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6 hours ago
Hi Guys
In an audio system there has to be a point in the signal path where the signal can be made smaller - all the way to zero - allowing the listener to regulate how loud the sound is. A voltage divider made from fixed resistors with the tap points selected by a switch will accomplish this goal, but not very gracefully. We can add more resistors in the divider string, creating more tap points and add more positions to the switch, for smaller loudness increments. Integrated forms of this feature highly in many hifi offerings.
We can expand this idea to as many points as we wish and there are many hifi systems that use relays to select the corresponding taps in a stereo system. The simplest use the panel switch to control the relays directly, while others replace the switch with a vector control and digital interface. Integrated circuits are available as 'electronic volume controls' where the resistive elements are selected by CMOS switches during the signal zero-cross to reduce switching noise.
A variable resistance will do this much more easily and with the potential for infinite resolution, i.e. equivalent to an infinite number of resistive taps and switch positions, and thus infinite loudness levels.
The potentiometer has a resistive track with a total specified resistance from end to end. There will be a tolerance for the value, which is usually 20%. This means a 10,000-ohm pot can be 8,000-ohms up to 12,000-ohms and still be within specification. Since the pot is the voltage divider in its entirety, this sloppiness of production does not matter, at least until you want to change more than one thing simultaneously.
A mechanical moveable contact can be wiped from one end of the track to the other. This contact is creatively called the 'wiper'.
The rate of change of resistance versus the rotation of the wiper is called the 'taper'. If the resistance varies in direct proportion with the percentage of wiper sweep, the taper is 'linear', denoted by a raised-letter 'B' in the value. A linear 10k pot is denoted as either B10k, or as 10kB.
For the loudness change to feel linear to our ear, the power has to change logarithmically. For it to seem half as loud, power must go down by ten times. A simple reduction by half would only feel "a little bit quieter'. Therefore, we need pots with a log taper, meaning that the signal must change very quickly in amplitude over the sweep of the pot. Where a linear pot set half way will have half the input voltage at its wiper, a log taper pot will have one-tenth. Log taper is denoted by a raised-letter 'A' as A10k or 10kA, for a 10k pot.
It is very difficult to lay down a logarithmic resistive track, so instead, small linear sections are laid down and the hope is that the transition points are not too audible.
In a mono audio system, the issues of pot value tolerance and taper irregularities can be minimized, and are generally non-problematic.The taper of the pot is selected for each pot position in the circuit, based on the circuit itself, and how the sonic result should be over the sweep of the pot. The basic division by circuit is 'passive' or 'active'.
Passive controls work freely comprised of resistors, capacitors and maybe chokes, but are otherwise independent of gain elements. All of these pots will be log taper, for Volume, level, and tone controls.
When a pot is inside the feedback loop of a gain stage, we may use linear or reverse audio, taper 'C' to achieve a linear loudness or effect change.
In a stereo system, or one with yet more audio channels, ganged pots with multiple sections are used. Each section is specified as for a single-section pot, but now we have a new parameter called 'tracking'. If you feed the same signal into all of the pot sections, will all the wipers have the same output voltage? For conventional pots, the answer will be 'no. Even for expensive pots of this type tracking will be within 3dB, which is quite noticeable.
ALPS from Japan has their famous 27mm stepped pots which are 42-position switches with resistors between the positions. Using 1% resistors allows these pots to track very well, within 1dB which is the nominal limit for Human perception of a loudness change. despite the opportunity to calculate the steps very accurately, many of these pots have a wide overall tolerance but do divide the signal properly. For modern equipment made ever smaller, this pot option is discarded.
The physical size of the pot has a relationship to its expected life time, measured as 'cycles', end-to-end wiper sweeps. Tiny pots like 6mm and 9mm diameter, may be rated for 1,000 to 10,000 cycles, or have no rating. 16mm are usually 15,000, as are average 24mm pots. 28mm mil-spec pots might be rated from 15,000 to 25,000. This is for carbon tracks.
Pots can be made with conductive plastic tracks and these generally have 100,000-cycle life ratings, at least down to 16mm.
All of the above are panel pots, with shafts that accept fixing a knob to, that can be easily manipulated by the equipment user and hopefully which matches the aesthetic of the equipment. A single rotation from CCW to CW is generally 300-degrees of the circle.
Wire-wound pots are made using a wire coil of resistance wire with a wiper that sweeps across one side of the coil. These or most often designed to handle high power, although there are precision units made by Bourns and Spectrol used for fine-setting of machine and instrument parameters. These are typically 10-turn pots, where the pot shaft is rotated ten times in the same direction for the wiper to move fully from one end of the track to the other. Life cycle is usually 1,000,000, but keep in mind it takes 10 turns to go end-to-end. These pots are not something that you make broad adjustments with, so they are unlikely to be used by hobbyists except for a bench power supply.
