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Old 27th June 2002, 09:07 AM   #31
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There's not a lot of values to calculate in diff amps themselves, just the tail current, half of which goes through each transistor. This can be determined by a simple resistor, or a constant current source of some sort. There is much controversy over which is better, but a resistor is sure simpler. You can also use two diff amps and two VA stages, called a mirror topology.

In this constant current source, the voltage drop of the diodes - the Vbe drop of Q3 = the total voltage drop across R7. The voltage drop across them would be about 1.34V. 1.34 - Q3's voltage drop of about 0.67 = 0.67V across R7. A simple ohms law calculation, 0.67/150 = 4.4mA. Half this is the quiescent current of Q1 or Q2.

The current that you would want here is based on how fast you can charge the dominant pole cap, C4. There's a formula for figuring out how much current you need.

I = 6.28 x f x C4(farrads) x peak VA output voltage.

So for example, at 50kHz, with C4 as 100pF, or 1e-10F, 6.28 x 1e-10 x 50,000 = 0.0000314.

If you have a 65V supply, you might expect maybe 60V peak from the VA, so 0.0000314 x 60 = 0.001884.
So you need about 1.9mA through the each leg, about 3.8mA tail current.
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Old 27th June 2002, 09:10 AM   #32
e96mlo is offline e96mlo  Sweden
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Kilowatt, what does your component names (C4 etc) refer to?

/Marcus
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Old 27th June 2002, 09:15 AM   #33
Duo is offline Duo  Canada
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C4 refers to the Dominant Pole cap, I think that's usually from the
base to the collector of the VAS transistor...
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Old 27th June 2002, 09:21 AM   #34
e96mlo is offline e96mlo  Sweden
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Yes, that is quite obvious as he said that in the post. But since he refers to components named Q1, Q2 etc I thought that he maybe had a specific schematic in mind.
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Old 27th June 2002, 09:23 AM   #35
Duo is offline Duo  Canada
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Yeah, I was thinking about that after I made the comment...
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Old 27th June 2002, 09:24 AM   #36
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Sorry about that, I forgot to post the image. Then I was trying to get back ASAP and my computer had to go and freeze up on me, and it's like 1:21 in the morning too. It's sooooo frustrating!!!!

Anyway, you've all been doing some posting as I type, so I might be a bit behind. I hope this helps.

P.S. Maybe someone should check me on this stuff, just in case.
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Old 27th June 2002, 09:29 AM   #37
e96mlo is offline e96mlo  Sweden
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I see that we all go to Rod Elliot's site for knowledge.
Haven't found anyone to beat it yet.

/Marcus
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Old 27th June 2002, 09:33 AM   #38
Duo is offline Duo  Canada
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Good point! I have scoured each and every one of his amplifier diagrams over and over. I learned quite a bit just by some reverse engineering of what he has done. If I had the parts(or could afford them) I would build some of his bigger amplifiers quite happily. Every time I need a pointer, I usually go to esp first!
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Old 27th June 2002, 10:32 AM   #39
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Yup, nothing beats project3a (or mods of it, like seen above) for educational purposes, or for a good, simple amp to build. When I needed plans for a switching supply for car amps, where did I turn? When I need plans for a VU/PPM meter, where did I turn? When I was first learning audio, where did I turn? The list goes on and on! One thing anwers all of these questions: Rod Elliot's site.
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