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Old 23rd August 2004, 05:05 AM   #1
jsd is offline jsd  United States
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Join Date: Mar 2004
Location: New York
Default Basic amplifier questions

Being new to amplifiers, I was wondering some things..

What would be a good website (or book!) to learn how amplifiers work, what common design criteria is used etc. I see references to long tailed pairs, and things of the like and I have practically no idea what's going on!

And for a more specific set of questions..

On ESP, power amp project #3:

If I wanted to replace the output transistors with MJ21193/MJ21194s, and raise the rail voltages to around 75v each what else would I have to change? I also have some 150w/180v/16 amp/6MHz devices laying around, so I could use them for the output transistors before actually buying something.

If the rail voltages were raised so much, I guess the driver transistors would probably have to be changed too? Would MJE340/MJE350 work? Those are 20w/300v/500mA devices.

How about the input transistors? Maybe MPSA92? 625mW/300v/500mA/50MHz devices..

I'd also have to increase the gain which I'm assuming is set by the 22k/1k resistors. Would it be alright to increase that to 22k to 56k?

What's the zener diode for in the schematic?

What else would have to change in order to increase the rail voltages that much? With 75v rails, how many pairs of output devices would I need? I plan on testing into 8 and 32 ohm loads, never anything below that. I'm guessing 2-3 pairs would be fine?
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Old 23rd August 2004, 05:17 AM   #2
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Join Date: Mar 2001
If you've plugged into ESP, then I suggest you start by building
up some of his simpler and less powerful offerings. As you
proceed with that, you'll find yourself answering a lot of your
own questions.
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Old 23rd August 2004, 10:19 AM   #3
MikeB is offline MikeB  Germany
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Join Date: Apr 2004
Location: GŁtersloh
hi jsd !

I you are new to amplifiers, do it like nelson said, build an existing
and proofed design like maybe project3a. Elliott says that you can use
Keep the supplyvoltage as is, do you really need that much power ?
+/- 75volt is very high, that makes 150volt ! (also getting dangerous)
Do you really need 300watts into 8ohm ?
Don't forget, you also need a 4times bigger heatsink.

If increasing the supplyvoltage that much, you need much more
outputdevices and other biasing and so on.
And NEVER change feedback on an existing amp, you might get into
some real troubles, as the design might rely on the exact values.
Correct feedback and adjusted caps are the heart of a proper design.

And big NO to changing the resistor from 22k to 56k, it is intended
that it has the same value as the one at input to ground. This
ensures for example low dc-offset. And this resistor is the most
critical one in the whole design. If you need increasing gain, then
you should reduce the 1k, and increase the cap below by the same amount.
But it's still not the best idea...

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