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Old 28th April 2009, 10:09 PM   #1
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Default DJK - Questions on old design.

I have a couple questions about this schematic posted awhile ago by DJK. I realize this is the basic circuit QSC has been using since the beginning, but I'm still a little shaky on the details.

This is the schematic/parts list i have:
http://i120.photobucket.com/albums/o...pampdriven.jpg

A) R6 sets the bias current right? How much current needs to be available from the -15V supply used there? Is that just from the +/-15V supply used for the opamp?

B) The transformer CT for the main supply is not connected to ground or anything, as the rails need to float with the signal, right? This also means the +/-15V for the opamp supply needs to come from a separate winding or another transformer?

C) Because of the floating rails each channel should have its own winding, or it's own transformer; dual mono is mandatory?

D) Even though the output transistor collectors are at ground they still need to be isolated from the heatsink to avoid multiple ground loops through the heatsink to chassis and the ground trace on the boards?

Lastly, is the schematic shown here a fully working/reliable design? Do you have any pictures of your layout that i can work off of? Thanks for any help.
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Old 28th April 2009, 10:42 PM   #2
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That cct digram looks very wrong somehow !

It looks like +15 volts is missing off the top end somewhere.


I would have thought the output would have gone into the inverting input not the non inverting. Makes a good oscilator !
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Old 28th April 2009, 10:52 PM   #3
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The output stage is inverting so the schematic is correct after all.
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Old 28th April 2009, 10:57 PM   #4
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I'm assuming the -15V node is just for bias. The +/-15V rails for the opamp and the main rails are not shown. C7 and C8 are the main filter caps.
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Old 28th April 2009, 10:57 PM   #5
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Quote:
Originally posted by blueskynis
The output stage is inverting so the schematic is correct after all.

Of course it is, I was looking at the center as being output as normally drawn.
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Old 29th April 2009, 02:33 AM   #6
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Quote:
Originally posted by Stormrider
I'm assuming the -15V node is just for bias.
Yup - this bias arrangement pulls current from the opamap output continuously - forcing the opamap's output stage to run class A. This should improve the sound compared to the usual bias scheme. Downside is not just any op amp can do this - you need one that can drive relatively low Z loads directly.
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Old 29th April 2009, 02:43 AM   #7
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Default Re: DJK - Questions on old design.

Quote:
Originally posted by Stormrider
D) Even though the output transistor collectors are at ground they still need to be isolated from the heatsink to avoid multiple ground loops through the heatsink to chassis and the ground trace on the boards?
The usual practice is to mount the trannies directly to the heatsinks. You just have to be careful about ground loops. That may mean isolating the HS from the chassis (but the voltage differential will be low so there is no shock hazard if the HS is exposed).

Quote:

Lastly, is the schematic shown here a fully working/reliable design?
No current limit if that's what you mean. If you do without that, use enough outputs so that the power supply collapsing under shorted conditions is enough to protect it before a fuse blows. That will take 8-10 MJ15024's per bank per 500VA of trafo. More if you oversize the reservoir caps.
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Old 29th April 2009, 03:04 AM   #8
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Thanks for the help wg ski.

That clears everything up except for the transformer CT part. I'm pretty sure there shouldn't be, as no QSC schematic I've seen shows one anywhere for this topology. Just looking for confirmation.

Isolating the heat sink should be easy enough for any layout i use. I'm betting the heat transfer will be much better with only grease.

Would current limiting be simple to add, or would it be a redesign? I'd be using a 500-600VA transformer per channel with +/-60 to 70Vdc rails. I was planning four pairs of MJ21193/4 or 5/6 originally. It will only be me operating the amp and shorts with signal would be unlikely, but not having to replace entire output stages because of a simple mistake would be nice.
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Old 29th April 2009, 03:30 AM   #9
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The trafo center tap isn't required, but advisable. If you don't use one, the put the usual equalizing resistors across the filter caps. If you do, it goes to the speaker output terminal (center point between the caps).

Current limiting is tricky with this topology. In the QSC amps, it uses two levels of current starvation to implement. The first is to limit the drive voltage from the opamp, with the trip point set by a pot. This limits instantly and is normally set to a very high value. The is second time-dependent. The QSC opamp supply is drived from the idle rail voltage - and thus relies on a couple 100uf or so bypass/bootstrap caps to supply current to the opamp during output swings above +/-15V. Where it limits depends on how long the conditions last. More base current = quicker limiting action.

If you wanted to use a standard load line limiter, it would take two transistors per rail. One to sense the voltage and current, which would be tied to the rail where the emitter resistors are. Then you'd have to use a second to translate the switching action back to where the output stage is driven from the opamp. Either that or put in "emitter resistors" in the collectors of the outputs, but that defeats the purpose of having the heatsink at ground.
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Old 29th April 2009, 07:03 AM   #10
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Default that is a very inetersting

circuit and even though was arround here since i dont remember any more but no one clone anything like that ....

of course this is no hi end amp but sure worth looking at it powerwise

my circuit ....the modified Hill chameleon is still working like hell serving small PA application ..... an AB +b design that doesnt say much quality wise but its solid rock and very ready to accept various forms of abuse ....
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