The 10-turn pot is also a panel-mount and requires a Vernier dial to be useful and for the operator to know where along the track the wiper is situated. A vernier dial shows which revolution the pot is at and the infinite division of that rotation. A small plate is secured under the nut that secures the pot to the panel; the plate has a tab that lines up with a slot in the knob and a set screw secures the knob to the shaft. These controls are surprisingly small, typically about 17mm diameter, not including the solder tab connections.
In an audio amp or musical instrument amp, even one with tubes, the signal voltages across the pots are generally small, less than 10V or so. This is good, because the pots used in these devices are often limited to 50-250Vdc. Linear pots have higher voltage ratings than log pots, where, for example, 16mm linear is rated at 250V and 16mm log is only 50V. This rating is based on the physical construction of the pot and has nothing to do with the resistive value. Moving up to 24mm pots gives more leeway at 500Vdc for linear and 100V for log.
Voltage ratings on pots is something that nearly all amp designers are unaware of. Even for those who know about it, they often make poor choices or assumptions for specific applications, such as when adding Power Scaling in MI amplifiers. In the basic form the pot sustains high voltage and must be of sturdy design, explaining why London Power used a mil-spec pot rated at 1,100V. This rating applied even to log taper pots, which turned out to be better for the application than linear pots. Copyists skimped and used standard 24mm pots with dire results. The present kits use less expensive 16mm pots that fit into smaller spaces, but they see much reduced voltage and will last their normal life.
Power ratings for pots are often routinely ignored. Again, linear pots have higher ratings than same-size same-value log pots due to the segmentation of the log pot resistive track. In most applications the power rating is not a limiting factor provided the resistive value is high. In guitar amps the lowest values might be 10k for a bias pot, or 25k for a midrange frequency control, with all others 100k or higher; majority values are 250k to 1M.
A 16mm linear pot is typically rated for 125mW and log is 60mW. If this is a 250k pot, the voltage across the track is limited to 176V and 122V, respectively, so not a big concern in common tube amp circuits. Hifi amps may use down to 1k pots, with 10k being the most common these days (and nights). Voltage limited by power is now 35V and just under 8V, respectively for linear and log at 1k.
In tube amplifiers that are fixed-biased, the bias pots should be not too high in value. London Power uses 25kB as standard.
In an audio system there has to be a point in the signal path where the signal can be made smaller - all the way to zero - allowing the listener to regulate how loud the sound is. A voltage divider made from fixed resistors with the tap points selected by a switch will accomplish this goal, but not very gracefully. We can add more resistors in the divider string, creating more tap points and add more positions to the switch, for smaller loudness increments. Integrated forms of this feature highly in many hifi offerings.
We can expand this idea to as many points as we wish and there are many hifi systems that use relays to select the corresponding taps in a stereo system. The simplest use the panel switch to control the relays directly, while others replace the switch with a vector control and digital interface. Integrated circuits are available as 'electronic volume controls' where the resistive elements are selected by CMOS switches during the signal zero-cross to reduce switching noise.
A variable resistance will do this much more easily and with the potential for infinite resolution, i.e. equivalent to an infinite number of resistive taps and switch positions, and thus infinite loudness levels.
The potentiometer has a resistive track with a total specified resistance from end to end. There will be a tolerance for the value, which is usually 20%. This means a 10,000-ohm pot can be 8,000-ohms up to 12,000-ohms and still be within specification. Since the pot is the voltage divider in its entirety, this sloppiness of production does not matter, at least until you want to change more than one thing simultaneously.
A mechanical moveable contact can be wiped from one end of the track to the other. This contact is creatively called the 'wiper'.
The rate of change of resistance versus the rotation of the wiper is called the 'taper'. If the resistance varies in direct proportion with the percentage of wiper sweep, the taper is 'linear', denoted by a raised-letter 'B' in the value. A linear 10k pot is denoted as either B10k, or as 10kB.
For the loudness change to feel linear to our ear, the power has to change logarithmically. For it to seem half as loud, power must go down by ten times. A simple reduction by half would only feel "a little bit quieter'. Therefore, we need pots with a log taper, meaning that the signal must change very quickly in amplitude over the sweep of the pot. Where a linear pot set half way will have half the input voltage at its wiper, a log taper pot will have one-tenth. Log taper is denoted by a raised-letter 'A' as A10k or 10kA, for a 10k pot.
It is very difficult to lay down a logarithmic resistive track, so instead, small linear sections are laid down and the hope is that the transition points are not too audible.
