[sdiy] Re: [AH] Re: Gate to Trigger conversion circuit

[Michael Bacich]

On Apr 2, 2006, at 4:13 PM, Jeffrey Needham wrote:

> Crap.
>
> ARP Trigger IN spec:
> +8V min.
> 10 usec min.
>
> I guess I still need to find at least 8V somewhere since the Pro-1  
> GATE OUT
> is 0-5V, so it needs to be level-shifted.
>
> I guess I could level-shift the Pro-1 and then add the cap to the  
> Avatar?
>
> OK, that's pretty ugly.  It has some appeal as it means I don't  
> have to
> build a circuit (providing I can level shift the Pro-1 GATE)

Hello Jeffrey,

If I was doing this, I would probably put the modification inside the  
ARP.  This is a problem for pretty much all ARP synths, and here is  
your chance to correct their "mistake".  Install the two-transistor  
level shifting circuit somewhere inside the ARP, and connect it  
internally to the ARP's Gate input (at the terminals of the Gate  
input jack).  That way, the ARP will always gate/trigger correctly no  
matter what you connect it to.  (BTW, you can use pretty much any  
small NPN transistor for that circuit.  If you can't find the 2N2222,  
look for a 2N3904 -- or a BC548 if you're outside the US.  Also, that  
5K resistor can be a 4.7K, since you'll likely have a hard time  
finding an actual 5K resistor)

Regarding adding a cap in series to differentiate the Gate signal  
into a trigger pulse:  It's a good idea to add a diode to ground  
after the cap before connecting it to the ARP's trigger input.  This  
will insure that the trigger spike only goes positive, and will also  
guard against possible double-triggering.  Otherwise, you'll get a  
bipolar double spike every time the cap receives a gate signal.

Use any regular small-signal diode, such as a 1N914 or 1N4148.   
Connect your level-shifted Gate to one side of the cap, and connect  
other side of the cap to both the ARP's trigger input and to the  
cathode side of a diode (the side with the little line mark).  The  
other side of the diode (the anode) should connect to ground.  This  
will eliminate the negative-going part of the spike that's created by  
the diode, leaving just a clean positive spike for your trigger.

Depending on the inout impedance of the ARP's trigger input, you may  
also have to add a resistor in series with the cap's output.  Sorry,  
I don't know what value.  The resistor will control the discharge  
rate of the cap, and thus control the duration of the trigger spike.   
You may have to experiment a bit to get it right.

Do look at Ken Stone's circuit -- it's more complex than what I've  
suggested, but it's really perfect.  See how he has the diode  
connected to ground in each of the two sections -- that's how the  
aforementioned diode would be connected.  You could also use one half  
of Ken's circuit to take care of the needed Gate level shifting stuff  
really nicely.   Simply eliminate the 10nF series cap in the first of  
the sections (just jumper across it).  You might have to juggle a few  
resistor values on that circuit half to get it right, but the end  
result would be a clean level-shifting comparator circuit for your  
low-voltage Gate.  You would then connect that shifted gate to the  
input of the second circuit section (with the cap in place) to derive  
the trigger.

Good luck!
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