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Old 30th November 2003, 12:53 PM   #1
K-amps is offline K-amps  United States
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Default Amplifier power supply question:

Please pardon the obviousness but this dodges moi. I was wondering if one of you guys could help me out here:

I have a 600 VA Toroid with dual 55v secondaries. I have it hooked up to a pair of 10,000mfd caps via a 25amp bridge and get about 77vdc.

When I test load the capacitor terminals with a 500watt Halogen Lamp (about 28 ohm load) The DC sags from 77 to about 62 volts (which me think is a huge drop but still fathomable) but the AC side (measured before the bridge) drops only from 55vac to 52vac.

Question why this huge drop in the DC voltage and negligent drop in the AC side?

Second question would be, assuming the Toroid is holding on, what do I need to do to get at least 72 vdc under load using the same AC voltage i.e. 55vac?

Sorry about beinga little vague. Any info is appreciated.

thanks!
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Old 30th November 2003, 04:35 PM   #2
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try adding some more capacitance to the power supply so that they are not drained as much between the charging pulses they recieve from the transformer.
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Old 30th November 2003, 04:46 PM   #3
K-amps is offline K-amps  United States
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Default Ok will try that..

Also do you think there would be abnormally high losses in the Bridge rectifier? Would a hi-speed discrete diode help or would I gain just 1 volt or so?
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Old 1st December 2003, 10:04 AM   #4
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The available voltage goes to 0V 120 times per second, it's AC.

I generally figure the DC voltage under load to be only about 10% higher than the no load AC voltage. In your case 55VAC + 10% = 60.5VDC, which is about what you have.

"try adding some more capacitance to the power supply so that they are not drained as much between the charging pulses they recieve from the transformer."

A 1F cap (1,000,000µF) cap puts out 1A for 1S. The point of diminishing returns is quickly reached. A 25A bridge rectifier only has about 300A surge capacity, filter caps above 30,000µF will generally require soft-start circuitry to avoid blowing the rectifiers. 'Fast' rectifiers have less surge rating than normal types. Consult the data sheets if you intend to use these types. People using stupidly large amounts of filtering may require a two-stage soft-start, it is possible to blow the rectifiers running at low powered normal use if the filter caps are too large.

The A40 article at the Pass Labs site has a power supply tutorial in it. Summary: use no less than 3,000µF per channel for an 8 ohm load, no more than 30,000µF. For 50hz use 4,000µF minimum and 40,000µF maximum, you will probably have rectifier problems near the maximum.

A Carver M1.5T puts out 600W per channel from only a single pair of 3,400µF filter caps.

A bigger amplifier does not need any more filter capacitance. If you double the power supply voltage for a larger amplifier the energy stored in the same sized filter cap quadruples.

Sonically, a regulated supply for the low current front end of your amplifier makes more sense than to try and psuedo-regulate the high current output stages with large filter caps.
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Old 1st December 2003, 10:14 AM   #5
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Default Thanks!

That was a great reply and a semi tutorial!

Since I am new, would you send me a link to the A40 PSU article you mention?

thanks again!

PS: What transformer rating/ rectifier and caps (in an unregulated PSU) would you have if you wanted 400w RMS into 8 ohms and 750 into 4 ohms?
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Old 1st December 2003, 10:25 AM   #6
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I 2nd djk on ps caps.

my rule of thumb is to use about 3300-6600 every 50woutput on 8ohm loads. It has worked very well for me.
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Old 1st December 2003, 11:04 AM   #7
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Default Re: Ok will try that..

Quote:
Originally posted by K-amps
Also do you think there would be abnormally high losses in the Bridge rectifier? Would a hi-speed discrete diode help or would I gain just 1 volt or so?
Think around 2 volts * Current, when you calculate. You must have a heatsink to the rectifier bridge. 2-3 A is max without heatsink. The only way to reduce power is to use schottky diodes but I don't think it's worth the trouble.
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Old 1st December 2003, 12:24 PM   #8
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The capacitance numbers suggested here for the A40 do not necessarily apply to all Class A electrical amps as I understand it. The amount of filter capacitance is highly dependent on the amount of current bias used in the amp. The Aleph-X built with 250 milliohm source resistors would need at least 100mF per channel to tame the ripple. Look here for more information on this topic. For corroboration, ask Nelson Pass how much capacitance is included in the AX series of amps?
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Old 1st December 2003, 01:47 PM   #9
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Obtaining DC by rectifiying a 50Hz sine wave is an inefficient and crappy power conversion method

Current is only drawn from mains during 15..30% of time so the current drawn in this interval is 3..7 times bigger than the DC current you are demanding and so you will be getting 3 to 7 times greater voltage drop and 3 to 7 times grater heating in the transformer and bridge rectifier

Peak voltage drop in mains line will also be 3 to 7 times bigger tan expected [the sine wave will appear clipped]

In your case, the RMS rectified voltage you get on the *output* of the bridge rectifier with constant load is almost the same for 1.000uF or 1.000.000uF

In other words, adding more capacitance past some value won't reduce RMS voltage drop, only ripple will be reduced

If somebody doesn't belive this, just simulate it and cry [I have assumed 1 ohm mains resistance and 100uH leakage inductance]

You can reduce voltage drop by using a tranformer rated at 3 to 7 times the power you will be drawing, but this is obviously not practical

Fast recovery rectifiers are a nonsense solution since they would have almost double voltage drop when conducting [making a rectifier ultrafast allways means increasing its conduction losses]

What you have seen is the crude reality, as I've said, 50Hz rectified supplies are crappy and huge voltage drop is one of their fundamental properties [If there weren't voltage drop then it wouln't be a 50Hz supply]

Fortunately audio signals have high crest factor so maximum power consumption only will happen during short periods of time and this will reduce effective voltage drop to some extent and will also benefit to some extent of extra storage capacitance [actually due to the fact that a 750Wrms class B, AB or D amplifier driven just under clipping will usually draw 250W or less of average power]

If you don't like voltage drop nor ripple, think that SMPS have regulated outputs ...

PD: I don't understand why high quality SMPS units or kits of 0,5 to 2Kw with good shielding and filtering, symmetrical adjustable outputs and ready to use for a reasonable pice aren't available for DIY
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Old 1st December 2003, 03:02 PM   #10
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Quote:
Originally posted by Eva
PD: I don't understand why high quality SMPS units or kits of 0,5 to 2Kw with good shielding and filtering, symmetrical adjustable outputs and ready to use for a reasonable pice aren't available for DIY
I do! The market is limited. A 2 kW SMPS for industrial use cost A 2 kW audiophile SMPS with proper filtering costs not less
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