Unusual pan-pot solution

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HAHA!

Anyway thanks alot!

Maybe i can wrap may head around the formula-math!

At least i will try!

regards,
Wolfgang

 
I have never quite understood the crosstalk argument against the wiper to ground cheap pan pot circuit. The only time it matters is when panned fully left ot right and the wiper to end stop resistances of most pots is less than an ohm. With a 10K pot the crosstalk is -80dB.

Cheers

Ian
 
ruffrecords said:
I have never quite understood the crosstalk argument against the wiper to ground cheap pan pot circuit. The only time it matters is when panned fully left ot right and the wiper to end stop resistances of most pots is less than an ohm. With a 10K pot the crosstalk is -80dB.

Cheers

Ian

The common pan circuit (I built many many that way), does not have a crosstalk problem, but there is arguably a kill issue.

Kill may be important for some broadcast consoles where you might use L and R for two completely unrelated feeds.  For stereo mixes separation is much ado about nothing.

For extra credit what was the nominal separation of a record cartridge .

JR
 
ruffrecords said:
I have never quite understood the crosstalk argument against the wiper to ground cheap pan pot circuit. The only time it matters is when panned fully left ot right and the wiper to end stop resistances of most pots is less than an ohm. With a 10K pot the crosstalk is -80dB.

Cheers

Ian
No. There are two resistors that drive the pot. It's the ratio of the residual to these combined res that govern the xtalk figure. My sim shows -65dB for off.
When the pan is used in conjunction with assign buttons for routing, this is not acceptable, particularly considering that it is bound to degrade over time.
 

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abbey road d enfer said:
No. There are two resistors that drive the pot. It's the ratio of the residual to these combined res that govern the xtalk figure. My sim shows -65dB for off.

I think it is not quite as bad as that - the upper output on your sim should be -3dB not -7dB making the crosstalk -69dB which for stereo work is perfectly acceptable.

When the pan is used in conjunction with assign buttons for routing, this is not acceptable, particularly considering that it is bound to degrade over time.

Agreed but that's more a matter of system design. Many designs have a pan in/out switch for just this reason though it is arguable whether the extra expense is less than using a dual pot in the first place.

Cheers

Ian
 
ruffrecords said:
abbey road d enfer said:
No. There are two resistors that drive the pot. It's the ratio of the residual to these combined res that govern the xtalk figure. My sim shows -65dB for off.

I think it is not quite as bad as that - the upper output on your sim should be -3dB not -7dB making the crosstalk -69dB which for stereo work is perfectly acceptable.
My schemo was not accurate, pls check new one. Pls note that I had to alter the pot value in order to have ca. 3dB pan law.
When the pan is used in conjunction with assign buttons for routing, this is not acceptable, particularly considering that it is bound to degrade over time.
Agreed but that's more a matter of system design. Many designs have a pan in/out switch for just this reason though it is arguable whether the extra expense is less than using a dual pot in the first place.
Rather than adding a PP defeat, which takes up real estate and defeats the economic relevance, I would rather use a proper PP design that doesn't degrade the mixer's S/N by 3dB for a start.
In fact, in order to have 3dB pan law, the basic attenuation must be 10 dB, instead of 3 for the standard dual-pot PP and your choice for the active one.
And BTW (and slightly OT), I think 3dB is not a proper pan law. The constant-power criterium is not valid in view of the radiation pattern of mirror speakers.
Constant SPL is a solid criterium.
 

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abbey road d enfer said:
ruffrecords said:
abbey road d enfer said:
No. There are two resistors that drive the pot. It's the ratio of the residual to these combined res that govern the xtalk figure. My sim shows -65dB for off.

I think it is not quite as bad as that - the upper output on your sim should be -3dB not -7dB making the crosstalk -69dB which for stereo work is perfectly acceptable.
My schemo was not accurate, pls check new one. Pls note that I had to alter the pot value in order to have ca. 3dB pan law.
When the pan is used in conjunction with assign buttons for routing, this is not acceptable, particularly considering that it is bound to degrade over time.
Agreed but that's more a matter of system design. Many designs have a pan in/out switch for just this reason though it is arguable whether the extra expense is less than using a dual pot in the first place.
Rather than adding a PP defeat, which takes up real estate and defeats the economic relevance, I would rather use a proper PP design that doesn't degrade the mixer's S/N by 3dB for a start.

Agreed. I am not so much trying to defend the cheap pan pot just point out that it is not quite as bad as it seems to have been painted. Not sure about your 3dB S/N degradation because in most cases S/N ratio has been determined by prior stages in the mixer and 3dB loss here has less effect on S/N than the typical 10dB loss in the channel fader that precedes the pan.

In fact, in order to have 3dB pan law, the basic attenuation must be 10 dB, instead of 3 for the standard dual-pot PP and your choice for the active one.
And BTW (and slightly OT), I think 3dB is not a proper pan law. The constant-power criterium is not valid in view of the radiation pattern of mirror speakers.
Constant SPL is a solid criterium.

This is a continuing debate. A mono source issuing from two speakers is 100% correlated so for constant power you would argue for -6dB at the centre position but this tends  to sound like the source moves back in the centre. True constant power of uncorrelated sources would of course suggest -3dB in the centre. Many designs seem to compromise somewhere between these two, typically in the region of -4.5dB.

What I dislike about both techniques, cheap and dual pot, is that they present a varying load to the driving electronics. This is not so much of a problem with op amps with their huge NFB and apparently very low output impedance (though I am constantly amazed by how much some mixer manufacturers load these op amps). For tube designs where the load driving capability is much less, it can be problematic.

