Discrete Op Amp with the FET/BJT differential stage

[DDD]

Here's simple discrete Op Amp schematic for guitar electronic gadgets below.
The original topic with much more info and waveforms (in Russian):   http://forum.gtlab.net/cgi-bin/yabb2/YaBB.pl?num=1424862539

The Op Amp itself consists of 3 transistors (J1, Q1, Q2) and 4 resistors (R2, R3, R6 and R7). Other stuff is biasing, clipping and negative feedback components.
The enclosed schematic diagram shows something like Tube Screamer clipping stage using the subject.
The circuit is quite flexible and can be easily adjusted for specific needs if any. For example, just reduce R2, R3, R6 and R7 proportionally if more "output power capacity" is needed, etc.
Quiescent current is 150 uA.

[DDD]

Some more transient plots with brief description:

[bool]

Are these sim waveforms done with the 36K "load"?

If so, I suspect that the ugly negative side flat-top clipping (unless that's a "feature") could be greatly improved with either reducing the both 22K resistors ... or increasing the 36K "load".

As-is, is it "close enough" output impedance to be able to drive a classic FMV stack with little-to-no change in tonal response (assuming the original, "valve" values)?

Otherwise, it looks like a very "clever circuit". Technically, it's a Bi-Fet, haha.

[DDD]

Also, please find below Input Filter schematics and gain control curves.
Filter has 950 Hz controllable hump that is usual for heavy-sounding stompboxes.

[DDD]

1) "Are these sim waveforms done with the 36K "load"? - Yes, just 36 kOhm resistive load, that is close enough to the "input impedance" of the classic tone-stack with 36 kOhm resistor.
2) "... flat-top clipping (unless that's a "feature") could be greatly improved with either reducing the both 22K resistors..." - Yes, you're right. Reducing both 22k resistors makes more symmetric clipping at the given input voltage. Also, in this case you may reduce both 120k resistors to give more base current to Q2.
At the same time heavily oderdriven input stages usually produce asymmetric waves, so it may be a "feature", especially if you like even harmonics at the attack portion of a string sound.

[DDD]

The Op Amp can also process NEGATIVE input voltage in spite of single +9V battery used for its supply.
It's quite surprising feature by the by.
Please see the test circuit and DC transient plot enclosed below.

[bool]

Yeah that's a feature of the JFET's Vgs ... I can't speak for this one opamp, but I did stuff with bi-fet CFP's (actually with a "picked" BF245C and a BC560 so ... not really that far off, depending on how you look at it, of course). But I used higher currents throughout, a little over 1,5mA for the whole thing.