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Old 27th April 2002, 06:01 PM   #21
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I've run across this topology in Rockford Fosgate car amplifiers, specifically the Punch 500. The "output" capacitors are also the supply's filter capacitors. If you note the output's collectors are grounded while the emitters are connected to the rails. The resulting "swing" is actually a modulation of the power supply. Since DC is the lowest frequency the amplifier will ever (rather, never) reproduce, using the capacitors in this form is actually ingenious. Crazy, but it works and works well. The amplifier is inherently self-protecting, to a degree, against short circuits and is capable of driving very low impedances without placing undue stress on the outputs. Inductive spiking is reduced to a minimum because of the capacitive "cushion" the filter caps provide.

My brother runs QSC amplifiers for PA work and once had an MX700 running into a dead short for a 2 hour period at full power. It got hot but didn't cause any damage!
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Old 27th April 2002, 08:16 PM   #22
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I was also kind of surprised when I saw an output grounded amp for the first time. The topology has one big disadvantage (according to a skilled designer), the amp has little rejection of noise from the mains if you don't have a very advanced (shielded) transformer.
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Old 29th April 2002, 07:41 PM   #23
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Naaah, there's nothing special about the PSRR of the
topology either way.

It's just a conjugate complementary Common Source
design with the power supply pulled inside out.

You can design it well, or poorly, and the PSRR will
probably reflect that.
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Old 1st May 2003, 07:12 PM   #24
gwolf is offline gwolf  Austria
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(A bit late to enter this thread)

Peranders wrote:

I was also kind of surprised when I saw an output grounded amp for the first time. The topology has one big disadvantage (according to a skilled designer), the amp has little rejection of noise from the mains if you don't have a very advanced (shielded) transformer.

Then Nelson Pass:

Naaah, there's nothing special about the PSRR of the
topology either way.
...

To my understanding this is not a question of PSRR, but the fact that the power supply is not grounded. Therefore any capacitive leakage current is injected in the amplifier and creates a voltage across the feedback resistor. I use this topology and had some difficulties at the beginning with this fact.
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Old 2nd May 2003, 03:02 AM   #25
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Quote:
Originally posted by peranders
The topology has one big disadvantage (according to a skilled designer), the amp has little rejection of noise from the mains if you don't have a very advanced (shielded) transformer.
The (small "c") circlotron topology has the same problem, so my detractors tell me. Another "feature" they both share is that you can't easily run multiple channels from one psu. Or did someone already say that? I didn't read the thread thoroughly...
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Old 2nd May 2003, 06:57 AM   #26
gwolf is offline gwolf  Austria
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I am actually also using the circlotron topology. I reinvented it (as many others, I see) by wanting to have a fully symmetrical circuit using only NMOS fets and by shifting the + and - power supplies around.

There was a lot of power supply noise at the beginning, but now it is nearly gone (even without using a shield winding in the transformers). I don't know exactly why; probably because I changed the feedback resistors to low values (4k7).
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