I'm building a Combo Amp for someone.

The cabinet is Pine 13" wide x 15" tall and 11" deep. All glued together and used a 1/2" roundover bit in my router for the edges.

The speaker is a vintage 10" Utah 15W 8ohm. The amp is my dual Im386 chip bridged amp design. Can run on 9volt battery or walwart and output is 1 to 2 watts.

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Sure. Any cap with equal values will work fine. I use small electrolytic polarized versions in 220uF/35v. You could use anything from 100uF for a bright sound, and up to 250uF for more low end sounds.

Or even add a switch between two values.

Voltage rating can be from 16v-50v. *( If using the LM386N-4 chip at the 18v max., the cap should be at least 35v.)

Well the DUH award of the week goes to me. Turns out that my schematic shows grounding the wrong side of the pots and I thought they were turned all the way up, but they were all the way down. DUHHHH

The 2 input jacks being tied together at the volume pot isn't working well. The switch option for either this cap or that one was better.

2 volumes pots aren't needed since both end up being turned all the way up anyway. 1 pot is all that's needed.

Need to draw up a new schematic. One that's correct wouldn't hurt either.

The first version of this amp that I made was perfect, this build has fought me at every turn.

Got everything working. It's got some hum coming through the speaker. Put some caps in the output lines to the speaker(2-100uf and then 2- 220uf), that got rid of the hum for the most part and added some volume. Also distorted the speaker too much. Tried a couple of other speakers to rule out a speaker problem, same results.

I'll do some more trouble shooting and try some other things this afternoon.

That was bad. LOL

I'm going to recheck all the grounds, think that has a lot to do with most if not all the hum.

May also try some smaller caps in the output circuit. Got some 10uf, .047 and .022. just have to find the right ones.

The little things can be a real pain. Checked all the solder joints and they're good.

I had some 3.3uf caps that are working very good. With all 4 controls turned all the way up, the speaker still distorts more than it should. Now I'm beginning to think the speaker cone is half shot. The speaker is rated at 15 t0 20 watts at 8ohms and should be able to handle this 8ohm 1 to 2 watt amp without issues.


Thinking this will do nicely.

WGS also has a 8" for 31 dollars so I may go that route and check it with the 220uf caps that the Ruby amp uses.

I've listened to the Jensen Mod speakers and the WGS speakers sound bites/videos on youtube and various other sources over the past couple years. The WGS speakers sound much better and the price is close to the same.

Thanks for your help looking those up, I appreciate it.

Hey Finder,

Yes, a 2 or 3 watt amp will drive a 20 watt speaker. Even my small 1/2 watt circuits can do that.

The second and third units you linked are 2 channel (stereo) circuits designed to process the signal from MP3, hi-fi, etc. and into dual independent speakers (stereo).

They will not distort like a guitar amp circuit without using an effects pedal. Instead, their tone/sound is clean.

The first unit is not an amp, but a bass/treble/tone adjustment circuit designed to be connected to another amp board.

As to your last question, a volume pot in an amp simply regulates how much of the guitar pickup signal enters the amplifier, or in the case of tube amps, they regulate the signal flow after the first pre-amp stage. Both the volume and tone pot will often have a capacitor to bleed off certain frequencies.

Often, these pre-made boards will have a range of operating voltage recommended. (i.e. 4-12vdc). One can choose to go with the minimum or maximum voltage. But in many cases, a mid-range of power will result in the amp sounding it's best. So, for example, the LM386N-1 IC chip is rated at 4-12vdc. But it sounds best overall at 9vdc, due to how the IC reacts to different voltages and it's designed output.

Thanks for that Scott.

The watt ratting for a speaker is the suggested limit of output wattage. So they will handle anything from 1/2 watt to 20 watts, anymore than 20 will damage the speaker.

No, the PCB board is not under production nor is it available for sale.

The circuit Paul is using is like the Little Gem MK2, which uses the two LM386 IC chips. The schematic is owned by Runoffgroove.com, and is made public only for single use personal projects, not for commercial production. There is a PCB layout available by Pablo de Luca, for those who want to make a professional etched PCB.

For those who wish to pursue building these circuits to make amps for sale, permission can be obtained by contacting the host at runoffgroove.com

I noticed the Little Gem MK2 doesn't use buffer caps in the output circuits either.

When I get the new speaker, I'm going to back through all the cap values again to get the right ones. The 220uf like the Ruby uses was good till the second gain was dialed up, just not sure how much of the bad was cause of the speaker.


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