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Old 7th March 2005, 11:11 AM   #11
MikeB is offline MikeB  Germany
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Default Re: What do uncompensated amps behave?

Quote:
Originally posted by tmblack
My power amp does not oscillate but if sounds overly crisp and detailed
in a bad way.
I used a current mirror instead of resistors in a LTP.

What tests can be applied to test the compensation of an amp?

Tom

Hi Tom !
You ran into some typical trap. You increased openloopgain to
lower thd, but at the same time you reduced openloopbandwidth...
This results typically in exactly this sound you described.
You can remove this by adding a resistorload to the vas, try a 22k
to ground. Check your amp that it has openloopbandwidth flat
up to ~20khz.
Don't forget, low THD only means low distortion on sinus-signals.
Often low THD results in high distortions for complex signals
like music. But an amp is intended for music...
This crisp sound is normally heavy intermodulationdistortion,
it's not caused by instability.

Mike
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Old 7th March 2005, 01:52 PM   #12
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Quote:
Inevitably, someone will pop up at this point and start screaming about how it lowers distortion, etc.
No one seams to have popped up so I thought I would. NFB does lower distortion, etc.
Quote:
Your proper response to such a post is to ask them how often they listen to live, unamplified music. Amplified music doesn't count--it has to be unamplified, elseways you're listening to the PA system, and PAs make lousy stereos.
Quite often down my local Jazz pub.

Just to apply some balance here:
If you really understand what you are doing...
a) Feedback is extremely good - as much as possible.
b) Bandwidth beyond the audible range is unecessary
c) Miller compensation is just fine, better than many methods.

I'm not trying to be controversial but rather I wouldn't want budding designers to adopt assumptions that limit the potential of their designs.
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Old 7th March 2005, 01:57 PM   #13
MikeB is offline MikeB  Germany
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Quote:
Originally posted by traderbam

a) Feedback is extremely good - as much as possible.
As much as POSSIBLE, not as much as DOABLE !

Mike
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Old 7th March 2005, 03:31 PM   #14
tmblack is offline tmblack  United Kingdom
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Hi MikeB,

I like to test your theory, so how can I measure the IM with my computer and soundcard?
Is there any free program?

Tom
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Old 7th March 2005, 04:11 PM   #15
MikeB is offline MikeB  Germany
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Hi Tom !

There are programs for this, but i don't know jack...
Maybe someone else can tell ?
I'm not sure if this kind of problem shows up in normal IM-tests,
i tried to reproduce this problem in sims, but with the normal
2freqs-im test it did not show up.
But you could try adding such a resistor in the vas, if it's this
OLBW-problem, the sound chould change immediately.

Mike
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Old 7th March 2005, 06:27 PM   #16
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Default Re: Re: What do uncompensated amps behave?

Quote:
Originally posted by MikeB



Hi Tom !
You ran into some typical trap. You increased openloopgain to
lower thd, but at the same time you reduced openloopbandwidth...
[snip]Mike
Mike,

This is misleading. The open loop bandwidth is reduced, but only because the gain at frequencies below the original cross-over frequency is increased. The gain above the original cross-over frequency is unchanged. So overall, there is more gain available between DC and the original cross-over frequency.

Jan Didden
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Old 7th March 2005, 08:32 PM   #17
MikeB is offline MikeB  Germany
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Hi Jan !

Yes, you're right, my point was more like openloopfrequencyresponse
has to be flat within audioband... I don't know why...
But in every single amp i've constructed it was absolutely necessary
to keep it flat, or sound was "trash".

Mike
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Old 8th March 2005, 01:57 AM   #18
tmblack is offline tmblack  United Kingdom
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I am thinking current mirrors as active loads sound bad.
My LM3886 chip amp sounds extremely harsh.

Tom
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Old 8th March 2005, 02:19 AM   #19
anatech is offline anatech  Canada
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Hi Tom,
Not neccessarily. Current sources help isolate the signal from supply noise and make the circuit more stable if designed properly. Degeneration helps linearise the circuit and burn off extra gain so the amount of feedback can be reduced. Too much feedback absolutely sounds bad.
Besides, I really like all the lights from my LED biased current sources.
-Chris
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Old 8th March 2005, 05:19 AM   #20
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Quote:
Originally posted by traderbam
b) Bandwidth beyond the audible range is unecessary
Well if you want decent phase response anywhere near the edges of the audible range then it's pretty much manditory.
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