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Old 17th November 2005, 02:44 PM   #1
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Unhappy Take a look on amplifier schematic please

I am currently building this amplifier and I asked Rod Elliot too tell me something more about this amplifier.

He wrote this:

Victor,
I had a quick look - I would not use that amplifier. While it might work, IMO there are too many things wrong with the design. I suggest you find something better. Bear in mind that I am not a free information service, and I will not assist you in improving the design.

Cheers, Rod

What is wrong with the design? I will use for PA.

Your comment…
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Old 17th November 2005, 02:54 PM   #2
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Q13 concerns me a bit. makes it hard for me to analyze by inspection. i'm not overly trusting it at all. looks like signifigant DC current would flow into the base and the device would just saturate. or in best case the amp becomes beta dependent.
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Old 17th November 2005, 03:15 PM   #3
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Why do you think significant DC current would flow into Q13 base? The design looks OK to me as a starting point, but I haven't spent more than a handfull of seconds analysing it.
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Old 17th November 2005, 03:56 PM   #4
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Default !

Please more....

Design is OK???
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Old 17th November 2005, 04:35 PM   #5
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Hi Viktor,

Whether this amplifier is O.K. for you or not depends on what level of performance you are after.

I looked at it for about a minute, It would probably work but would have poor CMRR and PSRR, and possibly issues with slew-rate limiting and VAS headroom.

These problems could be overcome by using a current source and current mirror in the input stage, and a current source in the VAS stage, but then you'd need to look at the frequency compensation to ensure that the design was stable.

If you can, I would strongly recommend picking up the following books:

The Art of Electronics by Horowitz and Hill, High-Power Audio Amplifier Construction Manual by G. Randy Slone, and (you guessed it) Audio Power Amplifier Design Handbook by Douglas Self.
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Old 17th November 2005, 04:36 PM   #6
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bases on LTP are grounded (i hope.)

thus you have at least 50V across the 18k resistor. this gives something like 2.7mA

per branch this is 1.3mA

this is 1V across the resistor at Q13, well, that can't happen. so it'd be more like 1mA through the resistor, and 0.3mA into the base of Q13.

0.3mA. ok, that might be fine. but now you've got some unknown current flowing in that part of the circuit. it is dependent upon beta, which isn't ideal.

0.3mA is likely conservative. and without knowing beta of the mje off the top of my head, i can't continue. it is very possible that the device will saturate.

edit -- mje is between 30 and 240 for Hfe. with 0.3mA at base, 9mA at collector, you would drop 90V across the 10k resistance. 300V if hfe is 100. 600V if it were 200. this is no good.
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Old 17th November 2005, 08:01 PM   #7
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Hi Victor,
the LTP is not balanced.
Q13 defines the voltage across R21. This becomes the current in Q11 (650mV/680r=960uA).
r22+r23 define the LTP tail current ([60V-0.7V]/[18k+680]=3170uA), this is approximate due to leakage through C4.
All the remaining tail current goes through Q12.
Iq12=2210uA Iq11=960uA. OOPS no good!!!!

Change r21 to 430r and the LTP is just about balanced.

You could (should) improve the amp by adding emitter resistors to EVERY transistor EXCEPT Q10.

Do NOT buy Horowitz & Hill instead try to find a copy of "The art of linear electronics" by John Linsley Hood published by Newnes Butterworth Heinemann. This AND Douglas Self will teach you a lot.
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Old 17th November 2005, 08:03 PM   #8
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Most severe design misses in my eyes are (rest is value tweaking):

Purpose of R12???

Purpose of R9???
...should there be an electrolyte voltage rail filtering cap on the diffpari side of R9 or not for some unknown "reason"???

Very heavy capacitive load for the VAS by the amount of output FET's, add drivers after VAS!

Good luck!

Cheers Michael
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Old 17th November 2005, 08:34 PM   #9
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Why you not use a current mirror for the LTP?
As far a I am concerned it is always better than the single resistor. It will mostly solve the balance problem too...

Doug
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Old 17th November 2005, 08:38 PM   #10
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Also, what the is purpose of R18? R12 seems superflous to me too. I think the amp would be just fine without it. Also the connection of C3 seems strange. Is this intended as a compensation cap? Usually they go coll-base of the VAS...

Hope this helps

Doug
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