See here:
http://www.geofex.com/Article_Folders/potsecrets/potscret.htm
Like the other guy says, politics aside, google is a wonderful thing...
If you have a spreadsheet program you can easily calculate and plot the curve that you will get- it's all Ohms law...
I_F
http://www.geofex.com/Article_Folders/potsecrets/potscret.htm
Like the other guy says, politics aside, google is a wonderful thing...
If you have a spreadsheet program you can easily calculate and plot the curve that you will get- it's all Ohms law...
I_F
Go to http://sound.westhost.com and look up Project 01. Not only can you make a linear pot behave like a log pot, it is argued that the results are moe accurate than a real log pot. This may be especially true with a double-ganged pot.
BTW, I only buy linear pots any more. It's not really all that big a deal in normal use but I definately discern that the "fake" log-pots respond more evenly than "real" ones.
BTW, I only buy linear pots any more. It's not really all that big a deal in normal use but I definately discern that the "fake" log-pots respond more evenly than "real" ones.
Hi,
Please note that any 'reverse log' pot made in this way (law-faking) will only work as a full 3 terminal voltage-divider arrangement.
It doesn't work properly as a simple two terminal 'variable resistor' arrangement as the change in resistance which is more pronounced per unit of rotation at one extreme, will be at the 'wrong end' of rotation, unfortunately.
I know because I tried it out quite recently, myself, much to my disappointment, and when I subsequently made some suitable calculations of the parallel resistances involved at different points of rotation, it was quite clear to me why this is the case.
Also, it cannot simply be corrected by reversing the connections for a two terminal arrangement, either, as this makes no difference, of course.
Please note that any 'reverse log' pot made in this way (law-faking) will only work as a full 3 terminal voltage-divider arrangement.
It doesn't work properly as a simple two terminal 'variable resistor' arrangement as the change in resistance which is more pronounced per unit of rotation at one extreme, will be at the 'wrong end' of rotation, unfortunately.
I know because I tried it out quite recently, myself, much to my disappointment, and when I subsequently made some suitable calculations of the parallel resistances involved at different points of rotation, it was quite clear to me why this is the case.
Also, it cannot simply be corrected by reversing the connections for a two terminal arrangement, either, as this makes no difference, of course.
Notes on Audio Attenuators worth reading.
No it isn't. I really don't know how he was wiring his tapering resistors, or where he was connecting them, but his results are both bizarre and wrong. Many manufacturers use tapering resistors with linear pots in their higher quality equipment - it's a much more accurate way to achieve a logarithmic response.
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No it isn't. I really don't know how he was wiring his tapering resistors, or where he was connecting them, but his results are both bizarre and wrong. Many manufacturers use tapering resistors with linear pots in their higher quality equipment - it's a much more accurate way to achieve a logarithmic response.
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The current increase through the pot by adding a tapering resistor is insignificantly small. The change in current may be a few µA at most.
There really seems to be a paucity of basic electronic knowledge on this site! I've been successfully designing high quality recording and broadcasting studio equipment for over 40 years, and I have never had any professional engineer object to the use of tapering resistors because of the increased current through the wiper of the pot!
I suppose you believe in "musical" capacitors and high-priced "magic" interconnection leads too!
There really seems to be a paucity of basic electronic knowledge on this site! I've been successfully designing high quality recording and broadcasting studio equipment for over 40 years, and I have never had any professional engineer object to the use of tapering resistors because of the increased current through the wiper of the pot!
I suppose you believe in "musical" capacitors and high-priced "magic" interconnection leads too!
The emulation of a log law (or any other transcendental response) using simple polynomials involved in parallel or dividers networks is at best very crude; that's exactly what is shown graphically.
Another question is: is it necessary to have an exact log law for a volume pot?
I don't think so, precisely because the law is logarithmic, and tracking is probably much more important because any error will readily be detected, even if it is small.
For applications where absolute attenuation matters, a stepped attenuator is the solution.
That said, loading the wiper of a pot is not necessarily a good idea: to get a pseudo-log behavior, the burden needs to be significant, and taking into account the fact that the wiper to track resistance is the weak point of all pots, it is probably preferable to try avoiding it.
In principle, decent circuits are designed to ensure zero DC through the pot, meaning noise shouldn't be an issue, except the wiper resistance introduces a noise modulation to the signal. The effect is more subtle, and therefore nastier than straightforward noise: it causes a broadening of all spectral lines, which is difficult to identify, but definitely unpleasant
A better mousetrap!
The distortion introduced by the contact of a potentiometer wiper to the track is vanishingly small when compared to the non-linearities introduced by semiconductors (or valves, come to that).
I professional design, it is the accepted norm to use padded linear pots rather than the nasty pseudo-logarithmic ones used in domestic equipment. Linear pots even when padded are generally much better better matched when it comes to stereo pairs, and it's possible to calibrate the pot in actual dB!
When it comes to gain controls - rather than attenuators - I really like (and frequently use) the Baxandall gain control circuit, using a op-amp and a grounded wiper for the gain pot. Again, this uses linear pots, but uses the geometric change in gain introduced by the variation of both the negative feedback resistor and the inverting input to ground resistor as the wiper moves along the track. The subjective level versus pot position is superior to any other configuration that I've ever tried, and I've recently used this configuration in a prototype broadcast desk.
The distortion introduced by the contact of a potentiometer wiper to the track is vanishingly small when compared to the non-linearities introduced by semiconductors (or valves, come to that).
