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Old 9th September 2005, 02:06 AM   #1
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Cool external power supply, rectifier in or out?

If I build an amp with an external supply, should the rectification and cap bank be inside the amp chassis or outside with the transformer? I have room for the caps but not the tranny.

I was not sure if there was an advantage in a power cable with AC or DC.
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Old 9th September 2005, 11:18 AM   #2
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I face the same problem, and that's quite a dilemma by the way...

Rectifiers must be together with the caps. High AC fluctuations (I mean in the cable between the amp and PSU) produce more EM-interferences, than high DC fluctuations. DC heats up wires more than AC with the same rate... Then this star-ground part...

For now I've decided to keep my tranny (2500 VA, weighs 20KG) with the rest of the stuff. Just have to find a proper power cable
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Old 9th September 2005, 11:52 AM   #3
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i dont know an answer to the question sorry, but whatever you do watch out for parsaitic inductance and capacitance in the long PSU lines it really can cause nasty oscillation (use the thickest wire you can, preferably stranded to avoid skinning).
Also check out snubbers on DIY audio if you do get oscillation

That is one -mutha- of a transformer where'd you get it and whats it going to power?
What current can it put out continuously?

Craig
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Old 9th September 2005, 12:19 PM   #4
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The caps must be near the amp of the length of the leads will introduce too much inductance leading to instability and poor bass response.

The bridge must be near the caps because long wires will emit significant interference with the short charging pulses. Ideally the transformer should be close as well.
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Old 9th September 2005, 12:19 PM   #5
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I already know about the cable trick, and i think I'll use something like 3x 3mm cross-wound for +- and ground. I hope nothing will oscillate, cause the wire will only be about a meter long... If yes, I was going to use snubbers on DC-inlets anyhow

The transformer is custom-wound 2500 VA toroid with 2*50-60-70-80-90-100 V outlets. I'm intending to use it in a test stand for different 8-Ohm power amplifier projects. The thing is, that it can only supply about 13 Amperes without overheating. In future I'm planning to wind a 5000VA version for 4-Ohmic loads (who knows, maybe I'll use it for arc-welding as well ).
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Old 9th September 2005, 12:39 PM   #6
real is offline real  Australia
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Hi,

If I were you I would recitify and use a few capacitors at the transformer end, then use more capacitors inside the amp to null any effects of the cable resistance/inductance. This way there is minimal AC on the umbilical cord, which means less chance to induce any noise into nearby circuitry (especially from the sharp current peaks from charging the capacitors at the top of each cycle).

Chris.
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Old 9th September 2005, 12:45 PM   #7
mzzj is offline mzzj  Finland
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I would transfer dc, with caps in powerbox. Maybe somewhat larger caps than usually in amp pcb's
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Old 9th September 2005, 12:57 PM   #8
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Default Re: external power supply, rectifier in or out?

Konnichiwa,

Quote:
Originally posted by poolorpond
If I build an amp with an external supply, should the rectification and cap bank be inside the amp chassis or outside with the transformer? I have room for the caps but not the tranny.

I was not sure if there was an advantage in a power cable with AC or DC.
The answer was given elsewhere, but not quiet outlined well in terms of reasoning.

1) You MUST have a reasonable amount of capacitance near the actual amplifier circuit (how much depends on many issues).

2) You must keep rectifier and reservoir capacitor in close proximity, to keep the current loop tight.

This implies to place the rectification and the bulk capacitance either in the amplifier or in the supply box, but under all conditions a considerable amount of capacitance should also be in the amplifier chassis.

An obvious option is to use multiple reservoir capacitors with some extra choke filtering inbetween, where rectifier, input cap and filter chokes are in the PSU chassis and the amplifiers PSU capacitors in in the Amplfier chassis.

Sayonara
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Old 9th September 2005, 01:29 PM   #9
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Looks like the best option up to now is to use 2 capacitor banks (or to build a 10U rack to fit everything ).

The fact is that I've successfully tested a power amplifier design by my friend (a 300W sub amp) using a ~1m cable between the PSU and the DC inlets without any parasitic oscillations, although there were only two 4700uF caps on the PCB (and my PSU has 22000uF ones + 2 ultrafast bridges).

Anyway, I have the time to try all the available options and choose the best performing one. Life is hard without a proper test stand...
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Old 9th September 2005, 08:20 PM   #10
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This is in line with what I thought. My supply should look like the jpg below. The cord will be at the break in the schematic. I also planned on using chokes for the +/- rails inside the amp chassis.

You have all been helpful.
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File Type: jpg psu4-1.jpg (10.1 KB, 456 views)
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