1/4” phone plug for a balanced input source, or a 2-cir- cuit (Tip/Sleeve) 1/4” phone plug for an unbalanced source. The XLR-type jack is wired pin 2 HOT (+), pin 3 COLD (-) and pin 1 GROUND.


    The 160A's OUTPUT (BALANCED) jack is driven by a floating active-balanced amplifier that simulates a true trans- former output. This allows for the load at the OUTPUT to be either balanced with respect to ground, or single-ended to ground (unbalanced) with very little difference (less than 0.5dB) in output level.
    For proper unbalanced operation, the unused pin (either pin 2 or 3) MUST be grounded.

    and this about the level and ohms:

    Nominal output signal level is +4dBu into 600Ω, and typical maximum output level is +25dBu into 600Ω. This jack accepts XLR-type connectors, wired pin 2 HOT (+), pin 3 COLD (-), and pin 1 chassis ground.

    OUTPUT (UNBAL) Jack:

    The 160A has a separate single-ended (unbalanced) output amplifier capable of driving a 600Ω (or greater) load to +24dBu. This jack accepts 1/4” phone plugs. In normal operation the TRS phone jack is internally wired with the TIP HOT (+), and the RING and SLEEVE connected to ground. In Bypass mode, however, the RING is connected to the COLD (-) leads of the input jacks, to allow for balanced connections.

    The 800RB has this about the effects loop:

    Effects Send: Level(Boost off) - 1.5V rms 100k ohm min

    As agedhorse says, there were several versions of the 160A, so I can't say for sure if you have an impedance mismatch going unbalanced or if the unbalanced to input ground wiring isn't right, but compare yours to your manual.
  8. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    The input impedance on the DBX-160A is ~40k, the send impedance on the 800RB is 4.7k, that's a plenty good match IME and not going to be the cause of the symptoms.

    Ground loops between the two devices cab be an issue, some of the DBX comps had a chassis ground link that could be opened up to minimize this impact.
  9. Josue Cabalceta

    Josue Cabalceta

    Sep 11, 2019

    Thank you for your answer!

    I don't see any +4/-10 or -20dB switch(s) on the back, there's only a Lift switch.

    I'll attached some photos of the DBX, I don't know if it is one of the early versions...

    Here is a video of what the hum sounds like: https://photos.app.goo.gl/91A8Swt6Xdg6Y6H19

    Attached Files:

  10. Josue Cabalceta

    Josue Cabalceta

    Sep 11, 2019
    This is in the DBX 160A manual:

    "GROUND (GND) Lift Switch: This switch allows for troubleshooting hum (line interference) caused by ground loops. In the “GND” position, pin 1 of the input XLR-type jack is connected to the 160A's chassis ground. In the “LIFT” position it is disconnected."

    But I guess it only refers to the XLR output (balance). I am using the TS output (unbalance), so this switch does not solve anything in this case, right? I tried it but the hum continues.

    Should I open the chassis?
    Last edited: Sep 12, 2019 at 2:57 PM
  11. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Yours is a later version, the ground lift was originally a link rather than a switch. I don't have the correct schematic, but historically the ground lift only applies to the XLR or terminal strip connections on DBX products.

    The workaround is an XLR to 1/4" TS connector wired per their instructions, and this might not work well since lifting that ground on a TS connection can cause instability in some I/O circuits.

    The pro workaround is to use 1:1 audio isolation transformers. When I toured (in pro audio), I carried a dozen for those situations where it was the only practical solution. It also tends to work about 95% of the time. If you are interested in this, PM me and I will look to see what I have.

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