Little Gem MKII

[GuitarLord5000]

Just a few quick questions about this one.  

For a volume control on the MKII, would the 25ohm rheostat from the Little Gem schem still be the choice for this one?  Or would a different pot be the choice for this build?

I will be drawing power from a 9V battery for testing,  but will implement an adapter once all the possible bugs are worked out.  Should I use a regulator for this?  I have some 5V regulators extra for the Rebote2 delay build, but dont think 5V would be enough for the MKII, would it?

And just for curiosity sake, what are the two 10uf caps between pins 1 and 8 of the 386 chips for?

Thanx

[RLBJR65]

Pins 1 and 8 are for gain controll (check out the app. notes) The gain is set internally by a 1.35k resistor across 1 and 8 for a gain of 20, the 10 uF cap bypasses the internal resistor for a gain of 200. You can use a pot like the Ruby has for a variable gain of 20 to 200.

(check App. notes section) http://cache.national.com/ds/LM/LM386.pdf
(bottom pg6. gain controll) http://www.njr.co.jp/pdf/de/de05001.pdf

A regulated PS is a must! 386's are a little noisy anyway and very suseptible to rf noise, so use good shielding practices (short as possible leads, shielded hookup wire etc. a none regulated PS would make it hum like crazy. 5v will not be enough because it starts to break up and distort (not in a good way) below around 7v Use 12v if your IC's are rated for it. It will sounds a heck of a lot better!

Mine really sounds great! I'm using 12v. with an old 8" Jenson P8 speaker it will run a lot bigger cab than that with no problem, some say their ruby's will drive a 4 X 10" cab! It handles most effects very well, but with the amps gain turned up some high gain effects make the bass distort and sound farty. I first thought it was the old speaker but it's not, I tried a friends new 10" and it did the same thing.

I built it using stock configuration except for replacing one 10uf gain cap with a 1k pot for a variable gain, might change that one day and replace the other cap with a pot as well for more controll, I'm thinking that may solve the farty bass problem as well. Since you have not built it yet you may want to consider using the pot's or use a dual ganged pot to controll the gain of both at the same time.

I also put a heat sink on the IC's they run a little hot if youre cranking it up, which BTW is when it sounds the best :twisted: The temp was well within operating range, but for the IC's life span I thought would be wise.

That's my 2 cents worth, hope it helps you!!

[GuitarLord5000]

Actually, the circuit is already built.  I layed it out on perfboard last night.  I'll be testing it out as soon as I put a volume pot on it.

So you're saying that the caps between pins 1 and 8 serve essentially the same purpose as having a pot between them?  I realize that they are there for gain control (I've built the Little Gem, Grace, and Big Daddy already).  However, I usually just put a switch between the pins that connects them directly.  I was just curious as to the function of the caps themselves.  I.E. more or less gain than a direct connection.

As for the power supply, I'll definately look for some 12V regulators.  I have the crappy Rat Shack 386N-1's.  Anything more than 12V and I fear they'll explode.

What kind of pot do you use for a volume control on yours?

[vanhansen]

Put the 25 ohm rheostat between the output cap and speaker.  I did that to my Ruby.  I can crank the gain and input volume all the way and not have my wife tell me to turn it down now.    Funny that she'll say a 1/2-1 watt amp is too loud.

[Peter Snowberg]

Funny that she'll say a 1/2-1 watt amp is too loud.

Remember that sound is measured logarithmically. 100 watts is only FOUR times louder than 1 watt.

[vanhansen]

Funny that she'll say a 1/2-1 watt amp is too loud.

Remember that sound is measured logarithmically. 100 watts is only FOUR times louder than 1 watt.

Very true.  She likes it when I play my acoustic more but sometimes you just gotta rock out with some good heavy overdrive.  

[RLBJR65]

I just use the vol. on my guitar.

Not sure if the 25 ohm rheostat will work in this case. Take a look at the MKII schem again, http://www.runoffgroove.com/littlegem.html the Ic's are bridged but you have 2 outs 1 for + speaker and 1 for - speaker.

I'm no EE but I fear you may destroy 1 or both Ic's maybe even speaker damage if you try to use just 1. You need someone smarter than me to answer that one

[R.G.]

I don't think the 25 ohm works...

[Peter Snowberg]

To use an output attenuator with the MK II you'll need to attach the 25 ohm pot across the two 386 outputs with the speaker connected to one of the 386 outputs and the pot wiper. It should work just fine .

I've used a little different volume control on my 386 amps.... I just put a 500 ohm pot in series with the speaker so the chip sees anywhere from 8 to 508 ohms depending on the setting. This way the attenuator pot doesn't have to dissipate nearly as much power. 508 ohms is really good for bedroom playing with a single 386 and a 12 inch speaker.

[R.G.]

Why not just turn the input signal down???

[Peter Snowberg]

Why not just turn the input signal down???

....overdriven 386 without overdriven neighbors.

[john3696]

can someone draw on the schematic where the volume pot should go??

I can't follow schematics too well, usually just follow a layout, and I can't seem to figure this out.   from the previous text I can figure that if I want to have a gain pot, I take out the 10uf caps between pins 1 and 8 and put in a pot.  but where does the volume go??  I coulldn't follow from the text on that one....

anyhelp would be very appreciated......thanks

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