Goldpoint SA2X - Nobsound NS-05P passive attenuator comparison technical measurements

Discussion in 'Portable and Other Gear Measurements' started by atomicbob, Jan 10, 2019.

  1. Shplad

    Shplad New

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    atomicbob:

    Due to chronic illness, I don't often get to return to these things. I measured again, and triple-checked.

    1-2: 8.89K
    1-3: 7.19K
    2-3: 16K

    What does this mean about the POT value I have?
     
  2. atomicbob

    atomicbob dScope Yoda

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    Your POT value is nominally 20K but low by 20%. Not a very tight component tolerance by the vendor manufacturing the POT.
     
  3. atomicbob

    atomicbob dScope Yoda

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    When the SA2X is at position 23 corresponding with 0 dB attenuation the input is connected to the output with zero ohms. No high frequency roll off added by the SA2X. See this post for a detailed explanation.
     
    Last edited: Jun 16, 2021
  4. Shplad

    Shplad New

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    Thanks atomicbob. Where can I get a drop-in (solder-in) replacement that's a lower value POT?
     
  5. atomicbob

    atomicbob dScope Yoda

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    The characteristics about to be described apply to all potentiometers and series resistor stepped attenuators using a potentiometer model. Examples include JBL Nano Patch, Nobsound, Goldpoint, Khozmo and many others.

    The same principle applies to balanced and unbalanced attenuators. For sake of simplicity unbalanced will be shown for the first three diagrams. Data will be from a balanced SA2X.

    01 Typical unBalanced passive attenuator - wiper full CCW.png
    At minimum volume the attenuator load presented to the DAC output is the maximum attenuator resistance. The resistance presented to the Amp is zero or nearly zero ohms. Cable capacitance has minimal impact at low volumes with attenuation of more than 20 dB as will be seen in the graphs below.

    02 Typical unBalanced passive attenuator - wiper at center.png
    At 6 dB attenuation the attenuator has equal resistance between ClockWise and CounterClockWise. This is the point where the highest attenuator output impedance occurs and the point where cable capacitance has greatest effect on high frequency roll off.

    03 Typical unBalanced passive attenuator - wiper full CW.png
    At maximum volume the attenuator load presented to the DAC output is the combination of the attenuator maximum resistance and the input impedance of the Amp. The resistance presented to the Amp is primarily the DAC output impedance with the additional load of the attenuator maximum resistance. Cable capacitance will have approximately the same effect whether the DAC is directly connected to the Amp or through the passive attenuator.

    04 SA2X-47  R - annotated.png
    In the graph above we observe resistance from DAC to Amp decreasing as attenuator volume is increased (blue trace). We also observe resistance from attenuator output to Amp increasing as attenuator volume is increased (orange trace). At -6 dB the two traces cross and equal resistance exists between input to output and from output to ground. Parallel of the two resistances is ½ of each and thus the output impedance is ¼ of the maximum resistance. Again, this works for all potentiometers and series resistor stepped attenuators.

    21 20210606 SA2X-47 10K FR 4M Canare L-4E6S  100K load - atten 0 to -30 dB - overview.png
    Frequency response sweeps were recorded for a Goldpoint SA2X-47 10K with 4 meters of Canare L-4E6S into a load of 100K. Attenuation of 0 to -30 dB at 3 dB intervals were recorded. Note the change in frequency response above 10 KHz between 0 and -3 dB, which is high frequency roll-off.

    22 20210606-1 SA2X-47 10K FR 4M Belden 1800F  100K load - atten 0 to -30 dB - overview.png
    Frequency sweeps repeated using 4 meters of Belden 1800F. Less deviation at high frequency is observed.

    23 20210606 SA2X-47 10K FR 4M Canare L-4E6S  100K load - atten 0 to -30 dB - zoom 4.png
    L-4E6S zoom 10KHz to 30KHz and 0 to -18 dB attenuation to visualize impact of high frequency roll-off.
    The deviation increases to maximum at -6 dB. By -21 dB of attenuation the deviation below 30 KHz is minimal compared to 0 dB attenuation.

