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.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
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.
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.
JohnRoberts said:For extra credit what was the nominal separation of a record cartridge .
JR
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.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.
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 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.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.
abbey road d enfer said: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.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.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 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.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.
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.
Agreed; I should have written "idle bus noise".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.
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?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.
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).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.
abbey road d enfer said:That's why I've always recommended pan-pot buffering. I know in valve equipment, it is not trivialruffrecords 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 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).
The output voltage is V1.(1 +R2/R3).
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.wolfgang said:One more Question about stability: Someone mentioned issues - is it suspect to oscillate or something else??
The arrangement works well enough for Soundcraft to continue to use it in its top-of-the-line MH series.I have seen this configuration in some Soundcraft mixers, so i suppose there are no problems?!
I don't think you do it the old way, with xformers, so you must use cath followers with substantial idle current, innit?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.
JohnRoberts said:For extra credit what was the nominal separation of a record cartridge .
abbey road d enfer said:I don't think you do it the old way, with xformers, so you must use cath followers with substantial idle current, innit?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.
abbey road d enfer said:What's the matter with separation at 40Hz?
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