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Old 10th April 2013, 03:12 AM   #1
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Default QSC A21 bias?

I recently picked up an old QSC A21 amplifier to rebuild, this is the schematic:

http://www.qscaudio.com/support/libr...ies/A21-22.pdf

Anyone have the adjustment info for the bias and current limiting?

This is the first amp that I have worked on with this sort of topology. I'm looking forward to learning more on the theory of operation, since I've never dealt with the floating power supply design. QSC seems to have used it in other models too, such as the older MX series.

I've seen that clipping indicator circuit with the bridge rectifier before too, but I'm not sure how it could work like that. Where are they getting the reference point to calibrate the LED from?
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Old 10th April 2013, 01:11 PM   #2
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Adjust the bias to whatever you want, 20mA per device would be fine.

The output of the opamp slams to either rail when the amplifier clips, it is below the diode threshold during normal operation.
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Old 10th April 2013, 02:52 PM   #3
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Quote:
Originally Posted by Maxxarcade View Post
...QSC seems to have used it in other models too, such as the older MX series.
I can't think of an amplifier QSC has built that doesn't use the floating rail topology. Certainly all of the Class AB and Class H units use it. The Powerlight amps seem to use some different variation of it.
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Old 14th April 2013, 05:56 AM   #4
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Got the amp yesterday. Found one shorted output transistor and removed it, and the amp powers up and sounds good.

I'm contemplating whether to replace the bad transistor (SJ9092) with an MJ15016, or if I should replace all 3 SJ9092 with MJ15016. Not sure how picky the amp is about matching gain on the transistors.

Ideally I want to replace all transistors in the affected channel, but it's probably not worth the cost. I'm just going to use it on a pair of bookshelf speakers for my workshop PC.
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Old 14th April 2013, 06:20 AM   #5
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The MJ15016 is a fine part, don't mix with any other PN.

Just replace all three at one time and you should be fine.
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Old 14th April 2013, 05:22 PM   #6
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Quote:
Originally Posted by djk View Post
The MJ15016 is a fine part, don't mix with any other PN.

Just replace all three at one time and you should be fine.
That's what I figured. At least they are not too expensive. I had considered going with MJ21193/21194, but it would cost a fair bit more having to change all 6 transistors.
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