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PS 400 BiAmped System


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#1 louishob

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Posted 14 December 2005 - 03:41 AM

I recently purchased two used Crown PS 400s to biamp my Martin Logan speakers. One PS 400 get's the left channel information, and the other the right. (I used a splitter for the input channels for each amp.) In this configuration I can keep my speaker cables short and can play with the channel gain to balance the woofer with respect to the electrostatic panel.
  So far so good. Once I had hooked everything up, I noticed the center of the soundstage was decidely to the left. I played with both gain controls for the left channel to center the sound stage. With my previous single Rotel amp setup, the sound stage was centered, so I theorize that the input levels for both left and right were pretty well matched. It appears that my PS 400s don't produce the same ouput levels with similar imput levels. Is this to be expected, or should I have these used amps serviced ?

  Help !

#2 DGlass

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Posted 14 December 2005 - 10:06 AM

I would first try switching the cables from the left side amp to the right side amp and see if the issue goes to the other amp. If it doesn’t than it is in the amplifier. If it does than the issue resides in the Preamp.

That amp does not use precision Input Level Attenuators and had a component tolerance in excess of 20% (for the attenuators). This means the level controls could be anywhere from 1 to 3 clicks different from amp channel to amp channel. The only place they were guaranteed to be the same was all the way up (0db of attenuation). So yes there could be a variance in knob positions.  smile.gif

If the difference is an excessive amount than you might want to have the amp looked at anyway.

#3 louishob

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Posted 15 December 2005 - 03:56 PM

QUOTE(DGlass @ Dec 14 2005, 10:06 AM)
I would first try switching the cables from the left side amp to the right side amp and see if the issue goes to the other amp. If it doesn’t than it is in the amplifier. If it does than the issue resides in the Preamp.

That amp does not use precision Input Level Attenuators and had a component tolerance in excess of 20% (for the attenuators). This means the level controls could be anywhere from 1 to 3 clicks different from amp channel to amp channel. The only place they were guaranteed to be the same was all the way up (0db of attenuation). So yes there could be a variance in knob positions.  smile.gif

If the difference is an excessive amount than you might want to have the amp looked at anyway.
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   Thank you for your response !

   The problem is in the amplifier. If I understand you correctly, with both amps at 0 db gain, you'd expect the same output levels with the same input levels from the preamp. Is this correct ?
   The problem was detected with both amps at 0 db gain. So I guess I have a real issue here. The question is which amp is the culprit. Am I getting too much amplification from one amp or not enough from the other ? I'll probably need to have a Crown certified technician look at both of them. Would you agree ?

#4 DGlass

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Posted 15 December 2005 - 04:26 PM

Yes with the amplifier turned all the way up (0db atten) the output should be the same.

Good point, yes you probibly will need to have them looked at to determine which one is correct and which one isn't isn't. smile.gif

#5 louishob

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Posted 27 December 2005 - 12:05 PM

QUOTE(DGlass @ Dec 15 2005, 04:26 PM)
Yes with the amplifier turned all the way up (0db atten) the output should be the same.

Good point, yes you probibly will need to have them looked at to determine which one is correct and which one isn't isn't. smile.gif
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So I took the Crown amps to a certified technician. On the bench, the Crowns put out the same output level at clipping, 38 VAC. I take them back home, and I have the same problem, and it follows the amp. One amp is overpowering (underpowering ?) the other. The center stage drastically moves toward one amp. That amp also has a nasty 60 cycle hum component and some popping that sounds like a light scratch on an LP. The technician doesn't have a clue. Do you have any ideas ? I'm baffled in Houston. These amps have a good reputation, but they are old.

#6 DGlass

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Posted 28 December 2005 - 01:16 PM

If the amps bench checked out alright and you have moved the input cables between the amps and have switched the amplifiers input feed from one side, of the preamp, to the other and have the input attenuators set the same and the issue is still there then I am baffled as well.
Something is going on and it is appears to be system related but I can not guess what it is. blink.gif