Volume potentiometer taper curve?

[Mico]

I have to replace volume pot on my Onkyo Tx-8500 and have two Bournes potentiometers in the Mouser cart.
Other than slightly different pin length (which should not be the issue anyway) I see that taper resistance curve is different. One is Type A and the other Type B, I do not get the concept and potential consequence if one or another is installed. Image of the taper resistance curve for type A and Type B attached, obviously need a translation in a layman`s language.

 

[fred soop]

You want taper A2 for a volume control. 50% of rotation should be 10% of voltage. Hearing is logarithmic (dB scale) and a 20 dB increase is 10 times the voltage. If you were to use any of the B taper pots, you would have similar performance up to 30-35% rotation, then it would suddenly get very loud. The upper 50% of the control would only be a 3 dB change, just barely audible.

Active tone controls (where the pots are connected in a feedback loop) should be linear while passive tone controls should be A2 taper, same as volume.

 

[Mico]

You want taper A2 for a volume control. 50% of rotation should be 10% of voltage. Hearing is logarithmic (dB scale) and a 20 dB increase is 10 times the voltage. If you were to use any of the B taper pots, you would have similar performance up to 30-35% rotation, then it would suddenly get very loud. The upper 50% of the control would only be a 3 dB change, just barely audible.
Active tone controls (where the pots are connected in a feedback loop) should be linear while passive tone controls should be A2 taper, same as volume.

Perfect explanation, thanks a lot. While I am here and with the expert: original volume pot has 4 pins vs 3 on the replacement. Fourth pin is loudness null point which is supposed to cut loudness compensation at about 50% of the total potentiometer rotation . Knowing my amp, if I had it cranked at 12 o`clock (50% rotation) the police would be on my door in no time. So I assume that installing the 3-pin potentiometer without loudness cut would not have any practical consequence at normal listening levels?

 

[elnaldo]

Installing a 3 pin potentiometer you loose the loudness feature. Finding 4 pins potentiometers is hard but possible. Check my recent post in the Pioneer sub-forum about a pioneer SX-650 with links to 2 parts.

You usually need a B potentiometer in Balance and tone controls. Some units use a B potentiometer in the volume control. I'd check the specs of the original part.

 

[toddalin]

For the Alps pieces on the "bay" (first place to look), the four pin units are actually easier to find than the 3 pin units and sometimes cheaper. I just purchased four of the 3 pin units for my ultimate switcher. Alps just lists them as linear or log.

 

[Mico]

Installing a 3 pin potentiometer you loose the loudness feature. Finding 4 pins potentiometers is hard but possible. Check my recent post in the Pioneer sub-forum about a pioneer SX-650 with links to 2 parts.You usually need a B potentiometer in Balance and tone controls. Some units use a B potentiometer in the volume control. I'd check the specs of the original part.

There is no way to find any detailed info on original Alps pot other than the part number. Even resistance had to be taken from the pot itself, no value written in the SM.
I do not care much about loudness anyway so three pin Bournes potentiometer from Mouser will do the trick.

 

[Mico]

By chance I have got a mint dual audio taper + loudness volume pot from Akai AA-1050 receiver.
Smooth operation and perfect fit, but resistance is 250Kohm vs 100Kohm. The only reason I am even considering it is
that now unobtainium pin for loudness compensation circuit...which I am not really dying after anyway. The question is, what I might be messing up with if I change the default
resistance? I think I have heard somewhere that it might affect the tone controls "flat" position, among few other things.

 

[elnaldo]

Increasing the resistance of the volume pot mostly adds noise, and could change the way the knob respond. It shouldn't affect the tone control is most of the circuits. A schematic of the unit should confirm this. The tone control is usually a different circuit.

 

[Mico]

Too much sacrifice just for the sake of loudness pins, I will stick with new Alps part and correct resistance value.

 

[elnaldo]

loudness tap pots are not unobtainium yet:
(NO AFFILIATION) https://www.radioshack.com/products/radioshack-100k-dual-ganged-stereo-volume-control

 

[Mico]

I got the new Bourns volume pot with correct value and dimensions. The problem is, it looks like a cheap toy compared to original ALPS .
Now I am thinking how to avoid using it there and might have a solution. All tone pots are the same value (100Kohm) and size as the volume except for missing Loudness pins.
Which I do not care about and loudness is missing from the new pot as well. I suppose there is nothing special to prevent any of them from being used as a Volume control?
In that case I would put Bourns as a Mid or Bass or whatever tone control and call it a day.

