Simplistic NJFET RIAA

[Salas]

Maybe you got a 47R to parallel the 22R R1 with so to get something like 15R? That will give some decent headroom CCS for when it will fully work and hopefully Vin will remain adequate.

Change a Q5 to see what happens now. Dummy on output, +f +s to one side of the dummy, -f -s to its other side.

[Doug R]

I think we're good now (or at least very close.)

Changed Q5
Vin 44 V, Vout adjustable with pot from 44 down. Set it at 28 V out.
New R1 = 16 ohm, got Vdrop 1.57 V = 100 mA

Put a big 50 ohm put in front of reg, and dialed down Vin to 38 volts
(about 35 ohms through pot.)
Voltage stayed at 28 volts even with drop of Vin from 44 to 38 V

Added another 47 ohm to R1, new R1 = 11.3 ohms. V drop now 1.52 V for about 130 mA. Could probably go back to original 10 ohm resistor now for R1.

Other readings:

Q1 G to S = 3.3 V
Q4 G to S = 2.5 V

Q3, Q6 and Q7 all give B to E = 0.6 V

Q2 G to D = 2.9 V
Q5 G to D = 0.6 V

R4 Vdrop = 44 V (when Vin was still 45 V)
R5 Vdrop = 0.6 2 V (when Vin was still 45 V)

Anything else I should do? Is a target of 150 mA reasonable for two channels?
My choke is rated 200 mA, and I don't really want to go there.

Thank you!
Doug

[Doug R]

This is all with a 500 ohm dummy load. 28 V/500 ohm = 56 mA.

[Salas]

Congratulations!

Go to 150mA is a nice enough value given the choke's spec.

When you had the MOSFETS reversed, much current found its path from rail to ground through Q2,3,5 is likely. All restored and even better balanced now. Thumbs up! A photo with the DMM showing 28V on its output and the CCS Leds glowing is the debug glory and in order I suppose.

[Salas]

P.S. If you got a scope, set it to AC input, 10mV & 50uS per division, probe x1, and check that the rail is a DC line without much noise fuss or riding oscillation waveforms.

[Doug R]

Oops. Spoke too soon. LEDs aren't glowing. I probably got the first one too hot when I kept soldering around R1.
WIll replace them and change R1 back to 10 ohm.
Still holds 28 V though.

[Salas]

Output holds anyway if the first MOSFET passes current but is not regulating it. Check VdropR1/R1=CCS 150mA after that again.

[Doug R]

I did recently get a scope, but haven't bought probes yet, and need ti learn how to use it.
I assume you are concerned about oscillation?

[Doug R]

Vdrop at R1 did not change appreciably. Still about 1.5 V.

[Salas]

Originally Posted by Doug R

I did recently get a scope, but haven't bought probes yet, and need ti learn how to use it.
I assume you are concerned about oscillation?

I am not particularly suspicious with that class of 1.X regs, they are fairly layout indifferent, and been made many times along this phono, but is always good to check when had damages before final good test.