Feasible? - Replacing multiple, analog pots with digital pots in analog EQ.

First time poster, here - thanks for keeping up this great resource!

I'll get right to it...

[u]My goals (using Arduino)[/u]

  1. Replacing 20 (40 for stereo) analog pots with digitally controlled pots
  2. Ability to read DCP settings on an LCD.
  3. Ability to store settings for recall.

I am only mentioning 2 and 3, in case they impact the solutions for 1. If not, Ill leave them out completely for now if not.

Status

I know nothing about digital, and I am just looking for advice to make some purchases and get started with some experiments.

[u]DCP[/u]

I'd like to use the AD5291 DCP. It seems to meet my basic requirements, which are precision (1%), and at least +/-15V operation.

Alternatives are welcome, but I figure Ill get a dosen of those to get me started.

[u]Choosing the right Arduino (and mystery parts)[/u]

I need to interface 20 pots. I am worried it will represent an awfully high capacitance build-up on the clock and data transfer ports.

Eight of the pots I am replacing are actually 4 x 2 gang pots, which means, they will never need separate control. At first, I thought this would mean I could just put them on shared SS pins to pull two low at once, but that was a dead end.

I considered daisy chaining all pots in pairs of two, so I am left with 10 x 2 in parallel. After doing a bit of reading, I get the impression that daisy chaining can be finicky, and I dont know if mixing parallel with serial that way is even possible - It also takes 32bits of code to interface the daisy chained pots.

For multiplying available ports, I have come across multiplexing, but I am blank as to which device to look at.


If I weren't worried about potential problems running 20 SPI devices in parallel, I would just get a MEGA board to have plenty of ports. I am not above brute force solutions.

I would really appreciate some help getting started, and getting some parts home.

Half a dosen AD5291 An Arduino xxx? And?

Gustav

What's the interface of the digital pots, I2C or SPI?

Assuming SPI, it should be possible to feed multiple pots in parallel. Show a circuit diagram of what you tried.

Connecting many devices, be parallel or daisy chained, may reduce the maximum clock rate, when the shared clock signal lines can not be driven fast enough by a digital output. Did you try a lower clock rate? Eventually line drivers can improve the signal shape.

Twisted pair or ribbon cable can help to reduce signal distortion.

Added caps to the power lines at every pot?

DrDiettrich: What's the interface of the digital pots, I2C or SPI?

Assuming SPI, it should be possible to feed multiple pots in parallel. Show a circuit diagram of what you tried.

Connecting many devices, be parallel or daisy chained, may reduce the maximum clock rate, when the shared clock signal lines can not be driven fast enough by a digital output. Did you try a lower clock rate? Eventually line drivers can improve the signal shape.

Twisted pair or ribbon cable can help to reduce signal distortion.

Added caps to the power lines at every pot?

Thanks so much for replying.

Pots are SPI.

I have done no practical experiments yet, so I only have a schematic of the EQ, which wouldn't make much of a difference.

I am mainly looking for the feasibility/which Arduino to buy, if theres a way to stack the multigang pots/if there exists a device Id call an "SPI hub", since I dont know any better.

Gustav

A hub is not normally required for using multiple SPI devices. Simply connect power and SCK to all devices, and MOSI and a digital output (slave select) to the first devices of every daisy chain. Every free pin can be used for slave select, for up to 17 distinct slaves or chains with an Uno, Pro Mini or Micro. Many more with a Mega, if you like.

DISCLAIMER - I've never used SPI (or a digital pot).

and get started with some experiments.

I'd say experimentation is the key. Try replacing one analog pot and see how it goes.

Analog audio circuits tend to be sensitive to noise and there is the potential for all those digital signals & clocks leaking into the analog.

There is also potential for turn-on, turn-off, and initialization glitches.

I am mainly looking for the feasibility/which Arduino to buy, if theres a way to stack the multigang pots/if there exists a device Id call an "SPI hub", since I dont know any better

I believe you can use a "address decoder" to enable one Slave Select pin on one chip at a time (or one pair of chips at one time). You can address (enable) 32 pins with 5 Arduino outputs working as address lines. Or, with 6 address lines you can independently enable 64 lines.

So, you can add-up the number of I/O lines you need for the digital pots, the LCD display, and whatever buttons/controls you want, and then decide which Arduino you want to use. (Memory or processing power shouldn't be an issue, so it's mostly a matter of how many I/O lines you need.)

  1. Replacing 20 (40 for stereo) analog pots with digitally controlled pots

Assuming you don't want to control left & right independently, you should be able to parallel the SPI lines for each pair of pots.

Eight of the pots I am replacing are actually 4 x 2 gang pots, which means, they will never need separate control. At first, I thought this would mean I could just put them on shared SS pins to pull two low at once, but that was a dead end.

Again, if you parallel the SPI lines all of the devices should behave the same.

...Are you sure you want to do this? ...It's a LOT of wiring so you'll probably want to have a PC board made, and the costs are going to add-up. You might look into the [u]Behringer Ultra Curve[/u] or the [u]MiniDSP[/u], etc.