Hi, I was wondering which circuit I should use for mixing two signals (noise and a VCO) in my planned drum module. I found a
single-gang pot crossfader by electric druid:
But I was wondering if I could not simply use a simple voltage divider pot like I have seen many times, like here with buffers at the input:
So, is there any benefit of using the more complex electric druid circuit?
I use this crossfader
Source :
http://www.la-roue-tourne.fr/index.php/modulaire/273-module-quad-crossfader
Given the title, I suspect the point of that article was to show how to build a voltage controlled cross-fader (or two). Your voltage divider isn’t voltage controlled, at least not in the same sense.
I see, so I will go with the simpler design then, thank you!
Something which is maybe related: The output of my filter is shifted a bit to the negative (almost -2V DC offset). This is because I use the buffer of the LM13700. I would like to remove this offset and I thought a simple AC coupling cap would do it. I tried different values for C8, but I can not remove the offset, only if I use extreme values which really change the signal I see any effect… Is it because of the mixer that follows? I thought by using buffers I have very high input impedance in the mixer which would be good in this case. What am I doing wrong?
1M and 10 µF have an RC time constant of 10 s, very slow. Have you tried more like 1 µF and 100k?
This is with 1 uF and 100k, it starts changing the signal. Maybe I need to replace the the LM13700 transistor buffer with a real opamp, but I am still wondering why this decoupling does not work. Some interference with the filter itself? Do I need a buffer before the cap?
edit: oh, this is a very slow signal btw. like 8Hz, maybe that is also a problem?
It’s hard to high pass filter square waves with an RC network and keep them fairly square. They look like DC, for half a cycle at a time, so if RC is short enough to get rid of the DC component quickly it’s short enough to distort the waveform. And yeah, if it’s 8 Hz then getting rid of the DC in 1 second means distorting each cycle by a lot.
Oh, yeah, sounds kind of obvious like this xD Thanks!
I have replaced the buffer with an opamp now, it is a bit better, but still not perfect. I am wondering if I need perfect though… I could just add a little bit offset in the opamp, but I am not sure if I need it. Also with higher frequencies I can probably get away with a AC coupling capacitor…
Does this cross fade from signal 1 to 2 entirely silencing signal 1 (or vice versa)? I expect there to be always be some of both signals given this circuit.
Yes entirely silencing, look at his demo vid, and my " this afternoon test " look doing the same result 
Assuming there’s no resistance between the end terminals and their signal sources, it will work. Typically the wiper of a pot when turned to one extreme or the other shorts to the corresponding terminal. In that case the voltage on the wiper must be identically equal to that on the terminal, and there can be no mixture of signal from the other terminal.
I used this for the wet/dry mix in the Megamodule. Works fine.
This is good to hear! Getting back at the DC offset (I know, not exactly the topic, but it came up with it 
): does anything speak against an inverting summing configuration to add a bit of voltage and get rid of the offset? I need negative voltage because of the inverting stuff, I guess and I would try a voltage divider from -12V… good plan? 
Each of the opamps has a worldly thus measurable output impedance. Connecting the outputs of both to one another with a resistor will allow for some signal from the one opamp to reach the output of the other and vice versa. So I expect there to be some possible signal bleed. I tried this configuration in a few designs and stopped using it because of that effect.
I just breadboarded this:
With a 10 Vpp audio wave (square or ramp) into the input, and with the wiper turned fully clockwise (to the 3 terminal), I looked at the output on the scope; there was no wave signal seen down to the level of the noise which was maybe 20 mV.
I also tried a wave on pin 5 instead of ground and a different wave or 10 Vpp noise on pin 3 and saw just the pin 5 wave with no distortion or added noise on the output.
(The closed loop output impedance for TL074 is of the order of 1mΩ.)
I was wondering if theres anyway to add control voltage to the potentiometer that is used to crossfade both signals? I know you can a really crude and cheap method is just to get a 100k potentiometer, put one signal at one end the other on the opposite, and using the wiper as the output. But would it be possible to control that with a control voltage?
Maybe “a bit more” elaborate, but if you want control over a cross fader, maybe this could interest you:
In the experiment described I used an arduino nano to control the duty cycle of a pulse that controls 2 analog switches, but that could be simplified by using a 555.
This is the crossfader version from Juanito Moore:
And this is his instructable for a passive version in deadbug mount mod:
The 20k resistors to ground are… a choice. 100k input impedance would be more typical of Euro/Kosmo designs. I know there are those who complain that’s too large, but it’s what’s commonly used and assumed to be used.
Yes 100k could be more adapted, thank you for the suggestion !
