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Old 29th December 2004, 11:35 AM   #1
jesper is offline jesper  Sweden
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Default One transformer dual PSU's

Hi,
What's your oppinion on using one transformer and dual PSU's. With PSU I mean rectifers and capacitors.
At the moment I'm trying to get an amplifier silent without success, it is powered with such a power supply, ie one PSU for each channel but only one transformer. It's a 25V 500VA transformer and the PSU's consist of a PCB with a dual bridge (out of 8 discrete shottky diodes), dual 10.000uF capacitors some 100nF filmcaps and bleeder resistors. I think the PSU bord itself is good with a low impedence groundplane.

I've been reducing/eliminiating groundloops and other sources of buzz/hum/noise for quite some time, but It's not silent enough yet.

/Jesper
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Old 29th December 2004, 05:32 PM   #2
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Though I am not really an expert, it was my impression that the use of a separate bridge and filter caps for individual amplifiers was as good as having separate and complete power supplies for each amplifier, because there is no feedback between the individual amps.

This is, of course, assuming that the one transformer is of suitable VA to handle the full load of both amps.
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Old 29th December 2004, 07:33 PM   #3
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If you are having humm etc it probably is not from the fact that you have one transformer, but of the interconnections and loops to the two amps. Can you draw the supply schematic with the wiring *as it is now*, not what you want it to be (no offense meant, but these two may be different).

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Old 29th December 2004, 08:41 PM   #4
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Quote:
Originally posted by SupraGuy
of suitable VA to handle the full load of both amps.
Could be that "suitable" is arbitrary.

A Burmester 850 power amplifier may be a good example: a fully symmetric mono amplifier consisting of two bridged amplifiers.
Each with their own powersupply; toroidal transformer, rectifier and capacitors.
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Old 29th December 2004, 10:05 PM   #5
jesper is offline jesper  Sweden
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I've attached a photo of the amp as it looks right now. I'll try to talk you trough the picture.

The power input is in the back, in the middle, it goes through a simple spike protection (MOV resistor and RC network) and then to a slow start sircuit/remote start (middle front).
From the transformer the two PSU's are connected (placed on the back, you can se the two larger caps on each board).
From the each PSU power is fead to each poweramp (each amp is dual channel) and to the active filter PSU (on each side of the slow start unit). The powercables to the amps and to the filter PSU is tied togeter to reduse this loop area (one previous hum source).
Finally the active filters (front in the corners) are conneted to the filter PSU's.

The signal is taken from the RCA inputs in the back, to the active filters and then to each amplifier. The inputs of each amplifier have a "groundloop breaker", menaning that the input sircuit ground is taken from the filter ground and the input ground and the power ground is connected to each other by a series resistance (10ohm) parallelled with a cap (100n).
Direct connection of the amp input ground and power ground increases the buzz.

Finally the amplifier output signal goes through a speaker protection and delay (back in the corners) and then to the speaker terminal. The speaker ground is taken directly from the power PSU.

As seen on the picture I've tried using signal transformers to isolate the amp fron the input source, but it doesnt change things much, the sourse of the buzz is internally. Allso all units are now "flowating", not screwed to the chassis, to prevent the groundplanes of the different units to get in touch the screws, creating loops.

/Jesper
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Old 29th December 2004, 11:36 PM   #6
jesper is offline jesper  Sweden
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Hi again,

Hocked up an oscilloscope and took a look.
The buzz itself is to low to be seen on the scope, the scope itself is to noisy.
The picture attached is measured at the positive supply without music playing. You see some 250mV ripple, noting strange, but there is some overshot when the diodes turn off. I don't know if this may cause enough ground disturbanse to create buzz but is sure have som HF contents. A snubber would be usefull here.

/Jesper
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Old 29th December 2004, 11:41 PM   #7
jesper is offline jesper  Sweden
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Same overshot zoomed.
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Old 30th December 2004, 12:10 PM   #8
jesper is offline jesper  Sweden
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I managed to reduse the the buzz to a faint hiss which is only hearable if I stick the tweeter into my ear. What I did was to take the ground starpoint to the filterPSU and filter from the amplifier instead of the powerPSU.

/Jesper
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Old 30th December 2004, 05:31 PM   #9
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Hi there---I'm about to put cold water on your ideas...I'm very weary about using more that one mains tranny in any amp.........more magnetics equals more stray magnetic fields which doubles more problem loops and background noises. If one is using toroid cores (doughnut) and stuck with a particular size then rotating them can make a big difference with both channels listened to. (the windings are never wound symmetrical and under load the leakage inductance spews out the side of the winding which has poor coupling to the primary in to the circuits ajacent).

To get the real performance from a dual solid state amp does require pretty careful placement of the power supply components.There isn't a more critical place than the common of the smoothing caps and how that earthing is made in relation to the LS return, PSU connects and input stages. Both must be completely separate.

Some years ago I did such a beast .......with partial success. The S/N ratio was never as low as the manufacturer claimed.....but it was fine for live perf.


rich
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Old 30th December 2004, 07:45 PM   #10
jesper is offline jesper  Sweden
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Hi Rich,
I only have one tranny, but two PSU's (bridges and caps).
Right now I have the smoothing caps sitting on a farly large groundplane to which the LS return is fead, maybe I should try putting the LS return to the amplifier power gound point (which is very local, I've put lot of effort making it so). The PS + and - is very close to the local power ground point and fead directly to the tansistors with short leads. The input ground is then taken physically away from that local power ground of the amp and even isolated by a 10ohm resistor in parallell with a 100nf cap. The PSU connections is located in the center of the board and the inputs on the left and right edges. The input ground point and feedback ground is also very physically close.

I'll post an image instead...

/Jesper
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