 

[elnaldo]

If the tone controls are linear and your volume pot is also linear, you can swap them.

I'd install the new pot at the volume and use it.

 

[Mico]

If the tone controls are linear and your volume pot is also linear, you can swap them.
I'd install the new pot at the volume and use it.

That is the catch, I have no idea if the tone controls are logarithmic or linear but the new pot is for sure type A, logarithmic.
It should possible to figure that out by measuring the resistance once the pot is out. New 100Kohm has approx. 74Kohms at 50% what should I expect at that point from the linear one,
perhaps 50K? Yes, using the new pot in the right spot should be the logical and simplest way, it just looks so small and frail.

 

[elnaldo]

If the new pot is A, you can't use it as a tone control potentiometer. Better leave them alone, use your new potentiometer as Volume control.

A linear potentiometer will have 1/2 of the resistance at the middle position.

You bought a replacement pot for the volume control I wouldn't overthink too much, I'd just install it and close the cover.

 

[Mico]

If the new pot is A, you can't use it as a tone control potentiometer. Better leave them alone, use your new potentiometer as Volume control.

A linear potentiometer will have 1/2 of the resistance at the middle position.

You bought a replacement pot for the volume control I wouldn't overthink too much, I'd just install it and close the cover.

The new pot is installed but it was a very tricky business to fit it in. A lot of twisting and pins sanding , and at the end quite a bit of disappointment.
It works but gets loud very quickly, at 7 o`clock it is already too much for casual everyday listening. To make things worse, left side gets louder faster
and by the time the both channels get completely balanced my neighbor is already at the door with the police. The overall size and particularly the diameter of the new pot is puny,
which might be the reason why it screams so loud so quickly. I still have that mint Alps vintage Volume pot from Akai receiver but resistance is 250Kohm.
At the end what could be worse that this messy little thing....

 

[elnaldo]

I didn't check the schematic, but not having the 4th lead could be a cause of going too loud so fast.

I'd try the Akai 250K with 4 pins, or add a resistor in parallel to your 100K.

Also, check the level of the input signal. If too high, it will be too loud faster. Modern devices have louder signals.

When I need to fit a potentiometers with sorter leads, or narrow width, I usually solder extensions using capacitor leads or solid wires, and bending to fit the holes.

 

[Mico]

I didn't check the schematic, but not having the 4th lead could be a cause of going too loud so fast.

I'd try the Akai 250K with 4 pins, or add a resistor in parallel to your 100K.

Also, check the level of the input signal. If too high, it will be too loud faster. Modern devices have louder signals.

When I need to fit a potentiometers with sorter leads, or narrow width, I usually solder extensions using capacitor leads or solid wires, and bending to fit the holes.

I did installation the hard way, fitting the pins directly to PCB but will not repeat that treat again. Akai 250Kohm pot did not work well, sounds and responds weird.
This might have been the reason for new pot`s quick response- quality control. The one I installed had nominal resistance of 98Kohm and difference between left and right side was 2Kohm .
The second one I have is 105Kohm with left/right side difference of 3Kohm. Pot which is already in has 74Kohm at 50% turn - but the spare one is 92Kohm at the same 50% turn!
As a comparison, old ALPS is 100Kohm with left/right side difference less than 0,5Kohm . Moral of the story is to buy a half dozen of them and choose the one with
best tolerance figures. I am sure that vintage Alps would cost at least 50$ these days, so 25$ for a bunch is a reasonable waste.

 

[Mico]

It seems that Mouser sells Bourns taper A pots without exact curve designation.
Two pots I got from there were both marked just "A" , without following number (from 1 to 6) which should indicate how steep the taper curve will be.
And they are acting quite differently for sure, the only other difference being the length of the installation pins.

 

[elnaldo]

Usually the complete part number tells you the exact curve, and usually you have just 1 or 2 curves to choose from, not all that models described in the datasheet. Many potentiometers are made under special orders for some factory or brand, and they are not available from sites like Mouser to the public.

The usual A curves I see on standard pots I buy are the A2 (10% of the resistance at 50% turn) and sometimes A3 (15% at 50% turn)

Can you post a crop of the schematic, to see the way that 4th tab is connected?

 

[Mico]

[QUOTE="elnaldo, post: 13661081, member: 81242"
Can you post a crop of the schematic, to see the way that 4th tab is connected?[/QUOTE]

The complete part number is missing the 1-6 digit at the end while all B type pots have, it that is weird. I doubt the fourth pin should affect anything if loudness button is not engaged, but here is the schematics.