adding cv in and out to tonepad fsh-1a

[fogwolf]

I've been combing through old posts on the Maestro Filter Sample & Hold project and would like to be able to add jacks for a CV in and a CV out. I'm specifically looking at the Tonepad project - the schematic at http://www.tonepad.com/getFile.asp?id=96 I also saw something related to this in this old posting - http://www.diystompboxes.com/smfforum/index.php?topic=56009.0

I think I have an idea of how the CV in mod might work. First, I've never used a switched jack, but funny enough I had coincidentally just read about them this morning in Electronic Projects For Musicians. So, looking at the example jack Craig Anderton provides there (on page 24), one terminal goes to ground and then there are 3 other terminals, two of which form a connection which gets overridden when a plug is inserted into the jack. I would think you would wire up either pin 1 or 16 of the LM13600 to one of these terminals and the other to the collector of Q3, then the other pin on the IC to the final jack terminal. Then when you plug in a jack connected to an external CV source, that would provide the CV, is that right? But would you need a DPST - the 2 poles being the 2 pins on the IC and the single throws being either the collector of Q3 or the input CV from the plug. Do they make a DPST switched jack? Am I understanding this correctly or do I have the CV flow backwards - would you instead need a SPDT from the transistor to the IC?

Also, I'm guessing that the Tonepad PCB would not work for this CV in mod as-is - would you have to change it to break the hard-wired connection between the IC and the transistor so this connection can be fully controlled by the switched jack? I'm sure this is covered somewhere so if that's the case I'll investigate further if it's necessary, but is there an easy way to load an image from a pdf into Eagle or some other circuit CAD software to make these changes or would I have to manually recreate the PCB in the software?

Then I have no idea where on the circuit I would break off an output CV. Would there need to be just one output connection from the circuit which is wired to the tip of a mono jack (with the sleeve of the jack connected to circuit ground)? If so, where exactly would this extra output pad go on the tonepad circuit?

Thanks!

[Paul Perry (Frostwave)]

The two CVs that go to the resistors to the OTA op amp sections - just tie those 2 together & then when you run an external voltage to them, you sweep the filter. (not sure what voltage range you would want - but remember that the control inputs have a current limit that MUST NOT be exceeded, even for a nanosecond.

[fogwolf]

So keep the resistors connected to both of these pins and tie just connect them together to a pad where I would connect an input CV jack?

As far as current goes, I must admit I'm not sure - it would be from the Moogerfooger CP-251 or from my analog synth or sequencer, so I would assume that should be fine but maybe that's not necessarily true?

Also, where would I tap the circuit to run CV out from the S&H?

Thanks.

The two CVs that go to the resistors to the OTA op amp sections - just tie those 2 together & then when you run an external voltage to them, you sweep the filter. (not sure what voltage range you would want - but remember that the control inputs have a current limit that MUST NOT be exceeded, even for a nanosecond.

[Paul Perry (Frostwave)]

If you want to get CV out from the S&H, the obvious point is the junction of the 33K resistor & the fet Q6.
But depending what you want to do wit the CV, you might need to amplify it and/or offset the voltage.

And similarly if you were running a CV from some external source to the CV point, I don't know exactly what range & offset is needed there either.

I can't say any more without actually building one & looking at the actual voltages.
It isn't exactly simple (hence that kludge of resistors around the env/s&h switch, which is intended to match range & offset from the two sources).
What range & offset does the CP-251 or your synth put out? I do not know.

[fogwolf]

The voltage range for the synth and the moogerfooger is -5V to 5V. Not sure what offset is exactly - what does that mean (still really new to this)? The CP-251 has a CV attenuator that takes an external CV input with a range from -5V to +5V, so I would think that should help some with fine tuning the CV out tapped from the pedal.

Thanks.

If you want to get CV out from the S&H, the obvious point is the junction of the 33K resistor & the fet Q6.
But depending what you want to do wit the CV, you might need to amplify it and/or offset the voltage.

And similarly if you were running a CV from some external source to the CV point, I don't know exactly what range & offset is needed there either.

I can't say any more without actually building one & looking at the actual voltages.
It isn't exactly simple (hence that kludge of resistors around the env/s&h switch, which is intended to match range & offset from the two sources).
What range & offset does the CP-251 or your synth put out? I do not know.

[Paul Perry (Frostwave)]

By 'offset' I meant where the control voltage starts from, in the case of the MF you say the range is -5 to +5, so that means that something might have to be done before you run this to the Maestro, unless the CV input there happens by luck to want a LFO centred on 0 volts and with that range.

When one is running audio signal between different units, the use of capacitors at the inputs and outputs makes life easy. But when you are using control voltages, the grounds of the two circuits must be connected. This can make life difficult.

[fogwolf]

Thanks for all the information. I found an older posting where someone said they got this to work - wonder how that turned out.

By 'offset' I meant where the control voltage starts from, in the case of the MF you say the range is -5 to +5, so that means that something might have to be done before you run this to the Maestro, unless the CV input there happens by luck to want a LFO centred on 0 volts and with that range.

When one is running audio signal between different units, the use of capacitors at the inputs and outputs makes life easy. But when you are using control voltages, the grounds of the two circuits must be connected. This can make life difficult.