Dbx286s - DC Offset?

Audio Equipment
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Hello Everyone,

I have recently noticed that my beloved dbx286s is introducing what I believe is DC offset (see attached image).

I know that Adobe Audition has a DC Bias Adjust correction as part of the normalise effect but it does not seem to help here as the DC offset is meandering on both sides of the null line from positive to negative almost like a sine wave…

I have no idea how to fix this in post and it’s really concerning/annoying. I hope my dbx isn’t defective.

Anyone out there who has encountered this before? How did you fix it?

Thanks in advance for your help,

Charles (a deeply worried voice over artist)

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Hi CharlesVO,

First of all, I think this is hard to diagnose without a test bed to help identify the problem. However I have seen a problem like this before so this might help you locate the fault for an engineer to fix…

As far as I can see you have what’s called a fluctuating DC offset. Your file shows the offset going from pole to pole and back again. Unlike a DC constant which would either be a fixed voltage (analog) or fixed value (digital) above the ‘zero crossing line’ all the time (positive) or below (negative).

The culprit is most likely to be a failing capacitor in the analog signal path/chain.

The real question is this… Where in the signal path is the problem?

Ignore this bit if you don’t use a condenser microphone…
If you are using a condenser mic (which can also be called a capacitor mic) your problem could be here in the capsule/body depending on the design. You would need to swap this microphone with another to prove it is working correctly. Don’t forget to observe the correct routine for swapping condenser microphones with each other. (Turn gain down and phantom off before swapping, then reverse the procedure when the microphone has been replaced). Once you have swapped microphones try recording a small amount of audio and then look at it in Audition to see if your problem is still present. If you still have the same DC problem then it looks like your Dbx286s could be where the fault is.

To help find the fault in the signal path/chain of the Dbx you need to isolate each section from each other. So first of all turn off the processing section and just leave the preamp active, also turn off the 80Hz high pass. Again record a small amount of audio and inspect the file in Audition. If the problem is still present then your fault is either in the mic preamp, 48v phantom power supply (if using a condenser mic), the output stage or possibly the main power supply. If your fault has vanished try again with the high pass switched on. Once again check in Audition, if the fault appears then the fault will be found in the high pass circuit. Continue working along the unit until you find a section that produces the DC problem.

Once you have identified it you can then get in touch with your equipment supplier or manufacturer, tell them what and where you suspect the problem is and they will be able to more easily locate and fix it for you.

Hope this helps…

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Hello @IanT and thank you for your detailed input.

Here’s what I have found.

Signal chain from mic, to interface, to DAW is flawless whether I use either my Neumann TLM103 or Sennheiser MKH416 and my Sound Device MixPre-3. (So this answers the condenser/dynamic mic question: I only use condensers).

So this tells me that the culprit must be the DBX.

Now, question was to find out where in the signal processing.

My finding are as follows.

Test 1

DBX signal processing stages are quite easy to follow as they go from left to right when you look at the panel itself.

I turned everything off on the interface and activated the process bypass button to eliminate that variable for my first test. I plugged in the mic with no 48V phantom power and 0 dB gain trim as I always do to protect my gear. Then turned on phantom. Counted to 5… Slowly turned the gain trim knob to 45 dB, which is quite typical for any preamp I use with these 2 mics.

80Hz High Pass… OFF
Process Bypass… ON

Results: Asymptomatic

Conclusion: Problem does not occur when the signal is only going through the mic preamp portion of the DBX.

Test 2

Same mic, same setup.

80Hz High Pass… ON
Process Bypass… ON

Results: a tiny bit of fluctuating DC Offset

Conclusion: There is a bit of DC Offset occurring in the 80Hz bass rolloff circuitry.

Test 3

Same mic, same setup.

80Hz High Pass… OFF
Process Bypass… OFF
All other knobs in the signal chain… OFF or ZERO (Compressor, De-Esser, EQ, Expander/Gate)
Output Gain… CENTERED ON 0 dB

Results… Considerable fluctuating DC Offset.

Conclusion: Fluctuating DC Offset occurs in the process portion even if all the effects are turned off.

Test 4

Same tests as above 1-2-3 using different XLR cable (same brand and length https://digiflexcables.com/ which is a brand I like and trust).

Results: Asymptomatic no matter what combination of mic or DBX settings. (!!!)

Conclusion: Faulty XLR cable.

Overall Conclusion

Funny that a simple cable will react differently whether it is being used with a certain preamp with certain settings.

I have never experienced this particular problem before using this cable and the DBX processor… Interesting. Problem must have crept in over time.

Now, will it be worth my time and money to send in the cable and ask for a new one… Good question. Probably not.

Thanks for your help again!

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Hi CharlesVO,

Thank you for the update and the detailed logging of your fault finding…

I’m glad that it only appears to be a XLR cable which has been causing your troubles.

If it was up to me, I would send the cable back to have it checked if only to confirm the findings from the tests performed. It is strange that it has changed its ‘behaviour’ over a period of time, why?.

Anyway… for the moment you are able to work without a DC problem… this is good news. However I recommend that you keep a careful ear and eye on all the audio you produce for at least the time period that the faulty XLR cable was in the signal chain. If the problem starts to appear again, get your 48v and main power supply checked on the Dbx.

Here’s hoping that this is the end of your problems…

Cheers.

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