Does this qualify as X?

[vossie]

Comparing the circuit below with the X-BOSOZ it looks very much like an Pass X configuration. Is this true?

Cheers,
SV

[Nelson Pass]

Yes, and a fine one at that.

[Bricolo]

Nelson, you should write an entiere book about X
I stinn don't understant it

[Nelson Pass]

It's not an intuitive circuit. I took me many years to get
a proper handle on it, assuming that I have.

[Henrik]

Hi vossie

As I see your circiut, which looks nice and simple, you might have some DC at the inputterminals.
The Base-emitter (diffpair) current draw will give you a couple of volts over the feedbackresistors, wich will be seen either at the input or the outputterminals. I am not quite shure if you can get around this problem with BJT´s without capacitors, but I have tried to change the diffpair to some ZVN3310 fets, an then it looks as if the DC problem is gone.
I have only tried this in the simulator.

With your approach, it´s possible to make some verry compact design at lover voltage level.

I have attached the Simetrix files.
Best regards

[Nelson Pass]

I think it's fair to point out that the circuit shown is slightly
simplified, particularly regarding those elements which will
ensure the proper biasing of the DC output.

[vossie]

Thanks Henrik for your input.

I have mailed Nelson privately with a comment and some further details regarding this schematic.

I will wait for his reply before I comment further on the circuit.

Hope to hear from you soon Nelson.
Vossie

[GRollins]

For what it's worth, this is essentially the same circuit that I executed with tubes. I took the feedback from after the DC blocking caps at the outputs.
It's about as simple a practical circuit as you can do as an X. Yes, you can do a single differential as X, but you pay a penalty in gain or output Z.

Grey

[Nelson Pass]

Originally posted by GRollins
It's about as simple a practical circuit as you can do as an X. Yes, you can do a single differential as X, but you pay a penalty in gain or output Z.

To throw in my 2 cents, this is the limit you get from the devices
themselves, essentially their transconductance versus the output
load. If you can get a high enough transconductance figure out
of the diff pair, it goes a long way toward solving the problem.

[GRollins]

Actually, now that I've had supper and rolled it around in my head for a while, the gain question might not be so important anyway. I nearly always use phono as input (can't afford a good CD player and refuse to put up with the sonics of the cheaper ones [anyone got a decent CD player they're throwing out with the trash?]--besides, at this point I'd like to get one that would do SACD and DVD-A), and need a little extra gain. Those who listen to high level sources (i.e. CD) need little gain, if any, and can afford to burn off all but 6-12 dB as feedback if they want. That should be well within the capability of a MOSFET design. With enough feedback, you can bring the Zout down, though higher levels of feedback tend to carry a penalty.
Want more gain and/or feedback? Use bipolars.
Tubes, however, are pretty much going to have to stick with the general topology posted above in order to bring the Zout down. There is, however, one loophole. If you were to go for a massively parallel design, using multiple differentials in parallel with each other, you could bring the Zout down to any arbitrary level that you might wish and get more gain in the bargain. Come to think of it, the same trick would work for solid state devices.
Hmmm...lemme think about this for a spell...I may have talked myself into something, here.

Grey