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Old 14th October 2012, 09:00 AM   #1
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Default converting PA amplifier into symetrical class A son of Zen amplifier

hi i have this pair of amplifiers originally intended for PA 100V system, its a 500W class B pull push through output transformer

I want to use it for class A amplifier.
Reasons.
Power supply, rectifiction wiring , chasis and Excellent heat sinks.
Input output connections all made.
Each amplifier has 2x10=20 TIP35C mounted and with drivers and emittter resistors.

since its a pull push both sides have 10 TIP35C NPN transistors and my plan is to use them like son of Zen with either resistive or acitve current sinks and get rid of output transformer (or may be could use for CLC filter) and also remove the input circuitry as it will be fed with fully balanced signals (as designed in son of Zen).

Problems. How to determine how much power can be extracted from the amplifiers , its a +34V single rail and there are two options either 4Ohm tap with minimum 2 ohm load or use the 100V line with minimum 20ohm load. The amp states its rated for 500W . As i understand 500W total would be 250W per side of pull push (10 TIP35C) and at 34 V rail its just about 7.5 ampere.

So if i have to bias the output stage through a 4 ohm emitter resistor to bring the emitter voltage at Vcc/2=34/2=17 volts i have to pass around 4 ampere =4ohmx4Amp=16V so it would be at the mid point and the quiscent current is half of what the amp can deliver at full RMS rating. Is this corrent.

Secondly as they are BJT in common collector mode how can i get voltage gain stage ??? can any one suggest any alteration from the current circuit diagram ??
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Old 14th October 2012, 10:38 AM   #2
sreten is offline sreten  United Kingdom
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Hi,

Assuming the amplifier can only swing 30V of the 34V supply, i.e. +/- 15V,
what is the maximum rms and peak, current and power into a 4 ohm load ?

What standing current is needed in push pull class A , single ended class A ?

What is the real output power of the amplifier as it stands into 2, 4 and 8 ohms ?

"So if i have to bias the output stage through a 4 ohm emitter resistor"
Why is this a clueless question ?

What is the estimated VA of the mains transformer ? the 100V line transformer ?
(Simply go on size compared to rated similar types in a catalogue).

What circuit diagram ?

Edit : you've double posted.
Seems the amp is bridged, so the question becomes +/- 32V swing as it is now.

rgds, sreten.
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Last edited by sreten; 14th October 2012 at 11:03 AM.
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Old 14th October 2012, 08:32 PM   #3
sreten is offline sreten  United Kingdom
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Hi, somebody delete this thread, the OP was double posted, rgds, sreten.
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Old 15th October 2012, 09:19 AM   #4
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sreten the design is for 8 ohm load.
for 32V available i can get maximum continous 4A in single ended thats what i want to achieve with 4 ohm resistor to set the mid point at resistor at 16V.
Now the single ended resistive amp could provided 2A and the balalnced could provided 4A conitnous that gives us I2R=4x4x4 =64Watts ????? is it correct (8 ohm load would be seen as 4 ohm load)

The power dissipiation per side is 4Ax32V=128A and for two sides =256 W which would give power efficiency of 25% for single ended resistive loaded fully balanced class A. Is it correct calculation ???
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Old 15th October 2012, 05:04 PM   #5
flg is offline flg  United States
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Are we all, or maybe just me, missing something here? You have a 32V supply(of what continuaous current capacity???), and you want your ouput node at 16V? with a Drain load of 4ohms and 4A? And, your thinking you can swing down to 0V and 4A, and up to 32V with 8A???
Or what?
Maybe a pic would help?
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Old 16th October 2012, 04:01 PM   #6
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flg you got it right.
up to 32V down to 0V differnetial between two amps at 16V node at 4A continous through each amp.
total available current 8A, Class A available 4A. voltage swing 32V load 8 ohm ???
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