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Help with a Simple Mixer

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5/13/2000 4:02 PM

Help with a Simple Mixer

Hey

I found a schematic a while ago for this mixer and i finnally got around to building it and now its all built but it doesnt seem to work.

Now before i get into this check out the schematic because i am going to be refering to it alot.

http://orlot.8m.com/loud/simplemixer.html

I checked all the curcuitry and it looks fine so i figured that the error must be in the only part i guessed at.

In the schematic it doesnt show the power sorce.

I assumed that i just needed to connect a 9volt sorce to the each of the opp amps. i have one 9 volt battery who's + goes to the + power for each dual opamp. then i connected the - side of each dual opamp to ground. battery - goes to ground too. and then every other thing in the diagram goes to ground.

I was thinking maybe there is some over place in the curcuit that needs power but im not sure.

Maybe one of you who read this can help.

Thanks

Andrew

5/13/2000 6:31 PM

Andrew I think you connected it correctly for a 9 volt swing.

Now my question is to R.G. or ....

If we compare that mixer to this one:

http://www.montagar.com/~patj/smixer.gif

The one thing you notice is the standard voltage divider to get 1/2 V+.

Could that be what's missing on this schematic?

http://orlot.8m.com/loud/simplemixer.html

I mean, when do you need it?

I gotta get out my books.

5/14/2000 3:30 AM

Aron is right. You either need a voltage divider to create a bias voltage on the + input of the opamps, or you need to have a bi-polar power supply. Almost all opamps require that you connect (in the case of your standard TL*2, etc. layout) that you connect pin 8 to the positive supply and pin 4 to the negative supply. Using a bias voltage or split supply simply moves the voltage swing from +/-9v to between 0v (ground) and 9v+. As the schematic is drawn, it requires a bi-polar power supply (the power supply connections on the opamps are not drawn but are implied).

Lorren
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5/14/2000 4:06 AM
Andrew	Thanks for telling me what it needs... now all i have to do is figure out what i need to do to fix it. could you tell me step by step what parts i need to add/change to make it work. Thank you Andrew

5/14/2000 5:26 AM
aron	Andrew, The easiest way is to use 2 batteries. Alternately, take a look at the Son of Screamer on Jack Orman's AMZ site. See R1 connected to VR? See VR which is made of two 10K resistors with the 47uF cap? That's how you do it.

5/14/2000 3:36 PM
Andrew	Well... I think ill use two batteries cause im still a little confused about the other way of solving the problem. Ill have 2 on/off switches so that i can use just the left channel, just the right or both. Thanks for the help. Ill post after i try the 2 battery aprouch. Andrew

5/14/2000 5:50 PM
Rob	That circuit is intended to run off 2 batteries. The input impedance is a bit too low for plugging a guitar straight into the inputs though. Regards Rob

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