Up/Down Switch for DOD FX25 Autowah?

[Gila_Crisis]

I'm gonna build a DOD FX25 clone (infos here http://topopiccione.atspace.com/PJ11DODfx25.html ).
I was just wondering if you could add a switch for up/down sweep direction. is this possible? any ideas?

here's the schem: http://topopiccione.atspace.com/pjimages/DODfx25.sch.gif

[frequencycentral]

I think you could add an opamp or transistor inverting buffer at the output of the envelope section.

[Gila_Crisis]

i think this is what zero of topopiccione told me to do once. any other ideas or comments on frequencycentral's solution?

[Mark Hammer]

I don't think that will work.  The thing abut switching between upwards and downwards sweep is that the starting point for downwards has to be thought out and deliberately created.  In the case of the MXR Envelope Filter (Tonepad A-Gua), it IS a simple matter of inverting something because that circuit uses the "on" time of a clock pulse to set the filter frequency. In other instances where the envelope amplitude itself directly controls the sweep, you need to dictate a starting point.  You will note, for instance, that the Mu-Tron/Q-Tron/Neutron inserts a preset bias voltage into the equation when switching between upwards and downwards sweep.

The FX-25 uses OTAs as filter sections, which means that sweep is dictated by the current fed to the OTAs.  So, if the sweep moves upward as current from the rectifier is increased, then downward means you need to start with a given default current level and subtract from there.  More complicated, I would think than simply electronically inverting the rectifier output.

Do I know how to do this?  No.  I just know that it needs to be done.

[frequencycentral]

I see what you're saying Mark.

On an analogue synth, when using a 'normal' envelope (with a lowpass filter), the filter frequency needs to be set low for the envelope to have an effect - you can't open a filter that's already open.

Conversley, on an analogue synth, when using an 'inverted' envelope (with a lowpass filter), the filter frequency needs to be set high for the envelope to have an effect - you can't close a filter that's already closed.

So, in the DOD circuit, in addition to feeding an inverted envelope into the OTA's CV input, it would also be neccesary to feed a fixed voltage (through a summing amp?) to set the filter 'high' so the inverted envelope would be able to lower it.

On an analogue synth it's as simple as just twisting the filter 'frequency' knob - which is just a variable voltage divider summed with all the other filter CV inputs.

[slacker]

If you take the 47k resistor on the + input of the additional opamp and connect it to the wiper of a pot connected across the supply rails instead of vb that will give you a variable bias. This basically works as a frequency control like Rick suggested and will let you set the resting high point of the filter, with a bit of tweaking that should do the job.

[Zero the hero]

As stated by gila, the solution Rick proposed is roughly the way I would implement the inverted sweep. An appropriate transfer function should be calculated in order to preseve the correct start (lowest voltage) and stop (maximum voltage) which are fed to the OTAs. If you don't want to do the math, a rough approach would be to measure both voltage and calculate the transfer function.
I've seen the Maestro Filter / Sample and Hold with Tonda's mod which includes a sweep inversion option, and it's implemented with an inverting op-amp and clever components ratings.

The FX-25 uses OTAs as filter sections, which means that sweep is dictated by the current fed to the OTAs.  So, if the sweep moves upward as current from the rectifier is increased, then downward means you need to start with a given default current level and subtract from there.  More complicated, I would think than simply electronically inverting the rectifier output.

Do I know how to do this?  No.  I just know that it needs to be done.

Feeding a norton amplifier with the inverted envelope? I've never used them, I don't really know if this wuold be a real solution...

[Gila_Crisis]

reversing the sweep like on the Maestro FSH coulöd work for the FX25 too then?