Bipolar/ combination with a "normal" 0v ground + 9v,anyone explain the pitfalls

[Skruffyhound]

         Hi, Im fiddling around on the breadboard with envelope followers and wah circuits to create autowah effects.
I'm using the coloursound inductorless wah and using different EF's to trigger it.
My problem is that I want to try Andertons EF which is bipolar and I'm really wondering what happens to ground, when the two circuits are connected. I have a breakout box on my breadboard with a MAX PS which gives me the required -9v, 0v, and +9.
Could anyone lay out for me in hackers terms what is going on with vbias/ ground in this situation, or just point me to the right reading material.
        I know many of you have tried this before, so if you have any suggestions for envelope follower designs/ wah circuits I can play with I'd love to hear them.
Thanks
-Aston

[JKowalski]

With  a bipolar supply, you would not have decoupling capacitors because the signal can ride around ground and the op amps will be able to amplify it using the + and - supplies. The Vref is there in non-bipolar circuits because there is not negative supply that the op amp can "draw"  from, it can only output voltages between 0-9 volts (assuming a ideal op amp, typically they can only go from like 1-8 volts). So if you tried to amplify a signal around ground, you would lose half of it since the negative portion of the signal has no available negative voltage to draw from.

Decoupling capacitors solve the problem by moving the signal from around ground to around 1/2 the supply, (for 9v, 4.5v) so that there is a sort of split supply in that the signal around 4.5V has a -4.5v (0v) and a +4.5 (9v) to draw from.

In your case, you could use decoupling capacitors to move the signal between the Vref and ground based on what the power supply capabilities are in the part of the circuit you are working in but I would suggest converting one of them to use the other's supply. If you have a bipolar supply in the circuit anyways use that for your whole circuit - it's easy to convert, just remove the decoupling capacitors that move the signal to Vref, remove the Vref supply (resistor voltage divider usually) and put all Vref connections to ground. Of course, there are things to think about:

- Whether the circuit utilizes the lower headroom to work (like op amp clippers) and increasing headroom will have a negative effect or not
- Whether the chips used can take the +/- 9v (18 volts total supply) though most op amps usually can take up to 36v total...
- Whether you want DC protection on the inputs or not (without DC-removing decoupling capacitors, any DC offset on the input will be amplified through the circuit along with the signal) - you may want to use them anyways.

You can also maybe convert it the other way round (split supply section to single supply) to get rid of the more complicated split supply, but in doing this you need to be sure the circuit doesn't need the extra 9V of headroom to function properly.

[edvard]

In a bipolar powered op-amp circuit, the ground (0 volt) is mainly there as a mid-point reference for the op-amp, and everything else happily uses it for ground too.
Going from bipolar-powered circuits to normal 0-9V circuits, you should be able take power from the +9 and 0 ends of the power supply and it'll work just fine.
I suppose you could power it from the +9 and -9 to get 18 volts of headroom, but most stompbox circuits (like the Colorsound) were designed for 9 volt operation and may work undesirably or you may need to alter some parts.

[Skruffyhound]

Thanks Chris, I'll try to change everything to bipolar I think. Still fighting with it. Aside from the power issues, I think matching of the envelope follower to the controlled circuit may be the problem.

[Skruffyhound]

Thanks Eddy, so it seems that either way power may not be a problem here since I originally powered it as you suggest.
The coloursound section worked fine with the LFO off a tremulous lune, so something is up with the the EF. Oh well, back to the breadboard.

[Skruffyhound]

Well now I've got some quack , albeit of limited range and with very interactive controls. Think it will take some work to get these two circuits to play nicely together, but not a total fail. Thanks for the help. Now I'm going to build some sections of the Mutron and see how it gets along with the other two.