Ampl. mod. LM386 via pin 8

"Bruce Morton"

** Not true.

** If all you want are two gain settings ( ie high and low) then a cap ( or cap plus resistor in series) from pin 8 to ground will boost the gain. An NPN transistor or JFET can be used to ground the network as needed by suitable voltage drive to the base or gate.

... Phil

RTFM! " Gain control can also be done by capacitively cou- pling a resistor (or FET) from pin 1 to ground."

-- from the datasheet for LM386.

Grant

Following that general scheme, if one _did_ want to do amplitude modulate the LM386 output with a signal other than a square wave, one could use the JFET to ground in its linear region as a voltage-controlled resistor, or connect an OTA as a current-controlled resistor to ground (such an application is in the LM13700 datasheet), so long as the restriction that the gain has to be greater than 9 is obeyed.

"Bitrex"

** There is no linear region.

JFETs must be "linearised" in order to use them as a voltage controlled resistive attenuator - eg part of the signal on the drain must be fed to the gate and even then the useful linear range is small.

... Phil

Phil specified pin 8. I will try both on the bench. I assume the remaining "gain" pin is NC.

Will settle for "on/off" modulation via squarewave input from LM555.

Thanks all,

Bruce

"Bruce Morton"

** Ambiguity is your forte - right ?

You ain't gonna get 100% gain modulation.

... Phil

Sorry, bad use of the term "linear." What I meant was the "ohmic" region of the JFET - in some material, like on Wikipedia they call that the "linear" region in reference to the relation between the drain to source voltage and the drain to source current, to distinguish from the "saturation" region. It's not really linear at all since it depends on the square of Vds, so again - poor terminology.

As you say, the relationship between Vgs and Rds is nowhere linear without compensatory feedback as described here:

In the MXR Phase 90 it looks like they just drove the gates of the FETs directly:

However, since in that application the FETs are used to control the center frequency of the allpass filter, the non-linearity of the control characteristic probably sounds more musically pleasing.

I did this sort of thing years ago. Start with a sine wave, not square, and apply to any pin that it wouldnt be completely silly to apply it to. Be prepared to crank the sine volts right up, and thus you'll find out how to use whatever amp chip you've got to amplitude modulate. IME its often necessary to use large voltage swings to get some bit of internal circuitry out of its strictly linear region.

NT

Almost makes electronics sound like fun again :-)

Well, I got it working. Transistor switch pin 1 or 8 to gnd. No gain, no pain ... or is it the other way around?

Bruce