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 Forum index » DIY Hardware and Software
Simple Envelope for Monotron?
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Cynosure
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Joined: Dec 11, 2010
Posts: 692
Location: Toronto, Ontario - Canada
Audio files: 39

PostPosted: Fri Aug 23, 2013 7:45 am    Post subject: Reply with quote  Mark this post and the followings unread

Starspawn wrote:
Hi whats the needed voltage to trigger the 7555 in this circuit?

The 7555 is triggered by 0V. The first opamp inverts the trigger signal.

Starspawn wrote:
With the monotron delay it doesnt seem to work (Gate -> circuit -> Cutoff)

This was only tested with the original Monotron. I do not know if there are any differences with the Monotron Delay.

Starspawn wrote:
The gate drops to 3.7 once I connect the circuit, and to 2.5 on the 324 (on all three +/- and out), which is what I get at the trigger pin as well and then nothing on output.

Double check your values and the way you wired everything up. Is the LM324 powered from +5V and 0V? Do you have the virtual ground wired up correctly on the + input on the LM324? Did you miss the diode?

Starspawn wrote:
That seems to be just the positive input there really, the trigger signal drops to a tiny 0.10 DC spike through the 0.1 uf ceramic cap.

Obvious problem?
Like what is the proper capacitor type for 0.1, I dont see why it should block DC at all, yet its not polarized like the other one?

It should just be a spike of voltage by the time it passes through the diode. It is changing a gate signal to a trigger pulse. However, 0.1V is too low. Maybe this is a difference with the Monotron Delay. Try adjusting the values of the 0.1uF cap and 100k resistor until you get a strong spike after the diode.
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Starspawn



Joined: Jun 14, 2013
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Location: Oslo

PostPosted: Fri Aug 23, 2013 8:02 am    Post subject: Reply with quote  Mark this post and the followings unread

Its as low as 0.01 actually, and thats straight after the cap.
Gate voltage is as low as 3v when circuit is connected.

If the + input of the LM324 is supposed to be a virtual ground its off as the 2.5 voltage is coming in there.
Could it be a problem that Im using the negative battery terminal as ground?
As the power is 5v instead of the 3v out from the batteries?
Is the ground TP better to use? As far as I can see though, its all connected to the negative terminal. So perhaps use the positive +3v directly from the battery instead of 5V for the virtual ground?
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Cynosure
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PostPosted: Fri Aug 23, 2013 8:27 am    Post subject: Reply with quote  Mark this post and the followings unread

Starspawn - It sounds like you might have many errors. Let me try to explain it.

I tested this circuit using a wallwart power supply and a 7805 regulator. It should all work from a battery too, but I have not test that.

The +Vdd pins on the the 7555 and LM324 should connect to the 5V tap point on the Monotron.

The Vss pins on the 7555 and LM324 should connect to the Ground (OV) tap point on the Monotron.

The + input pin on the first LM324 should connect to virtual grand (2.5V). The virtual ground is created by using the two 4.7uF caps and the two 10k resistors.
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Starspawn



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PostPosted: Fri Aug 23, 2013 8:50 am    Post subject: Reply with quote  Mark this post and the followings unread

Yep, thats all correct, I got confused by you calling it a virtual ground, thinking it should be 0v.
What seems to happen is that the moment you connect anything to the gate point it drops from 5v to 3v, might have something to do with running on batteries, I dont know, but that seems a part of the monotrons problem not the circuit.
That 3v is then to low to do anything usefull in this circuit it seems.

And on the 324 Im confused, as the 2.5 VG is correct, is it also supposed to output 2.5v?
Basically the 7555 is getting just the virtual ground input through the 324 and not reacting.

So would you suggest trying different values for the cap and resistor to ground (up/down?) or just dropping it as the gate wont get higher unless I input my own?

It actually works ONCE when the machine is turned on, clearly affecting pitch as well, but thats all I get.

This is also using the ribbon, so perhaps it works with external CV/Gate.
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Cynosure
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PostPosted: Sat Aug 24, 2013 1:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

Starspawn wrote:
This is also using the ribbon, so perhaps it works with external CV/Gate.

It only works with an external gate. You might be able to use a transistor to buffer the ribbon's gate signal, but I never tested it. I wasn't too concerned about getting an envelope with the ribbon. The only reason I even left the ribbon in place was for novelty sake. I never use it when actually playing the synth.
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dauserarms



Joined: Apr 25, 2012
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PostPosted: Thu Nov 14, 2013 1:09 am    Post subject: Reply with quote  Mark this post and the followings unread

Could it be possible to solve the lack of external gate with a manual retrigger? I am looking to re case my monotron with a key bed and was wondering if I wired up a manual trigger along with the pitch to the keys could I get around having ext. gate.
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Cynosure
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PostPosted: Thu Nov 14, 2013 9:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

dauserarms - that isn't a problem since you are replacing the ribbon.

The external gate only doesn't work with the ribbon still engaged.
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dauserarms



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PostPosted: Fri Nov 15, 2013 9:11 am    Post subject: Reply with quote  Mark this post and the followings unread

So with the ribbon removed the envelope circuit will work with ext. gate. But I replaced the ribbon with a series of resistors, would there be a way to trigger the envelope?
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Cynosure
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PostPosted: Fri Nov 15, 2013 12:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

There are two different things here that require a gate signal. The Monotron's VCO requires a gate to turn on. This is usually accomplished by the ribbon. In my mod, I removed a resistor shown in the instructions for the MIDI-IF kit that allows an external gate to be used properly, but also disables the ribbon. I added a switch to reinsert the resistor when I want to be able to use the ribbon again.

The envelope generator is a completely different part of the circuit. It actually requires a trigger pulse, so the first part of the circuit that JovianPyx posted in this thread changes a gate to a trigger. That way, I could use the gate from the MIDI-IF kit to trigger both the envelope and the Monotron's VCO output.

If you are building your own keyboard and not using the ribbon, then I suggest creating a trigger pulse each time a key is pressed and using that with your envelope generator. Hardwire the VCO's gate to +Vdd so the VCO output is always on. Then add a second envelope with a VCA to the final output. This method would also allow you to use the envelope schematic here as an AR just by changing the trigger signal to a gate.

I really want to do a big tutorial and video showing all this stuff, but I have been ill and not able to do all the work required. Hopefully I will be able to do it soon...
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Starspawn



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PostPosted: Mon Dec 09, 2013 5:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

Soooo it was working all along ... guess what ... batteries were low.

However the problem on a monotron delay is that the cutoff also cuts the delay, and also I think the envelope is to strong, so going to try and see if going into the leg of the cutoff pot instead of the pad or something helps.

Edit: Directly on to middle leg was best, can set the range with cutoff pot then.
On pad the pot was bypassed it seems.
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robman84



Joined: Jan 26, 2012
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PostPosted: Wed Jan 29, 2014 1:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'm a relative newbie to electronics, but could you send the output of this circuit to a vactrol and use that to control the volume instead (replacing or alongside the volume pot)?

Thanks
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Cynosure
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PostPosted: Thu Jan 30, 2014 10:51 am    Post subject: Reply with quote  Mark this post and the followings unread

robman84 wrote:
I'm a relative newbie to electronics, but could you send the output of this circuit to a vactrol and use that to control the volume instead (replacing or alongside the volume pot)?

Yes. In my monotron I used it for the filter cutoff, but you can use a vactrol as a vca too. That would be the simple method. You could also send it to a more complex vca that uses opamps and/or an ota, but that will be slightly more complex.

You can make a very simple VCA from one side of an LM13700. The JH Wasp filter uses two such VCA's on a 5V power supply if you are looking for a schematic and part values.
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