In a mono audio system, the issues of pot value tolerance and taper irregularities can be minimized, and are generally non-problematic.The taper of the pot is selected for each pot position in the circuit, based on the circuit itself, and how the sonic result should be over the sweep of the pot. The basic division by circuit is 'passive' or 'active'.
Passive controls work freely comprised of resistors, capacitors and maybe chokes, but are otherwise independent of gain elements. All of these pots will be log taper, for Volume, level, and tone controls.
When a pot is inside the feedback loop of a gain stage, we may use linear or reverse audio, taper 'C' to achieve a linear loudness or effect change.
In a stereo system, or one with yet more audio channels, ganged pots with multiple sections are used. Each section is specified as for a single-section pot, but now we have a new parameter called 'tracking'. If you feed the same signal into all of the pot sections, will all the wipers have the same output voltage? For conventional pots, the answer will be 'no. Even for expensive pots of this type tracking will be within 3dB, which is quite noticeable.
ALPS from Japan has their famous 27mm stepped pots which are 42-position switches with resistors between the positions. Using 1% resistors allows these pots to track very well, within 1dB which is the nominal limit for Human perception of a loudness change. despite the opportunity to calculate the steps very accurately, many of these pots have a wide overall tolerance but do divide the signal properly. For modern equipment made ever smaller, this pot option is discarded.
The physical size of the pot has a relationship to its expected life time, measured as 'cycles', end-to-end wiper sweeps. Tiny pots like 6mm and 9mm diameter, may be rated for 1,000 to 10,000 cycles, or have no rating. 16mm are usually 15,000, as are average 24mm pots. 28mm mil-spec pots might be rated from 15,000 to 25,000. This is for carbon tracks.
Pots can be made with conductive plastic tracks and these generally have 100,000-cycle life ratings, at least down to 16mm.
All of the above are panel pots, with shafts that accept fixing a knob to, that can be easily manipulated by the equipment user and hopefully which matches the aesthetic of the equipment. A single rotation from CCW to CW is generally 300-degrees of the circle.
Wire-wound pots are made using a wire coil of resistance wire with a wiper that sweeps across one side of the coil. These or most often designed to handle high power, although there are precision units made by Bourns and Spectrol used for fine-setting of machine and instrument parameters. These are typically 10-turn pots, where the pot shaft is rotated ten times in the same direction for the wiper to move fully from one end of the track to the other. Life cycle is usually 1,000,000, but keep in mind it takes 10 turns to go end-to-end. These pots are not something that you make broad adjustments with, so they are unlikely to be used by hobbyists except for a bench power supply.
The 10-turn pot is also a panel-mount and requires a Vernier dial to be useful and for the operator to know where along the track the wiper is situated. A vernier dial shows which revolution the pot is at and the infinite division of that rotation. A small plate is secured under the nut that secures the pot to the panel; the plate has a tab that lines up with a slot in the knob and a set screw secures the knob to the shaft. These controls are surprisingly small, typically about 17mm diameter, not including the solder tab connections.
In an audio amp or musical instrument amp, even one with tubes, the signal voltages across the pots are generally small, less than 10V or so. This is good, because the pots used in these devices are often limited to 50-250Vdc. Linear pots have higher voltage ratings than log pots, where, for example, 16mm linear is rated at 250V and 16mm log is only 50V. This rating is based on the physical construction of the pot and has nothing to do with the resistive value. Moving up to 24mm pots gives more leeway at 500Vdc for linear and 100V for log.
Voltage ratings on pots is something that nearly all amp designers are unaware of. Even for those who know about it, they often make poor choices or assumptions for specific applications, such as when adding Power Scaling in MI amplifiers. In the basic form the pot sustains high voltage and must be of sturdy design, explaining why London Power used a mil-spec pot rated at 1,100V. This rating applied even to log taper pots, which turned out to be better for the application than linear pots. Copyists skimped and used standard 24mm pots with dire results. The present kits use less expensive 16mm pots that fit into smaller spaces, but they see much reduced voltage and will last their normal life.
Power ratings for pots are often routinely ignored. Again, linear pots have higher ratings than same-size same-value log pots due to the segmentation of the log pot resistive track. In most applications the power rating is not a limiting factor provided the resistive value is high. In guitar amps the lowest values might be 10k for a bias pot, or 25k for a midrange frequency control, with all others 100k or higher; majority values are 250k to 1M.
A 16mm linear pot is typically rated for 125mW and log is 60mW. If this is a 250k pot, the voltage across the track is limited to 176V and 122V, respectively, so not a big concern in common tube amp circuits. Hifi amps may use down to 1k pots, with 10k being the most common these days (and nights). Voltage limited by power is now 35V and just under 8V, respectively for linear and log at 1k.
In tube amplifiers that are fixed-biased, the bias pots should be not too high in value. London Power uses 25kB as standard.
