1K gain control idea

[Gus]

1K linear control with AC coupled taper resistor


Discuss

[brett]

Hi
the gain control idea looks so good that it should have been there from day 1.

A couple of other ideas...
A low hFE transistor for Q1 gives low input impedance. That allows "Sag and Bloom" and allows clean-up with the volume pot of the guitar. These characteristics are very desirable IMO.
A low base-emitter voltage on Q1 makes the clipping more single-sided (for thicker, richer harmonics). About 0.3 volts is a time-honoured favourite and is available in a popular semiconductor material ( ).
cheers

[Bill Mountain]

I'd love to comment but I'm confused as to what it does.

Is it a better taper?  Smoother gain?

[slacker]

I might be looking at it wrong but it seems like it would spread the lower gain settings out over a great portion of the pot's rotation than the standard control, squeezing the higher gain settings even further towards the clockwise end.

What's C8 for, is it to bypass R5 so changing its resistance doesn't change the AC gain?

[brett]

Hi again
R5 sets the minimum gain (because it is always bypassed by C8). C8 separates the effects of R5 on DC bias from its effects on AC gain.
cheers

[Gus]

The taper resistor also controls the max gain setting.  Using ohms law(at a high enough frequency you can think of the caps as a short) the resistance at max gain(wiper at ground) is 220 in parallel with ( the bias resistor value + 1k the gain control)

Note the schematic is using higher hfe Si transistors.  It also has a 47K feedback/bias resistor.  The 47K and 220 help control the max circuit gain

The linear control can be wired so it still turns CW for max gain.

The taper value and C2 value can be changed

R5 is where I like to place a bias control on a FF like circuit I like to select/define the collector resistor and therefore collector current at a known idle voltage range.  I make the bias control an external control.

For a "standard" 1k control in a FF like circuit remove R5, C8 and connect the 'top" of R6 and C2 to the Q2 emitter feedback resistor node

The sim looked interesting