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Old 28th January 2003, 07:50 PM   #1
Bricolo is offline Bricolo  France
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Default Digital potentiometer as volume control

Hi all


I was wondering if a digital pot (a pu/down one) would be good as an input pot (for volume control)

Are they OK, or is there any problem with them? (too noisy, pops when changing volume...)
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Old 29th January 2003, 07:27 AM   #2
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No, there ren't any problems using digit pots.
You only should beware the maximum rating voltage of each leads, by exemple, using a DS1267 (ARC SP16), if the alim rails are +5V and Vb (the subtrat vaoltage lead) -5, you could only swing between this values.
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Old 29th January 2003, 07:35 AM   #3
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ok


and for the noise? are they worse than an analog pot?
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Old 29th January 2003, 08:07 AM   #4
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Right now you have a couple of good digital potentiometers to choose from. Some of them can handle more than +-5 Volts but at the moment I can't remember the name or number. The new chips have real resistive elements and have low noise and distortion, no pops, with or without memory, with or without microcontroller, worth checking out.
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Old 29th January 2003, 09:06 AM   #5
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You should look for the ds1802 some comments are made about these units:
http://www.diyaudio.com/forums/searc...der=descending

hth

Emiel
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Old 29th January 2003, 11:11 AM   #6
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Concerning sound, yhe DS1267 is used in the last Audio Research preamp SP16, which is not really bad

More using a µcontroler you can have a remote control

(Si tu veux plus d'info sur les microcontroleurs mail moi )
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Old 29th January 2003, 12:49 PM   #7
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Default the problems with digital pots

is their temperature dependency --

you would probably want two in series -- say a 100K and a 1K -- this will give you a lot of precision.

be mindful that the wiper resistance is usually 50 ohms, not zero.

you can write an algorithm for a logarithmic curve, or just go ahead and purchase the National LM1971, LM1972 or LM1973 series.

or you can fashion a quasi-lorgarithmic voltage controlled gain amplifier by connecting one arm to the output of an op-amp, the wiper to the inverting input and the other arm to the analog input.

THD on digital pots -- the manufacturers have stats -- but Iwould want to check these out myself.

Digital switches -- like the dual 4 channel units from Analog Devices and Maxim combined with discrete transistors might be a preferable from a thd standpoint. These units are used in high end avionics gear.
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Old 29th January 2003, 02:39 PM   #8
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I have used the LM1972 and 73 with excellent results. There is no noise at all during transitions, and 0.5 and 1 dB steps are pretty darned small- it doesn't sound like it is stepping. It just sounds like a smooth volume change.

I used a PIC16F84 microcontroller to bit bang the 3 wire signals into the attenuator chips and put the mpu to sleep (which also puts the clock to sleep when you use a ceramic resonator) when it wasn't changing volume. No digital stuff running at all means no digital noise leaking into analog circuits.

I used pushbuttons to control the volume setting, but if you like twisting knobs, you can use a rotary encoder instead. You can get cheap rotary encoders that will easily outlast any analog pot.

One nice thing you can do with digital pots and attenuators that is extremely hard/expensive to do with analog pots is setting balance by putting an offset on one of the digital pots. Also, channel to channel matching is usually excellent with digital pots so you can easily make multichannel volume pots that will track perfectly for the next 20 years- you can't do that with analog pots.

Another thing you can do if you make a preamp with multiple inputs is build in programmable volume offsets so that when you change inputs, if one source is "hotter" than another, the mpu will apply the offset to the volume setting so you don't get any blasting. Also, you can make nice ramp-down and ramp-up mute and unmute. It's all software, so it doesn't cost anything but time to write the code. The PIC mcus are reprogrammable, so you can create and add features as you think of them.

Check out the PGA2310 attenuator chip (Burr-Brown/Texas Instruments). It uses +/-15V supplies so it can handle just about any audio signal you want to give it.

MR
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Old 29th January 2003, 02:54 PM   #9
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Default Dig pots

Mark,

To keep THD down, you normally want to keep signal across the switches low, which would mean using them on the virtual ground input of an opamp or something similar. Do you have any experience in this area as well?

Jan Didden
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Old 29th January 2003, 02:59 PM   #10
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no, I don't want a microcontroller for the pot

Just a digital pot, with a +Vol and -Vol pushbutton
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