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Old 20th April 2009, 01:55 PM   #1
Bønna is offline Bønna  Norway
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Default High-rails Passlabs amplifier design ?

Hi there,

I have decided to go for a class-A design DIY power amp.

I was lucky to get hold of some good 500VA+ toroids at a good price.

The most powerful one has 2x55V secondaries, which will give roughly 75V DC under load.

However most Pass designs and many other DIY amp designs are specified to a rail voltage of around 30-40V.

Are there any Passlabs designs suitable for such high rails, or is it possible to make simple tweaks to accomodate it ?

Are there any other well proven designs for higher rails that you would reccomend ? (FET, Bipolar, or whatever)

I appreciate all comments and hints please

Regards,
Bjørn
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Old 20th April 2009, 06:40 PM   #2
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What will you do with a 500 VA transformer and such high rails?

Figure that a 500 VA will be good for about 100 watts/ch stereo,
what amplifier do you want to make that swings +/-75 volts and
only delivers 100 watts?

Perhaps you could tap the primary for 240V and use it at 120
for +/-35 volts or so, or perhaps you could use it to make a high
voltage front end for an output stage having a lower voltage supply,

or perhaps you could build an amp which will drive the crap out of
a 16 ohm speaker like a Lowther or Feastrex

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Old 21st April 2009, 06:40 AM   #3
Buhl is offline Buhl  Denmark
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Hi Bjørn
As Nelson writes - 55 volts rails is way to high for a 500VA trannie if it is to be used for a class A amp. 18-22 volt would be just nice for an Aleph 3, 30, Zen, F, .......

It is however a nice trannie for some of the high efficiency switching designs out there, UCD, IcePower, ZAP and what their names are - mayby you could have a look in the Class D forum on this site?

Kind regards
Hans
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Old 21st April 2009, 01:59 PM   #4
Bønna is offline Bønna  Norway
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Thanks to Nelson and Hans for kind and useful replies !

It confirms my suspicion, but I had to check it out before writing that nice powerful toroid off. I only paid a little more than domestic shipping for 6 transformers so never mind.

However I have noticed that some of the commercial amplifiers like the Denon POAs and such have 72V PA rails, but of course these are 200W amps and up.
Some of the Class-D modules are utilising higher voltage rails, so it might come to use some day

Some of the smaller toroids (250-400VA) have 2x24V and 2x31V and will probably be more suitable for a low power, good sounding amp design. Probably too small for a class-A, but well suited for a simple chipamp design (LM3886, etc)

I think I have found my new favourite community !

Regards,
Bjørn
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Old 21st April 2009, 02:39 PM   #5
Magura is offline Magura  Denmark
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Using it with an LC filter would be an option. That would give you like 35V rails.


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Old 22nd April 2009, 09:04 AM   #6
Bønna is offline Bønna  Norway
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Quote:
Originally posted by Magura
Using it with an LC filter would be an option. That would give you like 35V rails.


Magura
Hi Maguram, thanks for yopur reply !

Could you please explain how ? From my understanding I would then have to drop the remaining 40V over the L of the filter, and hence dissipate more then a 100W across it. Not very practical.

I have considered dropping a smaller portion of the voltage across a power resistor in a RC filter, but either way, the power potential of the big toroid will be wasted.

I studied a Pioneer SX-1250 schematic (the old monster receiver) and found that the power stage (BJT) had 70V rails. It is approx 150W per channel, and runs class A for lower listening volumes.
I think that the transformer was around 600VA.
Based on this example it should be possible to make something out of the 2x55V transformer.

Regards,
Bjørn
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Old 22nd April 2009, 10:25 AM   #7
Magura is offline Magura  Denmark
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A LC filter VS. a CLC filter, will give you an inductive load, and hence 0.7 x input voltage, instead of the 1.4 x voltage.

http://en.wikipedia.org/wiki/LC_circuit



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Old 22nd April 2009, 02:21 PM   #8
Bønna is offline Bønna  Norway
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Quote:
Originally posted by Magura
A LC filter VS. a CLC filter, will give you an inductive load, and hence 0.7 x input voltage, instead of the 1.4 x voltage.

http://en.wikipedia.org/wiki/LC_circuit



Magura
You mean that an L in series and a C i parallel after the diode bridge will transform the 55V AC into 35V DC with little loss ?

This is new to me, but nevertheless great news !

1) What will be an appropriate value of the L in such a circuit ? I know it has to handle high current.

2) Do I need ekstra filtering capacitors after the C, provided it is big (>10mF) ?

Regards,
Bjørn
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Old 22nd April 2009, 04:35 PM   #9
Magura is offline Magura  Denmark
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You can answer those questions yourself, by putting the numbers into this:

http://www.duncanamps.com/psud2/index.html


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Old 22nd April 2009, 05:07 PM   #10
flg is offline flg  United States
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I think an L input filter will give you .9 Vrms and 1.41 Irms
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