Rectifiers for Power Amps

[richidoo]

Hi guys,
Picking up where this thread left off with such great info about rectifiers for preamp....

I am building Rod Elliot's Project 101 power amp. The low power version with 2 pair output devices, and 56V rails. I am estimating it will make 100W 8ohms. I will be using a giant 1000VA toroid that I have, with softstart circuit from Elliot. How do I figure out the rect. diode amp rating I will need?  If I want a 25A bridge, do I use 4 diodes of 25A rating, or do I use 4 diodes of 8A rating?  I want to use Fairchild Stealth2 diodes again. 

The tnt-audio article about power supplies suggests using a separate bridge rectifier for each rail (4 rectfiers for stereo amp,) like this:


The Elliot suggested PS uses one rectifier with some RC filtering for the whole amp (picture attached.)  What's right for this project?

I have not yet decided whether to make them monoblocks or stereo amp. I have 2 toroids.
Thanks,
Rich

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[Carlman]

My only thoughts:
If you go stereo amp... you could do a dual mono design and combining the center tap of the output leads from both torroids..   I'd recommend 2 or even 4 caps for the PS.  That's how my AKSA was designed and is the only real experience I have with amp building.  The principle was good though and provided excellent sound I think you would like.  I think more smaller caps in a PS yields faster or 'prattier' sound than larger or singular PS caps.

-C

[mgalusha]

Rich, if you need a 25A rating then each diode in the bridge will need to be 25A as each leg carries the full current for 1/2 of the cycle. The current rating you need is related more to the amount of capacitance after the diodes than the amount of current the amp will draw as the charging current of the caps will almost certainly by greater than anything the amp will draw.

The easiest way to get a handle on this is to download Duncan Munroe's power supply designer from the Duncan Amps site. It will predict with decent accuracy what the rectifier current will be. The other thing to keep in mind with diodes is the voltage rating. The diodes (bridge) need a PIV rating of at least 3x (actually 2.83) the RMS voltage. This is because the PIV rating is peak volts and the diodes see peak to peak. So for 56V that would be 56 * 1.414 * 2 = 158V, so I'd go with at least a 200V diode to ensure some margin.

The thing with using dual bridges vs the full wave with the center tap grounded is the output voltage is different even with the same transformer. I suggest grabbing a copy of the Hammond Design Guide, this shows all sorts of connection schemes and the multiplication factors to determine the DC output in relation to the AC coming out of the transformer.

And I agree with Carl, multiple small caps are usually better than single large ones.

Mike

I am building Rod Elliot's Project 101 power amp. The low power version with 2 pair output devices, and 56V rails. I am estimating it will make 100W 8ohms. I will be using a giant 1000VA toroid that I have, with softstart circuit from Elliot. How do I figure out the rect. diode amp rating I will need?  If I want a 25A bridge, do I use 4 diodes of 25A rating, or do I use 4 diodes of 8A rating?  I want to use Fairchild Stealth2 diodes again. 

The tnt-audio article about power supplies suggests using a separate bridge rectifier for each rail (4 rectfiers for stereo amp,) like this:


The Elliot suggested PS uses one rectifier with some RC filtering for the whole amp (picture attached.)  What's right for this project?

[richidoo]

Thank you both very much.