Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Default Taper of Potentiometers

"And when he brought me this:"

They got me once at the TV parts store. I needed a 223" color CRT for a Motorola. The guy says he only had 25" CRTs right now. I said "How am I going to fit that ?" and he replied "A cabinet stretcher". I said "OK, where do I get one of those ?".

Once and only once...
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"A simple way to get *very repeatable* "log" type curve for audio is to wire the two sections of a dual linear pot in series. The wiper of the first half goes to the top of the second and its wiper becomes the output. Bottom ends are both grounded.

At the centre (50%) position, this reliably gives -15dB attenuation when loading effects are taken into account.

At the 75% setting, attenuation is -6.5dB, at 25% it is -25dB while down at 10%, attenuation is -40dB.


...... Phil"

That is what high end audio equipment does. Have seen it a few times. It does make for a nice smooth taper. Also in preamps it enhances the S/N ratio at lower volume settings. Usually the first pot is somewhere near the tone control circuit, if any. The second pot is right at the preamp outputs.

Actually if a volume control is "too fast" a resistor between the wiper and ground side makes a pretty decent taper.

One time a guy broings in a receiver, an upgrade from the one he had but oth the same brand. He asked why he had to turn it higher to get the same volume level even though it was more power. I had to explain it so I used the accelerator in a car. I told him they could make the linkage so it is just about floored when you give it half a pedal, but does that make the car go faster ? Nope, it has the same horsepower (kilowatts for youse across the pond) no matter how the accelerator linkage is designed. I think he understood. I told him it gives him more accurate control.
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Default Taper of Potentiometers

Clifford Heath wrote:

----------------------

A simple way to get *very repeatable* "log" type curve for audio is
to wire the two sections of a dual linear pot in series. The wiper
of the first half goes to the top of the second and its wiper becomes
the output. Bottom ends are both grounded.

At the centre (50%) position, this reliably gives -15dB attenuation
when loading effects are taken into account.



Hah! Square-root taper


** Is that what it's called ?


Probably good enough for rock-n-^H^H^H^H^Haudio


** It's so repeatable, unlike with some batch of low cost log pots, you could have the -dBs figures printed on a front panel and they would always align with the pot.

If you know the source and load impedance of the pot,
wiring a linear pot with the input to the wiper and
output from the top gets you somewhere close to the
same effect.



** Horrible idea - it shorts the signal at low settings and sends the output impedance high just when you want it low for noise/hum reasons.


...... Phil
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Default Taper of Potentiometers

"** Horrible idea - it shorts the signal at low settings and sends the output impedance high just when you want it low for noise/hum reasons. "

Isn't that how some guitar pickups are wired ? And don't ask me why.
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wrote:

-------------------------


"** Horrible idea - it shorts the signal at low settings and sends the
output impedance high just when you want it low for noise/hum reasons. "



Isn't that how some guitar pickups are wired ? And don't ask me why.



** Only very rarely.

That method has just *one* advantage where there are multiple PUs, each with its own volume pot. It forms a resistive mixer that allows signal to pass from any pickup to the output regardless of the other pot settings.

The usual method requires no control be zeroed when PUs are switched in parallel or the instrument becomes silent.

The wiring schemes used in most electric guitars are primitive, full of dodges and compromises - good examples of how NOT to do it.


See wiring for early Gibson models.

http://archive.gibson.com/Files/schematics/lespaul2.gif




..... Phil



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Default Taper of Potentiometers

On 26/01/18 19:19, Phil Allison wrote:
Clifford Heath wrote:
Hah! Square-root taper

** Is that what it's called ?


No idea what it's called, but if a log pot can be
called that (even though the intended operation is
exponential, i.e. opposite of log, to compensate
for the log response of the ear) then this should
be square-root, because the actual curve is the
square function.

Clifford Heath.
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Default Taper of Potentiometers

"Strangely, I have not come across a reverse linear pot anywhere. "

Now I wonder what kind of taper is used in the pots in a graphic equalizer. Double reverse anti-log or some bizarre **** like that ?
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Default Taper of Potentiometers

Clifford Heath wrote:

-----------------------


Hah! Square-root taper

** Is that what it's called ?



No idea what it's called, but if a log pot can be
called that (even though the intended operation is
exponential, i.e. opposite of log, to compensate
for the log response of the ear) then this should
be square-root, because the actual curve is the
square function.


** At least one source calls it a "square law attenuator" or SLA.

http://www.bernacomp.com/elec/og2/sq...attenuator.gif

A SLA closely follows the formula:

Gain = 40log(position ratio) where closed =0 and open =1



..... Phil



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wrote:

-------------------------

Now I wonder what kind of taper is used in the pots in a graphic
equalizer. Double reverse anti-log or some bizarre **** like that ?



** The vast majority use standard, linear slider pots - which tend to crowd most of the dB adjustment range in the last 25-30% of their travel.

A few special types have a small dead band in the middle of the track accompanied by a centre detent.

I recall seeing just one, up market graphic that had sliders with symmetrical non-linear tracks to compensate for the end crowding effect and give a linear result.


..... Phil
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