MIDI Talk !

[fikri]

Hello, im new to this digital stuff, and i need direction from you guys

I want to learn about MIDI stuff, i have done my tube preamp with the switching system in the preamp. I'm currently using relays for the switch (channel selector, bright switch on/off, FX loop on/off). My questions :

Can i drive the switching system with my MIDI footcontroller ? How ?

Is it possible to use the expression pedal in the MIDI footcontroller to drive the volume level on my preamp ?

sorry for the dumb question !

Thanks

[Peter Snowberg]

Excellent questions! 

Type midi switcher into the search function and click on the first result. 

Also try typing basic stamp midi receiver into Google and click on the first result.

The receiver end of a MIDI chain is just a serial port listening for certain values at 31,250 bps.

Driving a volume control on an amp can be done in a couple of ways. One way is to put an OTA or VCA between the volume control wiper and the next stage in the circuit. Feed that OTA or VCA with control voltage made by a D/A converter connected to the chip receiving the MIDI data. The Parallax BASIC Stamp is a great unit for experimenting with things like this. The normal pot becomes a "maximum volume" control instead. The MIDI data would select how much to chop off the signal.

Another way is to use a servo module out of the RC Hobby world to turn a real pot. You can also get pots that have servo motors built in. The servo module is fed by a PWM signal from the microcontroller which listens to the MIDI data. Fender has some amps with this feature built in. I've heard that Neil Young was the first person to have motorized controls hacked into his tube amp as he was singing about "Sample & Hold". 

It can be done. 

[The Tone God]

Boy its almost like someone should write a library for uCs to interface with MIDI.

Andrew

[David]

Driving a volume control on an amp can be done in a couple of ways. One way is to put an OTA or VCA between the volume control wiper and the next stage in the circuit. Feed that OTA or VCA with control voltage made by a D/A converter connected to the chip receiving the MIDI data.

What about just feeding the D/A output to the LED in an optocoupler, like in the Craig Anderton pot de-scratch hack?  I've been thinking about the feasibility of using something like this as a standardized approach to onboard pot override.

[Peter Snowberg]

What about just feeding the D/A output to the LED in an optocoupler, like in the Craig Anderton pot de-scratch hack?

The hard part is figuring out exactly how much drive to give the LED in order to get full scale control.

Mesa did a nice trick with dual vactrols where they use one of the two LDR elements as a feedback source for the LED driver. Very cleaver! Unfortunately this only works for controls which have one side connected to ground.

[David]

Okay, how about Andrew TTG's Rock'n' Control circuit for voltage-divider type pots?

[Peter Snowberg]

Look at figure 5. Three trimpots.  Once you set them you have a fantastic circuit element at your disposal. just drive it with a D/A or PWM+LPF.

[David]

While we're doing "feasibility studies", Joe Davisson posted a circuit that implemented a up/down pushbutton setup.  Of course, I can't find the schematic, but (assuming you can find it with your Moderator Magic) might you know if something like that would work?

[fikri]

Thanks for the reply guys ! 

O.k before i can continue, i guess i have to digest all slowly ! 

Motorized pots peter ? wow where can i get them ?, i guess soldano have that in their rackmount preamp. Well, it is always fun looking the knobs rotating for themselves ! spooky !  . But maybe this is the only one way to have it controlled without affecting the tone ?

Im not sure, but i have tried a Sans amp rackmount preamp (sorry, forgot the type) but they have something like a "programmable pot position ?" (without a motorized pot) how about that ?

Thanks !