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[Jonathan Woytek]

I worked a show about a month ago with a new-to-me dbx 166xl.  I installed the unit in my processing rack about two weeks before the show, and ran it through some tests to be sure that it was functioning as it should.  Everything seemed to be working fine in just about every configuration, so I put the lid on the rack and waited for this show to give it some live airtime.  

During sound check, I was tweaking the compressor a bit, and was about to apply appropriate make-up gain on one channel of the unit (I was using it dual-mono for vocal mic compression at this show).  I did, and everything sounded okay (or so I thought), until the band stopped for a few seconds.  In that interim, I heard a regular "click(pause)click(pause)click(pause)" continuously in the system.  I did the quick visual trace to see if I could track it, and I saw it on the compressor.  I turned the gain control back down near zero, and it went away.  I experimented a little bit, and turned the gain control up slowly, and found that the artifact just seemed to appear out of nowhere when the control was slightly past zero, but disappeared as soon as I set the control back to zero.  Turning it up further didn't make it louder or change the quality at all.  

I know that we had one grounding problem during this show that was causing some hum on the channel coming from the bassist's amp (this was fixed by taking his bass direct through my DI before going into his amp--we were previously using a direct-out that his amp supplied, then through my DI).  We also had the electricians fix another grounding problem on one of the five circuits that they supplied to us during the first break in the show.  I did try to apply the make-up gain later in the show, but the unit still did the same thing.  

This was an outdoor show on a trailer-bed stage (that was properly grounded, actually).  Power was via temporary distribution that the venue provided.  All outlets were metered and tested out OK (except for the one with the grounding problem mentioned earlier, but we had that fixed as soon as possible and did not use that circuit until it was fixed).  The click was fairly regular, at about 2Hz or so, and was primarily low frequency material (in the neighborhood of 200-300Hz or so--nothing in the subs or HF sections of the system).  The vocalist in question provided his own Beta58, and I did not have time to swap it with one of my mics to see if the problem was in the mic or not.  The compressor was used as an insert with a known-good insert cable.  The board was a Mackie SR24*4 (NOT the "VLZ Pro", with which I've had other noise issues in the past--this is the older model).  There were LOTS of other stuff on nearby circuits, including refrigerators, fryers, beer coolers, all the squint's stuff, etc.  I have NOT had the opportunity to test this piece of gear since I did the show.  

Anything sound familiar?  I'm going to be able to get this gear back this weekend to test this unit (that rack has been out on loan for a little while, with ample warning that I had problems with that unit), but I haven't had a chance to listen to it yet.  If there's anything I want to check when I get it back, let me know.  Thanks.  

jonathan

[Tim Padrick]

Sounds to me like the gate was on, and the extra gain boosted the signal enough that the stage noise into the mic came up over the gate's threshold sometimes.

[Drew Curtis]

i concur.

[Jonathan Woytek]

The gate was the first thing I checked here, thinking the same thing.  The gate was as "off" as it can get on these units without a switched bypass.  Also, I thought that the gate stage would occur prior to the compressor stage, which is itself prior to the make-up gain control on these units.  All that said, why would the make-up gain cause the gate to trigger differently?  In any case, the gate shouldn't have been triggering here at all.  I'll run a few tests to see if the "off" is actually in full bypass on this unit.  There is a chance that the pots are a little dirty, of course, which could still leave the gate enabled.  

jonathan

[Andy Peters]

woytek wrote on Thu, 08 September 2005 19:18

I thought that the gate stage would occur prior to the compressor stage, which is itself prior to the make-up gain control on these units.

Nope -- there's only one VCA.  There's detector logic for the gate, and some more for the compressor, and the VCA control voltage is derived from the sum of both.

Make-up gain is a control-voltage offset.

Quote:

All that said, why would the make-up gain cause the gate to trigger differently?

It shouldn't, unless an internal trim isn't properly adjusted.

-a

[Jonathan Woytek]

Andy Peters wrote on Fri, 09 September 2005 01:05

Nope -- there's only one VCA.  There's detector logic for the gate, and some more for the compressor, and the VCA control voltage is derived from the sum of both. Make-up gain is a control-voltage offset. ... It shouldn't, unless an internal trim isn't properly adjusted.

This all makes sense.  I pulled-out the schem for this unit the other day myself, as I was wondering exactly where the gain adjustment fell in the chain.  Thanks for clearing-up that point.  

As for the clicking, I can't seem to reproduce it now.  The next gig I have with that equipment is this Sunday, so I'll give it a shot during sound check to see if it does the same thing.  If it does, it is probably going to get the screwdriver treatment with a 'scope to see if I can track this one down.  

jonathan