Behringer DDX3216 mic input(s) crackling

I recently noticed a crackling issue on a few channels. Seems to be largely independent of signal level (low level = still some cracking; increases a bit with signal level, but not by much).

It's definitely not a clipping issue, the crackling's there with other mics and cables on the same channels, and irrespective of the preamp gain setting.

The design seems pretty standard (compound pair into a differential receiver opamp), i've seen it in several Mackie and Soundcraft mixers. Although in the service manual, the way they drew the schematic is a bit less intuitive to "get".

http://www.plzi.com/aanta/ddx3216/ANAIN_Mic_Line_Rev_E.pdf

Since it's quite a chore to open up and extract the boards, what might the most likely culprits be?
Crapped out input transistors? The DC-blocking electrolytic in the gain adjust?

You probably have DC somewhere, caused by leaky electrolytics. If your noise is while turning mic gain, I would start out by replacing C167-C168, that are the 1000uF/6V in series with the mic gain pot.

Jakob E.

No, i don't think i had "unreasonable" noise while operating the gain pot. That would, of course, point at those DC-blocking caps there (which may or may not be part of the overall issue, though).

The cracking is while there is an input signal above some arbitrary(?) level.
It's quiet when there's no level, but if i, say, lightly tap a dynamic mic (nowhere near clipping the preamp), there's crackling superimposed on top of the signal. That's why i thought it could also possibly be the input transistors.

I vaguely recall having read about such cases, somewhere (albeit not on this particular model, but in similar preamp designs).

It's not that hard to open it up, just a bunch of screws. Set it on one side and open like a clamshell. It's been a decade!?! since I worked at the service center, but I would swap cards to identify where the problems were in the DDX3216.  If there are two input cards, swap them to determine if the noise is in the input, or down the line. It may be a digital issue. The power supplies can be problematic as well. I had one golden unit and recall several times finding multiple cards cause a single problem.

Oh, i've had this one open a fair few times already - bought it with a crapped-out PSU ;D

It's just a real hassle, with so many screws "just" to remove the top, plus all the XLR's and the 1/4" jack nuts...
I was just hoping to be able to narrow things down somewhat

For what it's worth, ch1's really crackly, ch2 less so. Ch9 sounded pretty clean to me (moved the mic from ch1 to ch9), so whatever issue might then be on the first card(?).

The powersupply for this mixer is a little under designed. (I have one that Ive worked on fairly extensively).
Most of the Electrolytics will needs to be replaced by now and I suspect thats where your noise is coming from.
If you're already recapped the PS, Id replace the caps on a channel and see if that solves your issue. The Analog side is a pretty straight forward setup all the way to the ADC and from the DAC out.

Unclear to me what exactly "crackly" means. If you turn up gain high enough to saturate and get clipping does it sound different from that?

Is it signal related?  With all trim settings identical, does one channel pass signal cleanly while another not?  If yes you have a bad channel.

Could be a component , could be a solder connection, dirty contact (switch or jack) etc...

JR

Has the PSU ever been recapped........if not start there while you have it apart , then recap the analogue section,
Horrendous chinese cheap caps !!
Have 2 off these and upgraded the op amps , The power supplys have been recaped twice in 15 years, they run hot,
Even with extra cooling fans.
Old Thread
https://groupdiy.com/index.php?topic=29157.0

The PSU has indeed been recapped (minus the two 400v caps on the primary) with a complement of Rubycon / Panasonic / ChemiCon caps, and i mounted a (temperature-controlled) 92mm fan right on the grill area on the PSU lid.

I hadn't, though, bothered with recapping the analog area, but it's starting to sound like i should look into it, after all.

Thanks for the input so far, everyone I hope i can report back with (good) news, as soon as i get a chance to haul the thing back from the band's rehearsal place...

I am not sure how capacitors cause crackling. More likely symptom from old caps is weak bass response.

If you open it up you can test for bad solder joints percussively. i.e. tapping components with a pencil eraser while unit is turned on and passing signal...

JR

Well, i think i managed to isolate (and mitigate) the issue - dirty(?) pad switches 

On the bright side though, the shafts of the switches have a "channel" in them that reaches all the way inside.
A squirt of contact-cleaner and a couple dozen actuations seem to have cleaned up the contacts and cleared up the issue.
At least here at home, anyway.

During some testing with the two stacked input boards removed from the top case, but plugged into where they needed to go, inside the bottom half (sitting on one side, on the floor), i managed to toast the BD140 (T2, page 12 of the service manual) from the cap-multiplier on the negative rail for all the opamps. After replacing that too, the crackling had indeed subsided. I wanna say it was still there, but only if i cranked up the master and headphone output.

Just to "guild the lily", i replaced the wire-links L1 and L2 on the analog power input with a pair of 220uH / 1.5A inductors, and added a pair of 470u/35v caps (the footprints for C34 and C68 were unpopulated).

Pending testing at its usual resting place...

Well, that had kinda-sorta helped a bit, but some noise still managed to creep in.

After some further home-testing, i finally narrowed it down to channel 1. With the gain maxed and no input connected, the (pre-fader) level meter filled up about half-way (16 LEDs). By comparison, all the other channels only lit up 1-2 LEDs on the meter.

And sure enough, after replacing the pair of input PNP transistors (2SA970GR), it's now finally fine!.. In fact, a smidge quieter than the other channels, only lights up the first LED w/ maxed gain & no input.

I'm just glad they were near the edge of the board, so i managed to swap'em out without also removing the ADC board (that's mounted to the bottom of the input connector & preamp board).