# Amplifier for 4 piezo mics to 4 mono speakers

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#### (((o)))

##### New Member
Hello! I am an absolute beginner learning electronics by working my way through the Make:Electronics book by Charles Platt. I have a long way to go, so I am hoping someone can give me some guidance on a project I am working on now.

I would like to have 4 small piezo microphones independently amplified to 4 small mono speakers with volume control, mute, global volume, and a switching jack. Each microphone will be picking up a different sound, and I imagine the speakers will be 2-3" speakers (I've been told 8 ohm full range speakers might be a good choice). I have attached a block diagram of sorts, which I hope is clear. Most of the amplifier designs I have looked at online and in Forest Mims' book assume a stereo pair (or two), which is not what I want.

I could build 4 discrete amplifier units, but as you can see in the diagram, I would like to add a global volume and a switching jack. It seems like it would be more efficient to use a single amplifier, but I am getting stuck on maintaining 4 independent signals, except for the global volume and switching jack (which I suppose would require a summing mixer before the signal is sent to the jack?).

In a previous project I soldered a small impedance-matching preamp for a piezo disc mic that I plugged into a guitar amp. I am hoping the amp design I use in this project will match impedance without the use of a separate preamp.

Finally, the piezos will be primarily picking up percussive-type noises, so this does not need to be audiophile quality by any means, but good sound is important.

My specific questions are:

1. What type of amplifier design would work? Can I use a single amplifier? Any guidance on sources for schematics I could build or modify would be great.
2. Do I need a preamp?
3. How and where in the circuit do I add a global volume control and switching jack?
4. How do I add a mute switch?
5. Are 8 ohm speakers the way to go?

#### AnalogKid

##### Well-Known Member
You need to work through the consequences of your requirements. For example, the switching jack outputs "all 4" signals. Mixing the four signals to a single output is not difficult, but it is another layer of circuit complexity.

You don't say what the signal level is out of the microphones, or what power level you want to drive the speakers. Without those numbers, there really cannot be any circuit recommendations.

For small speakers and table radio loudness, there are many amplifier modules on ebay based on the LM386 audio power amp chip or some Texas Instruments class D audio amp chips. Since you want independent control of all 4 signals, you will need 4 mono amps instead of two stereo amps.

The global volume control can be tricky. The least complicated way is a 4-gang pot, but those are not common, especially in low quantities. An alternative is electronic pots using specialty chips from Maxim.

By tradition, signals on a schematic flow from inputs on the left to outputs on the right.

ak

#### audioguru

##### Well-Known Member
I would like to have 4 small piezo microphones. I imagine the speakers will be 2-3" speakers.
.... good sound is important.
A piezo disc is used on an e-drum to trigger an electronic sound, not as a microphone.
A 2-3" speaker is a squeaker and is not full range because low frequencies will not be produced.

#### (((o)))

##### New Member
Mixing the four signals to a single output is not difficult, but it is another layer of circuit complexity.

Hello AnalogKid, thanks so much for your reply! I do see the extra complexity, but I feel like your answer has helped clarify some things. It sounds like I need to build 4 separate mono amplifiers. I will look into designs based on the LM386 or the TI chips.

I'm not sure of the signal level of the microphones, or what power level I will use to drive the speakers. I think table radio loudness sounds about right, although I'm not sure how that translates into signal and power levels. I envision the built-in speakers to be used for, say, a living room. Then plug in for the loud stuff.

I think I see how a logarithmic 4-gang pot would fit into the circuit. I did find some 100kOhms 25mW 4-gang log pots. Would that be sufficient? The digital pots from Maxim look interesting, but I think I sort of grasp the 4-gang pot idea.

I have drawn a revised block diagram (more left to right-ish, but still not a legit schematic), but I still have trouble with the mixer. I'm not sure how to prevent the 4 signals from flowing into the mixer circuit unless a jack is plugged into the switching jack. I'm sure what I have drawn here is wrong, but does it seem like I'm getting closer? Any thoughts on how to implement a push button mute? Thanks again for the feedback!

#### (((o)))

##### New Member
A piezo disc is used on an e-drum to trigger an electronic sound, not as a microphone.
A 2-3" speaker is a squeaker and is not full range because low frequencies will not be produced.

Hi audioguru. I am thinking of a piezo contact mic. I have wired some up and they work great for picking up sounds in reverberate bodies. By full range I think they (speaker manufacturers) just mean that the frequency response extends across much of the audible range, and is not clustered at the high end as in a tweeter. Definitely not as robust as larger speaker, however.

#### AnalogKid

##### Well-Known Member
Fundamental shift in approach - do all of the volume, muting, and mixing *before* the power amp stages, at what is called "signal level" or "line level". The power amps drive the speakers directly at "speaker level". The lower impedances and higher currents make modifying the speaker signals much more difficult.

mic > mute switch > preamp > individual volume controls > gang volume control > power amp > speaker

A switching jack cannot mute or disconnect the four signals while maintaining isolation among them. It can control a 4 pole relay or four analog switches. Question - do you want the individual and gang controls to affect the PA output, or should the PA output be full loudness all the time?

ak

#### dr pepper

##### Well-Known Member
If you get a couple of stereo potentiometers you might be able to put togther a 4 gang pot, so you can control all 4 channels volume at once.
Certain makes you can take 2 or 3 of them apart and put them back to make more than 2 gangs.
Another way would be to use a jfet to control vume from a dc voltage, thats just a little more involved.

#### (((o)))

##### New Member
do all of the volume, muting, and mixing *before* the power amp stages, at what is called "signal level" or "line level".

