Adding recall to a Shure M68 mixer

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***** M68X *****

The M-68 has an unusual configuration of the mix pots, which lends itself to a simple implementation of in circuit position reading.

Except for the Aux Input, all the pots are DC isolated. A capacitor may be added to the Aux In line if it is necessary to record this value as well.

The mix pots are all commoned with respect to ground and "hot".

A small microprocessor such as a Microchip PIC 16 or PIC 18 series, some of which include 10 or 12 bit A/D converters could be connected to the pot wipers via a high value low pass filter to exclude the audio; Perhaps, 1M & 10uF.

The microprocessor power supply, which may also be the A/D reference, may then be connected to the "hot" pot common.

A small keypad and digital readout could be added to record multiple pot settings.

That's it for the hardware. Just a little assembly language software required, or a bushel of 'C'.

Recall would necessarily be manual via adjusting each pot while observing the readout, or in the simplest configuration Channel No. plus Up & Down LED's.
 
Hi
Putting DC onto the pots is rather nasty for the audio aspects, a;though presumably that supply would be 'cut' when actually using it as an audio mixer. A more workable solution would be to replace the existing ([resumably single gang pots) with dual gang and kep the position reading completely separate. This is how SSL and AMEK do this although even then there is no 'scanning' activity during normal audio use.
Matt S
 
Assuming that the pots are audio or log taper - there is an issue in reading the wiper directly in terms of resolution along the fader length. For this use you would usually have a track for audio then a separate linear track alongside for positional feedback.
But if you're happy with the results using a non linear track then you could snapshot the desk.
 
Reading and resolving a linear track is rather easier than a 'Log' taper' typical of audio controls as of course the pot will produce a log law of measurements. Cheap 12 or 14 bit convertors can handle this although may suffer from 'noise' towards the lower end of the track although arguably less important as cutoff from -60 dB and less is not relevant. Trying to 'read' a pot track resistance (position) in the presence of audio signal on the same trackwould be near impossible. Having 3-5 Volts of DC on an audio track would of course produce horrible noises in the wanted audio signal as I said earlier.
 
Reading and resolving a linear track is rather easier than a 'Log' taper' typical of audio controls as of course the pot will produce a log law of measurements. Cheap 12 or 14 bit convertors can handle this although may suffer from 'noise' towards the lower end of the track although arguably less important as cutoff from -60 dB and less is not relevant. Trying to 'read' a pot track resistance (position) in the presence of audio signal on the same trackwould be near impossible. Having 3-5 Volts of DC on an audio track would of course produce horrible noises in the wanted audio signal as I said earlier.

Yes re Taper. As I think I've mentioned previously I did work on a project where the manufacturer wanted to use only a single track fader that usually carried audio - so audio taper.
The 'answer' was to switch DC across the track and read the wiper via a logarithmic amplifier to linearise the result - then the ADC reads the linearised signal.
I didn't do this part myself and it was the most challenging part of the hardware design for the designer who did. Log Amps over several decades of signal magnitude aren't easy to 'keep in line' wrt temperature stability etc.
To be clear the fader had either Audio or DC on it - not both at same time.
In 'Automation' mode with DC on the faders the levels were controlled via VCAs (VCA Automation System).

But for the 'Snapshot' requirement of the OP you could design a system that switched to 'DC', read/store the fader positions, Switch back to Audio.

I think the suggestion to use ac signal is with intention to be able to use the existing circuitry rather than needing to inject dc post ac coupling ?
You'd then need to signal detect the ac level on the wiper (and optionally 'log amp') it. I suppose you could use a high frequency to aid level detection if required and the existing circuitry allowed for it.

It all seems a lot of hardware and bother :)
 
Log Amps over several decades of signal magnitude aren't easy to 'keep in line' wrt temperature stability etc.
Some of the AMEK fader automation used had 'log amplifiers (I suppose antilog amplifiers really) as the data was converted at 8 or ? bit resolution but later when A/D chips became more common and cheaper higher resolution (16,000 count) was possible without the 'unlogging' stage. Of course fortunately the fader area of most interest (top to about 40db attenuation) only needed relatively few 'counts' as the difference between 65db attenuation and 75db attenuation is insignificant in the grand scheme.
I added temperature stabilisation to the antilog amplifier circuit that was incorporated into the production desks.
 
Anyone have an idea on adding a phantom power circuit to this mixer? I have a couple of them and thought this would be a great way to repurpose these old mixers for “not so critical” applications.

thanks everyone.
There are a number of commercial phantom power accessories, that can be used externally.

It was common to incorporate low voltage phantom power (12-15V DC) inside fixed install mixers. Usually through lower value resistors than the 6.8K used with full 48V phantom.

Of course some premium recording mics won't work from 12V phantom, but many do. Hint: the phantom power accessories powered by single 9V batteries, do not supply 48V DC.

YMMV

JR
 
Some of the AMEK fader automation used had 'log amplifiers (I suppose antilog amplifiers really) as the data was converted at 8 or ? bit resolution but later when A/D chips became more common and cheaper higher resolution (16,000 count) was possible without the 'unlogging' stage. Of course fortunately the fader area of most interest (top to about 40db attenuation) only needed relatively few 'counts' as the difference between 65db attenuation and 75db attenuation is insignificant in the grand scheme.
I added temperature stabilisation to the antilog amplifier circuit that was incorporated into the production desks.
Yes - I always thought that the term "Antilog Amplifier" was more correct but it seems that I (We) were 'outvoted' on that generally.
I can't recall what resolution ADC we had on it (It was DDA Profile desk) but it sounds about right. And don't recall what was actually achieved in resolution / ENOB or whatever metric is applied.
re fader attenuation - IIRC correctly the taper law ,on P&G faders anyway, is specified down to -40dB. After that it's the 'offness' at -infinity that is key.
While we are on this - it was in the days when DAC outputs were at a premium. Not many/any multichannel DACs. Used SSM2300 8 channel Sample/Hold ICs to get the VCA control voltages out.
 
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There were many 'cunning wheezes' to make usable systems and 'modern' designers don't appreciate the 'ease' and choice of parts that are now available compared to the days when an 8 bit (slow) A/D D/A was a significant expense. The efforts to make 'slide' faders have produced interesting ideas such as the helical 'vane' on a 1960's Studer valve mixer so that it can use a high quality rotary pot as the actual audio element. I wonder what became of linear motors and 'optical grating' used by AMS around 1988?
 

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