Hi - I´m planning to build a mic system for my accordeon, and I think I may need some help to design a pre amp circuit.
The plan is to use 3 electret mic capsules on the treble side of the accordeon, and 2 on the bass side. The mics will be wired to a pre-amp/mixer/battery box that can be velcro´d on to the outside. I want to have 2 separate output channels, one for each side, so I will need one set of 3 imputs mixed to one channel, and one set of 2 inputs mixed to the other channel. I will need to use preset resistors to trim the levels of each mic to compensate for differences in sensitivity, and I want to have one volume control for each channel. Of course, all this needs to be in quite a small circuit. With that in mind, I´m thinking I might have to make do with one pre-amp for each channel, with a pot to control the gain, and just use a resistor mixer, with presets, to mix the different mics to each pre-amp. Would that be a bad idea bearing in mind the fairly low impedance of the mics though? Should I use another tranistor pre-amp stage for each mic before mixing?
Any help appreciated,
Wilf

 

Unless you play very softly I believe the resistive mixing will work fine. You will be close micing. 10,000 ohm log taper pots and 6800 to 10,000 ohm resistors in series to power each mic (9dvc) has worked for me in the past. I used 100uf isolation caps, if I remember correctly.

Cheap electret elements are fine but make sure they can take the input level when you play load without distortion.

 

Thanks flat5 -
I would hope to buld something that can handle a full range of playing levels, from 'very soft' to pretty damn loud, as I will be using it for studio work and experimenting with electronic effects, not just for gigs.
I would think that playing very soft will still produce a reasonable signal with close mics, but when I´m blasting it out it´s possible I´ll have problems overloading the electrets. I think I will try with the normal electret configuration first to see if there are any distortion issues, but I might consider also modifying the mics to a Linkwitz configuration to provide some source feedback resistance, which apparently increases the headroom considerably...

Linkwitz Cosine Burst Generator

It seems this will also reduce the output voltage though, so maybe that will then cause problems at lower levels... experimentation is the way I guess.

On second thoughts, I think it will be tidier to have a small internal mixer circuit in each side of the accordion. If I can get away with resistance mixing to combine the signals several mics on one side it should be easy to make a circuit small enough to fit inside, and then I can have just one coaxial cable from each side leading out to a battery box with 2-channel pre-amp and gain control. That´d be a nicer solution.

One thing, you said
"10,000 ohm log taper pots and 6800 to 10,000 ohm resistors in series to power each mic (9dvc) "
I´m slightly confused by this, you mean to put a pot in series with the power supply resistance? (from +v to signal out of the mic?) To mix the signals shouldn´t the pot be between mic out and pre-amp input (mixing point)?

By the way, I´m thinking of using this pre-amp circuit...

Wilf

 

Hi - thanks flat5...
I replied yesterday, but it doesn´t seem to have come up yet, so I´ll keep this brief incase my last note appears...
so, thanks for your tips... I reckon I´ll give it a shot with the resistive mixing, should still be enough signal at low playing levels. But there was one thing I was confused about... you said:- "10,000 ohm log taper pots and 6800 to 10,000 ohm resistors in series to power each mic (9dvc)"
shouldn´t the pots be connected between the mic outputs and the mixing point at the pre-amp input? You seem to be saying to put them from the output to the +v rail? which I suppose would also alter the output signal strength of the mics, but it doesn´t seem to me the best way to mix the signal. Or am I misunderstanding you?
Anyway, I´ve been sketching a circuit design, I´ll post it up for comments asap.
Cheers
Wilf

 

Something like this. Shown are two inputs one channel.
Three inputs per channel should be ok without too much interaction of the pots. It is a compromise design.

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Now I see your circuit. It looks good. I don't think I would do the gain the same way. I would put the pot on the output or input, depending on what follows. But the designer may know better.
Would you use an amp for each input or each channel?
My circuit may be all you need for club use. For recording you may want something quieter, like your circuit.

 

Had time to think and changed my terrible circuit.
I think this is about all you can expect from a 3 input resistive mic mixer. Just not sure what is the best pot value.

