DCA series distortion issues

[Jon Dent]

We have a bridged DCA1622 running to our JBL 4642a subwoofer. Tonight I sit down to watch Transformers. When the Flash trailer comes on at volume 4.5, I hear an easily audible, farty sounding distortion coming from the sub every time any kind of bass effect triggers, even a slight one. I go upstairs and turn the amp volume knob down 3 ticks and the distortion goes away immediately and I never hear it again through the rest of the trailers. I don't hear any distortion throughout the feature at volume 5.0 either, and if you've seen it you know that the LFE in Transformers is frequent and LOUD, significantly more so than any trailer.

We had a similar thing happen in our other theater two years ago with a DCA1222 (the 'Voyagers Blew our Sub?!?" thread), and the same volume knob tweaking fixed the problem there.

Are we looking at an amp default or is it possibly something stupidly simple like dirty pots? Looking at the schematic it seems the audio signal does pass through them, but my schematic decoding is a bit rusty so I'm not sure.

[Pete Naples]

I'd start with some maths.

Is the sub & amp combination appropriate for the room?

Sub might be damaged, something mechanically impeding it?

Fader at 4.5 is quite well below reference, shouldn't be anywhere near clipping at that, regardless of input level.

The pots on QSC amps are notorious for going bad, so a skoosh with de-oxit or similar and a good wiggle won't go amiss.

[Devin McNamara]

Not much I can say about the size of the room for the sub, but I’m with Pete on cleaning the potentiometers on those amps. They are most certainly known to go bad. Especially since they are usually set and left alone for years which is a breading ground for this. The pot’s handle the input signal even before the pre-amp stage of the amp so it doesn’t take much crap on the brushes to cause problems. I personally own five of these amps and every one of them did it. I was pretty shocked when I adjusted the gain on one and the speaker let out a loud scratching sound. Once I bottomed it out on both ends a few times it seemed to clear it up. Otherwise also check and make sure the wires didn’t fall out of the back of the sub partially l. These push posts kinda suck and it happened at our theater. There was still bass but it distorted easily because two strands were touching…

[Steve Guttag]

All of the above. The DCA line of amplifiers are not prone to distortion and are one of the best amps out there, particularly for cinema. The input attenuators are prone to going bad though the current generation are sealed and are available as a repair part, for not too much money (sold in pairs, naturally). What Devin states about the JBL push terminals is also true. Their spring can shoot out the back, blowing a hole through the color and leaving you with a loose connection. Oddly, for newer JBL LF cabinets, they went to terminal blocks but for subwoofers, they've kept with the push terminals.

I also wouldn't rule out driver damage on the subwoofer...which would show up more at higher volumes. Try turning the amplifier pot up/down rapidly to see if the problem goes away...see if turning it up a few clicks helps or not. Once you know which component is causing the grief, just home in on that.

[Jon Dent]

To the best of my experience, the sub/amp combo is okay for the volume we run at (usually 5.0 for feature and 4.5 for trailers in a 52x40 room), but I know it's underpowered for anything much higher. I was concerned about driver damage, but like I said, the distortion disappeared after I moved the volume pot, and the sub was rock solid all the way through Transformers, which had bass that was MUCH louder than the Flash trailer.

EDIT: The underpowered subwoofer set up was the fault of the equipment seller. I wasn't around back then to tell our owner otherwise.

[Mark Gulbrandsen]

I don't know how dusty it is there, I only did one install in Little Falls back in the mid-80's, and that's a ways from you. But what I have seen a lot of on DCA's is accumulation of dust on the input side of the heat sinks. Can't tell you how many amps I've had to pull around the Mountain States just to clean the heat sinks. Same goes for the USA Series that have been in racks a very long time, 15+ years. What happens when the sinks are clogged, is that when the amp is called on to deliver lots of sustained power, it can overheat and go into thermal overload, or shut down depending on the model. Amps with bad capacitors can cause a similar effect, so if your amps are 15 years old, consider re-capping them. The boards in the picture are out of a USA-1700 that was driving a sub on a screen over in Colorado for over 15 years. You can see where the heat sink is almost fully clogged, plus I rebuilt the amp, semiconductors and capacitors so it's good for another 15 years or so. Also, oil the felts at each blower motor bearing. You may need an audio oscillator to properly check your sub...

[Mark Gulbrandsen]

This is what can happen to a DCA that's been in ten years...

[Marco Giustini]

The gain knobs on those amps are known for getting "scratchy".

First: Do the math and see if the Amp-Sub combination is good for your room
Second: get some dry contact cleaner and then some potentiometer cleaner. Flood the potentiometers with the dry one, move the knobs back and forth several times to dislodge the grime. Then let them dry. Finally, spray the "wet" cleaner or whatever "potentiometer" cleaner you find: it will leave a mild lubricant behind and improve the conductivity.

I've had those pots going back before and moving them "back and forth" would only restore them for a day or so.

Worst case scenario, replace them

Those amps are "dust friendly". There are no filters. Blow compressed air from one and and catch the dust with a vacuum cleaner on the other end. Maybe do that before you clean the pots

Finally, three "clicks" on that amp could be quite a lot? You shouldn't have to turn anything down to be honest.
Also, the DCA series comes with limiters. The 1622 is 1600W at 4Ohm - the 4642A is a 1200W at 4Ohm. If the limiter is engaged, I feel you shouldn't be able to hear any distortion unless the sub is REALLY being overdriven, but still, I doubt you'd notice.

