Twisted X Overture

[tiltedhalo]

" The super-symmetry topology does not use operational amplifiers as building blocks. It has two negative inputs and two positive outputs and consists of two matched gain blocks coupled at one central point where the voltage is ideally zero. The topology is unique in that at this point the distortion contributed by each half appears out of phase with the signal, and we use this to reinforce the desired signal and cancel noise and distortion. This occurs mutually between the two havles of the circuit, and the result is signal symmetry with respect to both voltage and current axis, and anti-symmetry for distortion and noise. This means that distortion and noise of each half appears identcally and cancels."

(C) 1998 Nelson Pass, Pass Labs

How does this occur in your circuit, looks more like a small amount of coupled feed forward error correction?

[Russ White]

My Circuit works by utilizing the fully differential nature of the device. The dominating feedback loop is the THS4131. That quite simply is how if becomes SuSy. You will have to ask TI how they implemented the technology.

This circuit topology is in fact nearly identical in nature to the circuit Nelson himself proposed. It is just that I am using a fully differential opamp where he used a differential pair with CCS. The result is the same topology from a more complete device.

Look here. This thread should answer your questions.

http://www.diyaudio.com/forums/showt...299#post421299

Cheers!
Russ

[Russ White]

The key quote is here:
http://www.diyaudio.com/forums/showt...450#post421450

Setting the gain around each Chip Amp via the - input loop
allows quite a bit of adjustment in this regard. The Chip
Amps are of course enclosed in the loop formed by the diff
pair, and so their distortions will tend to be nulled, reducing the
overall effect toward the performance of the diff pair itself.

I hope you guys appreciate that I used Bipolar transistors
just for the occasion.

So you see, in my circuit it is just exactly the same, just the devices are different. The performance of the cirucit will be dominated by the THS4131(or say OPA1632).

In my circuit you could replace the THS4131 with a differential pair and a CCS add one capacitor to block DC and adjust a few resistors to account for the change in gain, and you would have something that would look shockingly (ok its not so shocking) like what Nelson proposed... I have built that version too, and it is very very good. In practice, I found the THS4131 version better for mostly for practical reasons. They both sound great.

Cheers!
Russ

[Carl_Huff]

Russ,

Here is my version of a simple double sided PCB of your design. The board is small measuring only 3.8 by 2.5 inches. Notice that I have included an AD8620 input buffer. I am about to send it out for fab. I am looking forward to building it and then doing some critical listening. It is a very interesting circuit!

[Russ White]

Hi Carl,

Cool, good luck!

I hope you enjoy it.

My DS layout is nearly done and I will be ordering PCBs soon.

Cheers!
Russ

[tiltedhalo]

You are missing my point, If you look at the internals for the THS4131 it uses a form of super symmetry useing a voltage controled differential amplifier for the cross cancellation to acheive the signal symmetry and anti-symmetry for the distortion and noise components.

By feeding a portion of the NFB back to the input you are reinforceing the input signal component with the feed back circuit, basically achieveing a feed forward effect , not cross differential cancellation.
The chip itself will see any reinforced distortion component as signal applied as forward feed compensation, not differential nulling.
If not can someone show me useing the actual super- symmetry circiut how yours works, because I've got to be thick.

[Russ White]

Originally posted by tiltedhalo
You are missing my point, If you look at the internals for the THS4131 it uses a form of super symmetry useing a voltage controled differential amplifier for the cross cancellation to acheive the signal symmetry and anti-symmetry for the distortion and noise components.

By feeding a portion of the NFB back to the input you are reinforceing the input signal component with the feed back circuit, basically achieveing a feed forward effect , not cross differential cancellation.

A portion?

The power stage is in the TSH4131 loop. The performance of the amp is dominated by THS4131.

Would you say that Nelson's proposed circuit is also not Super Symmetrical?

How is my scheme different from his? The THS4131 is no less a differential device than a differential pair. And I am applying feedback identically.

Cheers!
Russ

[kmj]

Hi Russ, what would it involve to use LM3875 insted of LM3886?
Just changing some resistors or something more drastic?

[Russ White]

Originally posted by kmj
Hi Russ, what would it involve to use LM3875 insted of LM3886?
Just changing some resistors or something more drastic?

It would need a new layout, but it would not be a lot different, and should work fine.

[kmj]

Ah, ok.
Then it will have to way a while
Thanks for the reply!