Still plugging away at getting my pedalboard pop free. I use a transistor to switch amp functions, as shown below:
| control voltage ___ |/ o--------o-|___|- -| 3k |>
| o | | | o GND
(created by AACircuit v1.28.6 beta 04/19/05
The 15v is from the amp, and switching it to ground turns the amp function on or off (e.g. channel switching, volume boost). It switches dead quiet in one direction, but there is a pop when I switch back. I put the circuit on the scope, and found that it switches cleanly to ground, but when I switch back, there is a little spike in the waveform just before the beam goes to a higher position on the screen. I assume that this is my audible pop.
Any ideas as to why it's there and how to get rid of it?
My best guess is that the entire circuit jumps to +15V when you kill the ground to it. It is still connected to the amp's ground and when the coupling caps charge to +15 the thing pops. Try using a high side switch and see if that helps (a PNP switching the +15V).
I'm gonna check and see if I have any PNPs sitting around here and give that a try. I wish I'd have thought of that before I built the circuit into my pedalboard - I wouldn't have needed to use an inverter to get the control voltage right.
Thanks again for the insight. I'll try a higher resistor while it's hooked up to the scope, but I can't actually use a higher resistor on the pedalboard. The 15v comes from the amp through a 100k resistor, and the Ron of the transistor is too high to actually fully switch functions if I use more than 3k (actually it still doesn't quite fully switch with 3K, but I cant go any lower because it's being driven by a 4049.).
OK I'm glad this came up because it's been puzzling me for a while. The amp has an LED indicator for each channel, so whichever one is on has its LED lit. If I ground the connector (i.e., 15v to ground) the channels switch and
1 LED is on. However, if I connect the 15v to ground using a 15ohm resistor, one LED will light fully, and the other will still be partially (or fully) on. The same goes when using a transistor as a switch. I originally used a
10k resistor for the base, but 1 LED remained partially on. Using a lower value resistor (like 3k) turns the LED almost completely off. I figured from this that driving the base with more current reduced the Ron to a low enough value that it switched completely. It does seem odd, though, with a 100k resistor in there that an extra few ohms would make that much of a difference. You may remember from my previous posts on this topic that I was going to try a low Ron N-channel MOSFET to get away from this problem. Also, I've tried using an optoisolator to do the switching, but it also leaves one LED partially or fully lit.
Can you shed some light on this (no pun intended).
Oh, one other quick question - I'm using a 4049 inverter set up to be a latching flipflop and it seems to have died on me. When the input of any of the inverters is grounded, its output should be a logic high shouldn't it? I've been testing the various inverters in the package to see if there is only one damaged or if the whole thing is shot, and none of them give a logic high with the input grounded, although they do show 0VDC when a high is applied to the input.