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.
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.
Quote from: kylecavanaugh on February 07, 2007, 04:25:25 PM
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
Quote from: kylecavanaugh on February 07, 2007, 04:25:25 PM
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.
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.
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 :icon_lol: