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trigger/gate input help needed
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grm



Joined: May 13, 2013
Posts: 13
Location: metsään

PostPosted: Wed Sep 15, 2021 3:11 pm    Post subject: trigger/gate input help needed Reply with quote  Mark this post and the followings unread

hi everybody.

i have been breadboarding something that was supposed to be a rather basic drone circuit, but somehow i ended up with a gate trigger on it, since this is something i am not very experienced with and my electronic understanding is still raw, i would like someone (or more) to take a look at my circuit and tell me what can be improved about it. especially the gate section, i think needs some form of protection diode, but i'm not sure how to implement it.

i had it running with 5-12V from a wallwart power supply, and i think that has introduced some noise, i was not able to filter out. any tricks for that are welcome. i tried with some caps and also with a voltage regulator, but it did not help much (but a little bit, when i added the 100nF cap at the trigger decay cap to ground.)

any tips, hints and help warmly welcomed. thank you.
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dk



Joined: Feb 12, 2019
Posts: 113
Location: Europe

PostPosted: Fri Sep 17, 2021 8:44 am    Post subject: Reply with quote  Mark this post and the followings unread

I might be putting my foot in my mouth sometime after writing this, but it looks like:

- the vactrol is backwards. The LED part (diode symbol) should be on the side where the gate circuitry is, and the resistive side should be in the signal path. If you're trying to use it to turn on and off the audio from the drones, I would think you'd need to add a resistor to ground after the vactrol on the audio side (before R11). Since you should have 3 extra 4069 gates available, you could use one as a unity gain buffer between that and the filter section.

- the decay pot seems like it would effect the amplitude of the gate along with the decay. If you want it to just effect the decay, you could just put it in series with the decay range switch. That said, the cap on the other side (C1) is always in parallel, which is a really weird place to put it. That whole trigger button looks suspicious period... are you trying to make the input a gate to trigger with that switch, or are you trying to have a manual push button?

- does the saw/tri switch after the oscillators actually work? I've used this LFO/Oscillator design a few times and never got the output from above the 10k resistor (R2/R6) to work properly. In fact, it might have been the 4069's I was using, but I needed to up the values of all the resistors, use bigger pots, and mess with cap values to get the whole thing working at all. If you've got it working, though, no need to mess with it.

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grm



Joined: May 13, 2013
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Location: metsään

PostPosted: Fri Sep 17, 2021 10:45 am    Post subject: Reply with quote  Mark this post and the followings unread

thank you fro your response and points. (no i hit submit instead of preview so will edit my response still.)

dk wrote:

the vactrol is backwards.


that is correct, thanks for pointing that out. must have blindly thrown it into the schematic. have flipped it now.

dk wrote:

I would think you'd need to add a resistor to ground after the vactrol on the audio side (before R11).


i will try it on the breadboard, but what exactly would that resistor do in that spot. would it eliminate 'leaking' of audiosignal through the vactrol's LDR? or is that to somehow 'silence' the input of the filter, by grounding it when the vactrol shuts off the audiosignal?

dk wrote:

Since you should have 3 extra 4069 gates available, you could use one as a unity gain buffer between that and the filter section.


thank you for this idea, how would a unity gain buffer look like in this implementation? could you point me to a schematic or describe it?

dk wrote:

the decay pot seems like it would effect the amplitude of the gate along with the decay. If you want it to just effect the decay, you could just put it in series with the decay range switch.


ok, i had no problems with it, if i recall it correct, but will try your proposal on the breadboard as well.

dk wrote:

That said, the cap on the other side (C1) is always in parallel, which is a really weird place to put it.


the C1 cap i had added in an experimental manner to lower the static noise the circuit was giving me, in that place it helped a bit. i can imagine that the gate part looks suspicious, it works somehow on breadboard. with manual gate/triggering, as well as with 5V gates from a sequencer.

dk wrote:

That whole trigger button looks suspicious period...
are you trying to make the input a gate to trigger with that switch, or are you trying to have a manual push button?


i can imagine that the gate part looks suspicious, it works somehow on breadboard. and was coming together based on schematic fragments and trial and error procedures. it works somehow on breadboard with the tact switch being a manual gate/trigger, as well as with 5V triggers/gates from a sequencer. i asked here about help because, i somehow remember that often on cate inputs are protection diodes, but i do not know whot to implement them and could not find a suitable schematic that could have been an example for me.

dk wrote:

- does the saw/tri switch after the oscillators actually work? I've used this LFO/Oscillator design a few times and never got the output from above the 10k resistor (R2/R6) to work properly. In fact, it might have been the 4069's I was using, but I needed to up the values of all the resistors, use bigger pots, and mess with cap values to get the whole thing working at all. If you've got it working, though, no need to mess with it.


the tri/saw switch works, i have checked with an oscilloscope. the crude BLEND pot works as well, but in both directions full rotation, the other oscillator still bleeds through a bit. i had also used the circuit for a simple LFO successfully.

thanks again for your help. much appreciated. when i get to try it on breadboard i will report back.
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dk



Joined: Feb 12, 2019
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PostPosted: Sun Sep 19, 2021 2:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
i will try it on the breadboard, but what exactly would that resistor do in that spot. would it eliminate 'leaking' of audiosignal through the vactrol's LDR? or is that to somehow 'silence' the input of the filter, by grounding it when the vactrol shuts off the audiosignal?


You can think of the LDR and resistor to ground after it sort of like a potentiometer setup as a volume control (see picture A). When the LDR is at low impedance (light shining on it), the signal flows relatively freely to the output and only a little bit trickles to ground. When the LDR is at high impedance (no light), the output "sees" ground much more than the signal, and therefore goes quiet. How freely the signal flows when there is light or how quiet it gets when there is no light is directly related to the ratio of your vactrol's impedance vs the resistor going to ground. You will probably need to experiment with the resistor value to find something that works for your vactrol (ignore the value I left in the picture).... if it's too small, you'll lose too much signal even when the vactrol is lit up; if it's too big, you won't lose as much signal when lit but it will not be completely quiet when the vactrol goes dark.


Quote:
thank you for this idea, how would a unity gain buffer look like in this implementation? could you point me to a schematic or describe it?


Sure! Picture B is of a 4069 in linear mode acting as a unity gain amplifier. You can also change the ratio between the two resistors to change the gain, exactly like a an op-amp in an inverting amp configuration. If you make the resistor in the feedback loop bigger than the resistor before the inverter, you'll add gain. If it's made smaller, you'll reduce gain.


Quote:
ok, i had no problems with it, if i recall it correct, but will try your proposal on the breadboard as well.


Ah, well it should work as is, but you'll end up with different amplitudes out depending on the setting. One of the first diode VCA's I made I set the side chain up exactly that way, and the short spikes were much louder than when the decay was set really long.


As for the gate part, you don't need a protection diode there. You only really need to add one if the input could be connected to something it can't handle, like to keep from sending a negative voltage to CMOS, etc.


Quote:
the tri/saw switch works, i have checked with an oscilloscope. the crude BLEND pot works as well, but in both directions full rotation, the other oscillator still bleeds through a bit. i had also used the circuit for a simple LFO successfully.


Awesome!


picture A.png
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Potentiometer as a volume control vs LDR + resistor
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picture B.png
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4069 in linear mode
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picture B.png



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grm



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PostPosted: Sun Oct 24, 2021 1:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

dk wrote:

You can think of the LDR and resistor to ground after it sort of like a potentiometer setup as a volume control (see picture A). When the LDR is at low impedance (light shining on it), the signal flows relatively freely to the output and only a little bit trickles to ground. When the LDR is at high impedance (no light), the output "sees" ground much more than the signal, and therefore goes quiet. How freely the signal flows when there is light or how quiet it gets when there is no light is directly related to the ratio of your vactrol's impedance vs the resistor going to ground. You will probably need to experiment with the resistor value to find something that works for your vactrol (ignore the value I left in the picture).... if it's too small, you'll lose too much signal even when the vactrol is lit up; if it's too big, you won't lose as much signal when lit but it will not be completely quiet when the vactrol goes dark.


thank you for your, detailed info and help. sorry, that i took me so long to respond. now when i got back to the circuit i have tried to do the 'silencing the vactrol' trick you have clearly described. it has not helped and only in the end, i have tried to disconnect the oscillator from the filter completely. which made no difference either.

the noise is actually controllable with the pots of the oscillators, so it leaks through the power supply into the filter. i managed to get it much quieter by putting a 100uF cap directly from + to GND pins of the filter IC.

then i tried to 'terminate' the unused gates of the filter IC, by connecting all of them to GND, but that did not help either.

but then i finally, tried the version you explained last. to use an unused gate as an op amp (not quite sure if it is that actually?) and a resistor to ground from the audiopath before that and that did the trick. now the noise is all gone, except when i trigger the gate.

initially i wanted to use the second half of the filter IC for a second filter for a second gated oscillator, but now i will most likely do something else with the other 2 gates (distortion!).

thanks a bunch.
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