Read pot value while in circuit?

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Golgoth

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Hi everyone,

I would like to read my mixer's volume potentiometers' values. Measuring a pot value off circuit seems pretty easy but I could not find any example about how to do it inside a circuit.

This is with the objective of taking snapshots of analog gear and build a basic "recall" functionnality on my mixer. How do these things work on an SSL desk or Amek Recall?

Any tips?

Thanks!
 

Brian Roth

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SSL and Amek used an extra pot section with a DC voltage applied across that extra section. For example, a mono level control uses one pot section to control the audio level. A second pot section on that same control will provide a DC voltage that varies with the position of the pot and used for recall.

Bri
 

Golgoth

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SSL and Amek used an extra pot section with a DC voltage applied across that extra section. For example, a mono level control uses one pot section to control the audio level. A second pot section on that same control will provide a DC voltage that varies with the position of the pot and used for recall.

Bri
Hi Brian, thank you for the reply!

Another user here told me about that technique! Could you elaborate about why it is needed to duplicate the pot and how it is technically done (wiring and such..)? I guess it is not as simple as buying the same pot that is in my mixer and simply connect it..

Thanks a lot!,
G
 
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ruffrecords

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Usually the pot is also motorised. The dc pot voltage is sampled and stored. When the setting is recalled, a circuit drives the motor until the dc pot voltage equals the stored value.

Cheers

IAn
 

Golgoth

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Hi Ian!

I would like a simpler solution: just being able to read the value so I can save it, then load it and read again to compare the two (and manually move the knobs to match and recall). I already handled the write and compare digital side of the project, and just need to read the pots now :)

Best,
G
 

Brian Roth

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SSL and Amek desks had a totally analog main signal path. So, when a pot was required for an analog function (level control, EQ, etc) the pot was part of the analog signal path. Some pots required one, others required a second section to perform their analog "duties".

For recall, an additional section was added to each pot. That section had nothing to do with the actual audio signal. Instead it created a DC voltage which varied as the pot was adjusted. That variable DC voltage supplied "positional" information which the desk's automation computer stored for recall.

To do a recall of previous settings, the operator had to manually move each pot until the "computer" indicated the current position of the pot matched what was stored. In other words, the DC voltage from the pot "today" matched the DC voltage stored "last week", thus allowing a recall of the pot's physical position stored "last week".

On a large desk, it was a time consuming process to manually adjust each and every pot to match what was stored. But, it was how things worked Back In The Day.

Ian brought up a useful point. The faders on many of those desks were motorized, so the "computer" told the fader where to go. In that case, the fader had an extra track that created a variable DC voltage in addition to the tracks that carried actual audio program.

Bri
 

Brian Roth

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Usually the pot is also motorised. The dc pot voltage is sampled and stored. When the setting is recalled, a circuit drives the motor until the dc pot voltage equals the stored value.

Cheers

IAn
Well, on the SSL and Amek desks I've seen the only motorized controls were the faders. Things like EQ, etc. didn't have motorized pots. You had to manually match their settings.

Bri
 

Brian Roth

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Hi Ian!

I would like a simpler solution: just being able to read the value so I can save it, then load it and read again to compare the two (and manually move the knobs to match and recall). I already handled the write and compare digital side of the project, and just need to read the pots now :)

Best,
G
Sorry, no such thing as a free lunch. The extra section added to every pot to generate the variable DC voltage was done on the SSL and Amek desks. If there was some other magic solution beside that, I know the smart folks at SSL and Amek would have implemented it versus having to add an extra section to each and every rotary pot.

Bri
 

Golgoth

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SSL and Amek desks had a totally analog main signal path. So, when a pot was required for an analog function (level control, EQ, etc) the pot was part of the analog signal path. Some pots required one, others required a second section to perform their analog "duties".

For recall, an additional section was added to each pot. That section had nothing to do with the actual audio signal. Instead it created a DC voltage which varied as the pot was adjusted. That variable DC voltage supplied "positional" information which the desk's automation computer stored for recall.

To do a recall of previous settings, the operator had to manually move each pot until the "computer" indicated the current position of the pot matched what was stored. In other words, the DC voltage from the pot "today" matched the DC voltage stored "last week", thus allowing a recall of the pot's physical position stored "last week".

On a large desk, it was a time consuming process to manually adjust each and every pot to match what was stored. But, it was how things worked Back In The Day.

Ian brought up a useful point. The faders on many of those desks were motorized, so the "computer" told the fader where to go. In that case, the fader had an extra track that created a variable DC voltage in addition to the tracks that carried actual audio program.

Bri
That "Back In The Day" solution is exactly what I had in mind! My mixer is 100% analog too, all the pots are part of the signal path.

I had in mind to have all the new circuitry outside of the mixer, would the additional section for recall simply connect to the original pot?

I will try and get my hands on Amek Recall schematics, hopefully the recall circuit will be shown so I can get a better idea of what is going on there!

Best,
G
 

Brian Roth

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That "Back In The Day" solution is exactly what I had in mind! My mixer is 100% analog too, all the pots are part of the signal path.

I had in mind to have all the new circuitry outside of the mixer, would the additional section for recall simply connect to the original pot?

