Re: How do i hook up a opamp with a voltage divider bias

[R.G.]

How do i hook up a opamp with a voltage divider bias / bi-polar suply to turn on like a pedal? Ya know... when you insert the jack it should turn on.

im having problems because i tried hooking up the negative side of the battery to the jack but at the same time the ground is connected so all i get is this distorted sound. I hope im making sense here

I popped this out here because it had nothing much to do with the thread it was hidden in.

It's not clear from the post, but I'm guessing that you mean that you have a single battery, and have a voltage divider across that battery, and then have connected signal ground to the voltage divider in the middle.

If that's what you meant:
You're right, you can't simply use the stereo jack trick to turn on the battery. It shorts the battery (-) to signal ground, and makes anything in the - voltage side irrelevant.

What you have to do is to use something that senses when ground is connected to the second signal lug on a stereo jack, and arrange that to connect the battery. You also have to do this without the sensing circuit pulling current all the time, defeating the purpose of the power turn-off.

I would make up a darlington PNP from a pair of ordinary PNP transistors like perhaps 2N3906 and tie a 1M from its base to the darlington emitter. I'd take another 1M from the base to the signal ground point of the bias divider. The emitter of the darlington PNP goes to battery (+) and the collector goes to the circuit power supply +.

If the divider resistors are significantly less than 100K, then when you plug in the mono jack, it shorts the 1M base resistor to the voltage divider ground. This is sitting at the same voltage as the battery (-) connection since the battery (+) is open because the darlington is not conducting. So it pulls down on the darlington base through the darlington emitter (which *IS* connected to the battery + ) and turns on the darlington. The voltage at the voltage divider ground rises to half the available volts, but there's still enough voltage there to keep the darlington PNP turned on.

You'll lose about 1V across the Darlinton.  If you are willing to use three transistors, a PNP => NPN => PNP with the final PNP being in the power supply path can be made to saturate down to under 50mv or so.

[davebungo]

but why can't you reference the input signal to the battery 0V and AC couple the input - i.e. like most circuits do?  So the input cap has half the supply across it.

[R.G.]

but why can't you reference the input signal to the battery 0V and AC couple the input - i.e. like most circuits do? So the input cap has half the supply across it.

Beats the dickens out of me. Maybe ebasses will pot by and tell us.

I was going on the assumption that he had a reason for what he was doing, being as he went to so much extra trouble to set it up that way.

[ejbasses]

R.G, sorry about that post. I was making an op amp booster and i was in the booster thread so i thought it was kind of okay. sorry  :wink:

You hit the spot. thats exactly what my problem is. but im trying out a few things befor i go with the ground sensing aproach.