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Old 11th February 2004, 04:49 PM   #1
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Default Digitally controlled Baxandall?

Hi,

I'm thinking about building a new preamp, and want it to be fully operatable by remote control. For this everything should be controlled by a uC.

I'll most probably use relays for input selection and CS3310/PGA2310/DS1808/WM8816 as volume control. Or maybe I'll use shunt resistors for attenuation, possibly using SSM2404 switches.

Anyways, the attenuator problem is throughly discussed in the forum, but I'd also like my preamp to offer tone controls.

How do I make this digitally controlled? If I was to use digital pots, what kind should I use? I can't use any of the volume chips as pots in a Baxandall circuit, can I? Maybe there are other kinds of circuits I should look into - like bandpass filters with separate attenuators? This, however, would make each band flat internally, as opposite to the curve given by Baxandall.

I hope you guys have a few good suggestions and views on this matter. Has anybody got any experience with remote controlled bass/treble circuits?

Regards,
Truls
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Old 11th February 2004, 09:48 PM   #2
Steven is offline Steven  Netherlands
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Have a look at http://www.maxim-ic.com/DigitalPotentiometers.cfm
They make digital linear potentiometers, like the (stereo) DS1867.
  • Nonvolatile version of the DS1267
  • Two 256-position digital potentiometers
  • Wipers digitally set and read through serial port
  • EEPROM maintains settings in the absence of power
  • Resistors can be connected in series for higher total resistance
  • Ultra low-power consumption, quiet, pumpless design
  • Three-wire serial control interface
  • Operating ranges:
  • Single +5V or 5V supplies
  • -40C to +85C

Steven
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Old 12th February 2004, 10:50 AM   #3
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Thanks, but will any digital pot be good enough for audio usage? I mean there has to be a reason why there are special chips for volume control (like CS3310).

Besides, all the digital pots from Maxim has 20% or worse resistance tolerance. That can't be good. How would AD5203 compare to DS1867?

So, is the preferred method for a digitally controlled tone control to use normal dig pots in a Baxandall circuit? What would the ultimate digital pot for this be?

Cheers,
Truls
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Old 12th February 2004, 11:47 AM   #4
Bricolo is offline Bricolo  France
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this one looks good
http://www.analog.com/Analog_Root/pr...AD5228,00.html
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Old 12th February 2004, 11:47 AM   #5
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Most circuits with dig pots or switches take care that the signal across the pot or switch is low to avaoid distortion due to on-resistance modulation. One way to do that is to have one terminal at gnd potential and/or using the variable element at the virtual gnd input of an opamp. Presumably, with tone controls, that may not be possible, meaning that the variable element will see widely varying signal levels wrt the supplies. So, you will have quite higher THD compared to the use as level/volume control.

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Old 12th February 2004, 12:35 PM   #6
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Right, and does anybody have a solution to this, or do I have to accept that the pots in a tone control circuit will make alot more noise than in an attenuator?

I was thinking of using this baxandall circuit by Rod Elliott because I like it for using two 100k pots. Most tone controls use different value pots, which makes it hard to implement with digital potmeters. I'm open for other types of tone controls as well, but this is the most appropriate baxandall circuit I've been able to find.

What should I look for when selecting a dig pot to use for this?

Any suggestions for a different approach?

-truls
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Old 12th February 2004, 12:43 PM   #7
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The tone controls used by QUAD (early models) were of the "shelving" type, they had a bit more flexibility and used grounded pots, IIRC. Don't know where to find info. Will look.

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Old 12th February 2004, 02:28 PM   #8
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... just found the QUAD 33 diagram, with the shelving tone controls, but the overall topology is the same: feedback based freq response manipulation, so the pots are floating with the signal. Bummer.

Jan Didden
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Old 12th February 2004, 02:50 PM   #9
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Hi, how about using an analoge mux to allow you to select from a number of fixed resistors in the circuit? This owuld depend on how the mux chip delt with the unselected inputs, and would only allow a few variables, but you should be bale to find some good quality ones out there which won't mess with the sound of the signal too much.
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Old 12th February 2004, 03:09 PM   #10
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Too bad Jan, but I found something that might look like what you were thinking of. Have a look at page 8 of the schematics document for Rane Corporation's MP 2016 Rotary Mixer. To me this looks like a low-pass, a band-pass and a high-pass filter, with one attenuator each, followed by a summing amplifier to mix the three parts together again.

This tone control is patent pending and is described in Dennis Bohn's article Accelerated Slope Tone Control Equalizers.

There's one thing about this tone control that troubles me, though. Since the filters can't be ideal, some parts of the signal will be removed even before the attenuators, or am I missing something here? Maybe I just revealed that I don't know the first thing about tone controls? :-/

-truls
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