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Old 14th March 2002, 01:18 AM   #1
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Question What?!

What's with this schematic?! It looks to me like the outputs are shorted directly to ground, and I see no connection from the speaker to the OPS! I don't believe that this schematic could be accurate. It's for the QSC 1700.

http://schematic.narod.ru/Files/Audio/semi/QSC/1700.jpg
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Old 14th March 2002, 01:50 AM   #2
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It's capacitor coupled. Odd way to design an amplifier these days, given that direct coupling works so well with complementary output devices and bipolar supplies.

The relay switches the speaker to ground and isolates it
from the output stage. I think it's right but it's certainly not
the way I would build an amplifier.
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Old 14th March 2002, 12:36 PM   #3
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You guys don't get around much, do you? The grounded output stage has many nice feaures.Crown used it in their best amp, the PSA-2. In the QSC note that the cases of the outputs are grounded, no insulators required.If you want DC coupling just hook up the center tap of the power transformer to the speaker hot lead.Particularly nice for MOSFET amps.With the 2SJ50/2SK135 types the driver circuit needs to swing an extra +/- 7V in the front end.The Hafler XL-600 had a +/- 100V tier for the front end.The new TransNova Haflers ground the outputs, just like the QSC.Real easy to build a front end with a regulated supply that only has to swing +/- 7V instead of +/- 100V. http://k-amps.8m.com/cgi-bin/i/Power...afler_915c.jpg
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Old 14th March 2002, 01:35 PM   #4
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Get a TIFF viewer and then click on the images http://164.195.100.11/netacgi/nph-Pa...&RS=PN/5029299 Those with a skilled eye will spot this as the same with the grounds swapped. http://164.195.100.11/netacgi/nph-Pa...S=PN/4,321,554 Just to fry your brain, a Sumo Model 9 http://164.195.100.11/netacgi/nph-Pa...&RS=PN/4229706 The op-amp runs on +/- 20V, the outputs on two floating 40V supplies.
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Old 14th March 2002, 04:33 PM   #5
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Nope, I don't get around much. I'm still skeptical, but I see
some practical benefits of the design and it's an opportunity
to learn. If it works for Crown, et al, it's worth a close
examination.
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Old 14th March 2002, 06:28 PM   #6
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AFAIK Pat Quilter (the "Q" in QSC Audio) was the first to come up with this output stage topology. It's used in most of their power amps. You can download a bunch of schematics as PDFs from the QSC support web pages if you're looking for more of this kind of thing.

Quilter's name is on a couple of interesting patents for switching power supplies too (the PowerLight patents). Check 'em out.
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Old 15th March 2002, 01:50 AM   #7
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Default Floating Bridge Amplifier

This is called floating bridge amplifier - allows easy stereo/bridge mono switching.
Notice that the power supply centers are not hardwired to ground.
This technique has been around a long time in Pro Audio.
Amcron VZ-5000 buts 5000w into 1 ohm in bridge mode.

Regards, Eric.
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Old 20th March 2002, 08:11 AM   #8
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It's giving me a little bit of a headache. I think it works by
modulating the power supply...
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Old 20th March 2002, 08:21 AM   #9
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I think I'll stick with normal OPS's.
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Old 20th March 2002, 09:56 AM   #10
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You guys are making this too hard! There is no difference between this and a "normal" output stage. In your "normal" amp the outputs are tied to one terminal of the speaker and the other terminal of the speaker is tied to the center tap on the power transformer. In the "wierd" amp the outputs are tied to one terminal of the speaker and the other terminal of the speaker is tied to the center tap on the power transformer. Uhh, that's exactly the same! What's the difference? The ONLY difference is which side of the speaker the feedback is taken from. One side is inverting, the other is non-inverting.
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