Carver M4.0t with weak channel

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sjdemark
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Carver M4.0t with weak channel

Post by sjdemark » Wed Feb 03, 2010 7:25 pm

I'm hoping someone has some experience with these amps and can give some good advice or point me in the right direction.

I have a Carver TFM-4.0, which is the same as the M4.0t and very similar to the TFM-42 and TFM45. The left channel is about 5-10dbs stronger then the right. I checked the amp boards and the components are within a couple of ohms of each other. Neither channel is offset more than 15mv.

I studied the schematic, and there is an op amp (to invert the signal for bridging) only on the left channel, not the right, which I find a very odd design. It seems strange to me that carver didn't use an identical design for each channel and put active circuitry in one channel but not the other. Basically, for the right channel, the signal goes straight through to the amp board, but for the left it goes through an op amp first. In my case the left channel has more gain than the right. I swapped out the op amp with a new one to see what would happen, but the problem remains.

The other thing I noticed it that there is a FET that is supposed to mute the signal (part of the protection) on each amp board at the beginning of the signal path. I did check using a transistor/FET checker and it seems good.

Anyone have any suggestions for where the problem might be?

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BillD
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Re: Carver M4.0t with weak channel

Post by BillD » Wed Feb 03, 2010 7:38 pm

This is an inverting amp. You will notice at the output, the hot signal will be to black on one side (usually the right). The benefit of an inverting amp is that you can source more power because when you are using the positive rail on one channel you are using the negative rail on the other.
It should sound like it isn't there!
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sjdemark
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Re: Carver M4.0t with weak channel

Post by sjdemark » Wed Feb 03, 2010 8:51 pm

Thanks for the response! Do you think that has any effect on the weaker channel (right side)? If so, what areas in the amp would you suspect first? Also, the sound is not distorted, just quieter.

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BillD
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Re: Carver M4.0t with weak channel

Post by BillD » Wed Feb 03, 2010 9:46 pm

If things are running properly, it should have no affect on the level. It probably runs the input signal into the negative input of an opamp and out the other side comes it's inverse.
It should sound like it isn't there!
There is a difference between hearing and listening...
Making life enjoyable through expensive electronics.
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Carver: C-4000 & C-1 preamps, PSC-60 preamp/tuner, TX-11a tuner, M-400 (2), C-500, M-500, M-500t, M-500t Mk.II, A-500x, AL-III loudspeakers (2 pr.)
Sunfire:Theater Grand III processor, Ultimate Receiver, Cinema Grand Signature 400 ~ seven, True Subwoofer Mk. II, D-10 Subwoofer

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Re: Carver M4.0t with weak channel

Post by sjdemark » Wed Apr 07, 2010 9:53 pm

In case anyone else has this similar issue, I found the problem. There is a 'damping' pot on the each of the amp boards that goes bad over time. The first symptoms are a weak channel, but apparently if the pot gets bad enough the amp can oscillate with full rail voltage going to the speakers. Yikes! There is a good writeup of a permanent fix on diyaudio.com.

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mbskeam
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Re: Carver M4.0t with weak channel

Post by mbskeam » Wed Apr 07, 2010 10:31 pm

There is a good writeup of a permanent fix on diyaudio.com.
got a link?
Mbskeam

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