Author Topic: DD-20 Tap Tempo interface  (Read 3186 times)

G. Hoffman

DD-20 Tap Tempo interface
« on: March 03, 2016, 08:26:15 PM »
So, I'm setting up to use MIDI Clock for my guitar rig, and I'm making a MIDI to tap interface (using Electric Druid's MIDI clock chip) for my DD-20.  The tap input looks like so...


(from the DD-20 service manual)


What I'm trying to decide is, can I just send the +5V output of the MIDI Clock (actually, I'm sending it through a couple sections of a Schmitt Trigger buffer, but still, +5V), relying on the 47kΩ resistor (R44) at the DD-20's input (and R51 and R74) to take the load (small though it maybe), or should I use a P-Channel JFET on the MIDI Clock output to act as a switch?  I'm a little nervous to try the 5V signal on the actual pedal, because while I THINK it should be OK, I'm not that strong on this stuff, and I REALLY don't want to mess up one of my favorite delay pedals!


Gabriel

free electron

Re: DD-20 Tap Tempo interface
« Reply #1 on: March 04, 2016, 03:41:12 AM »
The input has a pull up resistor, so any open collector output will drive it. That makes the level shifting quite easy.
Assuming MIDI clock module sends active high clock pulses (0->5V) I'd use a simple optocoupler (ie PC817). Not only it will take care about the level shifting, but also will separate grounds. One potential gnd loop source less.
Simplest way would be an NPN BJT, just drive it through 10k from the clock output and use the collector to pull down the tap tempo line to ground.

G. Hoffman

DD-20 Tap Tempo interface
« Reply #2 on: March 04, 2016, 05:57:40 AM »
Optos are too big - I'm trying to keep the board size tiny.  The power is transformer isolated, so I'm not worried about ground loops. 

I'll try some JFETs and BJTs tomorrow.

Thanks for the suggestions!

G. Hoffman

DD-20 Tap Tempo interface
« Reply #3 on: March 07, 2016, 06:26:18 PM »
The JFET worked great (J176), and I sent the board off for fab.  Thanks for the help.  I added a polarity switch to make sure it works right. 


Gabriel