Film-Tech Forum ARCHIVE: Bad amp?

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Posted by Tom Wienholt (Member # 1510) on 06-10-2004, 03:06 AM:

I was curious to know what happens when an amp goes bad. Does it just completely stop working or can it just start to cause distorted sound? If an amp starts going bad but still works, what kind of distortion would it cause?

Posted by Robert L. Fischer (Member # 2424) on 06-10-2004, 03:12 AM:

From my experiences, it almost sounds like a static problem -- only much worse. It's sort of a loud buzzing noise, like if you turned a guitar amp up really loud. If an amp is bad, you'll know it, and so will customers.

Posted by John Anastasio (Member # 607) on 06-10-2004, 06:30 AM:

It all depends upong which section of the amplifier has gone bad and whether or not it's a tube amp or solid state. A bad power supply, for instance, can manifest itself as either 1.)a loud hum or 2.) total non-functioning amp. A bad phase inverter on a push-pull amp will just make the amp sound very distorted. A bad power amp stage has yet another set of symptoms...so you can't just say "bad amp"...it's like going to the doctor and just saying "Something's not right".

Posted by Tom Wienholt (Member # 1510) on 06-10-2004, 11:58 AM:

Well the problem I am having is distortion in the sound. I checked the A chain which turned out fine. The sound is sort of scratchy(loud speaking parts) and it seems to be pulsing(like during a scene with an applauding crowd the clapping seems to fade in and out). The sound seems to also have too much trebble. I also checked the EQ which also seemed ok.

I am using a CP50 with a reverse scan Kelmar reader. Right now I only have one speaker behind the screen with the other speakers coming soon. So, I am running the CP50 in Dolby stereo with no surround mode so that the noise reduction cards are utilized(CAT280 SR cards). I tried replacing the CAT117 output card but the problem is still there. Any help would be appreciated. I think I will try another amp today (using Smart amps). Thanks.

Posted by Scott Norwood (Member # 30) on 06-10-2004, 12:04 PM:

The "pulsing" comment makes me wonder if you are playing a Dolby A print in Dolby SR. Are you?

Also, if you are using cat. 280 (not 280T) cards, they need to be modified for use in the CP50. Otherwise, they go into "encode" mode by default instead of "decode" mode.

Amp failures can be especially fun when you start hearing loud popping noises and find out later than a horn diaphragm has blown. [thumbsup]

I haven't seen the SMART amps, but I think that most people here would recommend that you check out the QSC USA/MX/DCA product lines instead.

Posted by Gordon McLeod (Member # 33) on 06-10-2004, 12:22 PM:

If you are playing in Stereo Mode with only a centre channel then you will be hearing all sorts of the gain riding artifacts of the matrix as it is steering portions of the audio the left and right speakers that are not there. Remember that matrix sound relies to a large extent on psycoacuostic masking effect
Smart Amplifiers are usually either a Hafler circuit or made by AB International and seem to stand up well

Posted by Tom Wienholt (Member # 1510) on 06-10-2004, 12:41 PM:

The SR cards have been modified for playback only. I will be installing the left and right speakers tonight and I will see if the problem stops. Thanks.

Posted by Paul G. Thompson (Member # 655) on 06-10-2004, 07:04 PM:

It can be from completely dead to maximum smoke. Everything in between is possible.

Posted by Tony Bandiera Jr (Member # 2365) on 06-15-2004, 11:06 PM:

You should also be sure your CP50 has the newer Power supply Module (Cat 114C I believe) as the older power supply can't handle the draw from the CAT 280T's. (Or the later Cat 150's)

Posted by Oscar Neundorfer (Member # 458) on 06-16-2004, 05:50 AM:

Tom,

Please tell me what model SMART amps you are using.

Posted by Steve Guttag (Member # 268) on 06-16-2004, 07:44 AM:

I've yet to have a problem with the Cat. 280s affecting CP-50 performance. The Cat. 280s draw from the +24 supply and not the +/-12 volt supply.

However, adding cards with LEDs on them will suck down a CP-50 supply in a hurry. The Cat. 150C or later, the Cat 517 and the Cat 560 will surely draw the supply down to below +/- 10VDC and a hum will develop. Also the Cat. 113C with its power supply LEDs will also suck some juice.

Steve

Posted by Gordon McLeod (Member # 33) on 06-16-2004, 02:44 PM:

Maybe the original poster has added the other channels and could report back soon

Posted by Tom Wienholt (Member # 1510) on 06-16-2004, 05:42 PM:

The other channels were added so now the CP50 is in dolby stereo with no surround mode.(L,C,R channels) The sound quality is much better but there still seems to be a pulsing problem where the sound seems to fade in and out slightly when there is a scene with applauding or with any other type of "continuous sound".(hopefully that makes sense)

The only test I did not perform for the LED alignment is with illumination test film. Could this be causing the problem.

I am using Smart TA240 amps.

Posted by Gordon McLeod (Member # 33) on 06-16-2004, 06:53 PM:

It might as uneven illumination will cause a compressor effect
did you set the dolby level with the processor in mono?

Posted by Tom Wienholt (Member # 1510) on 06-16-2004, 09:34 PM:

Yes, I did set Dolby tone with the processor in mono.

Today I checked the illumination of the LED and only had to make a very slight adjustment. The "fading" problem in the sound is still present. Could this have anything to do with the fact that the surround channel is not being used? The surround speakers are going to be put up soon and I can see if the problem stops.

[ 06-17-2004, 02:17 PM: Message edited by: Tom Wienholt ]