Jonathan_D
Member
- Joined
- Nov 23, 2015
- Messages
- 11
I have decided to bias my FET using the scope method outlined by zapnspark. This isn't the first schoeps type mic I have built, usually I bias the FET by twiddling the pot up and down as I talk into the mic... not very scientific but it generally sounds fine to my amateur musician non-sound engineer ears. My test circuit is the same as his (2k2, FET drain, FET source to another 2k2 to ground. 1 meg pot connected across source resistor, wiper via a 10k resistor to gate) except I'm using a 9v battery instead of a 12v supply. Also as his oscillator has an electrolytic capacitor coupling the output I haven't used the 10n capacitor to couple the input to the test circuit. I'm using his triangle wave oscillator which gives me a 1.4khz triangle wave with about 6v peak to peak.
I am using a pocket Velleman oscilloscope.
Connecting the input to the gate of the FET and the output to the source of the FET, I have the 1meg pot wound all the way down and I wind it up. Also I try winding it from up to down.
My problem is, I seem to get clipping only on the upper limit of the triangle wave, not ever a the lower limit. So I can't seem to bring about a situation where I have above and below clipping.
If I connect the scope to the drain, I have the opposite situation, clipping only visible on the lower part of the wave.
Am I doing something wrong? Is it a function of this particular type of FET? Does it matter that I'm using 9v in my test circuit rather than 12v? Is it just my cheap oscilloscope doesn't give me good enough visibility to visualise the clipping on the other end of the wave?
Finally how much does this really matter? I am recording acoustic instruments, grand piano, cello, violin. So the SPLs are probably never over 100dB. Also I have noticed others say that using the J305 results in less distortion but I'm not sure if they're biassing it carefully or just using a "one size fits all" fixed resistor bias network or using a listen and twiddle approach like min.
I am using a pocket Velleman oscilloscope.
Connecting the input to the gate of the FET and the output to the source of the FET, I have the 1meg pot wound all the way down and I wind it up. Also I try winding it from up to down.
My problem is, I seem to get clipping only on the upper limit of the triangle wave, not ever a the lower limit. So I can't seem to bring about a situation where I have above and below clipping.
If I connect the scope to the drain, I have the opposite situation, clipping only visible on the lower part of the wave.
Am I doing something wrong? Is it a function of this particular type of FET? Does it matter that I'm using 9v in my test circuit rather than 12v? Is it just my cheap oscilloscope doesn't give me good enough visibility to visualise the clipping on the other end of the wave?
Finally how much does this really matter? I am recording acoustic instruments, grand piano, cello, violin. So the SPLs are probably never over 100dB. Also I have noticed others say that using the J305 results in less distortion but I'm not sure if they're biassing it carefully or just using a "one size fits all" fixed resistor bias network or using a listen and twiddle approach like min.