Implementing a Eq/Volume Control on Little Gem II

[eliktronik]

I was thinking about just adding a stompbox eq ckt in front of the little gem. Is this the best way to do it? Also, how would I add a volume control to the amp? Just a pot in series w/ the signal? Am I way off base?

Anyway, the circuit I'm talking about is here (last image)

http://www.runoffgroove.com/littlegem.html

Maybe there's something else that will fit my needs and will sound as good...

[ExpAnonColin]

I just posted a little dingy about active and passive filters on another post asking about them...  In terms of the volume, make sure you put the 3rd terminal of the pot to ground.  Try messing around with resistors in front of the pot so you can get it tweaked just right.  Try http://www.diystompboxes.com/cnews/mods.html for some passive filter circuits, make sure you mess with the values to get it just right.

-Colin

[eliktronik]

Hmm, so should I just put one of those in series w/ the input? Is that the best place for something like that? Thanks.

[eliktronik]

Oh, and what FET should I use? Thanks.

[Peter Snowberg]

You have two options for volume. I'll call the first one "drive" which is a pot in front of the 386. There you would ground the CCW tab of the pot, feed the signal into the CW end of the pot and connect the 386 to the wiper. The second is an "attenuate" control that allows you to keep the hard driven tone, but reduces the sound going to the speaker. For that all you need to do is connect the 386 output (after the DC blocking cap) to the CW end of a 500 ohm pot and connect the wiper to the speaker. If you ground the CCW end, you can use the amp as an overdrive pedal too.

Adding some kind of input buffer is a good idea too because the impedance of a 386 is only 50K which is quite low and passive tone controls will eat signal. Any JFET or MOSFET booster will work great. A BJT booster is also great there. Anything will work really... even an opamp with a high impedance input.

Take a look at "tone stacks". That's the generic term for the tone controls used in most tube amps. Put any booster/buffer in front, and the 386 behind it and you'll have a scfeaming little amp.

-Peter

[petemoore]

Any way somehow boost the signal and then EQ it before the 386?
 While you're inproving the input impedance with a booster you could repplace the volume control of that booster with a tone control instead..
 Of course I've never tried this...but have seen OA's gain up a signal run into passives to control tone.
 A setup like this could be cut and spliced in [there's examples of this around, DK exactly where]....

[eliktronik]

For that all you need to do is connect the 386 output (after the DC blocking cap) to the CW end of a 500 ohm pot and connect the wiper to the speaker. If you ground the CCW end, you can use the amp as an overdrive pedal too.

-Peter

Hey Peter, thanks for the help. I have a question about the attenuating ckt. It seems like you're talking about the first little gem. Would that also work w/ the LG II? The one w/ two 386's and no dc blocking cap, but instead a resistor + cap in parallel w/ the speaker. Would I connect it the same way? Thanks.

[Peter Snowberg]

I have never built a BTL (bridge tied load) 386 amp, but it should work exactly the same. Sorry, I should have looked at your question more closely.

The 386 automatically biases its output to 1/2 of the supply voltage. In a regular appliaction, in order to stop DC from flowing through the speaker coil, a DC blocking cap is required.

If you use two 386s, each should have its output sitting at 1/2 of the supply voltage and in a perfect world, no DC would flow. If you use two 386s from the same batch, the static voltage on the output should be close enough that you can just ignore the tiny current that will flow and forget about the cap. Nothing is perfect, but the degrading effects of the cap are far worse than any stray DC from mismatched chips.

I guess that's a long way of saying, sure it will work great, just put the attenuator in line with one of the speaker leads and don't connect it to ground. Also forget about the comment of using it as a distortion pedal too. (just build another for that )

Take care,
-Peter

[eliktronik]

Got it! Thanks for the help.

Oh, and does anyone have a suggestion for the FET?

[Gary]

Any N channel jfet will do nicely.

[Peter Snowberg]

Use a socket and a pot for bias and then try out....

2N5457
2N5484
J201
MPF102

As Gary says.... any N channel. Experiment around. JFETs vary widely even between parts produced from the same wafer so I would get a bunch of any type you want to try out, use a socket, and select based on how it sounds to you.

-Peter