Volume Control Resistance Stability Technical Measurements

Discussion in 'Audio Science' started by atomicbob, Aug 18, 2024.

  1. atomicbob

    atomicbob dScope Yoda

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    Many audio amplifiers have the following gain configuration:
    00 20190110 Typical Amp block diagram.png

    The potentiometer or stepped attenuator resistance stability can influence the system low level gain accuracy and residual noise.

    A selection of four volume controls will be examined for resistance stability and compared to a stable laboratory reference standard resistor:
    https://www.ietlabs.com/esi-sr1-calibration-resistor.html

    Background information on resistors and measurements:
    https://www.vishay.com/docs/28771/basics.pdf

    Additional background information may be found in this IEC standard:
    IEC 60195 Method of measurement of current noise generated in fixed resistors

    Instruments used for resistance stability measurements:
    01 DMMs.jpg
    Volume controls will be measured using 4 wire Kelvin probes.

    In a Kelvin (4-wire) measurement two test points are connected to each side of the component measured. One point acts as the “force” that supplies the current and the second as the “sense” which is used to perform the voltage measurement. The place at which the force and sense paths meet is called a Kevin point.

    02 Controls.jpg
    The controls to be examined:
    1) Laboratory Standard Resistor SR1 for reference stability
    2) Goldpoint SA1X stepped attenuator
    3) JBL NanoPatch
    4) Alps RK27
    5) Nobsound Little Bear MC102

    Stability is measured with each control adjusted to 1000 ohms or as close as practical. Keysight 34465A DMM in 4 wire resistance mode sources 1 mA DC current which will be 1 Vdc across 1000 ohms. DMM is warmed up for 1 hour before measurements commence. Volume controls are allowed to stabilize as necessary with 1 mA current flowing through them. After stabilization 5 minutes of resistance data are collected at approximately 3 readings per second, NPLC set for 10. NPLC is Number of Power Line Cycles over which to integrate measurents.


    Laboratory Reference Standard 1KΩ
    03 20240815 std ref 1K resistance 1K stability 4W 5 min.png
    Resistance varies ~ 800 to 1000 uΩ over short periods while drifting over a range of 1600 uΩ with an absolute accuracy of 1.0000048 KΩ

    Goldpoint SA1X stepped attenuator:
    04 20240816 SA1X 20K resistance 1K stability 4W 5 min.png
    Similar to the reference standard resistance varies ~ 800 to 1000 uΩ over short periods while drifting over a range of 1600 uΩ with an absolute accuracy of 1.00049 KΩ

    JBL NanoPatch:
    05 20240816 NanoPatch 10K resistance 1K stability 4W 5 min.png
    NanoPatch has greater resistance variance and drift, 20x greater than the SA1X stepped attenuator.

    Alps RK27:
    06 20240816 Alps RK27 20K resistance 1K stability 4W 5 min t2.png
    RK27 has short term variance approximately 10x greater than the stepped attenuator with drift similar to the NanoPatch.

    Nobsound MC102:
    07 20240817 MC102 5K resistance 1K stability 4W 5 min.png
    MC102 behaves similar to the RK27 above in both short term variance and overall drift.


    08 20240816 multiple R 1K stability 4W 5 min.png
    This composite graph was created by normalizing the first data point from each of the three potentiometer volume controls to the first Reference resistor reading. Normalization was not performed on the SA1X as it wasn’t necessary.

    It is clear in this composite the SA1X stability is nearly identical to the Standard Reference SR1. All potentiometer based controls have similar behavior in short term and drift characteristics though the RK27 drifted in a positive direction while the NanoPatch and MC102 drifted in a negative direction.


    09 20240818 34465A 34461A std ref 1K stability 4W 5 min.png
    Comparing Keysight 34461A and 34465A demonstrates similar capability to perform this particular set of measurements. This particular 34461A has slightly better absolute accuracy.

    Stepped attenuators appeal to me for the ability to have repeatable, known volume levels which is very useful for estimating listening sound levels. They also appear to have better low level resistance stability.

    For other measurements on passive attenuators:
    https://www.superbestaudiofriends.o...uator-comparison-technical-measurements.7324/

    https://www.superbestaudiofriends.o...ssive-attenuator-technical-measurements.7303/
     
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  2. schiit

    schiit SchiitHead

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    Oh, you have no idea how important this is...................hints for the (near) future.
     
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  3. insidious meme

    insidious meme Ambivalent Kumquat

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    Lol, I think he probably does.
     
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  4. HotRatSalad

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    How about TKD ? Anyway you can get one of those in for comparison?
     
  5. atomicbob

    atomicbob dScope Yoda

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    TKD 6R25 10K will arrive next week.
    DACT 21 10K will arrive a week or two after the TKD.
     
