DIY Vactrol

[Matthew Sanford]

Ron, thank you again. I realized I was kinda going the wrong way, thinking it was about the resistances in the two lines joining at the output jack. In thinking, I have two tl07x buffering in and out with 3 inverters in-between, so with the input grounded it makes the first output VCC -1.5v (under 1 volt input on TL071 is under the minimum input voltage) driving inverters low->high->low putting low on output buffer creating a high output into the - side of the polarized cap (so many of those that seem backwards in the circuit to me, but trust in Tone God as they say in the church of silicone), so shouldn't go through, but would effect the signal as more of it from Bypass makes it through the cap against the 7.5v (being AC signal). So essentially the 200R off the push-pull output of the TL07x is the resistance you note for the divider, being reduced for the series resistance on that side of it as the level pot increases, creating a divider between the opamp and ground feeding the output.... or no, between opamp and output, meaning as there is little resistance in effectively the side going to lower voltage (on the output side of the cap) means it isn't the circuit side ground that is really coming in to play here but the seemingly ground or 0 volts (due to cap) of the opamp. I hope I am getting that right.

I'm still not quite understanding how a lit vactrol on the FX output when bypassing (I am guessing prior to cap and level pot) corrects that situation of the divider it is creating, I'll think on it more, hopefully my understanding is correct so far.... well, I guess moreover I am not sure how to implement the vactrol to disconnect the output from the effect. Is it more it should be on the FX vactrol on line between the ouput of the effect and the output jack in order to provide a high resistance to out or a very low resistance when the effect is on, thereby increasing the ground resistor in the voltage divider to force most of the bypass signal through, and essentially making the 100k level pot a moot point?

[amptramp]

The easiest way to look at your vactrol switching is to look at it as a replacement for a mechanical switch.  If you want to bypass an effect, you switch the input signal to the pedal output and disconnect the effect output.  If you want to use the effect, you switch the effect output to the pedal output and disconnect the pedal input from the pedal output.  If you do not have any memory in the effect, you ground the input when the effect is bypassed so the output will be zero when you switch to it.  If there is memory in the effect like a chorus / delay / reverb type of effect, you may want to keep the input of the effect circuitry connected to the pedal input even when it is bypassed or you will get zero output until the delay time has passed and this is heard as a 'gulp' where there is no output.

You should not have a situation where two outputs are being connected together because they will fight each other and their respective outputs will form a voltage divider and the proportion of the signal that comes out will be set by the respective output impedances.  In other words, you will get a mix of the signals.  If you have an effect with a 1000 ohm output impedance and the vactrol resistance from the input is 500 ohms, then 2/3 of the output will be from the input and 1/3 will be from the effect.  You want the output to be all one or all the other.

[Matthew Sanford]

Ok, makes sense, I will try to remember to use that perspective moving forward. So in this case, the bare wire from level pot to output returns the bypass signal (through around 400R) to the effect through the pot, the resistance going under 50k at midpoint moving up so starting the reduction of the bypass signal by about 1% at that point and increasing...which is why I more noticed the effect at about 2/3 or greater CW.

So for this to work well, I will now include 2 vactrols on each line; Bypass from in to out and FX in to ground, Fx from in to FX input and Fx output to output jack. That way the bypass signal will have Megs of resistance between the output jack and Fx out, making any signal sent that way negligible.

Thank you again for such detailed explanations and passing knowledge so freely!

[MP_Quokka]

My take on making diy vactrols is that back in the 60s and 70s the light source was either light bulbs or low output Red LED available at the time.  This meant the input sensitivity was low making them easier to drive.

I made some using modern 3mm green LED (LDR are most sensitive to green) and the resitive range was between about 50uA and 1mA LED current. This is hard to drive accurately due to opamp offsets etc.  I'm looking for old low output red LED to make it easier to drive.

https://www.diystompboxes.com/smfforum/index.php?topic=128995.0


I've used aluminium foil to light proof the vactrol as it has very high reflectivity.

[Matthew Sanford]

My take on making diy vactrols is that back in the 60s and 70s the light source was either light bulbs or low output Red LED available at the time.  This meant the input sensitivity was low making them easier to drive.

I made some using modern 3mm green LED (LDR are most sensitive to green) and the resitive range was between about 50uA and 1mA LED current. This is hard to drive accurately due to opamp offsets etc.  I'm looking for old low output red LED to make it easier to drive.

https://www.diystompboxes.com/smfforum/index.php?topic=128995.0


I've used aluminium foil to light proof the vactrol as it has very high reflectivity.

Thank you, I'll read through that thread. I've had pretty good success with the way I've made them, and for some things I like them to bottom out, but am currently experimenting with series resistance to raise that bottom end. Of course my mind is ridding flip-flap-flop ideas pretty loudly too...