Tonestack help, please.

[javacody]

Here is the schem (you might recognize it as a modified little gem and JFET preamp)

bufferedgem tonestack v1.2

I wired it up and the only thing that effects the tone is the treble pot.  I think I made a mistake in the schem. Any ideas?

[Chill]

Try a 33K-56K for the slope resistor.  The 100K shown will cut the low end a fair amount.  The duncanamps.com tone stack calculator (free!) will show you what the different parts values do in that tone stack and the muff tone stack.

[javacody]

Yeah, maybe I should replace that with a 100k trimmer?

But still, the bass and mid pots literally do nothing to the sound at all.

[brett]

This may be completely spurious, but shouldn't there be a cap (10 or 22uF) between pins 1 and 8 on the 386?

good luck!

[javacody]

I think its not needed when you use a pot. Using the pot allows you to bypass the lm386's built in resistor between pins 1 and 8 that set its gain to 20. With the pot, you should be able to adjust the gain from 20 to 200, correct? At least, from putting this together, that's how it seems to work.

Also, this is just a modified version of the Little Gem found on Runoff Groove. What a great design.

[javacody]

Update:

I tried shorting 1 on the mid pot to ground and then the mid pot started to work. I had a bad solder joint to ground. Its fixed now. Now if I can just figure out why the bass pot isn't working. This tone stack seems to have cut the volume in half. I'm really stumped.

Please help!

[Ansil]

tone controls eat mad power man i am telling you.. plus  a 5k pot on the 386 is redundant.. you dont' need more than 2k and you wont' get all of that. out of it..   pins 1 and 8 have a resistor in between them in the chip.. it is only 1.35k  to set the gain at 20  anything less more than 1.35k will not do jack crap

[javacody]

That 5K pot is taken directly from the design on Runoff Groove. Is their schem wrong?

[Ansil]

i am jsut preaching here..  why pt a 5k inthere when over half of the rotation won't do anything..


anyone wnat to throw their .02 in here.

http://cache.national.com/ds/LM/LM386.pdf

check out the data sheet on a 386

[Bernt]

The 386 has an input impedance of 50kOhms. It is probably putting a heavy load on the tonestack. Try a buffer after the tonestack.
Regards, Bernt.

[javacody]

So a buffer before and after the tone stack? Ok ,I'll give that a shot.

Also, I'm going to set that slope resistor to 33k and see if that will make the bass pot more responsive.

[javacody]

Ok, set the slope resistor to 33k. Huge improvement! I got my bass back. Unfortunately, it still sounds like the bass pot is doing nothing at all.

[javacody]

Ha! I accidentally used a 1k pot instead of a 1M pot! LOL No wonder I had no bass! I have a 500K pot that I'm going to use for now. Sheesh. Note to self, triple check your components from now on!

[brett]

Don't fret.  That kinda stuff happens to me so much... :oops:

[javacody]

OK, now my tone stack is working and not sucking so much juice. Very nice. I recommend the little schematic I put together (with much, much help from Peter Snowberg) if you want to buffer and add a tone control to the little gem.

[Ansil]

at least you weren't FUBAR.

things could have been worse

[javacody]

Yeah Ansil,  when I first got started in this "hobby", I tried to perf a TS. Ha ha. What a mess. I couldn't even solder half way decently. I got it all soldered up and it was a royal mess. The very definition of FUBAR.

[Peter Snowberg]

Cool!

I'm glad you got it going. I would check out Bernt's suggestion of adding another buffer after the stack. That 50K is a pretty low impedance to drive and as you now know.... that stack EATS signal.

I got a bit of a chuckle when I saw you post about the 1K <-> 1M mixup. I can't tell you how many times that has happened to me with a component. :oops:

I'm guessing you went for the MPF102 because of RS availability? I bought a couple there but they're still in the bubble packs. I like the Fairchild 2N5457. If you come across them, give them a try. There are lower noise JFETs, but that one seems to have a nice sound to it.

Congrats on your success!!!

Take care,
-Peter

[Peter Snowberg]

one more thing.....

Now that you have a stack in there, if you can, try playing with both linear and log pots in the Treb and Bass controls. I think Fender uses logs for both and Marshall uses a log on the bass.

-Peter

[javacody]

Peter, I don't mean to be a total gimp here, but  I have a question about the buffer I'm going to place after the tone stack. Do I still need the large value resistor on the input of the new buffer? Should I just duplicate the existing buffer?