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Old 25th November 2008, 10:53 PM   #171
lineup is offline lineup  Sweden
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Quote:
Lineup:
while for global feedback amps the VAS stage can have very high gain with not very linear operation
Ostripper:
Not linear, huh?? Model my amps, model everyone else's amps,
model "secret" amps that are IP (don't ask ).
ostripper.
It is not very unusual to see VAS like in attachement.
In fact, I guess you have seen plenty of them.

How linear is this VAS?
If it is linear, then it would work well as a no feedback standlone amplifier.
But .. at least me, should not want to use it as my amplifier.

Regards
Lineup
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Old 26th November 2008, 03:47 AM   #172
anatech is offline anatech  Canada
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Hi lineup,
Quote:
How linear is this VAS? If it is linear, then it would work well as a no feedback standlone amplifier.
No, not really. The output stage is where most of the distortion is generated. You still need feedback of some kind to linearize the current output stages, no matter how linear the Vas is.

-Chris
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Old 26th November 2008, 05:24 AM   #173
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Quote:
Have you done any linearity sims on Ltspice
Yes I have, it seems to be very device dependant , it was
mentioned that I was running my MJE350/40's "too hot"
at 12ma so I set them at 8mA ,it simmed perfectly.

upon making the changes to the real amp I noticed slight
VAS distortion on the picoscope (could hear it too, golden ears).
cranked up the CCS to increase the bias a tad..11ma is
the "sweet spot" for a 350/40 VAS as shown on the frugal1.

I still want more, having built a real "symasym' on a veroboard
I like the sound and it's sim is impressive. I want at least
125W so I'll have to design its "big brother"..
OS
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Old 26th November 2008, 06:35 AM   #174
lineup is offline lineup  Sweden
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Quote:
Originally posted by anatech
Hi lineup,
No, not really. The output stage is where most of the distortion is generated.
anatech
No not really what?
Seems like you do not agree with me. But you are not clear here.

My original thought was, that VAS stages can sometimes be very un-linear .. in global feedback amplifiers.
In contrast to the input stage,
where we can not temper with linearity too much.
Even if using plenty of global feedback.
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Old 26th November 2008, 07:49 AM   #175
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Quote:
by lineup - that VAS stages can sometimes be very un-linear
Man, you have no idea ,but I'm happy now Visited rhe CM
thread, and added the buffered widlar to my VAS...WOW
most linear thing you could see ..rock solid..
here is schema..
Click the image to open in full size.

All at reduced current , which was my goal...now I can build the dang thing..
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Old 26th November 2008, 08:39 AM   #176
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I can't wait to build it ... It smokes the symasym..
at least in simulation (at 100k with 5v ripple thrown in for good measure) OLG at 1M with good phase margin.


[IMG]Click the image to open in full size.[/IMG]

{bottom is LPT degen, middle is VAS mirrors]
late night, but I got it..
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Old 26th November 2008, 09:45 AM   #177
AKSA is offline AKSA  Australia
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OS,

Great to see you removed the EFs from the VAS drive; it removes one further source of phase shift.

Have you considered a simple half stage current CCS on the active side of the VAS, drone side connected simply to rail?

Hugh
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Old 26th November 2008, 10:05 AM   #178
GK is offline GK  Australia
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Quote:
Originally posted by AKSA
OS,

Great to see you removed the EFs from the VAS drive; it removes one further source of phase shift.

Wrong again. Unlikely that anyone is likely to pay attention, but running a pair of undegenerated MJE350's (even from resistive LTP loads as low as 1.2k) without EF buffers will significantly load the LTP reducing OLG, decreasing OL linearity and the phase shift will actually be worse due to the fact that the miller capacitance of Q4 is working against a relatively high drive impedance (R5) of 1.2k (Q5 doesn't matter because it is cascoded by Q6)

However, for a definative confirmation on the performance evils of EF VAS buffers, have a look at Bob Cordell's amp for example - 2MHz unity loop gain frequency, stable slewing 300V/us and 0.0006% THD-20.
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Old 26th November 2008, 10:34 AM   #179
sandyK is offline sandyK  Australia
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Default Frugalamp

"Wrong again. Unlikely that anyone is likely to pay attention, but running a pair of undegenerated MJE350's (even from resistive LTP loads as low as 1.2k) without EF buffers will significantly load the LTP reducing OLG,"

IMO, there are far too many designs,(even some of the highly thought of designs in DIYAudio) that heavily load the preceding stage. Some even use low HFE parallel devices at the output of an opamp. Many heavily load the LTP as well.

SandyK
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Old 26th November 2008, 01:14 PM   #180
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Quote:
By GK - Wrong again. Unlikely that anyone is likely to pay attention, but running a pair of undegenerated MJE350's (even from resistive LTP loads as low as 1.2k) without EF buffers will significantly load the LTP reducing OLG, decreasing OL linearity and the phase shift will actually be worse due to the fact that the miller capacitance of Q4 is working against a relatively high drive impedance (R5) of 1.2k (Q5 doesn't matter because it is cascoded by Q6)
This is just round 1 in development, the 350/40's are gone,
being replaced by 2SA1381/2SC3503 higher beta devices
and Cdom is gong down to 47pf. I will post all soon.

this is the best of all the popular AB amps Ive simmed so far
even beating the symasym.
stay tuned..
Ps - I need more models...(devices above)
OS
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