[sdiy] Audio taper pots - shunt resistors and contact resistance
Robin Whittle
rw at firstpr.com.au
Sat Aug 25 05:24:57 CEST 2012
Short version: Potentiometers have a contact resistance which
varies over time and with position of the wiper.
This contact resistance can be a source of audio
noise in any circuit where there is significant
wiper current. Therefore, the shunt resistor
approach to making an audio taper function with a
linear pot is likely to cause audio noise when
the pot is turned, either immediately or in the
future when the pot becomes worn and/or contaminated.
The same goes for using a low value resistor in the
wiper circuit to mixing point.
In summary, if you care about audio quality, your
circuits should minimise the current drawn from
the pot. The only way to do this properly is to
follow the wiper with its own high impedance
amplifier, rather than burden it with a shunt
resistor or an unreasonably low resistor to a
summing point.
I guess if you use 10k log/audio taper pots and
100k resistors to a summing point, you will
probably get away with it. ALPS advise a load
100 times the resistance of the pot, which would be
1 megohm.
However, with one pot in the Devil Fish, I completely
violate this guidance and get away with it, because
as far as I know, the particular ALPS pots I use
have never exhibited significant contact noise problems.
Using a shunt resistor to ground will create a reasonably good
"logarithmic" response from a linear pot. I think this response is much
more suitable to audio level control than a linear response.
However the resulting shunt resistor circuit will only work well as long
as the wiper's contact resistance with the resistive track (plus the
wiper's contact with its metal track) does not vary significantly, since
this "contact resistance" is in series with the shunt resistor, or
whatever load is placed on the wiper of the pot.
I believe the best way to achieve an audio attenuator circuit it is to
have a pot with a suitable "audio taper" or "log" response, and then to
drive the wiper voltage into a high impedance amplifier. The output of
the amplifier then would drive a mixing state or whatever else follows.
The idea is to eliminate any significant current flowing in or out of
the wiper.
A second best approach in a mixer is to have he wipers of multiple audio
taper pots for the different input channels each feeding a relatively
high value resistor, with the other end of all the resistors going to a
summing point. This could be a high impedance input of an amplifier
which follows the changing voltage at that point. More likely it will
be the inverting input of an op-amp which has its non-inverting input
grounded, with a resistor from the output to the inverting input to turn
the summed input current into a suitably high voltage.
If the second approach (in voltage or current mode) uses relatively low
value resistors, then we are back to the initial problem of a shunt
resistors: the circuit only works if the contact resistance is very low
compared to the resistance of the pot and the shunt resistor. How low
is "low"? It depends upon how fussy you are about noise when turning
the pot.
The contact resistance of interest is the variation in contact
resistance when turning the pot. If you don't intend turning the pot
when listening to the audio, then most or all of these concerns do not
apply.
Pots are imperfect devices.
One imperfection is the variation in contact resistance as the pot is
turned. This is the big problem with the shunt resistor approach (or
the second approach above, with relatively low resistances). If you
have any DC component in the drive to the pot, contact resistance
variations when turning will generate noise on the output. A good
circuit will eliminate all such DC components.
Still, the problem remains - variation in contact resistance imposes
itself as a gain change in the output signal, but one which varies at
audio rates when the pot is turned.
This means that when any audio signal is present, there will be noise
imposed when the pot is turned. If the signal is all mid-range, you may
not hear the noise because the signal masks the noise. If the signal is
purely bass, you will hear contact resistance variation noise clearly as
low to mid-range noise, because the low frequency bass waveforms are no
longer smooth curves on the output - the more the voltage deviates from
zero at that point in time, the greater the noise due to the the varying
contact resistance.
I think it is probably not good enough to test the pot and decide its
contact resistance variation is acceptably low for a shunt resistor
circuit and whatever fussiness you might have about audio signals,
depending on what signal you are testing it with. Someone else, or
perhaps yourself at a later time with different signals, may be fussier.
Pot contact resistance varies with the wear on the pot, dust, the creep
of grease which was intended to lubricate the shaft but which over years
or decades creeps across to the resistance track and/or the metal ring
which the other part of the wiper goes onto.
While it would be possible to rig a pot up to a reciprocating system to
give it a hammering and try to wear it out, there is no way you can
reliably predict its behaviour years from now, with the effects of
moisture, dust, usage with forces on the shaft you don't anticipate, the
drying of oil into something thicker and more likely to lift the wiper
from the surfaces etc.
