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Old 27th September 2002, 09:54 AM   #241
Henrik is offline Henrik  Denmark
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Default Test of X-BSOZ and XSOZ

Yesterday evening I finished my test setup.
"The one and only" is so right about the use of buffers.
This time the buffers didnīt add anything, not even in the high frequense area as it did in my first testsetup, on the contrary, a significant part of the directness of the sound and soundstage dissapeard.
The addition of X to the BSOZ was quit the same as what happend when adding X to the SOZ.

I really am happy with this result.

I will be back with some conclusions and more descriptions on the change of sound and some conciderations on the scematic.

Nelson, thanks for driving me to these tests by your short answers dropped at the right spots.
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Old 3rd October 2002, 03:55 PM   #242
Henrik is offline Henrik  Denmark
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I have finished my description of the X-BSOZ and X-SOZ for now, so this is my point of view.


Gain
The gain at the original BSOZ an SOZ was controlled by R115 and R207, now it is by R120, R121, R122, R123, and R212, R213, R214, R215, all resistors in the X-feedbackloop.
I have kept the original gainlevel, and also this gave the best frequency responce.

Impedance
I have mesured the impedance by placing a resistor in series with the impedance to measure, and then scaled this resistor until it lowered the signal to the half. Thus the scaled resistor shoud be equal to the imedance.
If this is correct, then my measurements are reliable with recpect to the simulator.
I wanted drastically to lower the output impedance when changing BSOZ to the X-BSOZ, actually this was an easy task, because I got that as a free sideeffect when adding the X-feedback, it was lowered from 1.5kOhm to 20 Ohm, and without changing the values of R101 and R102, so I kept the original values of 750 Ohm each.
The X-SOZ output impedance was lowered by the X-feedback, just like it happend for the X-BSOZ, only not so dramatically, it went down to 4.7 Ohm from 15 Ohm.
The dampingfactor at the X-SOZ is the load divided with the outputimpedance, and at a 15-Ohm load this factor will be about 3. If this is the main reason to the better bas control, I donīt know, but i theory it should, and ceartenly the bas is far more controled with this X-feedback.

Capacitors in the signalpath and X-feedback loop.
A great deal of concearn (read audiophile anxiety) is caused by the use of capacitors in the signalpath or in feedback loops.
When I added the ZV4 buffer to the X-SOZ I also added some highquality filmcaps in the signalpath, and despite of those, the buffer turned out to degrade the sound compared to the X-SOZ without buffer. But more important, this X-SOZ had some absolutely nonexotic 220uF electrolytics in the X-feedback loop. So the nonexotic high value electrolytics dosnīt seem to degrade the sound as much as even the best constructed buffer, at least not when placed in the X-feedback loop. The X-feedback wil minimise the noise and distortion from thise electrolytic capacitors since they are inside the loop, but if this alone can explain the sonic abcence of these caps, I donīt know.
Another thing is that the outputcaps in the X-BSOZ are inside the X-feedback wich will reduce theier sonic precense, in BSOZ they were directly in the signal path.
Nelson emphasises the use of simple circiuts with few gainstages, some times also if the price paid is electrolytic capasitors in the signalpath or in the feedbackloops, and he also points out, that his designs are much more a question of construction rather than exotic parts.
I really agree with him.

Critical values
C101 and C102 determine the low-end frequency roll off, both in the X-loop and at the output at the X-BSOZ.
At the X-SOZ it is a little different, C201 and C202 determines the lowend roll off only in the X-loop and not at the output, since the output is before the caps. The input signal at the X-SOZ will be lowered by the negative X-feedback, but as the lower end is “rolled off” at frequencies lower than that. So these low frequencies will pass the gainstage in open loop gain, and since there is no cap at the output, it will go directly to the speakers unaffected by the X-feedback. Avoiding any raise in the low-end response under 4 Hz, the preamp must start rolling off at about 2-4 Hz. The low-end response is formed by the relationship between C101/102 and C201/202.
R208 and R209 must be as small as possible, but still able to reject parasitic oscillation, a relatievly small change in value do have a significant influence on the high frequency roll off, if you raise the value, the point of roll off will go down.

Tests
Because I have to work for living (sort of freelancer), I havenīt had so much time lately to describe my experiences from the last testsetup. But it is verry clear to me that the addition of the X to both the BSOZ and the SOZ is remarkable positive in any acpects of the sonic performance, and the best result is most certainly without the buffer.
The sound is more controlled than ever, and it still have this easy and almost scilent flow, and when nessesary the dynamicks taks over. The soundstage is deeper, higher and wider.

I am shure that this X coud be applied in different ways, but this is how I did it until now, and Nelson and others are more than welcome to make their comments and suggestions.

I have attached the testfiles in pdf and scematics in pdf / SIMetrix simulator formats.

Thanks everyone for this nice thread on the X-SOZ.

Attached Files
File Type: zip henrik x-bsoz and x-soz.zip (27.5 KB, 3512 views)
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Old 3rd October 2002, 04:24 PM   #243
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Nice piece of work, Henrik.

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Old 3rd October 2002, 04:29 PM   #244
eLarson is offline eLarson  United States
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Isn't the original Zen Daughter-in-Law linestage pretty "X-ish" already?

It doesn't feature a folded cascode (mentioned in the patent), but it already has coupling between the noninverting ports of two identical amplifiers.

Maybe the question should be: How important is the folded cascode idea to the Super Symmetry concept?

Erik
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Old 3rd October 2002, 04:41 PM   #245
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Not important, but X does require the symmetric feedback.
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Old 3rd October 2002, 04:58 PM   #246
eLarson is offline eLarson  United States
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I was under the impression that the feedback from the drain of the output section back to the gate of the input section was just to allow the distortion products of the output section to be communicated through to the other side. (I'm thinking of the drawing from the cover sheet of the Super Symmetry patent.)

That is why I was left scratching my head about the need for a drain to gate feedback path when all there is in a Bride-of-Son-of-Zen are two common-source amps tied together: there is no second transistor to worry about distortion products from.

See what I mean?

Erik
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Old 3rd October 2002, 06:09 PM   #247
Henrik is offline Henrik  Denmark
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Thanks Nelson, this means a lot to me.

Erik
The BSOZ was born diffrential, and thus both mosfets reflects each others work. But the X is as Nelson said a symmetricl local feedback, wich more or less cancels the noice and distortion from the mosfets. The center for this communication is the diffrential behaviour of the mosfets. It is hard to get, an still, when I am tierd in my head I donīt quite get it.
It is like walking bacwards.
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Old 3rd October 2002, 06:58 PM   #248
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Thank You Henrik

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Old 3rd October 2002, 07:08 PM   #249
Henrik is offline Henrik  Denmark
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Stefano

You are more than welcome, You were actually the one to start all this, GREAT!

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Old 3rd October 2002, 07:47 PM   #250
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If all you have is a diff pair biased by a current source,
and there is no other source of noise and distortion (including
the load), then the feedback does not lower the distortion;
it only controls the gain and output impedance.

This is because any AC current flowing through the diff pair
is already identical.

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