This is topic Clipping on older QSC amp in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Cory Johnson (Member # 103) on 12-05-2000, 02:31 PM:
We are getting clipping on a QSC 1855 amp to the left screen channel connected to a CP 500 and JBL screen array speaker. Is it possible for amps to clip if they are going bad? If I remember correctly, this amp was marked by a technician as questionable due to it buzzing output on one of the channels, but we were told it would hold out for a bit longer. Well, that was over a year ago and now if we turn the fader past 5.5 it will clip on certain scenes (Pearl Harbor preview, etc). We are planning on replacing it, but I was just wondering if a sign of an amp going is it ability to begin to clip the channel. The amp is probably around 5-6 years old.
Posted by Tom Sauter (Member # 616) on 12-06-2000, 09:34 AM:
Since you've indicated a problem with the amp anyway, I'd say yours has had it.An amp channel that suddenly decides to clip definitely indicates a problem. Other than internal problems, if a connection between the amp and speaker degraded or the speaker itself is faulty the load on the amp can increase to the point of causing clipping. The output of your processor may have been ramped up for some reason, so you could really be sending the amp too much signal.
If you're bored you can swap the inputs and outputs to the amp (after carefully labelling everything). If the problem persists on the amp you have an amp problem, if the problem moves to another amp channel you have another problem.
Posted by Rory Burke (Member # 512) on 12-06-2000, 10:23 AM:
Well I hate to have to admit but I bet that that amp although through design was intentionally intended to last longer than old Sherman Tanks or DC3's and be as robust as a German weightlifter it has seen its day. However I bet, without looking at it, that none of the components were at fault but that dust killed you ulimately. Probably accumulated so much dust that after a long while it met with what i call and"Electrical lint bridge short" jejeRory
Posted by Jeff Stricker (Member # 224) on 12-06-2000, 11:31 AM:
Absolutely. I would think your amp has a problem. If clipping begins at some point on the volume control setting, I'd be suspicious of a bias voltage being wrong or missing, especially if the unit has both plus and minus supplies in it. Also a leaky electrolytic coupling capacitor would be high on my list.What's needed is apply a sine wave from an audio oscillator to the input and follow the signal using a scope through the circuit from input to output and see where the clipping occurs.
Posted by John Gordon (Member # 523) on 12-06-2000, 11:52 AM:
Hello Cory,
I'm sorry to hear you are having problems with your amp. One of the fastest ways to determine the cause of your problem would be to call our Tech. Service Dept at (800) QSC-Audio.
As a note, QSC does not have a model "QSC 1855" amplifier. Are you sure this is the correct model number? What is the serial number?------------------
John Gordon
Cinema Applications Engineer
QSC Audio Products, Inc.
1675 MacArthur Blvd.
Costa Mesa, CA 92626
(714)754-6175
john_gordon@qscaudio.com
http://www.qscaudio.com
Posted by Scott Norwood (Member # 30) on 12-06-2000, 12:27 PM:
BTW, if the amp _is_ suspected to be bad, don't swap it with another channel, since you run the risk of damaging another loudspeaker.What kind of crossover are you using? (Not sure if that would make a difference, but I figured I'd throw it out anyway.)
(I've personally had no problems at all with QSC amps. They are reliable and sound good. I am currently using an MX1500a as a home-stereo amp and, aside from the fan noise, am very happy with it, and it is a very good device with which to annoy the neighbors.) 
Posted by Ian Price (Member # 14) on 12-06-2000, 12:50 PM:
When we took over the Rialto we finaly discovered that there were three bad channels out of our 13 QSC 1400 amps. After asking where to repair them my tech told me that QSC likes to repair their own amps and install upgrades when they do. We sent each amp, turnaround was one week and each amp cost $90 to rapair.Such a deal. Thanks QSC.
Posted by Cory Johnson (Member # 103) on 12-06-2000, 02:13 PM:
After checking it is a "Series One 1400" model. It was installed in 1994 with a DTS and CP65 Unit, but moved recently to the a CP500 unit (due to rack depth problems in one of our racks).
Posted by Rory Burke (Member # 512) on 12-06-2000, 04:55 PM:
I bet the gain structure that the amp is being asked to endure is another possibility since you mentioned it was recently moved. The proper output voltage of the Cp500 should be around 300mv when checking pink noise. Improper gain structure will most certainly place and undue load on your amp and make it clip. Id check into this right away. Rory
Posted by John Walsh (Member # 168) on 12-06-2000, 05:17 PM:
What _exactly_ does the "CLIP" led indicate on QSC amps? Does it just light up when the input voltage goes too high; does it actually come on when there's too much DC at the output; does it do a compare on the input and output and light if there's a difference??
Posted by Steve Guttag (Member # 268) on 12-06-2000, 06:52 PM:
Cory,Since you say it is a 1400. One quick visual check that the 1400 is ok is to observe the clip LEDs at power on....when the anti-thump circuit times out, the clip LEDs with have a very quick "blip" in unison...if they don't or just one does, I have always found a problem inside.
Also, how does the speaker sound before the "clipping?" Clean or not? A premature clipping on a channel can also indicate a load problem...the clip LED will turn on when the channel distorts, regardless of the reason.
Steve
------------------
"Old projectionists never die, they just changeover!"
Posted by Cory Johnson (Member # 103) on 12-07-2000, 09:43 AM:
The clip lights up steadily for a few seconds on channel one (only channel in use). Sounds fine in channel and then it is just mute until the clip stops. The amp was hooked to a CP500 w/o problem since 97. Clipping just started occuring in last few weeks.
Posted by Rory Burke (Member # 512) on 12-08-2000, 07:25 PM:
Well then if it has been a while that the amp was moved over then it cant be a gain strucutre problem....try switching to the other channel and see if it clips...if the problem is in the amp it could be limited only to the circuitry of the chnl you are currently using thus problem eliminated using the other channel. At least for now then you wont risk damage to speakers as well.
Rory
Posted by John Eickhof (Member # 323) on 12-08-2000, 08:15 PM:
I would be sure and check to see if the 'stereo' - 'mono-bridge' switch is in STEREO mode (if you are using only one channel.
The idael thing would be to switch it to
'bridge' mode the attach speaker outputs to the two RED binding posts on the back. BE SURE THAT NEITHER OF THESE WIRES ARE CONNECTED TO THE CHASSIS< GROUND OR COMMON WITH ANY OTHER AMP OUTPUTS! What may be happening is the amp is in bridge mode but you have the output connected to one black & one red binding post...check it out! Good luck!
Posted by Steve Guttag (Member # 268) on 12-09-2000, 08:18 AM:
John even if the amp is in bridge mode and he connects the speaker to one red and one black terminal, it would still work fine.The bridge mode merely inverts phase on the input to create an inverted ouput on channel two. Channel one behaves the same regardless...you could even connect another speaker on channel 2 between the black and red posts and merely have an out of phase speaker.
So the question is....is it the speaker or the amp that is having difficulties.
Disconnect the load (speaker) from the amp. Turn the amp off...wait 20 seconds and then turn it on and observe the clip LEDs...if either stays on for any time or doesn't "blip" after about 5 seconds (they will do it in unison ) after turn on, the amp needs service.
Another thing you can try is to put your speaker on the other channel (move the input over as well and see if the problem moves.
Steve
------------------
"Old projectionists never die, they just changeover!"