[sdiy] Re: reset circuit for LFO

Mon Oct 19 20:35:38 CEST 2009

wow crazy formatting!

The LFO is definitely resetting in phase (I have been able to get
envelope style behavior from it). Also the circuit is running off of
+/-12V

However when the LFO is on a very slow rate and I mess with the
symmetry controls, the reset behavior is no longer as expected. You
can hear the dip caused by the reset pulse  but the LFO picks up to
the same place in the cycle that it was before.

> If you're using bipolar power=2C and shorting the cap to ground=2C you=20
> should be getting the desired effect (the reset LFO starts over at=20
> zero).=A0 If you've only got one supply rail=2C you should short the=20
> cap to a buffered Vcc/2 circuit.
>
> That actually raises an interesting question for me: if you're=20
> simply grounding this capacitor and=A0 resetting the LFO to its
> center value=2C isn't the "direction" of the LFO now the same as it
> was before it was reset?=A0 If you want the phase to be predictable=2C
> the comparator needs to be reset as well.=A0=20
>
> ----------------------------------------
>> Date: Mon=2C 19 Oct 2009 10:53:05 -0700
>> Subject: Re: [sdiy] Re: reset circuit for LFO
>> From: danjel at gmail.com
>> To: synth-diy at dropmix.xs4all.nl
>>
>> Here is another issue (and it may have to do with the point I picked
>> to ground the CAP)
>>
>> Every time the reset pulse is high=2C the CV value of the LFO is held
>> high. On normal LFOs when you reset the phase of the waveform seems to
>> be held at the zero point. So if I was using an LFO to modulate an
>> Oscillators pitch=2C at reset point there would be no modulation.
>> In my circuit the reset point creates a higher pitched beep (higher
>> than the max point of the modulation waveform). Now that I have
>> stretched out the pulses a bit that beep is fairly long and
>> significant. It kind of sounds cool since it creates a rhythmic melody
>> but it is not the desired effect.
>>
>> On the LFO pcb there are two timing caps which are for the two
>> different rate ranges (Hi/lo) these connect to a switch which selects
>> between the two caps and it is at this point that I am trying the
>> grounding.
>>
>> So first should I just try a smaller cap size for the pulse
>> stretching? (22pF is already pretty small)? Should I try some other
>> type of trigger processing?
>>
>>
>>
>> On Mon=2C Oct 19=2C 2009 at 10:25 AM=2C Danjel van Tijn  wrote:
>>> ok I put a 22pF cap to stretch out the pulse and this is working great
>>> when feeding a pulse signal from the Elf LFO. I also put a diode in
>>> series for protecting against negative voltage.
>>>
>>> However=2C my midi2cv converter clock and gate (which have no problem
>>> triggering things like the doepfer A160/161 or doepfer envelopes) are
>>> not triggering it at all.
>>>
>>> This goes back to my question on if I am powering the cd4053 at 12V
>>> does it matter that the gates/clk are 5V?
>>>
>>>
>>>
>>> cheers=2C
>>> Danjel
>>>
>>>
>>>
>>>
>>>
>>>
>>> On Mon=2C Oct 19=2C 2009 at 10:06 AM=2C Danjel van Tijn  wrote:
>>>> oops not a diode to ground=2C a diode inline!
>>>>
>>>>> Is it as simple as putting a small cap between signal and ground of my
>>>>> reset jack?
>>>>>
>>>>> Should I also be putting a diode to ground to protect the CMOS chip
>>>>> from any negative voltage?
>>>>>
>>>>> cheers=2C
>>>>>   Danjel
>>>>>
>>>>>>
>>>>>> You might try a "pulse stretcher" circuit essentially adding a small
>>>>>> something picofarad cap
>>>>>> to lag the incoming clock a bit... (would be simpler than adding a
>>>>>> comparator and pulse
>>>>>> conditioning circuit with debouncing etc?
>>>>>>
>>>>>> Best Wishes!
>>>>>>
>>>>>>
>>>>>> Cynthia
>>>>>>
>>>>>>
>>>>>>
>>>>>> Danjel van Tijn wrote:
>>>>>>>
>>>>>>> Someone suggested using the circuit that Ken Stone used on the clock
>>>>>>> input here: http://www.cgs.synth.net/modules/cgs13v2_gated_comparato=
> r.html
>>>>>>>
>>>>>>> A comparator with a diode like this might help me out.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Mon=2C Oct 19=2C 2009 at 8:16 AM=2C Danjel van Tijn  wrote:
>>>>>>>
>>>>>>>>
>>>>>>>> A while ago I posted asking for advice on modding a Livewire Vulcan
>>>>>>>> lfo so that it could be reset.
>>>>>>>> Amos suggested using a CD4053 switch to control the timing cap bein=
> g
>>>>>>>> shorted to ground temporarily to cause a reset.
>>>>>>>>
>>>>>>>> As first experiment I had two wires: one to the timing cap and one =
> to
>>>>>>>> ground. Whenever I connected these together temporarily it
>>>>>>>> successfully reset the LFO.
>>>>>>>>
>>>>>>>> Next step was  wiring up a little board with the switch circuit. Th=
> e
>>>>>>>> reset jack was connected to one of the control inputs of the cd4053=
>  so
>>>>>>>> that a short pulse would temporarily connect the LFO timing cap to
>>>>>>>> ground.
>>>>>>>>
>>>>>>>> So far it kind of works but not well:  If I feed at an LFO squarewa=
> ve
>>>>>>>> from a Plan B ELF LFO  it holds the Vulcan in reset while it is hig=
> h
>>>>>>>> and then allows it to cycle when it is low. However when I switch t=
> o
>>>>>>>> the alternative narrow pulse square waveform the pulses seem to be =
> too
>>>>>>>> short to cause a reset.
>>>>>>>> Also when I connect a midi2cv converter and generate clock pulses o=
> r
>>>>>>>> gate signals they are unable to cause a trigger.
>>>>>>>>
>>>>>>>> Right now the 4053 is being powered by the 12V rail... should I be
>>>>>>>> using a lower voltage so that the logic signals from other devices
>>>>>>>> like my midi2cv can actually drive the 40530 controls? i.e. Should =
> I
>>>>>>>> run it at 5V instead?
>>>>>>>>
>>>>>>>> Do I need any kind of trigger conditioning off of the reset jack?
>>>>>>>> e.g. filter? schmitt trigger?
>>>>>>>>
>>>>>>>> cheers=2C
>>>>>>>> Danjel
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> Synth-diy mailing list
>>>>>>> Synth-diy at dropmix.xs4all.nl
>>>>>>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
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>                                         =20
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