Critique my onboard project

As the title suggests, I'm wondering if a few interested folk could have a look at the preamp I've 'designed' (flogged from various sources). The idea is a differential buffer for each pup into a JFET gain stage, volume for each pup, no tone controls.

Currently the JFET stage is the same as Kreuzer's, so I want to modify it a little, to operate at 18V being the main motivation (partly to be different, partly because I'd prefer to run the opamps at 18V).

Its about 1:30am at the moment, so I'm going to bed. Tomorrow I'm going to have another look at my EE textbook and see if I can figure out the JFET stage, then I'll SPICE it up and see what happens.

Any comments?

Josh D

 

Hmm, might attach the schematic...

Sorry about the big watermark in the corner but like I said, its the same as Kreuzer's . Does anybody have a reasonable freeware schematic drawing program?

 

I haven't drawn in the supply for the opamps yet, it was getting a little crowded and I'm not that familiar with the program to make it neat.

The source resistors you mentioned are for the opamps, right? Everything I've seen has the inputs symmetrical for a differential amp...Unless of course your talking about the JFET, in which case one of the source resistors does have a cap in series with it.

I've removed the coupling caps. And coupled the JFET output, I forgot that one.

Fixed the volume pots.

The covered up part is a 2k2 resistor in parallel with a 1k2 resistor and 100uF in series (1k2 and 100uF are in series). But like I said, I'm going to change the JFET stage, currently its mostly a filler to demonstrate the idea. I'm opting for the JFET mostly because I've never used one before. Its becoming evident that its a tad more complicated than I thought it would be.

For the opamp it'll probly be the TL072.

Josh D

 

I know little but many semesters of sleeping through lectures has forced me to learn quickly and guess well.

When I hear 'source' my default meaning for source is the source of the sound. Which was why my first guess was towards the opamp. However, I've been reading a fair bit on electronics lately, so I also know where the source is on a JFET.

It turns out that it doesn't seem I can easily get ahold of the BF245 (they're not in the catalogue), so I'm going to use an MPF102. Of the options I have, it appears to be the most similar.

I've got an EE textbook, which I've been trying to use to figure out drain and source resistances but its not working particularly well. Using this site and using Vcc = 18 and Ids = 8mA, I calculated 560 for the source and 1.8k for the drain, which seem a little wrong...

Thats it for now.

Josh D

 

Thats something I'd also gathered from Kreuzer' and Till's websites.

I was also thinking a pot on that last resistor, probly a little trimpot though.

What Tim said. There are a number of methods for achieving gain at that stage, the problem is they were a bit too involved for what I want out of this (ie different but simple). I'm taking a shining to the overdrive idea though, I'd be thinking more along the lines of another JFET stage though. Problem is, won't I be starting to draw a bit of current if I add that?

To actually wiring it up, I was thinking of a modular approach. Three boards, two identical ones with the differential stages (including their respective volume controls) and the JFET on the third. It would make easier to contain physically, and would mean that if I wanted to add the extra stage for overdrive, its just an extra module, same if I wanted to add piezo's or something. It would also make a JFET bypass switch a snap to wire.

Josh D