mixer using jfets j201

[numpty]

I downloaded a simple mixer design using j201 jfets on vero board which i had planned to use with my effects. Can jfets such as these be considered studio quality in sound equipment terms or are they noisy ,distort easily or low fi in this sort of application and would op amps be a better choice in this case ? Some people prefer tubes in mixers to op amps and jfets are supposed to exhibit similar characteristics to tubes

[zachomega]

I'd think it really depends on the design, but the J201 doesn't have much of a swing on the input side of the transistor and the curves for it aren't particularly linear in my opinion.  Looks more like a tube triode set of curves (12ax7ish?). 

However, depending on what you are feeding the input with and how much gain you are looking to achieve, it will probably sound just fine.  What is hifi isn't always what we think sounds best. 

If you are concerned about input headroom, you can change the Jfet to another type.  I'm sure somebody on the forum will have a much better idea of suitable types.  But as I said, it really depends on what you plan to feed it with.

-Zach Omega

[numpty]

thanks
    I intend to feed it with the stereo output from my yamaha gep50 which i use for reverb for my FX pedals, tape recorder, CD player and computer sound card.

[R.G.]

Can jfets such as these be considered studio quality in sound equipment terms

No. A bare JFET which is not part of a larger feedback circuit is not "studio quality" as the term is used today.

are they noisy ,

Not particularly. They are the quietest solution for high source impedances. But anything can be made noisy by bad design.

distort easily or

Yes. By itelf, a JFET without other devices and feedback will produce about 1% -2% distortion before clipping. They clip easily when you exceed a signal voltage which makes the gate more positive than the source, and may clip hard at other places if they are misbiased. It is particularly  easy to misbias JFETs because the individual devices vary so much. The only good way to get them biased properly is to put in a bias trimmer for each JFET. That gets them to be as well biased as they can be, but since the gains vary a lot too, several sections in parallel - as in a mixer - will all have different gains unless you use feedback to linearize them. That throws away gain, and JFETs are not particularly high gain to start with.

low fi in this sort of application and

Yes, as I understand your question.

would op amps be a better choice in this case ?

Yes.

Some people prefer tubes in mixers to op amps and jfets are supposed to exhibit similar characteristics to tubes

This is a bit of misdirected optimism that simply won't die. Tubes and JFETs are fundamentally different. They share some similarities, and are more alike than tubes and bipolars, but a JFET is not a tube. The people who prefer tubes generally don't prefer JFETs.

Taking a step up to the bigger question of what to use to make a mixer, a mixer really should have a way to have one input not affect another input. In the classical opamp circuit, input pots allow signal voltage to be turned into a signal current which is summed into the virtual ground of an opamp (or other feedback amplifier) at its inverting input. The virtual ground characteristic "eats" all of the signal currents placed there, so the signal currents cannot interact. A virtual ground is an almost ideal mixing node because of its very low input impedance.

JFETs are inherently high input impedance devices. Inputs mix as voltages, so it's hard to get them to not crossbleed and affect one another.

If you're speaking the words "studio quality mixer" then you have some significant studying to do to get to the place where you can build mixer sections. I recommend Craig Anderton's excellent "Home Recording for Musicians" for some understanding of studio mixers and the kind of design that goes into them.

[numpty]

Thanks RG -what would you recommend as a resonable, may be not quite studio quality schematic to follow?

[numpty]

http://www.tubecad.com/april_may2001/page22.html

what about these ?