Will this schematic work? (preamp/distortion box)

[jaywalker512]

Ok, first off, im no expert in the field of electronics/design/any of that. This whole schematic is basically a couple of different designs combined and some values changed in an attempt to get the result I want. It is based off of these 2 designs:

http://www.till.com/articles/GuitarPreamp/

and the "big daddy" circuit:

http://runoffgroove.com/grace.html

I liked the way the "big daddy" sounded, but I would like it to have a little more chunk/bottom end for metal-ish stuff. Hence why I put the booster in front of it to drive it a little harder (3 band EQ is just kind of an extra feature I thought could be handy). There arent many changes, but first off can anyone who is knowledgable in the field tell me if this will even work and if it will give me anywhere near the results I want? I have not built it yet, because I dont have the parts and dont want to buy them if what I have wont even work. It is intended to be a preamp/distortion box to be run into a larger amp. Any suggestions/pointers are welcome and appreciated. And so, here is my schematic:

http://img386.imageshack.us/img386/6...hematicut4.jpg

[chipaudette]

Couple of comments:

(1) Your tone stack looks funny. It might just be how it's drawn, but something doesn't sit right. It looks like you're going for a tone stack out of a tube amp...but this is a solid state pedal. Can that work? Are these impedances allowed in a pedal? Assuming that the overall tone stack concept works in a pedal, I've got a few concerns. First, your "mid" pot probably wants to be 25K (or 10K) and not 250K. I think at 250K, none of your 3 tone stack knobs will have any effect. Second, I don't know about your numbering on the pots. Is the wiper "2" or "3"? Third (this could be the drawing tricking my eye), I feel that the 250pF treble cap needs to be on the other side of the treble pot. Then, the 100K slope resistor also needs to be on the other side of the pot as well. Check out the traditional Fender tone stack here. Your arrangement might work, it's just never drawn that way...so I'm suspect.

2) The tone stack will have a massive signal loss (especially if you swap your "mid" pot to a 25K or 10K pot). I don't know how to calculate the gain of that first JFET, but I'm worried it won't have enough gain to drive feed all the losses that will occur in the tone stack. You can get more gain (if you need it) by putting cap across the 2.2k resistor on the first J201. Try a 2u2 cap or something.

3) You have no "volume" control stage before the 2nd J201. Like I said, I don't know how to calculate the gain of the first JFET but there's the chance your signal be too low (see previous comment) and there's the chance your signal will be too high. If it is too high you'll want to put in a pot prior to the J201. Then, you can dial down the signal level a little to get it where the circuit sounds best. If you don't put it in, your overly-hot signal might distort something that you don't want to distort.

Good luck!

Chip

[jaywalker512]

Okay, here is my slightly modified schematic.

http://img74.imageshack.us/img74/292...preamp2pl3.jpg

the gain of the first booster is 3db according to the page I got it from. I replaced the tone stack with a simple treble/bass tone control. much easier, and avoids the problems (I think) you mentioned with the original tone stack. Also added in the volume controller after first boost and a switchable gain boost with the cap you mentioned. Also not sure about those cap values for treble/bass (they are simply what I used in my smokey tone/gain amp I built), I know the 47pf lets most bass through, but would going lower allow even more through? I have yet to fully understand how cap values affect guitar frequencies. If you or anyone else knows of any web pages/resources I could look at that would help me understand all this stuff on my own rather than having to ask here, thatd be great So anyway, does the new version look a little more workable?

[loudthud]

In the first schematic, you had the tone stack hooked up backwards, the input and output were reversed. In both schematics there is no DC path to ground for the second J201. You need something like a 1meg resistor to ground. In the second schematic, three capacitors have wrong values. In the US, since we don't always have a way to type Greek characters on a computer, u is used for microFarad, 10E-6 Farad and p is used for picoFarad, 10E-12 Farad. The tone control in the second schematic won't do very much the way you have it wired. I suggest you download the Duncan Tone Stack calculator here:

http://www.duncanamps.com/tsc/index.html

Look at the Big Muff tone control. You can try it as is or tweek on the cap and resistor values until it does what you think it should.

[Mark Hammer]

Consider this tested and verified project: http://hammer.ampage.org/files/PocketRockit.PDF

It does not have the identical type of tonestack, but it runs nicely off a 9v battery (and even better off an AA 8-pack 12vdc supply), provides 3-band EQ, uses the same power amp chip, and comes with a layout AND explanation. With a nice sturdy 12v battery supply, you'd be pleasantly surprised by how much output this sucker has into a reasonably efficient speaker.

The one caveat I will draw to your attention is that the article was published in 1987, and I scanned it some time in the 90's, so it was PDF'd and posted before I was ever aware of the wonders of press-n-peel toner-transfer. As a result, if you have any intention of using the layout to etch a board, note the following:
1) The scan is not 100% to scale, so you may have to resize or enlarge/reduce on a photcopier to nail the spacing for the IC pins.
2) The image is presently s to use with toner-transfer.