I professional design, it is the accepted norm to use padded linear pots rather than the nasty pseudo-logarithmic ones used in domestic equipment. Linear pots even when padded are generally much better better matched when it comes to stereo pairs, and it's possible to calibrate the pot in actual dB!
When it comes to gain controls - rather than attenuators - I really like (and frequently use) the Baxandall gain control circuit, using a op-amp and a grounded wiper for the gain pot. Again, this uses linear pots, but uses the geometric change in gain introduced by the variation of both the negative feedback resistor and the inverting input to ground resistor as the wiper moves along the track. The subjective level versus pot position is superior to any other configuration that I've ever tried, and I've recently used this configuration in a prototype broadcast desk.
No. I am from the other end of the spectrum: firm believer in Maxwell's equations etc.mictester said:
My point was that the weak part of a conventional pot is the wiper-track interface. If you can minimise current here by giving it a high impedance load then many of the (real or alleged) problems of 'ordinary pots' should disappear. Curious then that some audio people want to make things worse.
Two things to say about that:
1. pots age, and then the contact gets worse
2. active devices generally add low order distortion in a consistent way, poor contacts can add higher order distortion in an inconsistent way
The fact that something is the "accepted norm" in some area of human endeavour is not in itself evidence that it is a good idea. I'm not saying that loading resistors are bad, just that like all engineering they involve a compromise: you exchange tracking for potential distortion.
Goodness - we agree!
DF96 - you're entirely right. It is a compromise. A dual linear pot and associated padding resistor(s) is the least worst option.
Using the Baxandall variable gain op-amp approach has its possible problems too, (especially if the wiper to track contact is less than ideal) but has proved to be an excellent method in high quality, low noise analogue recording and broadcasting mixers.
The failings of the potentiometers are mitigated by using high quality components - modern parts are very good, with their conductive plastic tracks and so on.
DF96 - you're entirely right. It is a compromise. A dual linear pot and associated padding resistor(s) is the least worst option.
Using the Baxandall variable gain op-amp approach has its possible problems too, (especially if the wiper to track contact is less than ideal) but has proved to be an excellent method in high quality, low noise analogue recording and broadcasting mixers.
The failings of the potentiometers are mitigated by using high quality components - modern parts are very good, with their conductive plastic tracks and so on.
Yes. I find it amusing that music mixed using pots (albeit reasonably high quality ones) often has to be listened to using switched attenuators with expensive resistors in order to avoid 'degrading' the sound. Then, to make matters worse, these people are astonished to discover that they must use evil coupling capacitors (which 'smear' the sound) in order to avoid switching clicks. I won't say anything about coupling capacitors and recording studios as that might be cruel.
I just measured informally the intrinsic distortion of the wiper to track resistance for a few random samples: pot midway, wiper grounded, 3Vrms injected on one side of the track, distortion measured on the other side.
The values range between 0.02 and 0.1%.
That's not what I call "vanishingly small", when today's audio circuits routinely reach the ppm THD level or lower (not valve ones, obviously).
Of course here the offending resistance is measured in isolation, and in a real application, it would be diluted into other resistances, like the track resistance, but I wouldn't call it negligible.
Just for the sake of amusement, I cast my Audio Precision test set to examining the distortion through a couple of pots:
The cheap "Alps" stereo pot exhibited nothing measurable with 2V p-p and the wiper halfway along the track.
The cheap "no-name" cermet Chinese 10k linear fader exhibited nothing measurable under tha same conditions.
An even cheaper "Alpha" brand pot showed 0.002% distortion - predominantly second harmonic.
I really don't know what or how you were measuring your nasty pot distortion, but I just can't see it.
Next you'll claim to be able to hear <1% distortion.....
FYI: The "cleanest" recording chain I've ever worked on exhibited a cumulative ~0.15% THD from microphone input to tape recorder input, and was used to make the best analogue orchestral recordings I've ever heard. These were made in the late 1970s and have never been surpassed (or even equalled). Every stage of the process - from microphone to tape head - used a combination of bipolar transistors, a few FETs and lots of op-amps. The noise floor was largely determined by the inherent tape noise - my electronics certainly didn't add (or subtract) anything from that. Thinking about the circuit configurations involved, there would have been between 8 and 12 pots in the audio paths according to routing....
Now - back to designing a PWM broadcast limiter........
The cheap "Alps" stereo pot exhibited nothing measurable with 2V p-p and the wiper halfway along the track.
The cheap "no-name" cermet Chinese 10k linear fader exhibited nothing measurable under tha same conditions.
An even cheaper "Alpha" brand pot showed 0.002% distortion - predominantly second harmonic.
I really don't know what or how you were measuring your nasty pot distortion, but I just can't see it.
Next you'll claim to be able to hear <1% distortion.....
FYI: The "cleanest" recording chain I've ever worked on exhibited a cumulative ~0.15% THD from microphone input to tape recorder input, and was used to make the best analogue orchestral recordings I've ever heard. These were made in the late 1970s and have never been surpassed (or even equalled). Every stage of the process - from microphone to tape head - used a combination of bipolar transistors, a few FETs and lots of op-amps. The noise floor was largely determined by the inherent tape noise - my electronics certainly didn't add (or subtract) anything from that. Thinking about the circuit configurations involved, there would have been between 8 and 12 pots in the audio paths according to routing....
Now - back to designing a PWM broadcast limiter........
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