JFET buffer and more

[PRR]

> Do you think then that ... would be better ..., than

It's what? $5-$10 of parts?

And neither is "wrong".

Build both.

Listen.

Report back.

[Gus]

I have been posting sims of different gain stages.  I am doing this to show there are different ways to build a buffer or gain stage etc.  I posted this follower because I have not seen it on this forum before.  If one searches for a Schoeps microphone schematics one might find this follower design in one of the schematics on the web.

I am trying to show there are different buffer circuits one can build.  With a well designed buffer for the application you should not hear them, unless you want to by designing in some distortion or EQ.

Here is an older NS app note with interesting circuits
http://www.national.com/an/AN/AN-32.pdf

There is also bootstrapping an JFET input opamp buffer or gain stage one can find in books.  Maybe I will post a sim.

[JRay]

I knocked up both buffer configs on a breadboard for a listening test. The first was a classic Source follower using a single J201 FET and a 10K source resistor, 9V battery for supply and 47nF input cap and 33K gate resistor as Paul. R suggested, 1uF output cap and 100K output load resistor. Input was my Martin acoustic with LR Baggs M1 pickup . The output went to my mixer input with about 30dB of gain. I listened using a good pair of headphones which will not give any idea what it will sound like through sound reinforcement gear, but good for direct comparison.

The classic source follower circuit above sounded good but the bass was a little indistinct. I reduced the source resistor to 4.7K and this improved the bass slightly but not by a huge amount. The sound was good but lacked sparkle and detail, which I like to hear with acoustic guitar preamps etc. It had a tendency to sound a bit harsh when played loud but overall the sound was good

I replaced the source resister with a further J201 with Gate and Source commoned. The sound was very much improved. The detail was vastly better and the bass became punchier . The sound improvement was quite marked and the sound has far more definition and sparkle.  The sound did not change when played hard. I then swapped the JFET pair with two J201 with matched Idss (612uA and 615uA measured). I could not detect any difference in the sound with the matched versions, so I doubt this is essential. The 2 FET config started to clip at around 2.5 - 3V rms input and so plenty of headroom for my use

Only a quick and dirty test, but I would say that for electric guitar buffers, the standard resistor based source follower circuit will be fine, but for acoustic guitar amplification or hifi use, the 2 JFET config is definitely worth the extra FET as the sound quality is far superior!