Sansamp Programmable Bass Driver - how do they do it?? explained

[crane]

This is a great pedal and I've always wondered how exactly does it function...

So a friend of mine brought this unit to me and said that two of three buttons do not function any more. Although I was able to change the tact switches without taking apart the whole unit, I just couldn't resist...
There is this neat little chip called AD5206. DATASHEET  + a PIC controller. That's it.
I hope this was interesting and useful to someone.
Too bad I'm so busy right now with my two bands and hunder electronic projects as this chip puts ideas into my head.

[Morocotopo]

Programmable pots eh? This means that we can have presets in honest-to-god all analog pedals, right?

Lord, what else will they come out with? Electric cars?

Of course, looking at the datasheet most of it is chinese to me. I mean, I still have trouble biasing a diode! Or was it a transistor? Never mind. 

Nice thing is, it´s available in DIP.

Can anyone versed in this type of technology give us guys whose knowledge limit is a Tube Screamer and a BBD chorus a little primer on what´s needed to use this? Auxiliary chips and/or hardware? Programming skills?

[FiveseveN]

This means that we can have presets in honest-to-god all analog pedals, right?

Plenty have been made (though overpriced Electra and Fuzz Face variations seem to be more popular than such flexible effects), and this is just one way to do it.

Auxiliary chips and/or hardware?

No, it's actually fairly simple. The "normal" pots on the unit work as simple voltage dividers. For each control, this voltage (indicating the position of each analog pot) is read by the PIC through ADC, "translated" into an 8-bit value and sent to the corresponding digital pot via SPI. It's the setting of these digital pots that affects the analog part of the circuit (gain, volume, filters).
When pressing "save" (or holding down a preset button or whatever the sequence is), the current set of six 8-bit values (representing the position of each control) is saved in the PIC's memory, at a particular location.
And when pressing a preset recall button, the values are read from memory from the appropriate location and sent to the AD5206 again.

Programming skills?

Yes, and programming hardware. But one could sell preprogrammed kits.

[crane]

It's a shame you cannot program the value of the "pot". This means that Schematics have to be adapted to using single value pots (like some mass producted products have).

[FiveseveN]

That's a significant downside but only when you're striving for simplicity (basic discrete designs). E.g. you can scale gain and volume controls and use active filters.

[Morocotopo]

Five, thanks for the explanation.

A preprogrammed PIC or even a complete board would be great for those of us whose knowledge doesn´t include programming, wich seems quite an involved area of study...

[slacker]

It's a shame you cannot program the value of the "pot".

There's nothing to stop you putting two or more of the digital pots in series to make a variable resistor of a bigger maximum value. You could even make a larger value voltage divider by using two or more of the pots, some for the top half of the divider and some for the bottom, then make your software increase the value of one half whilst making the other smaller to put the wiper where ever you want. Easier to design/redesign things to use the available pot values though.

[oskar]

I started on a programmable pedal once but that stuff is packed down in boxes stored in some 'clever place so I won't forget it...'  and on crashed hard disks.
For a beginner it looks like a good alternative to use programmable pots because you could just start without much re-engineering. But I would rather use a DA converter (tlc7628 is the one I've used) and then distribute control voltages with analog switches. If you look at the FX schematics you've got lying around there is probably plenty of places where the potentiometer already is providing a reference voltage or being an attenuator (CV/VCA) and that is pretty easy to adapt to a programmable version.

[gritz]

Five, thanks for the explanation.

A preprogrammed PIC or even a complete board would be great for those of us whose knowledge doesn´t include programming, wich seems quite an involved area of study...

Most people don't build their first analogue stompbox by learning lots of theory and designing from scratch - they take an existing design. Then (when the urge to learn / explore comes upon them) they play with different circuit values, look for information on the interwebs, or ask questions on forums. It's the same with digital - no need to learn all about programming first - just get an Arduino (it's probably the best choice for a beginner because of the huge knowledge base), download some "cookbook" code, plug in your digital potentiometer and play / ask questions.

[Morocotopo]

Gritz, you are right. But somehow for me (and I guess many other builders too) there´s a mental leap involved in going from transistors, tubes and IC´s to programming. It´s like "huh, computers, that´s only for the geeks. I´m not one of them (yeah, sure), for me the older the technology the better! "
I guess in time I´ll have to delve into it. Time marches on, in a few years through hole stuff will be unobtainable, etc etc. But no way I´ll let go of my noisy, sputtery, crackly Fuzz Face!

[gritz]

...and there are probably a lot of gifted programmers who feel daunted by the sight of a soldering iron, or the thought of putting a project into a proper enclosure that can be stomped on (rather than an Altoids tin!)

Go for it Morocotopo!

[Morocotopo]

You know what? This afternoon I started reading about arduinos and PICs...

Oh, Lord... I feel as I did the first day in college... newbie, lost and frightened.

[gritz]

You know what? This afternoon I started reading about arduinos and PICs...

Oh, Lord... I feel as I did the first day in college... newbie, lost and frightened.

It's not fear - it's the thrill of the chase!