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Old 11th November 2009, 04:04 AM   #1
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Default Leach amp questions

Hi,

A couple of questions regarding the small, low-TIM Leach. Schematic is here:
http://users.ece.gatech.edu/mleach/l...aphics/ckt.pdf

1. R39/C19 and R40/C20 are used to sense voltage drop across the emitter resistors and shutdown the drive if the voltage drop gets too high. How come the other output transistors do not have the cap in parallel with the sense resistor?

2. For a configuration with separate supplies for driver and outputs, what should the driver +/- power supply voltage be?

3. I have a nice transformer, which after rectification yields +/- 48Vdc and +/- 42Vdc. What should I expect if I use this transformer?
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Old 11th November 2009, 06:42 AM   #2
jaycee is offline jaycee  United Kingdom
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I'm not sure about the first one - I can only guess it is some kind of compensation.

For the frontend voltage it is usually best if it is a little higher than the output stage voltage, although that does depend on the topology used. The main benefit is that it should be regulated. If you're not planning to use Leach's PCB's, I would suggest powering the predriver transistors Q14/Q15 from the frontend voltage also. If you are, then simply leave out R32 and R33 and connect the frontend voltage there.

With 42V DC rectified you can expect about 90W of power into 8 ohms. You may need to recalculate some parts of the circuit for this voltage. Leach has some guideline to this on his site. You may need to substitute some lower voltage zeners in the diff pairs.
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Old 11th November 2009, 09:55 AM   #3
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In an older version of the article Leach said the protection circuit behaved better with only one cap in parallel with the sense resistor - with multiples the circuit tended to oscillate.

Can you tell I built my Leach Amos a LONG time ago?
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Old 11th November 2009, 10:23 AM   #4
djk is offline djk
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"With 42V DC rectified you can expect about 90W of power into 8 ohms. "

Not even with a regulated supply, the circuit cannot drive rail-to-rail. Depending on how 'stiff ' the supply is, I would guess more like 60W. Hooking the front end up to the ±48V would help a lot. I would add a Baker Clamp if I did this.
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Old 11th November 2009, 11:12 AM   #5
AndrewT is offline AndrewT  Scotland
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a 40Vac transformer gives 170W maximum output power.
a 35Vac gives 115W maximum output power.
a 30Vac should give about 80 to 90W maximum output power.

the Zener and it's feed resistor must be recalculated to pass the correct quiescent currents at your supply voltage.
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Old 11th November 2009, 01:33 PM   #6
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Quote:
Originally Posted by djk View Post
"With 42V DC rectified you can expect about 90W of power into 8 ohms. "

Not even with a regulated supply, the circuit cannot drive rail-to-rail. Depending on how 'stiff ' the supply is, I would guess more like 60W. Hooking the front end up to the 48V would help a lot. I would add a Baker Clamp if I did this.
I thought the Leach circuit had one. Can you kindly point me to a schematic that has a useful Baker clamp circuit?
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Old 11th November 2009, 01:37 PM   #7
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Quote:
Originally Posted by AndrewT View Post
the Zener and it's feed resistor must be recalculated to pass the correct quiescent currents at your supply voltage.
Thanks for the reminder.
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Old 11th November 2009, 01:40 PM   #8
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Quote:
Originally Posted by djk View Post
"With 42V DC rectified you can expect about 90W of power into 8 ohms. "

Not even with a regulated supply, the circuit cannot drive rail-to-rail. Depending on how 'stiff ' the supply is, I would guess more like 60W. Hooking the front end up to the 48V would help a lot.
One small detail I omitted from the original post is that I intend to operate this in bridged mode. So really it's two channels, bridged, each operating from +/-42Vdc.
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Old 11th November 2009, 01:42 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by weinstro View Post
One small detail I omitted from the original post is that I intend to operate this in bridged mode. So really it's two channels, bridged, each operating from +/-42Vdc.
that means the 80 to 90W into 8r0 becomes 160 to 180 W into 16r0.

Why don't you just use a 40Vac transformer and get 170W into 8r0?
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Old 11th November 2009, 01:52 PM   #10
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Quote:
Originally Posted by jaycee View Post
I'm not sure about the first one - I can only guess it is some kind of compensation.

If you're not planning to use Leach's PCB's, I would suggest powering the predriver transistors Q14/Q15 from the frontend voltage also.
Thanks for this tip. I'm re-using an existing amplifier chassis, and the standard PCB's won't fit for what I want to do. I'll probably need to look at how that thing is biased.
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