John Curl amp

[PMA]

and -

(runs hot - Iq = 1.45A)

 

[PMA]

You have bright ideas, John!

 

[john curl]

PMA, I would lower R1 to 50 ohms or less Then R2 will have to be adjusted to keep the ouput current at a reasonable place. The second stage should be operating between 30-50ma.

 

[PMA]

John, thanks for the comment, I will try it. Please look how it is biased now:

 

[roender]

PMA, please increase LTP current to min. 7mA per jfet. Also, do not increase folded cascode "VAS" current more than 2mA.
That way, you will maximize OLG

 

[PMA]

For potential builders, I would like to mention possible stability problems (see attached image).

The amp has wide open loop gain and enough high loopgain. It would be possible to increase R15 to some 200k to get more closed loop gain (with 80k only -8 x) and more stability margin.

The very low distortion is obtained only for high I(q), like mentioned 1.5A. It should be 1A at least.

The amp inverts signal polarity and has quite low input impedance (10k). The input cap should be at least 10uF, polypropylene.

 

[PMA]

That way, you will maximize OLG

Not recommended to maximize OLG (see my previous post)

I came into serious stability problem when I followed John's recommendation (R1 = 50 ohm)

 

[roender]

PMA, decrease R18 and increase LTP current (more folded cascode than normal "VAS")

 

[PMA]

For those who wanted to play with the circuit in MC:

 

[john curl]

PMA, input current set too low for input stage load resistor. Raise current to 6ma, each input device, minimum. Second stage is medium current 30ma-50ma.

 

[john curl]

Driver devices too small. You can't replace a TO-5 device with a TO-92, and get the heat out.

 

[PMA]

Driver devices too small. You can't replace a TO-5 device with a TO-92, and get the heat out.

??
They are in TO-126.

 

[john curl]

BC556 is TO-92 That is what needs upgrading. I originally used a 2A, TO-5 with a heatsink. 30-50ma, not 1 ma.

 

[john curl]

R7 should be 10 ohm, not 100 ohm.

 

[PMA]

R7 should be 10 ohm, not 100 ohm.

I seen it, but thought it was a typo. What is the reason for 10 ohm, to speed up charge offtake from output devices?

 

[john curl]

Linearity?

 

[PMA]

Linearity?

.... is much worse with R7 = 10 ohm.

 

[john curl]

Wrong parts?

 

[john curl]

Pole splitting compensation caps should be added 10-100pf. Add to B-C junctions of VAS transistors. I used 50 MHz Ft device for VAS transistors They apparently had MORE Cbe than replacement devices.

 

[PMA]

Already done a while ago.