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Old 22nd May 2002, 12:09 AM   #21
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Another thought strikes me: the output stage
which is used as the "dummy" stage could be a single
device while the speaker powering device could be
a number of parallel devices. This way, the "dummy"
output stage could replicate the real amplifier if it's
load is a multiple of the impedance of the speaker load.

In that case the "dummy" stage would not have to be as
large, saving energy and expense.
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Old 22nd May 2002, 01:51 AM   #22
AKSA is offline AKSA  Australia
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Default SPLIF Topology

As usual, and with minimal words, Nelson goes straight to the nub of the matter.

Take it further; if the purpose is to buffer the feedback from a real world load, why not just take the feedback from the drivers? If we use a simple double emitter follower output stage, and we join the two driver emitters with a simple resistor (common practice, typically 220R), why not split the resistor and take the feedback for the amp from the midpoint of this resistor?

Then the output devices see the load, the feedback sees the crossover disjunction and at least partly corrects for it, and the phase anomalies of the output devices as they work into an extremely reactive speaker crossover are not seen by the global feedback network.

Of course, the load now suffers the intrinsic source impedance of the output devices, and this could be highish, largely depending on the size of emitter resistors, but if it is true that the imaging and fine sonic detail is lost in the phase distortions of crossover interaction with a conventional feedback network, then this little number should do the trick.

In truth, I have tried all this. The result: the amp sounds very twee, delicate and with a ravishing sound stage. It presents very like a 2A3 SET amp on steroids. But it lacks slam and impact, with a slightly wooly but musical bass, and really is only suitable for small jazz combos and chamber music. It was completely and utterly unsuited to rock 'n roll, and eventually I came to the conclusion that a combination of voltage amp and output stage feedback, in the ratio 1:2, was the optimum mix, but even then, the sound was still a little precious, at least to my battle-hardened ears.

Those interested in an even more detailed analysis than this, go to www.harmonicdiscord.com, and look at the thread entitled 'Tweak that works for every SS PP amp'.

Cheers,

Hugh R. Dean
www.printedelectronics.com
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Old 22nd May 2002, 02:37 AM   #23
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Thumbs up AKSA

This idea of spliting the feedback has been used by several commercial designs. I have built circuits with class A mosfets outputs with feedback taken at the gates of the output stage mosfets and noticed some of the sonic attributes Mr. Dean has outlined. The first use that I am aware of of the dual feedback loop would be John Curl's JC3. Definitely an idea worthy of some experiments.

H.H.

http://marklev.com/marklev/JC3/jc3schematics.jpg
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Old 22nd May 2002, 03:32 AM   #24
jam is offline jam  United States
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Ah Ha! Nested feedback loops an area I want to experiment more with.
Harry and Dean would you care to elarobate more on the designs you are familiar with and if you played with changing the raitos of those loops, or is it better not to nest the loops and have the voltage amplilier have its own loop and a seperate output stage with gain having its own loop.

Jam
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Old 22nd May 2002, 10:53 AM   #25
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http://users.ece.gatech.edu/~mleach/.../outstage1.gif
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Old 22nd May 2002, 02:27 PM   #26
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yes there are some comercial amplifiers that use the spliting feedback system. I remenber the Densen MA10 that is publicited
of a nom negativ feedback amplifier and i have found that it uses a normal voltage feadback satge and only the outputs followers
are out of the feedback loop...(it woud be advertised as a overal
feedback free amplifier..a la Mister Pass...)
I have also try that topology and my findings are the same...speakers d'ont like be left floating in the breeze...the sound
is very sweet but d'ont have guts...the bass became slow and
d'ont stop in time...specialy the timing of music became dirty... everything nice but not exciting...
Just my opinion
Regards
Jorge
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Old 22nd May 2002, 08:16 PM   #27
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I have often taken feedback from the Gates of the
output stage, and it works well, but this would be
a whole other technique.

As to nested feedback, it works particularly well if
you have an IC op amp for a front end, as you can
set the open loop gain of that stage, and thus define
just how many decibels of feedback you want around
the output stage.
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