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I'm planning to make some mini amps out of old tins and I've been looking at kits based around the LM386 chip. I don't want to buy them from the US or China (I'm in the UK) as I'll have to wait a while and pay import duty. I've come across a UK company which do them very cheaply, £2.60 including a speaker and battery connector.


The problem is they don't have volume or gain controls. Would it be possible to wire in potentiometers to them to do the job, and if so, how?

I've attached their circuit diagram for reference

Any help would be gratefully received!

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A volume control is nothing more than a potentiometer with a knob.  So no problem there.  Typically the volume knob goes between the other circuitry and the speaker.  The thought is that the amp is actually always running at "full volume".  The pot, rather than increasing volume, is actually used to decrease it.

As far as gain... that's a separate circuit, with a secondary amplifier.  That's a little more work.  In a way, a preamp is basically a "gain module".

You can wire in volume for sure, and if you have specs you can surely wire in a gain pot, at the least just a tone pot.  For that rig, you cut the wires between the board and speaker, and wire volume and tone in between.   Cool circuit you found Mark, I might try one

Thanks for the advice guys, much appreciated. The company I'm getting these from sell mainly to schools for electronics courses on the national curriculum. As such their website has lots of resources like circuit diagrams and troubleshooting flowcharts. Very handy for electronics newbies like me! Someone posted plans for adding a pot on the audio input for volume control and I've also found some plans for wiring a pot and capacitor between pins 1 & 8 for gain control, so lots to play with! It's also nice to be able to buy local, especially when the price is so good!

As a note:  a good source for amp circuit boards is often Thrift store cheapie amps (even toys).  Sometimes the amp circuit is far better than the cheap speaker... and if you rip out the circuit board and hook it up to a larger, better speaker within the same ohm and wattage range, you can get some incredible sound for cheap money.

I'm always on the lookout, Wayfinder! This obsession really turns you in to a scavenger, doesn't it?

Oh, scavenger isn't even the word.  We commonly use "dumpster diver".   If you've ever found yourself waist-balanced and feet in the air head-first into a dumpster to grab a prime piece of wood or discarded speaker... you're part of the "club". :D

But yeah, thrift stores and rummage sales are where we live. 

On another forum we have a thread, "You know you're addicted when..."

My favorite thus far:  "You think of Home Depot as a music store."

Hi Mark, the kit you mentioned looks like it's designed for an MP3 player. Which is why the 1 & 8 pins are not shown on the diagram/schematic. The kit amp is meant to sound as clean as possible without distortion or gain. No worries. You can either add a 10uF/25volt (or even 35-50v) electrolytic capacitor between pin 1 & 8 (+ lead to 1, negative to 8), or you can connect a 1k-B potentiometer for a variable gain at that point. Look at the Ruby amp schematics on runoffgroove.com for details/diagrams.
For volume, remove R3 (10 ohm) and replace with a 10k-b potentiometer. Guitar signal from jack + to pot lug 3, and connect pot lug 2 to pin 3 on the LM386. Don't forget to connect lug 1 of the pot to the back of pot, along with your jack, battery, and other ground points.

Geeminy Scott.   I envy you that level of knowledge.   I keep trying to learn electronics... but then something else gets in the way.   I am impressed.

Thanks Wayfinder. Lots of info on the Internet about the little LM386 amps. Amazing sounds from such a simple circuit. I've built and sold many of them, including two I sent off this morning. Decided to step things up a bit, and am currently building tube amplifiers. One from an old Hammond organ reverb driver shown here...

She puts out around 17 watts.

Right now I am working on three other tube amps...a Fender 5F1 Champ clone, a Fender Princeton Tweed 5F2-a, and a Kalamazoo Model 1.

DANG Scott!!!!!

Yeah, crazy to think I started with a stick in a box, and wound up making these. Thanks Ted.

Ted Crocker said:

DANG Scott!!!!!

Thanks for the great advice, Scott. My kits arrived in the week so I've been having a bit of a play with them. I've made one up as per their instructions, without volume or gain controls just to test it and I've mocked the circuit up on a breadboard to tinker with it. My only issue with it is that the clean output is pretty weedy. I know it's not going to have a lot of volume due to the size of the speaker but it's not much use as an amp if my CBG is acoustically louder than it! As soon as I connect the gain, either with a pot or capacitor, I get a decent amount of volume with a nice bit of crunch to it. But volume and gain controls would be a bit redundant if you've got to have it up full to get anything out of it.

So I'm trying to find out if the clean volume is the best it can be or if I can improve on it. As I see it there are three things which might be the cause:-
The voltage of the power supply. I'm currently using a 9 volt battery
The design of the circuit itself
The fact I was testing it with a piezo pickup in my CBG. I can test it with a coil pickup just as soon as I finish building the next guitar!

Any thoughts?

Mightily impressed with your monster amp! Did you not consider a huge great Frankenstein style throw switch though, just to give it a sense of occasion when you turn it on?


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