Assuming you care about audio quality and want your machine to sound
good in years to come, I suggest you avoid all shunt resistor approaches
and use very light loading indeed on the wipers of all pots which handle
audio signals.
Another problem with pots is that the resistance element has a non-zero
width and the wiper fingers are not all touching a point on the
resistive track with exactly the same voltage divider factor.
Therefore, as you turn the pot, the various wiper fingers are traversing
their own little tracks, each of which has variations in voltage divider
factor with respect to the other.
This is particularly the case where the "log" or "audio" taper is
achieved by printing a thicker layer of carbon on top of a thinner
layer, with a diagonal border to smooth out the transition. Then, an
outer wiper finger can be in the thick area and an inner wiper finger on
the thin area, with these points of contact having radically different
voltage divider factors, despite the fingers being correctly on-axis
with the rotational centre of the shaft. Then, for any input voltage to
the CW terminal, significant voltage differences would occur between
these contact points, so current would flow from one wiper finger to
another, with the final wiper voltage being some mix of the four contact
voltages, depending on the impedance of those contact points and the
contact resistance of each of the wiper fingers.
The output voltage of the pot, even with a infinite impedance voltage
following amplifier, represents the sum of, for instance, four contact
wipers, each on their own imperfectly linear (or logarithmic or
whatever) voltage divider curve, each with their own varying contact
resistance (which varies at a potentially audio frequency rate
especially when turning), in series with the impedance of that part of
the track itself, which would be difficult to calculate, due to
resistance along the track and across it to the four points of contact
by the wiper.
Furthermore, the contact wiper has a non-zero length - it shorts out
some length of the resistive track, which varies with the topology of
the surface, the pressure on the wiper and how worn the wiper is (the
wiper will get flatter over time, with less contact pressure at any one
point, and so will be more subject to being lifted from the resistive
surface by grease, dust or whatever). There's not much which can be
done about this. Fortunately, as far as I know, this is unlikely to
produce any audible noise unless you go looking for it, with a DC
voltage across the pot.
I have a Behringer Eurorack MX802A mixer. It is compact and very neat
looking. The faders are rotary pots - Panasonic 9mm pots EVUE or EVUF
if I remember correctly, similar to these:
http://au.mouser.com/Search/ProductDetail.aspx?qs=WwqriLBepZulX6QrHnqNpg==
It appears that these are no longer made. I couldn't find them at:
http://www.panasonic.com/industrial/electronic-components/electromechanical/potentiometers.aspx
There's a March 2012 notice at:
http://www.panasonic.com/industrial/electronic-components/tertiary-navigation/product-information/discontinuation-notices.aspx#PotentiometersEncoders
"Discontinuation of Select EVA-J, EVA-NA, EVA-NE, EVJ-C/Y, EVN-D, EVU-F,
EWA-P/Q Part Numbers" which links straight to the datasheet:
http://www.panasonic.com/industrial/components/pdf/aok0000ce4.pdf
Indeed they are no longer made.
The previous owner evidently only turned the faders of channels 1 and 2.
These pots, whenever there is audio present - or at least audio which
is primarily bass - are very noisy when turned. There must be something
about these pots which makes the contact resistance become noisy with
use. Because of this I would never use these pots in anything I make.
The variations in contact resistance are not a problem when the pots
haven't been used much, but with some use it turns the mixer into what
Neil Young would describe as "a piece of crap".
I have never had any such problems with the little 9mm ALPS pots I use
in the Devil Fish. These are truly remarkable little pots:
http://www.alps.com/WebObjects/catalog.woa/E/HTML/Potentiometer/RotaryPotentiometers/RK09K/RK09K_list.html
There have been 270 Devil Fishes since 1992 with 7 such pots (6 in the
first 18 DFs). I have never heard of any trouble at all with these
pots. None of them have failed mechanically or electrically, as far as
I know. They get a *lot* of use. Two of the pots directly handle audio
- the Filter FM pot (10k log with a 10k load) and the Overdrive pot,
which is a 100k log (audio taper) being driven from the VCO and going
into a 3.3k resistor to what is effectively a low impedance summing
point, the base of one of the Q12A transistors at the bottom of the
diode ladder filter. This use of a 100k pot with a 3.3k resistor is an
extreme violation of the guidance I have given above. ALPS suggests:
http://www.alps.com/WebObjects/catalog.woa/E/HTML/Potentiometer/RotaryPotentiometers/RK09K/RK09K1130A5R.html
[Impedance on the Output Side]
As when the impedance on the output side for the voltage
adjusting circuit is low, it may receive the affect of
contact resistance between the resistor and the movable
slice, please set the impedance to the value as much as
100 times of the total resistance value or more.