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  6. Armaegis

    Armaegis Friend

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    Evil-me wants to try and convince the power hungry nutjobs* who run speaker amps for their headphones that they need to build their L-pads out of these for optimal sound quality...


    *yes this is the pot calling the kettle black considering I used to run mine off monoblocks...
     
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  7. atomicbob

    atomicbob dScope Yoda

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    In the original post resistance stability of four volume controls were compared to a laboratory standard reference resistor. This post adds a “vintage” Penny & Giles RF11/2/CA obtained from ebay to the comparison.

    See the first post for measurement methods employed and data collected.

    Important Notes
    The data below is NOT representative of a proper NOS P&G rotary fader.
    Vintage and Brand Name do not automagically confer godlike audio quality upon the particular component.


    21 P+G RF11-2-CA edited.jpg

    The data from this specific P&G rotary fader should serve as a warning to the perils of acquiring used unobtainium.
    There is a reason these components are available. They were likely removed from a functioning console as having become noisy.

    Penny & Giles RF11 / 2 / CA test setup
    22 P+G RF11-2-CA test setup.jpg


    Penny & Giles RF11 / 2 / CA adjusted for approximately 1K
    23 20240823 P+G 5K R gang 1 1K stability 4W 50 min.png
    After several days of measurements it became clear this potentiometer had issues. Exceptionally difficult to adjust for a specific resistance as it drifts for more than 35 minutes before settling after every adjustment. Every damn time the pot is turned. There are 2 gangs on this pot. The second gang was even worse with dead zones.


    24 20240823 P+G 5K R gang 1 1K stability 4W 5 min excerpt.png
    Once settled the stability observed varies 600 mΩ over short periods and 1.6Ω overall. This is orders of magnitude greater than the four controls originally tested.


    25 20240816 multiple R 1K stability 4W 5 min.png
    Composite chart of the original four controls and lab reference resistor measured. Note y-axis is 0.005Ω per minor division.

    P&G RF11 data overlaid on data from previous graph
    26 20240823 multiple R 1K stability 4W 5 min v2.png
    y-axis is 0.1Ω per minor division. 20x greater than the previous graph.

    This specific P&G rotary fader data overlaid on data from the previous graph. Definitely a defective potentiometer. The other four controls look very stable in comparison. Again, note this is not representative of a functioning P&G control. This highlights the problems with acquiring “vintage” components.
     
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  8. Armaegis

    Armaegis Friend

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    The sad part is this particular pot has probably been pulled and resold numerous times simply due to its branding.
     
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  9. JK47

    JK47 Friend

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    Kind of like tubes...
     
  10. Priidik

    Priidik MOT: Estelon

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    This is in category of what lay audio people think dark magic is.
    So simple and yet so little known mechanism.
    What use of 16 bit microdynamic resolution when attenuator buldozes over this.
    Well done investigation, ser.

    Followup investigation idea - drift in sub second scale.
     
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  11. Armaegis

    Armaegis Friend

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    Since we're looking at stepped pots, what about IC volume chips? I believe that's how Jan Meier handles it in his amps. Granted, that adds a whole layer of complexity on top since they are active components, though I'm not entirely certain if some of them separate the signal path from the rest.
     
  12. atomicbob

    atomicbob dScope Yoda

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    In the original post resistance stability of four volume controls were compared to a laboratory standard reference resistor. This post adds a TKD 2CP-601 100K potentiometer to the comparison.

    See the first post for measurement methods employed and data collected.

    Important Notes
    This is an interim report. This was intended to be a 10K rather than a 100K TKD 2CP-601 potentiometer. Measurements were collected with the pot set for 10K out of 100K and scaling the results by a factor of 10 which may or may not be representative of a 10K pot set for 1K out of 10K.

    In a week or two I will receive the following which will be added to the comparison:
    TKD 2CP-601 10K
    TKD 2CP-2511 10K
    DACT 21 10K

    4 wire measurements typically involve commercial test leads similar to those shown in the following picture:
    31 K4 leads.jpg
    When making sensitive, low level measurements, influences from the environment may introduce unwanted contamination to the data collected.

    4-wire Kelvin method along with 10 NPLC is very robust at minimizing unwanted contamination. However, movement near the cables can still introduce contamination. Typically I must sit very still during a data collection, or delay start it and leave the room.

    I decided to create a custom 4-wire test leads using Canare L-4E6S starquad cable, hopefully to further minimize issues such as cable movement contamination.
    32 C4 leads DMM end.jpg
    DMM end of custom test leads. The shield is connected to earth ground as an additional guard.

    33 C4 leads DUT end.jpg
    DUT end of custom test leads. In this case the DUT is my SR1 laboratory standard reference 1K resistor.

