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DIY Stompboxes => Building your own stompbox => Topic started by: aeiou on October 02, 2012, 03:49:22 AM

Title: Can I build a simple active mixer using just MPSA18s?
Post by: aeiou on October 02, 2012, 03:49:22 AM
Is there anyway to build an active 4 channel mixer using just MPSA18 NPN transistors? Reason being -- It's all i've got on my desk at the moment.. that or a bunch of PNP Germanium transistors, which I'd be open to using as well.
Title: Re: Can I build a simple active mixer using just MPSA18s?
Post by: ~arph on October 02, 2012, 03:58:44 AM
Depends if you have any caps or resistors laying around  ::)
Title: Re: Can I build a simple active mixer using just MPSA18s?
Post by: aeiou on October 02, 2012, 04:00:12 AM
yes yes, plenty of those here -- what i should have said is that those are the only transistors i have on tap.
Title: Re: Can I build a simple active mixer using just MPSA18s?
Post by: ~arph on October 02, 2012, 04:02:52 AM
 ;D
Yeah I thought so..

I see no problems building this mixer..

Build five buffers.. four of them each have their own volume control and then a cap. 
All outputs of those buffers (after the cap) connect to the input of the fifth (no input cap needed ).
You can even add a little gain at the end.
Title: Re: Can I build a simple active mixer using just MPSA18s?
Post by: aeiou on October 02, 2012, 04:26:07 AM
so i wouldn't encounter any issues dropping MPSA18 (bjt) into this JFET circuit?

http://www.muzique.com/schem/eagle.htm
Title: Re: Can I build a simple active mixer using just MPSA18s?
Post by: ~arph on October 02, 2012, 04:33:36 AM
I don't think that will work directly as that is a BJT for FET substitution and you might run in biasing issues. I suggest you use a simple BJT buffer instead. But it should be very similar.
Title: Re: Can I build a simple active mixer using just MPSA18s?
Post by: aeiou on October 02, 2012, 04:43:21 AM
here's one i found that calls for a bjt --

http://www.hawestv.com/mtv_faq/amplifier_art/buffers_bjt_fet_5v.png

any good?
Title: Re: Can I build a simple active mixer using just MPSA18s?
Post by: ~arph on October 02, 2012, 04:45:46 AM
Bit low on the input impedance.. try the third one from the bottom..:

http://www.muzique.com/lab/buffers.htm
Title: Re: Can I build a simple active mixer using just MPSA18s?
Post by: Nasse on October 02, 2012, 01:50:08 PM
http://electroschematics.com/97/simple-mixer-circuit-schematic-common-base/ (http://electroschematics.com/97/simple-mixer-circuit-schematic-common-base/)

old elektor application

edit...

http://electroschematics.com/2917/audio-mixer-with-one-transistor/ (http://electroschematics.com/2917/audio-mixer-with-one-transistor/) and another, looks like one that I built many years ago
Title: Re: Can I build a simple active mixer using just MPSA18s?
Post by: PRR on October 02, 2012, 11:51:21 PM
Four resistors will mix.

But the sum is weaker than the sources.

If your system has gain in reserve someplace else, no "active" is needed.

If not, you want a gain of 4 or so. Easily gotten with any BJT/FET in an appropriate circuit.
Title: Re: Can I build a simple active mixer using just MPSA18s?
Post by: Jdansti on October 03, 2012, 06:49:38 AM
Would four resistors going into a Tillman work well?
Title: Re: Can I build a simple active mixer using just MPSA18s?
Post by: Perrow on October 03, 2012, 06:55:02 AM
If you happen to have an dual op-amp laying about...

http://generalguitargadgets.com/projects/15-boostersrouters/68-mini-mixer-project (http://generalguitargadgets.com/projects/15-boostersrouters/68-mini-mixer-project)
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