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Old 16th July 2004, 01:13 AM   #21
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Quote:
Originally posted by jh6you
What is the practical problem in use for audio amplifiers???

its specs are no good, it is not modern, and it would faithly produce 100mhz signals (our inability to hear that is irrelevant, ).

Sometimes, I wonder why we care so little how those devices actually sound.
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Old 16th July 2004, 01:18 AM   #22
sam9 is offline sam9  United States
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My impression is the "speed" (as in fT?) of a device becomes more important as power and rail voltage increases, i.e., slew limit.

However, just upgrading to a newer device may not be a slam dunk. I've read from a Phase Linear tech that substituting the certain newer MJ15xxx devices sets off oscillations in the old PL700s and PL400s. This sounds to me like combining slow drivers with faster output devices presents a problem. I could also believe that the gain and feedback network might create problems as well.

I may be off base on this, so don't take as truth.
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Old 16th July 2004, 03:10 AM   #23
anatech is offline anatech  Canada
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Hi sam9,
Right you are as far as oscillation is concerned. But, you may end up with oscillations in a low voltage amp just as easily. So, the voltage & current only makes the problem more spectacular. Any make of amplifier may suffer from this if your original outputs were much slower. The same thing used to happen when a Japanese output set where older JAN types were used.
If you blow the outputs, it's always a good idea to change the drivers as well.
-Chris
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Old 16th July 2004, 03:33 AM   #24
sam9 is offline sam9  United States
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"But, you may end up with oscillations in a low voltage amp just as easily."

<Sigh> I'm olny too painfully aware of this.


"makes the problem more spectacular."

I'm afraid I can help thinking about the old sci-fi TV shows where the contol pannels on the "bridge" would go up in smoke and sparks every other episode. Imagine a rack of PL700's going poof all at once!
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Old 16th July 2004, 03:46 AM   #25
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Quote:
Originally posted by sam9
My impression is the "speed" (as in fT?) of a device becomes more important as power and rail voltage increases, i.e., slew limit.
Your impression is well aligned with this:

http://www.labgruppen.se/Files/TechN...note_02-11.pdf
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Old 16th July 2004, 02:32 PM   #26
sam9 is offline sam9  United States
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I had calculated about the same figures via a slightly different route. Walter Jung published some articles in the mid 70's that were based on the required v/uS at 80khz in order to set a safety factor of 4 and I think Nelson Pass may have recommended slew rates adequate for 200kHz as desirable.

I can understand designing for 2 to 4 times beyond what is needed for 20kHz, but I don't know the reasons why even higher might be thought beneficial.
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Old 16th July 2004, 03:10 PM   #27
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Anyone have any suggestions for a faster output transistor than mj802 that is linear up to 7A and is not a darlington of course. BJT preferably TO-3 case.
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Old 16th July 2004, 03:58 PM   #28
sam9 is offline sam9  United States
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MJ15022/13, MJ15024/25, MJ21193/94
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Old 16th July 2004, 04:03 PM   #29
x-pro is offline x-pro  United Kingdom
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HUF76639
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Old 16th July 2004, 11:05 PM   #30
anatech is offline anatech  Canada
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I second the MJ's, not familiar with HUF76639. Very flat hFE curves as IC changes.
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