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Old 17th February 2004, 09:55 PM   #1
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Default A40 Rail Voltage

Hello,

I'm building an A40 and would like to use crc filtering in the power supply. I'm wondering if using a 30-0-30 650 VA X-Former would be OK as the no load voltage would be about +-42 volts DC which is 10 over the orginal specs. I'm figuring about 5 volts will be lost to bridge, caps, resistors etc. Bringing the rails down to around +- 38. Thanks in advance for your input.
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Old 18th February 2004, 06:25 PM   #2
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With that much voltage to waste why not just adapt the power
supply from the Pass ZV5 ?

Woody
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Old 18th February 2004, 06:44 PM   #3
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gpsmithii,

You can try a CRC+RC = CRCRC to lover the ripple and loose a few more volts.

tell us about the current needed for the finals ?

Alain.
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Old 18th February 2004, 07:29 PM   #4
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Their no need to use a CRC in the supply it will only decrease the performance of the power supply. Also, 650w is a minimum a 1Kw would be much better.

I have built several of these amps and using a 650W transformer the DC will droop or sag from the load. You can run these amps from +/-30V to +/-45V dc with no problem.

I also have some of extra Lamda output transistors.
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Old 19th February 2004, 03:23 AM   #5
grataku is offline grataku  United States
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Quote:
Originally posted by jewilson
Their no need to use a CRC in the supply it will only decrease the performance of the power supply.
I don't think so. Not in classA amps.
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Old 19th February 2004, 05:17 AM   #6
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grataku,

Anytime you add resistance in the supplies lead it's burn up energy and increases the impedance of the supply. Ideally, you want the supply impedance to be as close to zero over the frequency range. If you run a FFT with either resistors or inductors you will find that slew rate under load can be effected, even in class A.

Another way to look at this is hang a resistive load in series with the speaker, does that sound like a good thing to do? Well you could add a .5 ohm but going higher would not be cool.

In a tube amps you can get away with doing this since the device are not low impedance and are voltage devices.

Next, why do this the amp work great as Nelson designed it, with out resistor in series with the supply.

If you want to change something the amp sound better with match FETs and some source degeneration.

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Old 19th February 2004, 09:06 AM   #7
akb1212 is offline akb1212  Norway
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Quote:
Originally posted by jewilson
grataku,

If you want to change something the amp sound better with match FETs and some source degeneration.


FETs????? You haven't built the A40 I see..... BJT's all the way!

I have built several of them using +/-45 V, and have never had problems with it. All you do is make the amp have more watts in class AB. Mine were able to give out 450 W bridged in 4 ohm with a 1KVA trafo. I used one of these for each selfmade Apoge (Lafolia presented in the Danich magazine High-Fidelity) copy for a long time with good results (except for burning tweeters a few times).

All you have to take care of is the voltage accross the current source.

For bass these amps are marvelous!

I'm using the one I have left for highs now, while I'm waiting to finich my Alep-X amps (I HAVE cards ).

Anders
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Old 19th February 2004, 03:28 PM   #8
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Anders, Yes I have built several of them. there nothing special about making this change and their nothing special about the small transistor used in the design.

You can repalce just the differential pair, which are the input transistors. There no need to replace other components. The part I used was a 2N5566 which is the same part Nelson used in the 2nd run of the Threshold amps. You should also add some source degeneration about 100 ohm should work fine on each source.

Work great Try it
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Old 19th February 2004, 06:13 PM   #9
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Default 2n5248

Quote: You can repalce just the differential pair, which are the input transistors. There no need to replace other components. The part I used was a 2N5566 which is the same part Nelson used in the 2nd run of the Threshold amps. You should also add some source degeneration about 100 ohm should work fine on each source.

Jim,

Can you recommend a replacement for the 2n5248?
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Old 19th February 2004, 07:38 PM   #10
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For the 2n5248 you can use a number of fet these also will work
J204 - BF245C
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