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how to combile outouts ? Answered

Hello, i made a LittleGem and a LittleGemMK2 .But i only have one speaker, is there a way to make speaker play 2 signal from 2 input at a time ??
Thanks !
sorry for my bad English.


Quercus austrina

7 years ago

The problem with trying to do what you want is that you have 2 different output topologies - standard single ended (original Little Gem) and bridged (MkII). The first sends the signal to ground while the second sends the signal between 2 opposite signal outputs from the amps. There is no speaker ground on the second one. If you try to combine them as you show, or even as CTS-Chris shows, is that you will ground 1/2 of the MKII amp. This will make the MKII essentially a regular Little Gem where only 1/2 runs the speaker while the other half shorts directly to ground (could ruin that half).

What I would do is use just the MKII to power the speaker. Make a small mixer, this would work with your setup, and make a total of 3 inputs for it. The first two would be for the left and right channels of the MP3 player, You could use a stereo jack and dual-ganged potentiometer for that input. The last one would be for the guitar. Now here comes the fun part. Use the original Little Gem to drive that last mixer input. Put a potentiometer on the gain pins to adjust the gain and then put a pair of 1N914 diodes (head to foot, between the output and ground) at the output of the Little Gem (before the mixer). This will give you a variable gain booster to go from clean (any output below about 0.7 volts) to massive metal distortion.

Now you can listen to the MP3 player and the guitar at the same time if you so desire. Or you can listen to either one alone. Plus, the inputs of both can be set with the mixer pots.

Hope that helps.


PS - Here is a list of substitute transistors if you cannot find the one listed in the link I provided above.

Quercus austrina

Best Answer 7 years ago

Typically, you mix the signals previous to any amplification. This would be done with a mixer. You can search instructables for quite a few or look at some of these sites:

Red Circuits - has a few 9V mixers, though they might be a bit more complicate than you want at this point.

ESP - has some higher end stuff, but  a lot of info is there.

Electro Schematics - type in "audio mixer" in the search box provided near the top of the page and you'll have quite a number of mixer schematics to look at ranging from simple to complex.

Circuit Exchange International - this is a 6 input (3 mic, 3 line level) mixer. Pretty simple if you only need the line level inputs.

Now, you could mix the outputs together as has been suggested, but please be careful. Even though the LM386 is a robust chip, it might be damaged by doing things to it that it wasn't designed for.

If you have a long range plan for all this construction, why not let us in on it so as to give advice / guidance on standard procedures and alternative views. It could save a lot of grief and help you along your path to the audio knowledge you seek.


login721Quercus austrina

Answer 7 years ago

I use 2 Gem, the LittleGem for my mp3 player and MK2 for my guitar.Both amps use a same power source(9v 1A).
i current use a switch to switch between output to speaker.but I want to find a safe way to let the speaker play both outputs from mp3 and guitar at same time.
thanks in advance


7 years ago

There should be no problem in combining the amplified sound of both the little gems. and the only problem will be distortion and "noise " from mixing the signals if you wand you can insert a shorting selector switching device similar to this one
link here
this will automatically play sound from whatever source is active.
I have included 2 schematics for suggested wiring to accomplish your dual feed speaker one including a shorting selector switching device and one using a simple bridging method. ( also depending on the amplitude of your speaker signal gain OUT you may damage a small speaker by bridging ( y-splice) two sources.
good luck and I hope you had fun building those Gems.


7 years ago

generally, we do so by using a mixer.

the most simple form of modern mixer is to use an opamp in summing mode.