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Amp out grounded, power supply as output, why?
Amp out grounded, power supply as output, why?
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Old 3rd January 2020, 07:50 PM   #11
indianajo is offline indianajo  United States
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Advantage is that it makes bridging the two halves of a stereo amp cheaper. Very common practice in the PA market. Peavey made a lot of money selling this pattern. Disadvantage it requires 2 rail windings in the transformer, 2 $2 bridge rectifiers and 4 rail filter caps. As 2 x 10000 uf filter caps are cheaper than one 22000 one, probably saved money right there. Kept design from requiring screw terminal rail caps which are extremely expensive these days. Another advantage wiring is 10 ga yellow ring connectors, instead of 6 ga wire and expensive 6 ga crimp terminals. Another advantage on 8 to 20 output transistor models, output transistor PCB traces can be thinner.
I just rebuilt two DC power supplies for organs, 15 v 30 A and 50 A. Those screw terminal caps are expensive, the 8 ga wiring is difficult to handle. those 1/4-28 screw terminal diodes are nearly extinct now, and were expensive in 1980 when these were built.
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Old 3rd January 2020, 08:16 PM   #12
JMFahey is offline JMFahey  Argentina
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Quote:
it requires 2 rail windings in the transformer, 2 $2 bridge rectifiers and 4 rail filter caps. As 2 x 10000 uf filter caps are cheaper than one 22000 one, probably saved money right there.
That.
And the "disadvantage" is even smaller

1) when winding *large* power transformers, way too thick wire becomes unconteollable , do not bend well at corners, etc, so itīs *common** practice bifilar winding using two half section wires, then soldered in parallel.
Nothing stops sending them to separate terminals and voila! , now you have "free" dual windings.

2) some manufqcturers use two power transformers (and supplies) to keep amp height inside a 2U rack amp.
Again, using them as grounded output amps means no extra cost.
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Old 3rd January 2020, 08:58 PM   #13
Mark Tillotson is online now Mark Tillotson
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Bridging two amps just requires an inverter - which doesn't change cost between conventional and this scheme. Two half-sized supplies is usually more cost than one large supply unless you've crossed a threshold such as needing a soft-start circuit, or 3-phase line input or some such. And you still need the separate low voltage supply, ie three separate supplies to operate a bridged pair of amps with this scheme, v. one supply conventionally (assuming the low voltage is dropped using zeners/regulators from the main supply).
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Old 3rd January 2020, 09:00 PM   #14
rayma is offline rayma  United States
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The output stage has voltage gain, since the output devices are operated in the common source mode.
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Old 3rd January 2020, 11:14 PM   #15
DuncanV is offline DuncanV  Netherlands
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Quote:
Originally Posted by JonSnell Electronic View Post
QSC or Shure have used this system for many years, it helps when bridging power stages together.

This particular example also shows the output devices can probably be bolted onto the heat sink without insulating thermal pads.
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Old 3rd January 2020, 11:31 PM   #16
Mark Tillotson is online now Mark Tillotson
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Originally Posted by DuncanV View Post
This particular example also shows the output devices can probably be bolted onto the heat sink without insulating thermal pads.

You mean the CFP output devices in posting #5? Yes, that's an advantage. Also lateral MOSFETs have source as the substrate rather than drain so could be similarly shorted to heatsink. A bipolar EF style output-section would still need isolation.
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Old 14th January 2020, 03:28 PM   #17
adason is offline adason  United States
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Thanks everyone for interesting discussion. I learned new stuff. Just wondering, can anyone explain the function of four trim pots? I suppose vr1 sets dc on the output, vr4 perhaps bias, but not certain. Thanks agan.
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Old 14th January 2020, 03:41 PM   #18
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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VR1 and VR3 set the DC balance, VR2 and VR4 the bias of each stage. Surely they are interdependent, so an adjust of one of them implies the readjust of the others.
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Old 14th January 2020, 06:10 PM   #19
adason is offline adason  United States
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thanks!
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