FET's? A Tube Screamer?

[kylecavanaugh]

Hi, I'm new here.  Been modding pedals and such for 2 or 3 years.

I was wondering if anyone had tried making the discrete op-amp out of J-FET's or MOSFET's?  If so, how would they compare to the BJT version?

Also, has anyone tried this in their Tube Screamer or similar, I'm curious as to how it affects the tone.

[Northwave]

Hi! I'm kinda new here too  Not a lot of post at the least.

Seeing as the OPamp is (close too) a perfect gain/block, swapping them should be of no major consequence. Of course there will be some difference, whether its clearly audible or not would depend on the OPs swapped and the application. I`d try to swap the stock ones for other types of monolithic OPs first, before venturing into constructing your own. There`s both JFET and CMOS low noise/audio OPs commercially available. TL7x/6x/8x are popular JFET types, but cant think of any CMOS right now.

[johngreene]

Hi, I'm new here.  Been modding pedals and such for 2 or 3 years.

I was wondering if anyone had tried making the discrete op-amp out of J-FET's or MOSFET's?  If so, how would they compare to the BJT version?

Also, has anyone tried this in their Tube Screamer or similar, I'm curious as to how it affects the tone.

Look at a Boss Blues Driver schematic.

--john

[blackcorvo]

Hi, I'm new here.  Been modding pedals and such for 2 or 3 years.

I was wondering if anyone had tried making the discrete op-amp out of J-FET's or MOSFET's?  If so, how would they compare to the BJT version?

Also, has anyone tried this in their Tube Screamer or similar, I'm curious as to how it affects the tone.

i have tried a 2N7000 in place of Q3 (the output transistor) for a simple Dist+ like distortion, and it worked just fine.
All you need is to change the 6k8 resistor with a 25k trimpot and adjust it until you get 1/2 of the V+ in Q3's collector.

[Transmogrifox]

The way to get interesting clipping out of JFETs & MOSFETs is to make the differential amplifier found in the first basic op amp gain block and omit the feedback.  You can drive that with a common source/emitter type transistor amplifier for more gain.  I had a schematic for this on my geocities page, but geocities shut down.  I have not yet posted my stuff back up yet

Either way, Northwave is right.  The discrete op amp using FETs does not make  a drastically different sound.  However.... reduce the feedback and/or internal gain of the op amp and its colors will start to show. 

[Baran Ismen]

I just tried this yesterday and put 2x 2sk30a instead of 2n5088's, as the output seemed a bit hi-gain for me. The result is not bad at all.

I hope I haven't done anything unorthodox