I will try and get my hands on Amek Recall schematics, hopefully the recall circuit will be shown so I can get a better idea of what is going on there!

Best,
G
You will have to replace each and every pot to add the required additional section that creates the variable DC "positional" info.

Bri
 

ruffrecords

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x

Ian brought up a useful point. The faders on many of those desks were motorized, so the "computer" told the fader where to go. In that case, the fader had an extra track that created a variable DC voltage in addition to the tracks that carried actual audio program.

Bri
Neve had this back in the mid 70s when I was there. The 'Necam' flying faders caused quite a sensation at the time. The principle was later applied to rotary controls but I do not know how successful that was. There was a big market in broadcast and FOH applications where you needed to rapidly change between setups so muting and group assignment were soon added.

Cheers

Ian
 

Brian Roth

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Ouch, that what @krabbencutter told me :( What is that section made of exactly? I don't understand why it is needed to replace the pots..

Best,
G
As an example, a mono send pot has only a single section (a single gang pot) to vary the analog audio level for that send. For recall, the manufacturers would add a second gang to that pot to create the variable DC voltage for recall.

In some cases, the pot would require two gangs to perform their analog audio task.....like a stereo level pot, parts of a EQ or filter. In those situations, they used a three gang pot; two gangs in the actual audio path, and the third gang to create the variable DC positional info.

Bri
 

Golgoth

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For recall, the manufacturers would add a second gang to that pot to create the variable DC voltage for recall.
This is the part I was missing! You can't just plug a cable on the volume pot center pin while in circuit and expect to read the correct gain value.. The pot itself has to have a dedicated "gang" (another row of pins correct?) to read from. That explains a lot!

Thanks again,
G
 

Brian Roth

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This is the part I was missing! You can't just plug a cable on the volume pot center pin while in circuit and expect to read the correct gain value.. The pot itself has to have a dedicated "gang" (another row of pins correct?) to read from. That explains a lot!

Thanks again,
G
Exactly! Another row of three pins on that extra gang to create the variable DC voltage positional info.

Bri
 

sahib

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Ouch, that what @krabbencutter told me :( What is that section made of exactly? I don't understand why it is needed to replace the pots..

Best,
G

The second section, which is used for positional control, is the same as the first section which is used for audio. Might be carbon or conductive plastic track. Except that the positional control second section is always linear.

If the potentiometers on your console do not happen to have a spare linear gang then you will need to replace them with the ones that do.
 

Golgoth

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The second section, which is used for positional control, is the same as the first section which is used for audio. Might be carbon or conductive plastic track. Except that the positional control second section is always linear.

If the potentiometers on your console do not happen to have a spare linear gang then you will need to replace them with the ones that do.
It's getting clearer and clearer, thanks a lot everyone for taking time to explain!

What is going on inside a pot that has 2 gangs like this? The two rows are isolated from each other but connected to the pot shaft?
 

Newmarket

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I would like to read my mixer's volume potentiometers' values. ...

You can't determine the pot position by measuring the audio output, Well you could work it out if you referred it to the input level as a reference but that is more complex to do than alternative solutions and would give problems when there was no or little signal.
You can have a VCA solution where the fader handles only DC voltage and uses DC to control the VCA. Obvs downside to this if you don't want all audio going through VCAs.
Audio faders (and rotaries in many places) are not linear. Faders will usually have an 'Audio Taper'. Sort of a modified log law that may depend on the manufacturer. Giving maximum sensitivity around the '0db' point - top of fader is usually '+10dB'. If you use this for reading position via DC you are likely to run into problems.
There IS a way to have audio via faders whilst tracking (non-automated) and automated for mixdown (via VCA) with a single track audio fader.
For automation purposes the fader is used with DC across it to read position and control a VCA.
The 'Audio Taper' problem is handled by reading the DC voltage at the wiper via a Log Amp to give an approximately constant resolution over the whole fader length - remembering that VCAs are usually dB/Volt response. When I was working on a project using this technique, a (then far more experienced) colleague was developing this part of the system. I wouldn't recommend it - log amps are difficult !
 

Brian Roth

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It's getting clearer and clearer, thanks a lot everyone for taking time to explain!

What is going on inside a pot that has 2 gangs like this? The two rows are isolated from each other but connected to the pot shaft?
Yes indeed! In a purely analog desk you might have a pot which adjusts two audio levels at the same time.....like a stereo send for an aux. In that case, the shaft of the control physically moves (ganged) independent sections of the control at the same time. Each gang is its own independent chunk of three pins, yet each gang's movements are physically moved at the same time by the single pot shaft.

Bri
 

Brian Roth

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It's been 10+ years so exact memory isn't accurate. I did work with two Amek 9098 series desks which had recall of every pot and switch. As I remember the automation system provided an on-screen replica of each channel...one at a time...and if any control needed to be manually adjusted, the graphics would show that particular pot in red color on the computer's display. When the physical position of the pot matched what was stored, the graphic of that pot would change to green on the screen.

I don't know if this is fact or legend, but allegedly an option for the 9098 desks provided a speech audio with Rupert himself saying something like "up...up" or "down....down" as you manually turned each pot.

Bri
 
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