    34 20240827 SR1 1K R stability K4 4W leads 60S - cable hit 3x.png
    Commercial 4-wire Kelvin test leads connected to the SR1 for residual noise testing.
    The cable was touched three times at
    T = 12, 26 and 42 seconds. Observe the contamination introduced to the resistance stability measurement.

    35 20240827 SR1 1K R stability C4 4W leads 60S - cable hit 3x.png
    Using the custom starquad 4-wire cable with guard, the cable was touched again 3 times at T = 12, 26 and 42 seconds.
    Observe no contamination introduced.

    Details matter when making low level, real time measurments.

    The following measurements were made with the custom starquad test leads abbreviated C4 hereafter.
    41 20240826 SR1 SA1 1K R stability.png
    Goldpoint SA1 compared to SR1 laboratory reference.
    Performance is substantially similar over a 60 second period.

    42 20240826 SR1 SA1 RK27 1K R stability.png
    Alps RK27 potentiometer data added. Note y-axis has changed to accommodate the increased variance of the RK27 pot.

    TKD 2CP-601 100K pot adjusted to 10K data:
    43 20240827 2CP-601 100K 10K R stability K4 leads w gnd2 4W 10 NPLC 60S annotated.png
    This is the original data (not scaled) with pot adjusted to 10K out of 100K.

    There was a problem with this data collection. More noise than expected is observed, approximately 1 Ω of variance. I will post an update when the issue has been resolved.


    44 20240827 SR1 SA1 RK27 2CP-601 1K R stability C4 leads w gnd2 4W 10 NPLC 60S annotated..png
    Scaling the data by a factor of 10 and normalizing to 1000 Ω.

    If a linear scaling of the data is valid, then the TKD pot looks very promising as appearing to have lower resistance variance, better stability, than the RK27 pot.

    There was a problem with a cut and paste operation between two spread sheets in which the formulae were pasted, not the values. If one looks carefully the data purported to be from the 2CP-601 is actually that of the RK27 scaled downward by a factor of 10.

    This chart too will be updated when both of the above issues have been resolved.


    When the 10K versions arrive, they will be added to this examination of passive volume controls.
     
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    Last edited: Aug 28, 2024
  13. Beefy

    Beefy Friend

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    Awesome!

    Not surprised by the superior measurement of the TKD. Conductive plastic is a superior material to carbon film in just about every technical metric.
     
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  14. peef

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    This is great data, thank you for putting these pots through such rigors and sharing the results! I've found it exceptionally difficult to measure long-term drift; awesome to see that you've been successful.

    I'm curious if you considered either applying a very low frequency (say 0.01Hz or 0.1Hz), or looking at the data as an amplitude spectral density plot. My thinking is the stability of the resistance is effectively the stability of the voltage drop across the DUT under a 1mA stimulus. Removing the very low frequency information will help differentiate issues like self-heating of the smaller pots from other phenomena.

    The idea with the amplitude spectral density plots is that you could then easily compare the noise density to the theoretical Johnson noise of the resistor, and get an understanding of what kind of excess noise we're dealing with-- broadband, 1/f, popcorn noise from intermittent wiper contact, thermocouple effects within the pot itself, something else? It's also possible to make this kind of measurement without applying a DC bias, so self-heating effects are reduced.

    I attached an amplitude spectral density of a 20k RK27. My rig is not capable of sufficiently low noise measurement <5Hz or >1kHz as it's optimized for measuring low impedance noise sources. It does suggest that the noise has a 1/f spectrum.

    Here are the APX settings. The lower trace is the noise floor of my measurement preamp.

    upload_2024-8-28_14-4-24.png

    And comparing that to the theoretical resistor noise:

    upload_2024-8-28_14-0-44.png

    Here's the computed excess noise. I subtracted the theoretical noise for a 20k pot from the measured value. The noise increases at lower frequencies, but the slope is flatter than typical for 1/f noise. The peaks all seem to be harmonics of 60Hz.

    upload_2024-8-28_14-0-30.png
     
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  15. Biodegraded

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  16. Pancakes

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  17. atomicbob

    atomicbob dScope Yoda

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    Corrections necessary. Please re-read post 12 above

    I am very sorry for any misrepresentations this may have given.
    Updates will occur when issues have been resolved.
     
  18. Priidik

    Priidik MOT: Estelon

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    Engineers (the ones with imagination and capacity to invent) and spreadsheets do not get along all the time.
     
  19. atomicbob

    atomicbob dScope Yoda

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    Especially tired ones that should have gone to bed instead of pushing through to see and share results.
     
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  20. CEE TEE

    CEE TEE MOT: NITSCH

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    In the first week of October, @atomicbob , I should have a Spectral Super Fader and another Penny&Giles pot to lend to the investigation. Will send to you as soon as I have them. Thanks for this work!
     
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