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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. enter image description here

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  • \$\begingroup\$ Your 15VAC signal requirement doesn't stack up with your existing +24v amplifier power supply (ah! didn't spot the transformer at first) All IC switches graph a non-linearity of resistance versus input voltage, from which you will be able to derive a distortion figure if you use them in pot mode, though you might consider 'BBC mode' which avoids switch resistance issues (it's the off switches that cause the limitation). From RF experience, their noise is within a few dB of their resistance, audio end may or may not be the same, buy some and try them. \$\endgroup\$ – Neil_UK Jul 16 '16 at 4:59
  • \$\begingroup\$ Do you want to use standard audio potentiometers with stepper motors controlling them? I think it is kind of ridiculous but your customers might love it. \$\endgroup\$ – mkeith Jul 16 '16 at 5:11
  • \$\begingroup\$ Give them fixed gain. Many of the high-end systems have no tone controls so why should they have volume controls? Just tell them that that's the right volume to listen at and anything else is another form of distortion. ;^) \$\endgroup\$ – Transistor Jul 16 '16 at 8:34
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    \$\begingroup\$ You seem to have a clue about things. Your only hope for survival is to distantiate yourself from these people. \$\endgroup\$ – pipe Jul 16 '16 at 9:54
  • \$\begingroup\$ you can use a multiplying DAC (MDAC) as a digitally-controlled gain or attenuation device. it is essentially an R-2R ladder. \$\endgroup\$ – robert bristow-johnson Jul 16 '16 at 20:12
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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.


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.

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  • \$\begingroup\$ Very interesting (and surprising - to me at least) idea. I'll certainly give this a try. Thanks! \$\endgroup\$ – MeatBallRagu Sep 23 '16 at 20:19
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I highly doubt that there is ANY argument you could make to the guys who buy 1073s that will convince them a digipot of the ordinary sort is harmless (One of the reasons people go the old Neve desks is that they do not have VCAs in the chain (Seems to me to be cutting off nose to spite face, but whatever makes them happy).

Have you had a look at the stuff from THAT Corp? They do some very nice digipots intended for use in the feedback of mic amps that you may be able to use.

Another possible approach would be discrete jfet switches, Doug Self has much interesting to say about these in 'small signal audio design', may be worth a look.

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  • \$\begingroup\$ Thanks, I bought Self's book (look's very informative) and ordered some THAT CORP ICs for testing. \$\endgroup\$ – MeatBallRagu Sep 23 '16 at 20:14

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