Film-Tech Forum ARCHIVE: Kelmar 150+ --- Sound is Not Good

This is topic Kelmar 150+ --- Sound is Not Good in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Cody Martin (Member # 2523) on 07-01-2006, 02:16 PM:

Hello again,

Just ordered a screen today, so in preparation I was cleaning up the booth and doing a little wiring and such. The only change I made was I ran some wires through a wall, so I'm pretty sure the wiring is fine.

I'm set up with 4 Kelmar 150+ amps(L, C, R, S) plus one backup, all ran with a CP50. I ran a test loop(trailer) through the projector today and noticed my L channel sound like crap. I switched the amp out and it sounded fine then... so the problem is with the amp. It sounds the way speakers do when you over drive them. I turned all the levels down so that I could make sure it wasn't over driving anything and when I did that it still sounds bad...just at a lower volume. I checked the fuses also and the speaker.

I was wondering if there is something inside the amp that may be proned to go bad or something I should be looking for. I haven't had a chance to pull the amp yet, but i will soon.

Hopefully someone may have a suggestion.

Thank you for all your help,
Cody

Posted by Cody Martin (Member # 2523) on 07-03-2006, 10:26 AM:

Any ideas on this? I'm not very electronics smart, so I'm at a total loss right now.

Thank you,

Cody

Posted by Robert Minichino (Member # 3495) on 07-03-2006, 12:00 PM:

Could be that one of the power supply rails bit the dust which, depending on the exact design of the amp, could give you heavily distorted (half-wave rectified) output or no output at all. It also could be that some of the output transistors are bad; they usually fail shorted, however, and that usually results in taking out the rest of the output transistors and/or getting DC on the output. If some failed open then that might also result in getting half-wave rectified output (only half of the signal, either above or below 0V). Also could just be very badly underbiased, meaning that there's a gap around 0V where there's no output.

These scenarios are all very easy to detect with a voltmeter and an oscilloscpe. Open up the amp, power it on, and (VERY CAREFULLY) check the +/- DC rails from the power supply to make sure they're both powered and at similar levels from ground. Then check the outputs with a voltmeter set to DC volts with no input; they should be very close to 0V. Then run a sine wave into the amp and check the outputs with an oscilloscope. If you see a flat spot around the zero-crossing, then you need to rebias or the bias circuit is borked. If there's a flat on the top and the bottom there might be an issue in the input section of the amp. If half the waveform is missing, then there's probably an issue with the output transistors.

See, it's very simple. [Wink]

Posted by Cody Martin (Member # 2523) on 07-04-2006, 12:27 AM:

Sounds like this will be a little out of my league. may be sending this to someone to check out and repair if possible/feasible.

Thank you for your help, it is greatly appreciated.

thank you,
Cody

Posted by Mark Gulbrandsen (Member # 72) on 07-04-2006, 07:31 PM:

They are older amps and will need re-capping. This is also common on the AS-7200 series of Kelmar amps. Usually its the input cap that fails and the result is alot of distortion. A set of caps shouldn't set you back more than 25.00 per amp. If you can find them surplus then they will cost less. Call Kelmar and get a schematic then make a list of all the electrolytics and replace them.....

Mark

Posted by Gordon McLeod (Member # 33) on 07-04-2006, 08:09 PM:

With the low cost of some of the china amps I would ditch the Kelmars as I never found them that good sounding at best
for small screening rooms I have used the PRZ1000
http://www.pyleaudio.com/itemdetail.asp?brand=PylePro&cat1=Pro%20Audio&cat2=Power%20Amplifiers&model=PZR1000

Posted by Phil Hill (Member # 371) on 07-04-2006, 08:16 PM:

Cody, all these "experts" are guessing at what is wrong. If you want, email me for my address and if you send it to me, I'll look at it for free and see if I can help you out.

I have found those amps to be an economical theatre sound solution.

Posted by Richard Fowler (Member # 893) on 07-05-2006, 11:40 AM:

The slide in amplifier boxes which where popular in the 1980's most are very basic construction and easy and cheap to fix.
Gorden, that Chinese amplifier is pretty wimpy for the price: a few dollars more get Chinese amplifier product from Crown or QSC [Razz]

Posted by Mark Gulbrandsen (Member # 72) on 07-05-2006, 07:15 PM:

Well, they may be economical but I will certainly side with Richard on this. Thay never did sould that good and they have a class B output stage... old stuff. If the Philster is going to fix it free thats a nice gesture and I would send it off to him, the parts shouldn't cost you that much.

I couldn't agree more with Richard on the cheap Chineese amps! Gord, We got in a couple models of the Pyramyd amps that you reccomended to me and that you had installed a bunch of. Had you bench tested them before hand I doubt that you would have installed them... pretty awful performance and he is correct that for about 50.00 more the Chinese built Crown or QSC is alot better deal. To my ears the Crown amps also sound the best of the "Cheap" amps, very dynamic and with a warm midrange that is wonderful. We ended up giving both the Pyramyds to an employee for his home system.

Mark

Posted by Cody Martin (Member # 2523) on 07-05-2006, 09:13 PM:

Hello guys,

Thanks for all your replies. I'm going to go ahead and send the unit til Phil Hill and hope it's fixable. I would just purchase a newer amp, but I just purchased a screen and automation controller so funds are limited as usual.

Thank you for the amp advise though. I had though about the pyramid ones, but will stay away from those now.

Thank you,
Cody

Posted by Gordon McLeod (Member # 33) on 07-05-2006, 09:36 PM:

well Mark at last count we have about 200 of the pyramid pa1000 in service in 150 seat screens with A5 speakers and have had one fail they are capable of sustained 120watts RMS per channel and with an efficient speaker like the A5 that is all that is needed in many smaller rooms
they Pyle has a few extra features like a balanced input and less space and we have wuiet a few of those in service

I will never use a crown product on an installation as I have had just too much overall grief from their products

Larger rooms I use QSC

Posted by Louis Bornwasser (Member # 3063) on 07-06-2006, 09:09 AM:

Ashly FTX-2001 amplifiers in all theatres regardless of size including my living room. Louis

Posted by Mark Gulbrandsen (Member # 72) on 07-06-2006, 02:14 PM:

Gord,

I think in the long run you will find that they will not last anywhere near as long as the Chinese QSC or Crown amps will. Also they spec out really raunchy with an awful looking harmonic content, especially at the high end of the response.

Over the years I only had a little trouble with Crowns... Nothing that was a big deal and no more really that I've had with QSC. We are starting to see alot of older QSC's start to drop and requiring rebuild kits put in them. Grented these are well over 20 years old.

Louis,

Ashleys are good amps and have a MOSFET output stage that is more likely to generate even harmonic distortion which is not nearly as fatiguing as odd order distortion is. The transconductance of a properly biased MOSFET is also much higher than a similar sized bi-polor deivce and there are many other important advantages to MOSFETS. We have basically zero problems with the FET 2000's out here... a fan now and then is about it and they are built to at least an adaquate quality level. The FTX 2001 has been retired.

Mark

Posted by Louis Bornwasser (Member # 3063) on 07-06-2006, 08:27 PM:

Retired? officially, yes. They are still made for us when we order in quantity. Currently dealing with the "lead free" in Europe. Louis

Posted by Mark Gulbrandsen (Member # 72) on 07-06-2006, 11:07 PM:

I build alot of high end audio clones and I use alot of the new ROhS parts and have had zero problems with them. Here is the DATA on the maxlmum levels allowed just in case no one is interested.

Mark