Soft Power Up?

[dpresley58]

I've built a small transistor-based preamp and powered it with a standard 9v battery. Its using a SPST switch to which the ground is hooked to turn it on. The problem is that there is a substantial thump when the switch is thrown, and I'm looking for a solution with a small component count that would slowly ramp the power to the circuit in hopes that approach would eliminate the pop.

Is there a simple approach to this? 

 

[audioguru2]

I've built a small transistor-based preamp and powered it with a standard 9v battery. Its using a SPST switch to which the ground is hooked to turn it on. The problem is that there is a substantial thump when the switch is thrown, and I'm looking for a solution with a small component count that would slowly ramp the power to the circuit in hopes that approach would eliminate the pop.

Is there a simple approach to this? 

Post its schematic for us to see if its biasing can have a slow ramp-up.

 

[dpresley58]

Thanks. The schematic is under Topic 9 of this link.
http://runoffgroove.com/fetzervalve.html

Although the author used pots for dialing in the source and drain resistors, I've calc'd resistors for the individual transistors. He shows a small breadboard jig for measuring these values. Since my application is stereo, I had to find 2 trannies that were more or less matched. I'm using the J201, by the way.

 

[audioguru2]

The Jfet is self-biased so its output suddenly jumps up when its power supply voltage is applied.

If you slowly ramp up its power supply voltage then it should not make a POP sound.

 

[dpresley58]

What would be a good method of doing this while keeping component count down? A decent-sized cap (100uf) with a pull-down resistor to ground on the way from the switch to the board?

 

[audioguru2]

Since you are switching its ground instead of switching its positive supply to turn it on and off then ramping the power supply voltage must also be done backwards. A PNP emitter-follower transistor can provide the Jfet preamp with a ramping 0V.

 

[dpresley58]

I see what you're saying... Not particularly married to the switching situation - the positive could be used just as easily, since it only really amounts to breaking the battery leads with a switch. I'll look into the emitter-follower.

And thanks...

 

[audioguru2]

If you switch its positive supply then an NPN emitter-follower can provide a slowly ramping positive supply to the Jfets.