If the 1622 limiter is NOT engaged and the 4642A is severely underspecced, then there is a chance the amplifier is clipping (or the drivers are bottoming).

But I'd focus on the pots.

[Mark Gulbrandsen]

I don't really agree with the compressed air theory on the DCA amps. Mainly because the power supply heat sink is at quite an angle compared to the other four, and if the dust is a bit greasy, then it's not going to necessarily get completely blown out. The picture above was taken in a theater in central Nevada. It's very dry there and so is the dust, but I also cleaned the level pots. The only difference between the older pots that become crackly is that the newer pots have a better seal so dust can't get in. But the older pots clean up fine. The amp can also get a general internal inspection. Not specifically in DCA amps, but doing this to older USA QSC's, I have found leaky capacitors necessitating a rebuild. Of course this much work would have to be done after the place closes for the day.

[Steve Guttag]

QSC amplifiers have always drawn air from rear to front (the correct way, in my opinion). As such, I try to pressurize my racks with a filtered blower at the bottom that pushes filtered air into the rack and let the amplifiers draw the air out. This allows the rack to run cooler too as the heat generating amplifiers are pushing their heat into the booth/room rather than into the rack for the upper equipment to start with "pre-heated" air. I've found that racks with filtered blowers in the bottom also tend to run MUCH cleaner as dust doesn't tend to settle onto the tops of things either. Air comes out either via equipment's fans, like the amps or the vents in the top portion of the rack though I use a solid top in most applications to avoid dust settling in that way either.

The DCA 1622 does not really do 1600 W/bridged into 4Ω for subwoofer use. That is a 1KHz/1% number. At what point is a subwoofer playing a single frequency and when is that frequency 1KHz? For cinemas, in particular, when looking at subwoofer power ratings, one really has to look at the continuous power and one has to look in the frequency range from 20Hz - 120Hz (with an emphasis below 80Hz). The DCA 1622 is really only supplying about 1000W, bridged, into a JBL 4642A. Odds are, unless the room is quite small, the system is under powered/under subbed. An often misunderstood aspect to subwoofers is that money is almost always better spent on more subwoofers rather than bigger amplifiers. For instance, doubling the subwoofers on the same amplifier (so running 500W/CH) would output as much level as a theoretical 2000W mono amplifier into a single subwoofer (presuming the drivers withstood 1000W each...which the 4641 drivers would not do for any length of time in a subwoofer application).

Back on topic. If the pots are found to be the source of the distortion, I'd replace them. The front panel board come off and the pots are easy enough to change. Digi-key does not carry those pots...sorry.

[Marco Giustini]

Mark,
Fair enough, My experience is in a booth with HEPA filtered supply so very little dust and not greasy at all. Still, if removing the amps is not feasible, compressed air should keep them in reasonable shape. Proper compressed air, not canned air

Steve,
Of course, I got the wrong number apologies. With the limiter on though, there shouldn't be any audible distortion. I've used those amps in situations where they were limiting hard and they seemed to be able to do that job properly.
Sure, chances are the whole system is underspec'ed but on this occasion I put my bets on the pots

[Jon Dent]

Digi-key does not carry those pots...sorry.

That's true, we don't. Already checked...


Regarding the rest, I'm very well aware that we are seriously under spec on our subwoofer setup, not even going to try to argue. Shit, the only part that looks up to spec is the screen channels. According to the Dolby Room Design Tool (I know it's not without it's issues, but given our location, I sure can't get a competent installer to visit and weigh in) we'd max out our current subwoofer at about 5.3. So at 5.0 it's definitely being driven hard, but not quite to it's limits, at least on paper.

Poor decisions were made when the systems were purchased ten years ago. The owner loves movies but he's not an audio nerd, so when the reseller told him "this is all you'll ever need" he didn't have anyone around to disagree. He's willing to let me add additional subs, but we keep getting hit with large expenses over the last year or so (shitty accountant gets us in trouble with the IRS, water well goes out, servers needed to be replaced, etc) so I haven't been able to get the funds to do it.

Oh, and I do have the clip limiters and 30Hz filters on for the sub amp.

[Marco Giustini]

I still feel that with the limiter on you shouldn't be hearing anything nasty from that sub. I had those scratchy pots before, I did move them back and forth and thought I'd fixed them as the noise from the LFE was fine. Following day I went to watch Star Wars in that very auditorium and during loud passages I was getting "distortion" (I knew it was the pots). So during intermission I rushed to the booth, turned them back and forth again and the second part of the movie was fine. The following day I sourced the contact cleaners and fully serviced them.

Try running the Flash trailer again: do the CLIP lights light up on the amp?

[Jon Dent]

Try running the Flash trailer again: do the CLIP lights light up on the amp?

No. I've never seen the clip lights come on, even on movies that feature some pretty heavy bass like The Suicide Squad or TG Maverick. Can't say that it never happens.

After everything here, it sounds like it's the volume pot, so I'll have to work on it.

[Marco Giustini]

In the US I think you can buy De-Oxit for potentiometers. I would still give it a good wash with any dry contact cleaner first.