Battery wiring question: attn R.G. Keen

[C Bradley]

I built the opamp wah from the Geofex "Technology of the Wah" article. I used a 9V battery with two 10k resistors to create a +/- 4.5V supply. The two 10k's are in series from the + terminal of the 9V to the - terminal, with the junction of the two grounded. If I use the normal method of using a stereo jack to make the ground connection when a mono plug is inserted, then when the plug is out I've still got 20k from the + terminal of the 9V to the - terminal draining the battery.

How can I wire this so that there's no load on the battery when the plug is out?

[C Bradley]

It appears that I may not need a dual supply. I tried lifting one of the 10k resistors from ground, but the circuit would sweep even with no plug in the input jack.  :?

Also, I used a LM741 and I get lots of feedback when the effect sweeps to the treble position. What can I do to fix that?

[C Bradley]

bump

[petemoore]

I used a 9V battery with two 10k resistors to create a  The two 10k's are in series from the + terminal of the 9V to the - terminal,   +/- 4.5V supply.
 if you're hookin' up n opamp and using two 10k divider resistors, the junction of the two will be in the middle of V+ and V_.  I don't understand "with the junction of the two grounded'.
 two 10k's seriesed and connected to V+ and V- at the ends of the chain of the two seriesed resistors, 4.5v is what apears at the junction of the two resistors when the difference bethween V_ and V+ is 9V.
 What voltages does the DMM read at the opamp when the plug is inserted and removed?
 removing the plug, removes the sleeve, if the battery clip black is connected to a (*Stereo jack [it's easy to mistake this for////word...the kind of jack you can't use for IJ power cut trick]
 Black to stereo jack Ring [middle connection of a stereo plug or jack], circuit board ground wire only from sleeve,if the jack is good the battery will be an incomplete circuit.

[niftydog]

quite simply you are not breaking the circuit in the correct place.

The input jack should completely disconnect one of the battery terminals from everything in the circuit - including those two 10k resistors.

[petemoore]

Where is that input jack trick?

[niftydog]

there's a reasonable explanation and diagram in the DIY FAQ, see link at top of this page.

[C Bradley]

if you're hookin' up n opamp and using two 10k divider resistors, the junction of the two will be in the middle of V+ and V_. I don't understand "with the junction of the two grounded'.
two 10k's seriesed and connected to V+ and V- at the ends of the chain of the two seriesed resistors, 4.5v is what apears at the junction of the two resistors when the difference bethween V_ and V+ is 9V.

If you ground the junction (4.5V) of the two 10k resistors, you will see +4.5V at the positive terminal of the 9V battery and -4.5V at the - terminal in relation to ground. I'm thinking that I can eliminate the two 10k's and just wire the supply like a single 9V supply with ground.

[R.G.]

Grounding at the middle of a resistor-created bias string is possible, but it does keep you from using the stereo jack trick to turn the battery on and off unless you use some circuitry to turn off both + and - to the circuit. There is a circuit for this at GEO, but as you note, you don't have to do that.

You can use a big capacitor at the input and output of the circuit and use the - supply for ground, just like 99% of all single-supply pedals do, using the 4.5V reference for "ground" inside the capacitors. Then the stereo jack trick works. This is the idea you were flirting with in your last post.

Hook up the circuit just like you have it, 10K reference and all. Use a DC-blocking, but signal-passing capacitor at the circuit input and output, with pull-down resistors outside the caps to ground. The battery + is connected to the circuit and the - is connected to the stereo jack. The 10K/10K creates an internal "ground" if you bypass it to the battery - with a 10uF or bigger capacitor. The circuit should then operate without these funnies.

If you eliminate the 10Ks, the circuit will work, but the "ground" inside will wobble around with respect to the battery as the currents pulled between the + and - change. They won't remain at +/- 4.5V all the time.

[C Bradley]

Let me get this straight. You're saying that I can put a 10uF at both the input and output of the wah circuit with a 1M or so pull down resistor on the jack side of the cap. Then, just disconnect the - battery wire from the circuit board and connect it to the ring connector of the TRS jack?

[C Bradley]

Can I wire the opamp wah circuit with a 4.5V bias supply to the non-inverting (+) input of the 741 opamp and use the 9V like a single polarity supply? In other words, supply 9V to the positive supply pin (7), ground the negative supply pin (4), and supply 4.5V to the + input.  Then I could lift the battery ground to turn the power off, I think.