Sneak Current?

[Palbert]

Hello all,

I just finished building the phuncgnosis effects pedal (http://folkurban.com/Site/Phuncgnosis-719.html) on a breadboard.

For some reason, not just with this pedal but with every op-amp circuit I have built on a breadboard, I am still getting sound when the op amp is turned off and the 9V supply is physically removed from the circuit. I read Analog Device's article on "off amps" and I was thinking about adding a series resistor like they suggested to see if that helps. I want to put it in series with the .01uF cap and the non-inverting terminal if U1A. Would this cause me problems? Is there another solution for the issue of getting sound without power?

 

[alec_t]

If the switch for the 9V supply were a DPDT type perhaps you could use one pole to short the output of U1a (or U1b, depending which sound source you want to blank) to ground when the 9V is off?

 

[kubeek]

Shorting outputs of opamps is not a very bright idea, shorting inputs will be much better.
As for the sound staying there, have you tried discharging any large caps you have in the circuit?

 

[Roff]

I'm pretty sure your blow-by is directly through (and/or around) U1b. I don't think U1a is involved. I have added a JFET shunt switch which is off then power is on, and on when power is off. I ran some sims on it, and got about 50dB attenuation at the junction of the two 680pF caps. Give it a try.

 

[alec_t]

Shorting outputs of opamps is not a very bright idea, shorting inputs will be much better.

Yes, that would be preferable. I should have specified an AC short (as Roff's mod provides); not a DC one.

 

[Palbert]

I gave the JFET setup a try ROff and it works pretty well. If I turn my bass up and the amplifier up both to max, I still get sound with power off but its definitely better than it was at normal volumes. Thanks for the suggestion!

 

[Roff]

I gave the JFET setup a try ROff and it works pretty well. If I turn my bass up and the amplifier up both to max, I still get sound with power off but its definitely better than it was at normal volumes. Thanks for the suggestion!

What JFET part number did you use? It matters.

 

[Palbert]

I was trying to figure out which parameters of the JFET were important for this implementation. I'm using the MPF102 JFET.

 

[Palbert]

Also, if you could explain how the JFET is accomplishing this, I would appreciate it. I'm just confused as to what connecting the drain and the source to the same voltage does.

 

[Roff]

I was trying to figure out which parameters of the JFET were important for this implementation. I'm using the MPF102 JFET.

The important parameters are Vgs(off) and Rds(on). MPF102 is not suitable. It will never turn completely off when power is on, which will reduce the gain and alter the filter response. You may have invented a new wah pedal!
I did a fair amount of research to find a FET that would work for you, be available, and be in a thru-hole package. J113 was the only one I found. There may be others. Jameco has them at $2.79 for 30 of them ($.093 each). Mouser has them for $0.19. Digikey doesn't have any thru-holes in stock.

 

[Roff]

Also, if you could explain how the JFET is accomplishing this, I would appreciate it. I'm just confused as to what connecting the drain and the source to the same voltage does.

The J113 is a switch. When the power is off, Vgs=0, and the ON resistance is typically 100 ohms, acting as an attenuator in conjunction with the 22k resistor. When power is on, Vgs=-4.5V (gate at GND, source and drain at +4.5V), and the J113 is OFF, so does not interact with your circuit. The 1Meg resistor from drain to Vr is just there to establish a known voltage across the cap. Otherwise, it might slowly charge through drain-source leakage, slightly altering the current through the LEDs (which act as current-controlled resistors in your filter). You could probably leave out the 1Meg resistor and see no difference in performance.

 

[Palbert]

Haha I wish, I accidentally connected it at the non inverting pin of U1a and I forgot the 1uF capacitor so it was just too quick of reading on my part. I'll snag some from Mouser since I have to order other parts for pedals from there as well. Thanks for the help, I really appreciate it.

 

[Palbert]

Thanks for the explanation as well, this makes much more sense to me now.