2 LM386 amplifiers to 1 speaker

[Jack.Straw]

Hello. I'm wouldlike to take two of **broken link removed** and mix the outputs to one speaker. One of them would amplify a piezo mic'd guitar. The other would amplify a media player via a 1/8" phone jack. Would i need to make a buffer/preamp for the piezo? Would i use diodes to prevent output form one amp from feeding back into the other amp?

Thanks for any advice you can offer,
-Scott

 

[audioguru]

Audio signals are mixed in an audio mixer that feeds a single power amplifier. A very simple audio mixer is two resistors.
A piezo pickup needs a preamp with enough gain and a very high input impedance.
Diodes are never used with audio signals because they pass only one polarity and the missing polarity would cause severe distortion.

A speaker has two wires. You could feed one wire with a signal from one power amplifier and feed the other wire with a different signal from another power amplifier. The signals do not feed from one amplifier back into the other amplifier because the speaker is in between.

 

[Jack.Straw]

AG, thanks for the response. I'm confused by it though. When you say that a speaker has two wires, i assume you mean the positive and negative terminals. You're saying that I could wire the positive output wire from one amplifier to the positive speaker terminal, and the positive output wire from the other amplifier to the negative speaker terminal? Would that really work? I assume i would attach the negative output of both amplifiers together?

Thanks again,
-Scott

 

[audioguru]

When you say that a speaker has two wires, i assume you mean the positive and negative terminals. You're saying that I could wire the positive output wire from one amplifier to the positive speaker terminal, and the positive output wire from the other amplifier to the negative speaker terminal? Would that really work?

Yes.

I assume i would attach the negative output of both amplifiers together?

Most amplifiers have one output signal wire and one output ground wire. Of course all the grounds must be connected together.
Some bridged amplifiers (especially in car radios) have a negative output signal wire that is alive with a powerful signal. Those negative output signal wires must never be connected together.

 

[Jack.Straw]

Thanks for the clarification... is this be correct?

**broken link removed**

Thanks for your time,
-Scott

 

[audioguru]

Instead of using two power amplifiers, I would use two simple resistors as an audio mixer that feed a single power amplifier.

 

[Jack.Straw]

I tried this already:
**broken link removed**

Do I just need a couple resistors after between pin 3 on both the pots and the amp input?

 

[audioguru]

If the volume control for the guitar is turned down then the tiny 47nf capacitor is driving a low resistance which cuts low frequencies.

 

[ben7]

I tried this already:
**broken link removed**

Do I just need a couple resistors after between pin 3 on both the pots and the amp input?

How loud is the amplifier?

I find that if you drive a class B transistor output stage (comprised of 2N3904/2N2222 transistors) with an lm386, you can get loud, clear, sound!
I don't usually use this chip though cause when you increase the gain to 200, which most lm386 circuits are wired for, you get a terrible amount of distortion, and mostly high pitched,low quality sound.

 

[audioguru]

How loud is the amplifier?

It is as loud as a cheap clock radio. The datasheet for the LM386 shows that its output power into an 8 ohm speaker when it has a 9V supply is 0.7W when its input level is way too high so that the output is clipping its head off with a horrible-sounding 10% distortion. The output is only 0.45W when the output is barely clipping and the distortion is low.

I find that if you drive a class B transistor output stage (comprised of 2N3904/2N2222 transistors) with an lm386, you can get loud, clear, sound!

No.
A class-B output stage produces crossover distortion and reduces the 0.45W output to only 0.25W. The LM386 and every other power amplifier IC have a class-AB output for clear sound.

I don't usually use this chip though cause when you increase the gain to 200, which most lm386 circuits are wired for, you get a terrible amount of distortion, and mostly high pitched,low quality sound.

Most LM386 circuits use a gain of 20, not 200. The amount of gain does not affect distortion much and has nothing to do with reducing low frequencies output.
Maybe you have a line level input and the gain of 200 is too much and is causing clipping distortion.
Maybe you have the gain at 200 because you use a microphone at the input and the mic can hear the speaker which causes acoustical feedback and the high pitch is the sound going around and around. Turn down the volume or move the mic away from the speaker.

 

[Jack.Straw]

If the volume control for the guitar is turned down then the tiny 47nf capacitor is driving a low resistance which cuts low frequencies.

Guru, could make a recommendation on how to improve the above circuit?

 

[Dragon Tamer]

I would recommend using the circuit that I have attached. It has a gain of 200 but since both of the inputs are connected to pots then you can vary the input voltage so that there is no clipping on the signals. You don't have to have a preamp to the mic, but you can. I made the schematic so that it would not need a preamp. Even with this circuit, your guitar may be louder than the actual amp. You could connect it to a larger amp.

 

[Nigel Goodwin]

I would recommend using the circuit that I have attached. It has a gain of 200 but since both of the inputs are connected to pots then you can vary the input voltage so that there is no clipping on the signals. You don't have to have a preamp to the mic, but you can. I made the schematic so that it would not need a preamp. Even with this circuit, your guitar may be louder than the actual amp. You could connect it to a larger amp.

Sorry, but that's even worse than his was

You can't just connect the sliders of the pots together like that.

 

[Dragon Tamer]

It worked just fine when I did it. But then again my 386's are in very bad shape. Either way, a DC blocking capacitor, or 2 10k resistors on the wiper of each pot should resolve the issue.

 

[Jack.Straw]

So you're saying it should look like this?

**broken link removed**


Thanks again for all the help!!
-Scott

 

[Nigel Goodwin]

So you're saying it should look like this?

**broken link removed**

That's closer, but still isn't a true virtual earth mixer, but at least it will work, where as without those resistors it wouldn't (or at least not acceptably).

 

[audioguru]

The value of C5 is much too high. Use a 220nf (0.22uF) film capacitor.

 

[Jack.Straw]

Ok, revised again. Does everything else look OK before i build this thing? Should the pots really be 100k?

**broken link removed**

Thanks again for all your help!!
-Scott

 

[audioguru]

A film capacitor (for C5) is not polarized.
If a 10k volume control is used then it will load down the electret mic which reduces its output level, and the coupling cap C5 must be 2.2uf.

 

[Jack.Straw]

Thanks Guru... the "mic" in the diagram will actually be a piezo. Does that change things?