Looking at MP-1's JFET input stage.

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Author: Kaz Kylheku
To: ada-mp1
Subject: Looking at MP-1's JFET input stage.
Hi all,

I'm poking at the ADA MP-1 again. It's been in the back of my mind whether
the MP-1's input stage might not be a place for some kind of modification,
such as for more gain.

I started looking at the circuit schematic and ended up reproducing
it in LTSpice to do some simulations. (Some of the part values are hard
to read, and I didn't open up the MP-1 to double-check; for instance
the series resistor in the input, I'm guessing, is 910 ohms.

It's interesting how the input stage combines an N-channel JFET
with a PNP transistor to form something very similar to a Sziklai
(or complementary feedback) pair (with a bypass resistor: R86).


The circuit is a pure buffer: the voltage in is the voltage out. You can't get gain out of it by playing with resistor values.
The circuit uses the complementary transistor pair as
a source follower.

It's not immediately clear that the Q6 BJT is necessary.
However, if you take out Q6, some bad things happen. Firstly,
the circuit becomes more sensitive to JFET choice. With Q6
in place, it works with most of the available JFET models in LTSpice;
but without Q6 that is not the case.

Q6's current delivery improves the ability to pass through
higher voltage swings, and to handle lower load impedances.
(Though admittedly, high voltage swings won't come from a
guitar, even with monster active pickups.)

For instance, here is an X-Y graph of Vout versus Vin with Q6 in place.
The input is swinging +/- 12 volts. The output craps out a little bit
at around 11 volts. But otherwise it is symmetric and centered:


Now here is what happens if we take out Q6.


That's it for now. I will be experimenting (initially, in simulation) with possible mods which add gain.

Cheers ...