Pedal switching matrix with the AD75019

[Thewoodguy]

What socket did you get? That's one thing I'm looking for right now

[G. Hoffman]

Keep us updated! Wish my work bench was clean like that

An hour earlier, it wasn't!  But I just finished my last project over the weekend, so it seemed like a good time.


Gabriel

[G. Hoffman]

It's just a generic smd socket, which I soldered onto a Schmartboard.   It was a bit of a pain to solder, but I got there in the end.

[Thewoodguy]

Gab. What micro controller are you planning on using?

[G. Hoffman]

Gab. What micro controller are you planning on using?

I haven't really thought about it yet - right now I just want to see how the chip actually works, as I find the data sheet less than informative. 

Mostly, I've always used PIC16F's, and I've got a bunch of those around, so I'll probably find the smallest one I've got with both UART and SPI built in, and use that.  I'm pretty sure I've got some 28 pin 16F882 or 16F886.  I've used the 16F887 a fair bit, and they're the same data sheet, so I know it reasonably well.

I've got an Arduino, but that would mean learning a whole new thing for a project where, to be honest, the micro doesn't have a whole lot to do - receive MIDI, send out SPI, and turn on some LEDs.  Unless someone wants to do that part for me.....


Gabriel

[Thewoodguy]

   

Digital pots are prone to digital noise, but you can isolate them with a opto-resistor.  Just create a voltage divider with another resistor (or, if you prefer, use two optos).   I've got a tested schematic on the PC - I'll try to post sometime when I'm not on my phone.
[/quote]

Did you find this for me? [emoji4]

[G. Hoffman]

   

Digital pots are prone to digital noise, but you can isolate them with a opto-resistor.  Just create a voltage divider with another resistor (or, if you prefer, use two optos).   I've got a tested schematic on the PC - I'll try to post sometime when I'm not on my phone.

Did you find this for me? [emoji4]
[/quote]




(Click for a legible version.)

For the data input, the +5V goes to Pot A, the ground goes to Pot B, and pin 3 goes to the wiper.  This is actually set up to use two opto's, and two digi-pots, but you could do almost as well by replacing LD 1 with, say, a 1k resistor.  You'll loose a bit at the top of the range, but probably not enough to matter, and you can make it up on the output amplifier.


Gabriel

[PRR]

> (Click for a legible version.)

If U1B is wired as an earth-input amplifier, what does LD2 bring to the party?

[G. Hoffman]

> (Click for a legible version.)

If U1B is wired as an earth-input amplifier, what does LD2 bring to the party?

LD2 is hooked up to a digital pot.  It isolates the signal from any digital noise that might get through.


Gabriel

[PRR]

Input of U1B is about 1 Ohm.

LD2 is unlikely to get below 100 Ohms.

[G. Hoffman]

Input of U1B is about 1 Ohm.

LD2 is unlikely to get below 100 Ohms.

You're going to have to explain that one to me.  I'll admit, the input and output amps are taken from a Tremulus Lune, but the way I see it the input of U1B is mostly from R17, and will depend on where that is set - or am I missing something. 

I can say for sure it works as intended - i.e., MIDI control of the signals output level.  Of course, that's with both opto's in place. 

[Thewoodguy]

[/quote]


I can say for sure it works as intended - i.e., MIDI control of the signals output level.  Of course, that's with both opto's in place.
[/quote]
Are you using that as a midi controlled volume then?

Input of U1B is about 1 Ohm.

LD2 is unlikely to get below 100 Ohms.

You're going to have to explain that one to me.  I'll admit, the input and output amps are taken from a Tremulus Lune, but the way I see it the input of U1B is mostly from R17, and will depend on where that is set - or am I missing something. 

I can say for sure it works as intended - i.e., MIDI control of the signals output level.  Of course, that's with both opto's in place.

[G. Hoffman]

You're going to have to explain that one to me.  I'll admit, the input and output amps are taken from a Tremulus Lune, but the way I see it the input of U1B is mostly from R17, and will depend on where that is set - or am I missing something. 

