Tweakers' Asylum

Posted by BFitz (A) on April 29, 2003 at 07:41:45

"If you want to make a FET input 634, you could simply connect a N and P channel JFET as a complimentary follower pair directly in front of the BUF634. Connect a 200-500 ohm pot between and take the audio output from the center wiper (I would use a wirewound pot). Adjust the pot for no DC offset. Another take would be to stick the volume pot inbetween the jfet input and the BUF634 output, and make it a lower value."

FYI I made one of these last night from parts on hand, nothing special or even closed to matched. I used a J176 P channel FET, and a 2N5457 N channel FET, a 500 ohm pot, and +/- 12V supplies. I adjusted the pot for 0VDC offset. The fets were running at about 1ma each.
Measured output impedance was about 300ohms. To find this, I used 100k to gnd on input, and varied the output load to gnd with another pot. The point point on the pot where the level drops to 1/2 of the 200k setting is the circuit output impedance.

Measured distortion and noise at 1.5Vrms out was at the noise floor of my 16 bit FFT analyzer, about -96 dB. I could essentially se no distortion, or noise, nothing at this level. As I turned up the signal level, there was some 2nd harmonic that popped up at at 3vrms out, at -90dB. Pretty good for such a simple circuit with no feedback.
This will work well driving anything from about 10K up.

I would certainly get better parts if I was to make one for use use in my system. I'm sure with better matched parts, it would run better.

Bob