MOSFET current source as triode load

[Craig405]

Yes adding the caps helped it, it seems to settle down faster now but still swings about.
the reason there is so much feedback is i increased it to lower the gain for a CD player.
It used to be a 78k resistor in the loop but i lowered it to 27K.

(sorry the image is small i couldn't get rid of all the white space around it so i had to shrink it)

Thanks
Craig

 

[SY]

There are two reasons for all the feedback- the low closed loop gain and the high open loop gain. This is going to be a challenge to stabilize; 60dB is a LOT of feedback.

I think you should consider drastically reducing the open loop gain and a double ccs is making things worse. Save the ccs for another circuit and take some satisfaction in having another tool in your intellectual armory.

 

[Craig405]

Ok, thanks very much for all the help you have given it has been very useful.
I will look into reducing open loop gain of the circuit before i revert it back to its original state.
My main reason for attempting to add CCS's was to improve my poor PSRR, and i still intend to improve it but i have some research to do first .
Thanks again.

Regards
Craig

 

[diy_audio_fo]

Here's a sketch for a 1.25mA ccs suitable for your preamp. If you want to drop it to 1mA, change R1 to 1K. The reference string is set to pass 5mA through the LED voltage reference, which lowers its noise and impedance. You might want to bypass the LED with a cap- 470uF at 16V will be more than adequate.

The reference string can feed more than one ccs- if you're doing two, for example, all you need is a second pair of transistors and another R1. R2 should have a 2W rating; higher is better and wirewound will work fine. R2's value is not critical- a 47K will be just fine, as will a 51K.

Depending on your power supply, you might have to diddle the value of any series resistors to account for the extra 5mA of current burned up in the reference string. On the bright side, the LED gives a nice visual confirmation that the power supply is working. And if your supply drops under the additional 5mA, that will still be OK as long as the supply is 15 volts or more higher than the tube's plate voltage.

The transistor closest to the B+ rail does not need a high voltage rating- get a small signal PNP with a high hfe and ft. It's not critical. The transistor connected to the 12AX7 plate needs to have at least a 300V rating. If its power dissipation is about 1W or higher, it will be fine without a heatsink. As before, the particular type is not critical- use what's easy to find.

I was wandering: what about the output signal distortion if I replace the Red Led with a resistor of appropriate value and inject the input signal (through a cap) into the base of transistor closest to the B+ (instead of the 12AX7 grid)?

 

[DougL]

What do you hope to accomplish driving a 12AX7 "active load"?
I am sure it would do something, it just hurts my head.
I design things by using building blocks.
That's not a block I own.

Doug

 

[CBS240]

Hi

I don't normally post much in the tube forum but that doesn't mean I don't have interest, just lack of funds and actual experience with them. But, wouldn't it be possible to use a self bias J-fet, then cascode with a P-ch enhancment mode Mosfet to create the CCS? I would think the noise would be quite low. There are P-ch E Mosfets out there of at least 500V, TO-220.

 

[Gordy]

...inject the input signal (through a cap) into the base of transistor...

Input signal into the CCS? Maybe you want to use it as a mu-follower? If so then consider these links...
http://www.diyaudio.com/forums/blogs/geek/104-underappreciated-hybrid-mu-follower.html
and
http://www.diyaudio.com/forums/blogs/geek/166-soft-start-hybrid-mu-follower.html

Regards,
G.