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Old 25th March 2013, 04:46 AM   #1
tony Ma is offline tony Ma  United States
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Talking Need suggestion

Dear All,

I am looking to build SS class A amp from what ever left with my B&K ST-140

Here what I got

1. Case 16x13x5.25 inch
2. on the back there is a heat sink size 7.5x4 inch
3. Transformer with secondary wiring 46x2 (I measure it)

and there is writting on the transformer

-904 wattage 300 W

Any suggestions ??? I like to make Amp with heavenly sound.....
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Old 25th March 2013, 07:54 AM   #2
Jay is offline Jay  Indonesia
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Too high secondary voltage for a class A amp.

You don't have the output transistors?
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Old 25th March 2013, 08:49 AM   #3
JMFahey is offline JMFahey  Argentina
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Quote:
3. Transformer with secondary wiring 46x2 (I measure it)
Secondary wiring 2x46V AC or PSU rails 2x 46V DC?

Not the same by a huge margin.
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Old 25th March 2013, 02:31 PM   #4
tony Ma is offline tony Ma  United States
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Sorry it is AC
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Old 25th March 2013, 02:52 PM   #5
JMFahey is offline JMFahey  Argentina
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Weird
Because +/- 46V DC is right for a 70W/8r per channel amplifier (ST140 specs), while 46+46VAC means around +/- 63V rails, good for 180/200W/8r

Big mystery there.
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Old 25th March 2013, 05:23 PM   #6
tony Ma is offline tony Ma  United States
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Originally Posted by JMFahey View Post
Weird
Because +/- 46V DC is right for a 70W/8r per channel amplifier (ST140 specs), while 46+46VAC means around +/- 63V rails, good for 180/200W/8r

Big mystery there.
You are right

When it still used on B&K ST-140 the DC rail was 63V

so what Class A amp I can use?
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Old 25th March 2013, 06:08 PM   #7
JMFahey is offline JMFahey  Argentina
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Not recommended for class A amplifier.

Why? because Class A passes a lot of continuous current, always.
The necessary current for an 8 ohms load will be very high, pull more power from the PSU than it can deliver.
As Jay said in post #2, class A amplifiers use relatively low +/- rails, to keep power dissipation at bay.
Not the case here.
The original PSU was designed for a class AB2 amplifier, you can't veer away from that, unless you get a 300VA transformer but at a quite lower voltage.
So all you could use is the case and heat sink, mut notmuch more.

Personally I would rebuild or clone an ST-140 or build there a standard power amp (your choice) , which is happy with +/-V rails.
AB class, of course.

*If* you insist on a class A amp, fine with me, but pick any suitable schematic (lots of them here) and get a *new* transformer, which is more suitable.
Good luck.
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