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Old 24th April 2002, 01:38 PM   #1
AKSA is offline AKSA  Australia
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Default A good digitally controlled resistor pot

Folks,

For some time I have been trying to source quality pots for
audio. By the time you specify dual gang, audio taper and
audiophile quality, the field shrinks to perhaps two, and if you add to that motorized control, you attract a very high price.

I have looked at cermets, and auditioned them. They sound
terrific, particularly in law faked linear taper, but dual
gang is difficult, and the only one worth its salt is the
Vishay P11. But you try and get them; they are almost
unobtainium.....

Out of desperation I am now looking at digital volume
controls (bring out the wooden cross!). Dallas Semi have
some interesting designs, and here's a description from
their website:

**********************************
The DS1808 is a dual-channel, digitally controlled,
log-taper potentiometer. Each potentiometer is comprised of
32 wiper terminal positions plus a mute position. The device
has three accessible potentiometer terminals that include
the high-side terminal (H), the low-side terminal (L), and
the wiper terminal (W). The resolution of the DS1808 is
shown in Figure 8 and represents 1dB per step for the first
12 taps, 2dB per step for the next 12 taps and 3dB per step
for the bottom 8 taps, providing a total attenuation range
of 60dB. The mute position of the DS1808 provides greater
than 90dB of attenuation. The wiper position on the resistor
ladder is selected via a 6-bit register, whose value is
controlled by the industry-standard 2-wire interface. The
interface consists of two control signals: SDA and SCL. The
DS1808 is available in a standard 45k resistor value. The
DS1808 is specified to operate over the industrial
temperature range (-40C to +85C) and is available in the
16-pin SOIC package. The DS1808 was designed for low-cost,
stereo volume control applications. The device is specified
to operate from 12V 10% supplies and accept a maximum
input signal range of 12V.
************************************

These ICs have a listed THD of less than 0.001% and input
capacitance of 10pF. The switching of the resistor chain is done by CMOS in the cheaper ones, and bipolar transistor in the more expensive chips. This is all pretty good, but I'm interested to know if anyone has tried them and what they sound like. This forum is a wonderful resource for those choosing not to reinvent the wheel.....

Alternatively, does anyone own some ultra fi equipment which
uses a digital pot? It is reasonable to assume that these
companies have done their homework and selected the right
chip, and if this does apply to you, do you know the pot they chose?

I would be most grateful for any informed comments.

Cheers,

Hugh

Hugh R. Dean
Research/Technical Director
http://www.printedelectronics.com
Melbourne AUSTRALIA
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Old 24th April 2002, 02:24 PM   #2
paulb is offline paulb  Canada
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Hugh, there was a long thread a while back on (I think) the PGA2310, did you see it? Alaskanaudio may have evaluated some of these parts by now; I know he was looking into it.
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Old 24th April 2002, 02:44 PM   #3
SteveG is offline SteveG  United States
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Default tolerance...

Did you notice that the tolerance of the resistor is +/- 20%? That is a fairly big tolerance! If you're interested, I have a pdf of a volume control circuit that uses 16 relays to provide attenuation in steps of 1.25db... (thanks to ftorres) You just increment/decrement a 6 bit binary number, and it switches the relays. You control the accuracy with resistors. I was planning to use this for a 6 channel preamp I am building.
Steve
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Old 24th April 2002, 03:01 PM   #4
sonnya is offline sonnya  Denmark
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What ever you do Hugh, try if you can use parts with SPI interface. It is much easier to control than IIC bus.
If it is fine 2Vrms and at the same be cheap i would go for CS3310 (used in german www.audionet.de suround preamp).

Sonny
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Old 24th April 2002, 03:26 PM   #5
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Default Controlling digital pots

You don't have to get into the nitty gritty of IIC or SPI, it's easy to control a Dallas or Analogue Devices digital pot with plain old 3 wire -- clock, enable and data-in and even a BasicStamp or BX-24 have plenty of I/O, the SHIFT-OUT functions on these embedded compiler chips make implementation a breeze, or you can do as I do and use PIC-BASIC PRO from MELABS to emulate the BasicStamp instruction set. The devices are easily cascaded and for even finer control you can gang high and low value pots. I believe that Microchip (PIC controllers) also has a line of digital pots. The National Semi digital pots (Overture series) are a little expensive.

Of course, if you need a log or reverse log function, you can always write it in software, or use a look-up table.
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Old 24th April 2002, 03:28 PM   #6
sonnya is offline sonnya  Denmark
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Default Re: Controlling digital pots

Quote:
Originally posted by jackinnj
You don't have to get into the nitty gritty of IIC or SPI, it's easy to control a Dallas or Analogue Devices digital pot with plain old 3 wire -- clock, enable and data-in and even a BasicStamp or BX-24 have plenty of I/O, the SHIFT-OUT functions on these embedded compiler chips make implementation a breeze, or you can do as I do and use PIC-BASIC PRO from MELABS to emulate the BasicStamp instruction set. The devices are easily cascaded and for even finer control you can gang high and low value pots. I believe that Microchip (PIC controllers) also has a line of digital pots. The National Semi digital pots (Overture series) are a little expensive.

Of course, if you need a log or reverse log function, you can always write it in software, or use a look-up table.
plain old 3 wire = SPI.
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Old 24th April 2002, 03:44 PM   #7
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Taken from Sonic Frontiers printed material regarding their line preamplifiers: "Much research and development went into selecting a volume control system which provided the functionality audiophile consumer demanded while not compromising sound quality in any way. After reviewing several alternatives, the Crystal CS3310 "electronic attenuator" chipset was selected."
Me myself wouldn't select anything else over quality series resistor installed permanently in the signal path and shunt element to the ground (prefferably switched resistors). I thought that 24 positions wouldn't be enough but since I'm using it now I know it's more than required.
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Old 24th April 2002, 05:01 PM   #8
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Default SPI for the BX24

I guess it's a matter of semantics when it comes to these specialized MCU's like the BX-24. SPI and its command set is not conventional serial peripheral interface, "SPI", it's a specialized set of one pin commands (there are also Dallas One-Wire protocols and X-10 protocols for the BX24.) It's easy to freeze the BX24 if you incorrectly program the SPI and I have found SHIFTIN and SHIFTOUT painless.
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Old 24th April 2002, 05:15 PM   #9
sonnya is offline sonnya  Denmark
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Hi Jackinnj, it is a bit off the thread. But you can control 3-Wire devices from a SPI bus.
The main difference is that 3-Wire units only can recieve data where real SPI unit can recieve and transmit even be a master or slave. They can also tristate the output.
Most "SPI" or 3-Wire units except micro's are slave units. a CS3310,PGAXXXX is a "SPI" slave device and you can daisychain them.

The triggy part is to set the polarity of SDI,SDO,SCLK and phase of SCLK. But the AVR family (ATMEL) has a nice chart where you can see the different modes and how to set the register.

But i think this should be discussed in the Digital thread.

Sonny
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Old 24th April 2002, 05:18 PM   #10
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Ever try the DS1666? Sounds good, easy to work with. Maybe not the "Ulitamte", but ever take a conductive plastic pot apart?

Jocko
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