So for a 100k pot, according to ALPS I should have a load impedance of
10 megohm. Instead, my load resistance is ~3.3k ohm. This is a 3000:1
violation of the above guidance! I use it to get an extreme range of
drive for the VCO into the filter. The normal TB-303 level of drive
(via a 220k resistor) is achieved with the Overdrive pot at about 10
o'clock. With the pot fully clockwise, the drive is theoretically
220/3.3 times the usual level. I chose this by experimentation and
later realised that it the ratio incorporates the 666 Sign of the Devil:
220/3.3 = 66.666...
This is a case of combining the log/audio taper of the pot itself plus
the effect of the 3.3k resistor to give a very wide dynamic range as the
pot is turned. Even with the pot at about 2 o'clock, the signal to the
Filter is limited primarily by the impedance of the pot, which is
probably something like 20k to the CW terminal in parallel with 80k to
the ACW terminal. So this is 16k, in series with the 3.3k resistor =
19.3k. Turning the pot to the CW extreme reduces the total resistance
to 3.3k, so we get 5.85 times more signal.
The VCO is frequently putting out bass signals and if there were contact
noise problems as in the two Panasonic pots in my Behringer mixer, then
the Devil Fish too would be a "piece of crap" in that it would be
untenable to alter the Overdrive pot while listening to the machine.
The fact that this has not happened is attributable solely to the
exceptional quality of these small, inexpensive, ALPS pots. I designed
this circuit before I had bad experiences with pot contact noise problems.
The Panasonic pots in the Behringer mixer are difficult to access and I
haven't tried cleaning them. I doubt if "cleaning" is the fix for any
contact noise problem which was evidently caused by a moderate or even
extreme amount of use by the previous owner. The mixer looked brand new
when I bought it, so it can't have been used extensively. I assume this
contact noise problem is inherent in these pots, so I have not tried to
fix it.
In my experience a common form of contact noise is where the pot has
been sitting around and probably not used for a few years to a decade or
so. There seems to be some creep of grease or whatever. This can often
be reduced or perhaps eliminated for a while at least by turning the pot
a few dozen times. A better fix is to dismantle the pot can clean it
with isopropyl alcohol, including with little pieces of cardboard
between wipers and the metal and resistive tracks. I do this for the
Volume pots of TB-303s, TR-606s and TR-808s. For the many drum circuit
pots of the TR-808 it is too much work to desolder and dismantle them.
They are rather open to dust - but also to a quick squirt with isopropyl
alcohol. My current technique for dealing with these pots is to squirt
some isopropyl alcohol into them, turn them ten or so times, blow out
the isopropyl alcohol with compressed air while turning the pot, and
then allow any remaining isopropyl alcohol in the pots or on the
circuitry to dry before turning the machine on.
The pots I use for the Devil Fish Overdrive function is an ALPS
RK09K113-10KA (old part number) with a "knob" shaft with an indicator
slot. The only difficulty I have with these pots is that it is very
difficult to craft a specialised pen to paint the slot silver. (I do it
with a little piece of kynar wire-wrap-wire insulation inserted into the
nib of a large felt-tip silver pen.)
I was able to get these pots in 10k log and 100k log off the shelf from
my local ALPS distributor: http://www.switchesplus.com.au The 500k log
involved a special order from Japan. The closest I can see to these
pots in the current ALPS product line is:
http://www.alps.com/WebObjects/catalog.woa/E/HTML/Potentiometer/RotaryPotentiometers/RK09K/RK09K1130A5R.html
The drawing is for a 30mm shaft but this particular pot, as we use, is a
20mm shaft. This is a 1B taper - linear according to:
http://www.alps.com/WebObjects/catalog.woa/E/HTML/Potentiometer/RotaryPotentiometers/RK09K/RK09K1130A5R.html
The pots we use are log "A" in the old terminology. I think this is
A(15A) in the second graph on the above page.
- Robin http://www.firstpr.com.au/rwi/dfish/
On 2012-08-25 5:30 AM, mark verbos wrote:
> I think it's important, but you could use tapering resistors.
> i.e. run a resistor from the wiper to ground that is about double the value of the pot.
>
> http://www.geofex.com/article_folders/potsecrets/potscret.htm
>
> Mark
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