I can say for sure it works as intended - i.e., MIDI control of the signals output level.  Of course, that's with both opto's in place.

Yeah - the pot is from a Highly Liquid MPA - they don't sell the MPA's anymore, but have open sourced the code and the boards, so you can get the boards made and build them yourself fairly easily, assuming you have a way to program a PIC.  The boards are kind of big, in that they are through hole and not very tightly packed, so the boards are a bit pricey (from OSH Park, about $40 for three boards - yikes! - but no coding required!)


Gabriel

[G. Hoffman]

Input of U1B is about 1 Ohm.

LD2 is unlikely to get below 100 Ohms.

You're going to have to explain that one to me.  I'll admit, the input and output amps are taken from a Tremulus Lune, but the way I see it the input impedence of U1B is mostly from R17, and will depend on where that is set - or am I missing something. 

I can say for sure it works as intended - i.e., MIDI control of the signals output level.  Of course, that's with both opto's in place.

[free electron]

(Click for a legible version.)

For the data input, the +5V goes to Pot A, the ground goes to Pot B, and pin 3 goes to the wiper.  This is actually set up to use two opto's, and two digi-pots, but you could do almost as well by replacing LD 1 with, say, a 1k resistor.  You'll loose a bit at the top of the range, but probably not enough to matter, and you can make it up on the output amplifier.


Gabriel

Hint1: inverting opamp gain stage vs non inverting one. Basic opamp stuff. It looks like you are trying to use a typical voltage divider (3 terminal pot simulated by the LDs) and feed its wiper directly into the -IN of the opamp, which is a virtual ground point.
Hint2: biasing problems. Your opamp stages are referenced to Vbias, yet there is R23 biased to GND, serving actually no purpose (i'd put it before the C6 to keep that terminal at GND). LD2 is also referenced to GND instead of Vbias. LD2 brings actually nothing in terms of gain control. The AC signal gain of the U1B is determined by the R17/R13 ratio.

...what does LD2 bring to the party?

makes the U1B's output DC bias dance?

[Hatredman]

DC Bias Dance is a good band name. Or not.

[G. Hoffman]

Hint1: inverting opamp gain stage vs non inverting one. Basic opamp stuff. It looks like you are trying to use a typical voltage divider (3 terminal pot simulated by the LDs) and feed its wiper directly into the -IN of the opamp, which is a virtual ground point.
Hint2: biasing problems. Your opamp stages are referenced to Vbias, yet there is R23 biased to GND, serving actually no purpose (i'd put it before the C6 to keep that terminal at GND). LD2 is also referenced to GND instead of Vbias. LD2 brings actually nothing in terms of gain control. The AC signal gain of the U1B is determined by the R17/R13 ratio.

Got it.  So, get rid of LD2 if I make another one!


Gabriel

[G. Hoffman]

Still reading the data sheet, but I may have found a MUCH easier solution!


The AD8113. 

Pros:

• Output buffers built in
• Parallel data input - I can run it directly off of a logic pin out from one of several MIDI cards - NO PROGAMING REQUIRED!!!
• Easier (for me) package (QFP instead of PLCC)
• It's supposed to have less crosstalk

Cons:

• Almost 50% more expensive
• I/O is not bidirectional

That's all I can think of so far, though I only just printed out the data sheet, but I think this could make this thing a LOT easier to make.  You just send it MIDI data!  It will take 10 MIDI logic outs, so the open source MPA (which I have) isn't quite going to work (or it would take two...), but there are other options out there.


Gabriel

[G. Hoffman]

Nevermind - it's a 16 to one switch - it can't do parallel routing.


Gabriel

[PRR]

Gain of 2.

If you patched to cascade all 16 channels, gain would be 96dB!)

Their hack for unity gain is ugly.
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It IS 16 to 16. The decode has 256 outputs (+16 for enable). It is a crossbar where any intersection can have a cross-connect.