    24 20210606-1 SA2X-47 10K FR 4M Belden 1800F  100K load - atten 0 to -30 dB - zoom 4.png
    1800F zoom 10KHz to 30KHz and 0 to -18 dB attenuation to visualize impact of high frequency roll-off.
    Again the deviation increases to maximum at -6 dB. By -21 dB of attenuation the deviation below 30 KHz is minimal compared to 0 dB attenuation. However, far less deviation observed than for Canare L-4E6S.

    25 SA2X-47 20KHz deviation data.png
    Using dScope trace rulers the above data was measured for each cable type at 21 KHz from the previous frequency sweeps, with the noted attenuations.

    26 SA2X-47 20KHz deviation plot.png
    Graphing data from the previous table, effects of the two cables may be compared. Note maximum deviation at -6 dB with less deviation for the lower capacitance cable, Belden 1800F.

    Difference in cable distributed capacitance are responsible for the variation in results. L-4E6S is 150 pf / meter while 1800F is 39.4 pf / meter.

    Canare L-4E6S is intended for low impedance, balanced sources. Starquad dual twisted pair configuration enhances immunity to common mode interference, especially E-field noise from SCR light dimmers. Very desirable for long runs between microphones on stage and the FOH or recording mix console.

    This issue is worse for unBalanced signals. When L-4E6S is used in unbalanced configuration vendors typically connect all 4 wires together for the signal which really increases capacitance per meter. Belden 1800F is two wires with a characteristic 110 ohm impedance for digital data transmission. This also works well for DACs with higher source impedances, for example 500 to 1000 ohms, to amplifiers.

    If the DAC has a low impedance balanced output to a balanced amplifier input, then either L-4E6S or 1800F will be fine. However if a passive attenuator is inserted in the signal path then the 1800F will provide an advantage.
     
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    Last edited: Jun 16, 2021
  6. Shplad

    Shplad New

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    atomicbob:


    Hmmm....here's a photo of the internals of mine



    [​IMG]


    What does the "10" in the markings mean?

    IS this a genuine ALPS? I don't see any logo or brand name.

    Is there a specific name for this form factor of POT so I can find a lower resistance replacement?

    Thanks.
     
    Last edited: Jun 16, 2021
  7. atomicbob

    atomicbob dScope Yoda

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    The process I would follow:
    1. measure dimensions of present pot including solder tab spacing and pattern
    2. review datasheets for pots at mouser or digikey to find a mechanical match
    3. I wouldn't go lower than 10K per gang for this application

    I'm guessing10 probably means nominal of 10K
    No, if this was an ALPS it would be clearly marked as such. See examples at
    https://www.parts-express.com/ALPS-RK27-10KA-Audio-Taper-Potentiometer-Solid-Shaft-023-204

    The pot you have is nominally 10K per gang. Your measurements show approximately 8K per gang. I wouldn't venture lower.
     
  8. Shplad

    Shplad New

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    Wait, I'm confused...earlier you said I had a POT nominally rated at 20K but it was closer to 16K.
    Then, you said it was rated at 10K, according to the markings.
    Can you please clarify?

    Also, I'm a newbie and don't know what gangs means here. What does 10K per gang mean?
     
  9. atomicbob

    atomicbob dScope Yoda

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    A balanced passive attenuator comprises two individual potentiometer sections connected with a common shaft to rotate the two sections simultaneously. These sections are often referred to as gangs as annotated in your picture below:
    16K specific pot 2 gang - annotated - small.jpg

    A nominal balanced 20K passive attenuator has 10K per section:
    20190110 Typical Bal passive attenuator - 20K nominal.png

    Your specific balanced passive attenuator has 16 K with each section annotated per your measurements:
    20190110 Typical Bal passive attenuator - 16K specific.png
     
  10. Shplad

    Shplad New

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    Thank you. That was very helpful.
     

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