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Old 27th February 2005, 01:40 PM   #21
MikeB is offline MikeB  Germany
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And phaseshifts, the do not look too good.

Mike
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Old 27th February 2005, 01:59 PM   #22
sajti is offline sajti  Hungary
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MikeB,

sorry, I was not very careful, when I saw the schematics. I don't want to use cascode connection. I found that it sounds sharply in the HF region. Somebody feels it as clean sound, but I don't like it.
So, I try to use double differential, with low overall feedback. Something like Mark Levinson, but more simple. As I remember You try something same few weeks before...

sajti
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Old 27th February 2005, 02:02 PM   #23
sajti is offline sajti  Hungary
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Quote:
Originally posted by MikeB
And phaseshifts, the do not look too good.

Mike
Try to reduce the open loop gain. I think, that increasing the emitter resistors in the input stage helps to do it.

sajti
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Old 27th February 2005, 02:09 PM   #24
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Quote:
Originally posted by MikeB
Hi Jan !

I checked your suggestion, i replaced the 1.87 voltage with 3 diodes
in series fed with 15k, (I had no adequate LED-model)
there are only subtile changes.

I checked with different closedloopgains, with gain of 26 results
in acsweep look identical, with gain 1:10 the acsweep gets critical,
but no oscillation yet. I decreased timestep from 10ns to 2ns to
be sure.

Attached is acsweep, blue is output before coil, green left base
of diffamp, red right base of diffamp. Feedback keeps always
below input, so should be safe.

Mike
Looks pretty good to me. But to be sure, you should put couple of ohms in parallell with that output choke. As it is, it start to roll off the output to the 4 ohms load above 350kHz or so. I'm not sure what that does, and I can't think of a good reason (engineering wise) to do it, but it is customary and it may have an influence. Then again, you may be the first to prove that "what everybody does" is just so much BS... . Sheep following and all that.

OTOH, there may be a reason once you get into complex xover networks. Anyone?

Jan Didden
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Old 27th February 2005, 02:15 PM   #25
MikeB is offline MikeB  Germany
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Hi sajti !

This is folded cascode, not cascoded vas, or do you meant the FC ?
I did not observe/hear sharp sound, but you're right, this could be
about taste... (it does sound clean)

About the phaseshifts, i am not sure...
I took the phasehifts at diffampoutput (currents because of FC),
they stay in phase up to 100mhz... Don't know why...
The +180 is because they are inverted/mirrored. (left to right)

Attached the phasehifts...

Mike
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Old 27th February 2005, 02:31 PM   #26
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by MikeB
...it seems impossible to get a "standard" topology stable
without feedbackcompensation...
It is possible! You need to use very fast transistors in the input stage (very fast as in GHz+ ft) with copious amounts of emitter degeneration and a MOSFET or otherwise very fast output stage.
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Old 27th February 2005, 02:51 PM   #27
sajti is offline sajti  Hungary
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Quote:
Originally posted by MikeB
Hi sajti !

This is folded cascode, not cascoded vas, or do you meant the FC ?
I did not observe/hear sharp sound, but you're right, this could be
about taste... (it does sound clean)

About the phaseshifts, i am not sure...
I took the phasehifts at diffampoutput (currents because of FC),
they stay in phase up to 100mhz... Don't know why...
The +180 is because they are inverted/mirrored. (left to right)

Attached the phasehifts...

Mike
When I built cascode connected amplifier, I feel, that it very clean. After some listening I found that it "too clear". Than I started to find something about it. Than I found an article about this effect. The very heavy load of the bottom device can results this sound. The upper transistor has very low input impedance, and it provide this heavy load.
I will try the double differential amplifier in the next 1-2 weeks. I will report the results. I don't think that we need very fast amplifier topology, if we not use large overall feedback.

sajti
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Old 27th February 2005, 03:04 PM   #28
MikeB is offline MikeB  Germany
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Quote:
Originally posted by MikeB

they stay in phase up to 100mhz... Don't know why...
Oops, of course they do...

Mike
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Old 27th February 2005, 03:08 PM   #29
MikeB is offline MikeB  Germany
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Quote:
Originally posted by sajti

Than I found an article about this effect. The very heavy load of the bottom device can results this sound. The upper transistor has very low input impedance, and it provide this heavy load.
I will try the double differential amplifier in the next 1-2 weeks. I will report the results. I don't think that we need very fast amplifier topology, if we not use large overall feedback.

sajti
But you told me that for the cascoded vas ?
The other topology i posted a few weeks ago sounds very good,
just not outstanding, too "natural"/soft for my taste.

Mike
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Old 27th February 2005, 03:25 PM   #30
sajti is offline sajti  Hungary
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Quote:
Originally posted by MikeB


But you told me that for the cascoded vas ?
The other topology i posted a few weeks ago sounds very good,
just not outstanding, too "natural"/soft for my taste.

Mike
It can be true for any cascode connection. The common base stage has the low input impedance. Doesn't matter where we use it. But if You satisfied with this amplifier, why not? I don't want to let You, to leave this design.

sajti
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