I follow all of this (in principal) except the mixing before the amp. Once the 4 mono channels go into the mixer, they come out as a stereo signal, don't they? Wouldn't that prevent me from having 4 individual speakers, each outputting 1 mic?

Question - do you want the individual and gang controls to affect the PA output, or should the PA output be full loudness all the time?

I was hoping that the individual and gang controls could affect the signal before PA output. That way you could control the volume at the instrument without having to go to the PA or guitar amp (so it would be like using the volume knob on a guitar). I don't know if this makes things easier or more difficult!

Thanks for help with this!

#### (((o)))

##### New Member
you can take 2 or 3 of them apart and put them back to make more than 2 gangs.

Hey Dr. Pepper, thanks for your reply. When AnalogKid mentioned 4-gang pots I looked it up and saw a lot of people complaining online that they were hard to get, but it seems like I am able to find them, no problem. Here are a bunch under $3, for example (except that one that's$1,197.61, for some reason). Am I looking at the wrong thing?

#### audioguru

##### Well-Known Member
You wrongly show LM386 Power amplifier ICs instead of opamps to amplify the piezo sensors.
You wrongly show a 4 gang potentiometer feeding the speakers when the volume controls should be at the input of the power amplifiers that feed the speakers.
Why do you show a mixer when you do not want the 4 signals mixed together??

#### (((o)))

##### New Member
Why do you show a mixer when you do not want the 4 signals mixed together??

I would like to have a switching jack that feeds the combined signals from all four mics to an amp. I think this means they must be mixed. If there is no jack inserted, I would like each signal to travel from the mic to the speaker uncombined.

#### AnalogKid

##### Well-Known Member
I follow all of this (in principal) except the mixing before the amp. Once the 4 mono channels go into the mixer, they come out as a stereo signal, don't they?
No. The each of the outputs from the quad pots go to two places. Each one goes to a speaker amp, and they all go to four isolated inputs of a 4-channel mixer circuit. The single output of the mixer drives the PA.

ak

#### (((o)))

##### New Member
each of the outputs from the quad pots go to two places. Each one goes to a speaker amp, and they all go to four isolated inputs of a 4-channel mixer circuit.

By quad pots, do you mean the 4-gang pot? If you mean the individual volume controls, wouldn't the output to the PA lack the global volume level?

The single output of the mixer drives the PA.

So the switching jack must come after the mixer, then. When the jack is plugged in, the mixed signal goes to the PA. The rest of the time, the mixer just sits there.

I thinking I'm starting to get it, sort of.

#### audioguru

##### Well-Known Member
Since each mic needs its own preamp then if each preamp is an opamp that usually has a very low output impedance then the mixer can simply be 4 resistors.
It will be difficult to sense that a plug is inserted into the switching jack while the jack is feeding the mixed signals to a 5th amplifier. It is also difficult to mute the 4 amplifiers when the switching jack is switched.
The switching jack can activate relays or analog gates.

#### AnalogKid

##### Well-Known Member
mic > mute switch > preamp > individual volume controls > gang volume control ins >
> gang volume control outs > relay contacts > power amps > speakers
> gang volume control outs > 4 mixer inputs > 1 mixer output > PA jack

PA jack isolated switch > 4PST relay coil

When something is plugged into the PA jack, that switches on the relay. Each of the 4 poles shorts one audio signal to ground. This mutes all 4 amp outputs while maintaining signal isolation. If you don't need complete muting, but just very low volume, the 4PST relay can be replaced by analog switches. Depending on the audio quality you want to maintain, they can range from low cost CMOS 4000 series parts to high end audio switches from Analog Devices or Linear Technology.

ak

Last edited:

#### (((o)))

##### New Member
> gang volume control outs > relay contacts > power amps > speakers
> gang volume control outs > 4 mixer inputs > 1 mixer output > PA jack

PA jack isolated switch > 4PST relay coil

Hey AK, thanks for sticking with me. It's like one step forward one step sideways. So the relay contacts and the 4PST relay coil are part of the same component, similar to the one below?

The gang volume control outs > relay contacts would be 4 single wires going from the 4-gang pot outs to S1, S2, S3, and S4 above.

As for the switching jack, one terminal will be connected to the mixer output. Another terminal of the switching jack will be hooked up somewhere to the relay above, maybe into A1 (guess I'd have to figure out the data sheet to figure out exactly where).

Plugging into the jack will throw S1 - S4, muting the signal to the speakers. Meanwhile the mixed signal will flow through the other terminal of the jack to the PA.

Am I getting there?

#### audioguru

##### Well-Known Member
Microphones are not used in this project. They are piezo transducer discs glued to a guitar or speaker housing that vibrates and causes the piezo to produce a signal that probably has a pretty high voltage if the amplifier input resistance is high. The LM386 has an input resistance that is 50k ohms and is less if its input has a volume control. He said he tried an impedance matching preamp (a Jfet used for a guitar pickup?) to feed an amplifier.

#### AnalogKid

##### Well-Known Member
Not quite. The component you show is a 4-to-1 multiplexer, not a 4-pole relay. A0 and A1 are used to connect electronically *one* of the four inputs (S1-S4) to the output (D). Or, it is a de-multiplexer, where D is the input and S1-S4 are the selectable outputs.

ak

#### (((o)))

##### New Member
Shows the voltage gain

Thank you, ci. That PDF has a lot of great information. It does sound like the consensus is that a preamp is necessary, although I have seen schematics that say a preamp is necessary with their particular design.

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