Adding a "starve" control to a circuit with a voltage divider

[MikeH]

In this instance, the voltage divider is biasing a dual opamp.  Would it be better (proper, if you will) to "starve" the entire circuit, voltage division and all, or would you want to only starve the opamp and leave the voltage divider out of it.  I guess the bigger question is; "does one bias an opamp with a voltage divider in order to provide 1/2 the supply voltage that the opamp sees, or simply to deliver around 4.8 volts for biasing?"

[slacker]

You normally bias the opamp to get it to operate around half of the supply voltage, this means the signal can swing equally negative and positive before it clips.
If you "starved" the entire circuit the signal could still swing the same amount both ways before clipping but with less voltage it would clip sooner.
If you "starved" just the opamp's supply pin and left the voltage divider running off 9 volts then as the voltage reduced the bias point would move upwards so you'd get asymmetric clipping as the top half of the signal would clip earlier than the bottom. If you dropped the voltage far enough one side of the signal would run out of room altogether and be completely clipped.

As well as just getting different clipping by reducing the voltage, the reduction in current to the opamp might make it behave differently.

I don't know if either method is the correct one but the second one is probably more interesting.

[R.G.]

The bigger question's answer is it's done to bias the opamp input at half the power supply voltage.

Opamp outputs cannot swing further than the limits of the power supply voltage, right? In fact, unless you buy special opamps which are guaranteed to swing to the power supply voltages ("rail to rail output"), the opamp can only get to within a volt or two on each side. For instance, if the output of the opamp can only as close as 1.5V to the power supply, with a 9V supply the output will only swing between 1.5V and 7.5V. You lose 1/3 of the power supply to the swing limitations.

To get the biggest possible swing and not let the opamps's clipping intrude into the other things you have going on, you want the output of the opamp sitting dead in the middle of the available range, which for most opamps is dead in the middle of the power supply.

The simplest way to do that is to generate a voltage that's always half the power supply with resistors and bias with that.

There are exceptions. The Phase 90 has a fixed 3.xV above ground. That bias fixes the voltage on the phasing JFET sources so the LFO can swing properly below it. And the P90 has distortion issues. What a surprise.

In either case, the term "starving" does not apply.

[MikeH]

Thanks for the replies guys; exactly what I needed.

In either case, the term "starving" does not apply.

So what am I talking about then?

[earthtonesaudio]

So what am I talking about then?

Well, there's "starving" the voltage meaning just reducing the voltage below what it's designed to use...

...Then there's dying battery simulation, which is lowered voltage and added series resistance.

Interesting that you brought this up, because I'm in the middle of building a prototyping board that has adjustable voltage out, a pot wired as a voltage divider for the opamp biasing (so I can change from exactly half to asymmetrical), and in addition, a series resistance on the main supply.  I'm hoping it will work to simulate any situation I might want.

It's wired like this: DC in, LM317 adjustable regulator, then it splits and one side of the power goes to the "opamp bias" section, and the other gets the series resistance and the full voltage.  I suppose having the series resistance before the opamp bias might work better, to more accurately simulate a dying battery, but I haven't wired it up yet so I may change it.  Having the "master voltage" before the opamp biasing ensures that I can crank the master voltage up or down and the bias voltage will stay at 1/2 or whatever.

[squidsquad]

Again...someone correct me if I'm wrong...I just try to regurgitate things I've read here over the years:
Transistor distortion may benefit from from having a lower voltage supply.  *Benefit* is a subjective term...the sound will change.
See this on Jack's site:  http://www.muzique.com/lab/lowvolt.htm
However...his circuits have been modified to run at these voltages...so I may be a bit off.
But certain fuzzes seem to sound better as the battery is dying.
I believe this is also from the change in battery impedance...as stated in RG's site.
I've also read that people don't like the sound of an opamp clipping.
(Unless of course you WANT that harshness)
And that's why most of them use diode clipping.
Bottom line: things that may sound good in one type of circuit...may not sound good in another type.

[John Lyons]

Interesting stuff to try.
Some op amp circuits "starve" the circuit right at the power input by means of a small resistance. 100 to 390 or more ohms.
It's said to " sweeten" the sound....
Try a 5K pot or whatever you have handy, wired as a variable resistor.
I suppose that some asymmetrical voltage bias would be interesting as well. A 20K pot set up like a voltage divider would do it.
Try it out!

John