digital logic - digitaly controlled voltage divider for picky audio recording enthousiasts

I want to replace expansive multi-deck rotary switches with a micro-controller and digitally controlled (i2C or ISP) analog switches.

It will be used in a vintage (1972) discreet transistor microphone preamplifier design (Neve 1073).

The original used "noizy" carbon resistors dividers to attenuate signal between stages. I want to keep the signal as "clean" as the original, wich should be easy as far as I understand it.

I wonder if there are any drawbacks in using cheap ICs like the new Vishays DG series Analog switches IC. Distortion? Noise? Etc...

I need to pass low (100mVac) and hi(15Vac) level audio signals through the switches. A ROn of 120Ohms or less is ok (I guess) with this design.

serial communication would be great (I2C or ISP)

I'm not an expenrienced EE and I'm lost among the available choices. I have no experience passing audio signals through this kind of ICs.

My big vintage console is an important selling argument. ICs have a bad reputation among audio enthousiasts so I must be carefull if I break the "discreet rules" as I will need good arguments to convince my clients no harm will be made to their music.

Here is a simplified shematic of the circuit. The divider switches are symbolized by pots. I Hope I gave enough info. Thanks for your insights.

Answer

For a bargain in FET switches and precision analog resistor networks in the same package, take a look at MDACs like the AD7528. The signal path is just that : FET switches and resistors, nothing else.

They have been used in some quite high end audio products.

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Downsides:

• Conversion from gain settings (dB, log scale) to DAC codes is easiest with a microprocessor.


• The 8-bit AD7528 only covers a 48dB gain range, with coarse steps at the bottom end of the range.


• There is some crosstalk between channels in the same package, careful design can mitigate that to an extent.

One design approach, mitigating the second and eliminating the third, is to use one AD7528 per audio channel. Use both its channels to extend the attenuation range, e.g. using one as a coarse (6dB steps) and the other as fine gain adjustment, probably with a buffer (emitter follower) between them.