Controlling pots digitally

Digital potentiometers would be the preferred option. They can introduce noise if you don't think about what you are doing.

The catch with digital potentiometers is that people tend to drive them directly from a digital processor, which links to ground of your music electronics to the (noisy) ground of your digital processor.

The nervous designer will galvanically isolate the digital potentiometer from the processor that drives it, usually with an opto- isolator, though there are other options. Analog Devices has devised a chip-scale isolating transformer

and Texas Instruments have a capacitatively coupled range of parts like the ISO15 that thye got when they took over Burr-Brown.

You could also use a balun as a common-mode choke to keep the worst of the noise on the digital ground out of the ground wiring of the music electronics, but that does involve having a pretty good idea of what you are doing.

-- Bill Sloman, Nijmegen

"Barry"

** FFS - explain what speakers you have and what knobs you want to rotate. ** Sure thing - there must be millions who want to install motors to turn knobs on speaker boxes. ** FFS - explain what speakers you have and what knobs you want to rotate. ** Motors make noise.

..... Phil

It's possible. But you haven't said what type of pot/knob it is, or what type of speaker. Neither have you told us what the "functionality" is.

Maybe. You haven't told us any details, so that question can't be answered. Digital pots are quite limited in terms of their maximum voltage for instance.

It might. When you connect to a PC you start introducing another ground path. You'd might be looking at needing some isolation for starters.

Everything has noise. You haven't told us any application details or typical figures, that would help.

Dave.

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There are motorized pots available. You might be able to just substitute for the same value that you have now. The rate of change of resistance varies with pots though - linear taper, log taper, audio taper, etc. so watch what you get.

--
Jim

I have a JVC receiver/CD player combo in my office that has a motor-driven pot.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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They can introduce a lot of things if you're not careful. Mechanically turning the knobs on the pots that are there is always safe; replacing them with digital pots requires a knowledge of the circuits involved and circuit design.

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http://www.wescottdesign.com

:Hi, : :We have a pair of speakers which we need to control from a pc. The :functionality we wish to control is currently controlable via a set of :knobs on the speakers. Each knob controls a pot. : :From a non invasive perspective, could I turn the pot using a motor or :some similar mechanism? Could you perhaps direct me toward more info :on this subject. : :From an invasive perspective, would replacing the pot with a digital :equivalent be a workable option? Might it introduce noise into the :circuit? : :What ever the solution we choose, it should not introduce noise into :the cicuit. I would also be desirable also if the mechanism itself was :noise free since we work in a music environment. : :Thanks for your help, : :Barry

No matter whether the speaker in question is passive or active, you cannot do what you want without invasive "surgery" of some kind. Adding a motorized pot involves addition of a suitable power supply plus the necessary control circuitry. All the motorized pots I've seen are for low level signals with resistance at least 50Kohms and there is no way you could use one of these in a passive attenuator arrangement to vary volume.

Similarly, if you want to go the digital volume control route you will need a powered speaker so this means adding the necessary power amplifier and control interface circuitry. Then there is the question of generating noise in the low level signal interfacing circuitry, whether using cable control, IR or radio etc...

All in all, it isn't a simple or easy thing to do.

There are many possibilities. Motorized pots, digital pots, transistors, etc.

The problem here is it would require some electronic experience. The pot itself may actually be digitally sensed rather than attenuating the volume directly. In this case it would have no impact on noise. Adding a digital pot would be rather simple assuming you have an easy way to control the pot. A motorized pot would require a significant power source and would be expensive. A transistor can be used like a digital pot but requires more work in designing the circuit.

The main thing how much electronics experience you have. This would be a rather simple project for any EE graduate.

I have several pieces of JVC consumer-grade audio with motor-driven pots... like 15 year-old stuff... still working just fine. ...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
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One option that sometimes appears in high-end audio gear is a photoresistor/light source combination. The photoresistor goes in the audio path, and the lamp is controlled by a DC voltage from some external source. The minimum ON resistance is generally around 1K, and goes up to 2M when the lamp is dark. So if you can tolerate a minimum ON resistance of 1K, this could work. It's not expensive. Since there's no electrical connection between the audio side and the control side, and the lamp drive is DC, this shouldn't introduce noise.

Controlling the lamp power level is a separate problem. Use a linear drive circuit, not PWM. A D/A followed by a power transistor should work.

Motor driven pots are mostly for where you want to store the setups on a mix board and reset the sliders to stored values. In many modern audio boards, the audio doesn't go through the pots anyway; that's all done in the digital back end.

John Nagle

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

A lot of digital pots have non-volatile memory, of one sort or another. In the applications you mention there is usually a DSP, or at least a PIC, that can easily remember pot settings. That's how we do it, so don't bother with the NV memory in the pot itself.

Same.