Cheers

Ian
 
ruffrecords said:
Not sure about your 3dB S/N degradation because in most cases S/N ratio has been determined by prior stages in the mixer and 3dB loss here has less effect on S/N than the typical 10dB loss in the channel fader that precedes the pan.
Agreed; I should have written "idle bus noise".
This is a continuing debate. A mono source issuing from two speakers is 100% correlated so for constant power you would argue for -6dB at the centre position but this tends  to sound like the source moves back in the centre. True constant power of uncorrelated sources would of course suggest -3dB in the centre. Many designs seem to compromise somewhere between these two, typically in the region of -4.5dB.
I'm glad you agree on this point, because so many knowledgeable sound engineers and designers deny it. After all, 3dB is the broadcast norm, and who knows better than broadcast scientists?
What I dislike about both techniques, cheap and dual pot, is that they present a varying load to the driving electronics. This is not so much of a problem with op amps with their huge NFB and apparently very low output impedance (though I am constantly amazed by how much some mixer manufacturers load these op amps). For tube designs where the load driving capability is much less, it can be problematic.
That's why I've always recommended pan-pot buffering. I know in valve equipment, it is not trivial. That's one of the reasons why making a valve mixer and expecting the same amenities as a solid-state mixer is a receipe for disappointment (or a huge depletion of the bank account).
 
abbey road d enfer said:
ruffrecords said:
  What I dislike about both techniques, cheap and dual pot, is that they present a varying load to the driving electronics. This is not so much of a problem with op amps with their huge NFB and apparently very low output impedance (though I am constantly amazed by how much some mixer manufacturers load these op amps). For tube designs where the load driving capability is much less, it can be problematic.
That's why I've always recommended pan-pot buffering. I know in valve equipment, it is not trivial
That's one of the reasons why making a valve mixer and expecting the same amenities as a solid-state mixer is a receipe for disappointment (or a huge depletion of the bank account).

To a degree this is my mission. Very large valve mixers are impractical but a 12 channel 8 track with four buses and 4/6 aux sends is just about practical. The power requirement is containable, the size is surprisingly compact but the big technical difficulty is the routing. If all four auxes a switchable pre/post then pre fade you need to be able to drive 4 auxes plus the channel fader - if they are all 10K pots that's a possible load of under 2K. Post fade worst case you have the two pan pots plus their law slugging resistors plus up to four auxes which with 10K pots is even lower. Now, because the noise floor is generally higher in valve circuits it is a good idea to keep the internal operating level up - usually to +4dBu. This mainly no problem since it is fairly straightforward to design valves drivers that can output over +30dBu leaving 26dB of headroom. However, these drivers might drive 10K or even 5K at that level but they don't like loads of 2K or less. So you end up with higher value pots and bus resistors. The higher operating level mitigates against the small increase in bus noise due to these higher value resistors but it does represent a challenge in signal screening without HF loss.

Cheers

Ian
 
Hello!

Now i have my things together!!
After spending time with this circuit and abbey´s explanation i had a knot in my brain! i could not understand this line

The output voltage is V1.(1 +R2/R3).

And there i had problems to convert the formula and get the right results. Simulated it in LTSpice but no help. The Circuit works as advertised.

Today in the morning i had the idea:

It should be : output voltage is V1*(1+R2/R1)  according to abbeys schematic.

So if i look at the amp just as non inverting-amp plus R3 everything makes sense!
I made a little Excel-file to play with some values, you can do funny panning laws with this thing!

One more Question about stability: Someone mentioned issues - is it suspect to oscillate or something else??
I have seen this configuration in some Soundcraft mixers, so i suppose there are no problems?!

Thank you once more abbey!

regards,
Wolfgang


 
wolfgang said:
One more Question about stability: Someone mentioned issues - is it suspect to oscillate or something else??
There is a risk only if the wiper loses contact or the track of the potentiometer gets damaged. In a standard arrangement, this would cause drop-outs or complete extinction of signal, in the case of the active PP, the circuit may lose its balance and the output may stick to one of the rails.
I have seen this configuration in some Soundcraft mixers, so i suppose there are no problems?!
The arrangement works well enough for Soundcraft to continue to use it in its top-of-the-line MH series.
 
ruffrecords said:
This mainly no problem since it is fairly straightforward to design valves drivers that can output over +30dBu leaving 26dB of headroom. However, these drivers might drive 10K or even 5K at that level but they don't like loads of 2K or less. 
I don't think you do it the old way, with xformers, so you must use cath followers with substantial idle current, innit?
 
abbey road d enfer said:
ruffrecords said:
This mainly no problem since it is fairly straightforward to design valves drivers that can output over +30dBu leaving 26dB of headroom. However, these drivers might drive 10K or even 5K at that level but they don't like loads of 2K or less. 
I don't think you do it the old way, with xformers, so you must use cath followers with substantial idle current, innit?

Like most mixers you would aim to be unbalanced at this point and you certainly want to avoid any unnecessary expenditure on transformers (not to mention their weight and volume). Cathode followers, though have their own problems at large signal levels and low impedance loads so you would most likely be using be using improvements or variations on that theme such as the SRPP and as you say, substantial currents (for valves that is ) are involved.

Cheers

Ian
 
abbey road d enfer said:
What's the matter with separation at 40Hz?

The separation specs are usually quoted at 1K. 40dB is about average. At 10k 60dB separation isn't unusual. At 40Hz 20dB wouldn't be unusual.
 
In addition to Kambo's 'single-pot' posting, here is the active panning found in the Raindirk Symphony desks.
Center is -3db, max L/R is unity gain. Buffer-output's feed the routing-matrix.
The S-curve correction provides a nice positioning feel.

Does anyone have a LCR(S) panning example without pots with centre-taps and exotic multi-gang arrangements?

grT
 

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