ruffrecords
Well-known member
As a result of several enquiries I have for some time I have been investigating low cost ways of adding a pan control to a passive mixer. The initial idea for reducing cost came from what I call the 'Neve trick' that we used for stereo monitor level controls back in the 70s. The basic idea is shown in the top schematic here:
A single pot is wired across a balanced line. Its wiper becomes the hot output and the cold signal goes straight through. When the pot is fully off, although there is no signal between hot and cold, both of them get the full cold signal. This is effectively a common mode signal so how little of it reaches the final output depends on the common mode rejection ratio of the immediately following transformer. This allows a dual gang pot to be used to control the level in two balanced lines rather than the expensive quad pot that would normally be used. The fact that the fully off position is not quite fully off did not matter for monitors.
The middle schematic shows how you might apply this technique to a basic passive mixer. Not particularly useful but the bottom schematic shows how you could then use it to add a pan pot using only a dual pot instead of a quad pot. I even hand built a prototype for a client to try out although circumstances have so far prevented him from doing so. Listening tests indicated that this actually works reasonably well. I then received another request for much the same thing so this time I had some simple PCBs made as this client is handy with a soldering iron and was keen to build it himself. I built one of the PCBs and did some tests myself. Again listening tests indicated it worked but I did notice the all off position had some some residual signal. Barely audible but still there and this was using good quality Sowter 600:600 transformers so the CMRR should have been excellent. Also with the level pot fully open and the pan hard left, there was a similar residual in the right channel.
I then went round in circles for quite some time trying to measure the thing. Part of the problem is that there is 40dB loss to the bus so even if the residual was only 40dB below that it would be around -80dBu. So I fed +20dBu into it and found the residual was about -68dBu or only 48dB below the nominal signal. Probably OK for a pan pot but not good enough for a level control that is supposed to be turned off.
So I then looked for alternative balanced attenuators:
The standard method is shown on the left and uses a pair of pots. At least we don't need a quad pot but at least one of my clients wants slider faders on some stereo inputs to his passive mixer and quad slider faders don't exist. The second schematic shows a single pot fully balanced fader so a dual slider pot would do a stereo fader. The compromise here is that there is a minimum insertion loss of 4.4dB but I can probably live with that.
It then occurred to e that you could probably use the second technique to make a balanced pan pot:
To get a reasonable pan law from a linear pot you need to pad the input to the pot with resistor equal to the pot value. This means there is a loss of nearly 10dB when panned hard left or right. So there can easily be another 14dB loss in addition to the 40dB bus loss which possibly creates a noise issue - it certainly worsens it.
I need good attenuation from the level pot so it needs to be balanced. Four pots for stereo is not an option so the attenuator with a 4dB loss is my preferred route. I don't like the level drop cause by the balanced pan so the only remaining question is if the performance of the 'Neve trick' pan is good enough for a pan pot.
Thoughts, comments?
Cheers
Ian
A single pot is wired across a balanced line. Its wiper becomes the hot output and the cold signal goes straight through. When the pot is fully off, although there is no signal between hot and cold, both of them get the full cold signal. This is effectively a common mode signal so how little of it reaches the final output depends on the common mode rejection ratio of the immediately following transformer. This allows a dual gang pot to be used to control the level in two balanced lines rather than the expensive quad pot that would normally be used. The fact that the fully off position is not quite fully off did not matter for monitors.
The middle schematic shows how you might apply this technique to a basic passive mixer. Not particularly useful but the bottom schematic shows how you could then use it to add a pan pot using only a dual pot instead of a quad pot. I even hand built a prototype for a client to try out although circumstances have so far prevented him from doing so. Listening tests indicated that this actually works reasonably well. I then received another request for much the same thing so this time I had some simple PCBs made as this client is handy with a soldering iron and was keen to build it himself. I built one of the PCBs and did some tests myself. Again listening tests indicated it worked but I did notice the all off position had some some residual signal. Barely audible but still there and this was using good quality Sowter 600:600 transformers so the CMRR should have been excellent. Also with the level pot fully open and the pan hard left, there was a similar residual in the right channel.
I then went round in circles for quite some time trying to measure the thing. Part of the problem is that there is 40dB loss to the bus so even if the residual was only 40dB below that it would be around -80dBu. So I fed +20dBu into it and found the residual was about -68dBu or only 48dB below the nominal signal. Probably OK for a pan pot but not good enough for a level control that is supposed to be turned off.
So I then looked for alternative balanced attenuators:
The standard method is shown on the left and uses a pair of pots. At least we don't need a quad pot but at least one of my clients wants slider faders on some stereo inputs to his passive mixer and quad slider faders don't exist. The second schematic shows a single pot fully balanced fader so a dual slider pot would do a stereo fader. The compromise here is that there is a minimum insertion loss of 4.4dB but I can probably live with that.
It then occurred to e that you could probably use the second technique to make a balanced pan pot:
To get a reasonable pan law from a linear pot you need to pad the input to the pot with resistor equal to the pot value. This means there is a loss of nearly 10dB when panned hard left or right. So there can easily be another 14dB loss in addition to the 40dB bus loss which possibly creates a noise issue - it certainly worsens it.
I need good attenuation from the level pot so it needs to be balanced. Four pots for stereo is not an option so the attenuator with a 4dB loss is my preferred route. I don't like the level drop cause by the balanced pan so the only remaining question is if the performance of the 'Neve trick' pan is good enough for a pan pot.
Thoughts, comments?
Cheers
Ian
![Electronics Soldering Iron Kit, [Upgraded] Soldering Iron 110V 90W LCD Digital Portable Soldering Kit 180-480℃(356-896℉), Welding Tool with ON/OFF Switch, Auto-sleep, Thermostatic Design](https://m.media-amazon.com/images/I/41gRDnlyfJS._SL500_.jpg)