 

It's a really horrible design - I hate to see pots used the wrong way like that

Why not use a simple virtual earth mixer?, one opamp, job done (and you can get the added gain you're looking for), although personally I'd like to see a preamp following each mike before it's mixed.

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The pots have to be backwards or they interact to much.
He has provided an active mixer I wanted to show a passive one that very well may do the job required.

I will Google on your virtual ground mixer and see if I can absorb some o-that learnin' thingie stuff :-)

Edit:
Nigel, this might look familiar to you.
Mobile audio mixer, w/ high level inputs

mixer circuit:

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Nasty!

That's the sort of thing.

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Hi again -
The passive mixer circuit you sent looks like the sort of thing I was thinking of, Flat5, to feed into a 2 channel pre-amp. The circuit I posted was what I intended for one channel of the pre-amp, not for each input, so the mic input in that diagram would come from a passive mixer like the one you showed.
I´m not sure about what you were saying :
The pots have to be backwards or they interact to much.
Can you explain that a bit? If I set up the pots with the wiper to the mixer output what problems does that cause? I don´t think I need to worry about crossfeed at all as this mixing stage is just to level out each mic by a few dB with trims.

The virtual earth active circuit looks good too. I think active mixing of the mics would be advantageous for sound quality, but as I say, I have to be careful for space if the mic mixing stage is going to fit inside the cover of my accordion. Would something like this with transistors instead of op-amps work?

 

You can't just join pots together, they short each other out, by fitting them backwards it helps reduce this, but they don't really work correctly.

Sound quality, and it would mix properly - opamps take little room, one quad opamp would do. If you're really tight for room then use a surfacemount device.

BTW, my daughter plays a little accordion, but she's never done much with it - but at least she bought it with her own money. She's played it at one gig, in one song - and the sound engineer wasn't happy

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I envision you are going to install the mics permanently on the body of the instrument.
You could use small trim pots which will be first set to pass as much signal as possible. You then lower some of them a little to achieve a balanced sound from the instrument, as natural as possible. Then you leave the pots alone. I suspect the fully passive mixer will work for this. You will be playing in a somewhat noisy environment and a very low noise mixer is not required. Try to shock mount the mics so they do not pick up key movement or other possible sounds like the bellows. Good luck with that. Hopefully the reeds will be so much louder there will be no problem.
You don't need to adjust the output side of the mixer because it is connected to a level control at the amp. I hope there is room to put this in a shielded box. Use a good alkaline battery and a switch and the battery will last years.

 

yep, thats pretty much what i was thinking.
I would like to mount the mics permanently inside the grille which covers the working parts of the accordion, although another idea might be to have the mics mounted in an external bar which velcro´s onto the grille. Either way I´ll be careful to avoid vibrations.
I´ll balance the mics with trims exactly as you said. As it should only need a small adjustment, it might be an idea to use a fixed resistor in series with the pots so that the pots can adjust more sensitively over a smaller range.
Here´s a sketch I did of the circuit a few days ago.. The trim pots are still the normal way round, so taking into account what you said about them interacting should redraw this as in your passive mixer design...



I´m going to stick with having a level control on the pre-amp box, I know I could leave that to someone at the mixing desk , but I think it´s my prerogative as a player, you don´t see an electric guitar without a volume knob after all. Definitely going to shield the box for that.

I still want to go as low noise as possible on this, as near as I can get to a professional mic system. As I said, it isn´t just for stage work and noisy environments, I´ll want to do stuff in the studio too. So if theres any issues there I´ll try the active mixer with a quad amp as Nigel suggested.
I do imagine I´ll have problems with key noise, but then I did recording with room mics too... I just need to work on my technique for that!

Nigel, glad to hear you daughters been playing a bit! I do sypathize with the sound engineer, it´s a bugger trying to get a mic close enough to the moving side of the accordion on the stage!
Cheers
Wilf

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That's a VERY strange circuit

Isn't the TDA2003 a power amplifier?, what's that doing there?.

There are no mixing resistors from the trim pots, the resistor in the bottom end of the pots will make the pots do nothing, and why are there capacitors in the bottom?.

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