Bi-Polar Madness!

[GrindCustoms]

Hi guys, lately in my «science» activities i've been playing a lot with bi-polar supply using various device to do so, charge pumps, DC-DC converters and the likes.

I've pretty much understand how to benefit at max of that powering option with OpAmp OD, LFO and other stuff that is based around IC.... seemed like the easiest way to get into at first.

Maybe i'm simply not using the right keywords in my researches but i can't find any information that i can use toward the application of bi-polar psu with JFET, BJT, Darlington and other semi-conductors.

Any guidelines about that matter would be greatly appreciated.

Thanks

Rej

[midwayfair]

Use a resistor of similar value on the emitter or source as you would on the collector or source. The source/emitter output will still be a buffer, of course, but with higher headroom (in fact, I think a bipolar supply will net you more headroom on a JFET buffer than a higher + supply, but don't quote me on that). You may need to adjust it. The bypass capacitor still works.

Gus on DIYSB designed a JFET booster with +9V/-9V supply rails.

I almost did +18V/-9V on the Snow Day overdrive, but it would have involved more design time and less common JFETs.

Oh, and it's not too many extra parts to get +18V/-18V if you're already doing the +18V. That's fun on op amps because you can omit input and output caps!

[GrindCustoms]

Wow! Thanks Jon! That single thread answers lots of my questions.

I really like the preamp/tone shaping ability of the Bax30 tube pedal and thought i could put something similar together using JFETs. Increased headroom for that type of circuit is a must.

So far i'm pretty much fonded on the +15/-15V DC-DC converter. In what i've tried so far, i can accept the compromise of not going up to Bi-18V... i could'nt really hear a difference either between the 2.

Thanks!

Rej

[midwayfair]

I really like the preamp/tone shaping ability of the Bax30 tube pedal and thought i could put something similar together using JFETs. Increased headroom for that type of circuit is a must.

Keep in mind that cascaded FETs have a headroom limit independent of the supply voltages: The |Vp| is the maximum voltage swing at the gate before the FET distorts. It's inversely proportional to the FET's gain. Therefore, increasing the supply rails with multiple FETs will produce MORE distortion, not less. However, in  a roundabout way, it CAN increase your dynamic range; you can use lower gain FETs that would normally be below unity volume (which will have a higher |Vp|), but get more gain out of them to maintain unity volume or higher.

But if you need maximum headroom, it's op amps every time. A high-gain BJT is probably the next best thing, though a case could be made for MOSFETs instead. (The BJT is usually going to be a little brighter, so you are more likely to hear distortion when it happens, but the MOSFET can probably provide more clean gain. It really depends on the circuit.)

The best use of FETs is to sound FET-y.

[GrindCustoms]

What i'm looking for is not to replicate the Bax30 toneprint but more of his functions.

I basically need a strong clean boost that will then drive a 2 band Eq. Reason why i was thinking of using JFET at the input stage is mainly for their «musical» sound flavor wich to my ear sound less sterile than their opamp counter part (note that this is personnal perception of sound). That would also act as the Level/Boost control.

Having the Eq section sandwiched between opamps buffer to assure less tone/gain loss would'nt be a bad idea either.

Attached is the MPF102 boost/Pre that i favor.