THIS output stage. For your consideration. - diyAudio
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Old 28th December 2007, 12:46 AM   #1
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Default THIS output stage. For your consideration.

Hi,
Playing around with difference output stages, I tried this.

Comments? POS or worth trying?
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Old 28th December 2007, 01:47 AM   #2
kaos is offline kaos  United States
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You seem to be burning a lot of power in the drivers. Idle alone appears to be around 70 watts total in Q2,5,6,8,9,&12 for a standard AB output condition. Canít tell were you want to idle the actual outputs at from the schematic given, that would make for additional heat as well. Better have some big heatsinks if you try it.
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Old 28th December 2007, 02:25 AM   #3
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Quote:
Originally posted by kaos
You seem to be burning a lot of power in the drivers. Idle alone appears to be around 70 watts total in Q2,5,6,8,9,&12 for a standard AB output condition.

Hi,
It was actually for a class B amp.
My idea was (probably silly) to have these driver transistors share about 10% (or more) of the load.
It works fine in the simulation, but it's nice to have those who are more educated give an opinion.

Am I wrong in thinking it has some voltage gain? Seems so in the sim.
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Old 28th December 2007, 04:26 AM   #4
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John,

You probably need some sort of temp feedback to prevent thermal run-away.

A3K
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Old 28th December 2007, 04:57 AM   #5
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These bias generator diodes (circled in pic) will track if mounted on the drivers heat sink, right? Should the drivers be mounted on a separate heatsink from these CFP outputs?
It's a little confusing, since I'm using driver transistors as outputs, but I'm referring to the actual drivers.
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Old 28th December 2007, 03:22 PM   #6
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Any more opinions?
A simple "yeah" or "nay" will do.
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Old 28th December 2007, 04:01 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
look at the first CFP pair, Q2, Q3 & R2.
The Vbe of Q3 is about 600mV.
R2 sees all this voltage and tries to pass 300mA. That's your problem!
Change R2 and all the others from 2r0 to between 100r to 200r.

If you want to run the drivers hard, to keep up their speed, then maybe try as low as 30r.
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Old 29th December 2007, 03:17 AM   #8
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Quote:
Originally posted by AndrewT

If you want to run the drivers hard, to keep up their speed, then maybe try as low as 30r.

Hi Andrew,
Remember, I know not what I do.

It is my idea here to use these "drivers" as outputs, that they would share a substantial part of the output current. That's the reason for the low value collector resistors.
Simulation says..(that sounds Richard Dawson from Family feud "survey says..") that it will work. Simulation actually says it will work very well.
You know the simulator is my new best friend.

I attach the full schematic to put things in context. This would be a subwoofer amp. Maximum 280 watts into 8 ohms.
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Old 29th December 2007, 03:34 AM   #9
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Going back to the simulator and changing the collector resistors to 4 ohms works a bit better. I am now seeing ~78mA through these drivers at max output.
The interesting thing is these drivers maintain a near constant current as the output power drops. The outputs (MLJ21193/94 will drop below these at low level.
This is what I see from the sim.
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Old 29th December 2007, 08:47 AM   #10
AndrewT is offline AndrewT  Scotland
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of course constant current, that's the way CFP works.

It's the 600mVbe across the resistor that determines the value of the almost constant current.
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