Goldpoint SA1X and SA2X passive attenuator technical measurements

Discussion in 'Portable and Other Gear Measurements' started by atomicbob, Jan 6, 2019.

  1. Ash1412

    Ash1412 Friend

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    Hey @atomicbob is the input impedance for Liquid Platinum at 48K Ohms for balanced input or SE?
     
  2. atomicbob

    atomicbob dScope Yoda

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    Balanced.
     
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  3. Ash1412

    Ash1412 Friend

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    So half for SE at 24k?
     
  4. atomicbob

    atomicbob dScope Yoda

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    Never Assume. You know the rest. To be clear I prepared the following pseudo schematic to indicate what I measured with a Fluke 189 DMM:
    20190228 Typical Bal and unBal amp input.png

    More precise Balanced measurements (Left Channel):
    45.61K between pins 3 and 1
    45.69K between pins 2 and 1
    91.30K between pins 2 and 3

    More precise Unbalanced measurements (Left Channel):
    445.3K between Center and Shield
     
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  5. Ash1412

    Ash1412 Friend

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    Thats interesting. The Jotunheim which doesn't use a phase splitter has SE input impedance at half that of balanced. Maybe the phase splitting circuit is whats causing the extraordinarily high input impedance?
     
  6. GoodEnoughGear

    GoodEnoughGear Evil Dr. Shultz‎

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    I have a set of actives at 10kOhm ZIn with a set of 10m balanced interconnects, using digital attenuation form DAC1541.

    I used this calculator from DACT: www.dact.com/html/ac_calculator.html and modeled the scenario using a 10k attenuator, with the shield+conductor capacitance data for the interconnects I'm using. This is the result:

    DACTCalc-Cordial10mXLR-10k-10k.jpg

    Unless I'm missing something, even at small levels of attenuation I shouldn't really be seeing much in the way of treble rolloff. Given that I would attenuate 20-50dB I should be in the clear using a GoldPoint right after the DAC. Am I missing something here?
     
  7. rlow

    rlow A happy woofer

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    With the Goldpoint in the middle, your speakers will now see the Zout from that preamp, which is now variable because it’s a variable passive resistor. So the Zout from the Goldpoint will change depending on the volume level, and is calculated based on the Zout from the DAC. To avoid issues, you want the lowest output from the DAC so that, even though the Zout your speakers will see from the Goldpoint will vary, it won’t get high enough to cause problems at all/most attenuation levels.

    Without getting into the math, with a passive preamp in the middle, the rule of thumb I’ve read is 100:1 ratio of the two ends (ignoring the preamp in the middle). In other words, 100:1 ratio between your source and your power amp. The is will give you relatively low output impedance from the passive preamp at almost all attenuation levels.

    Your DAC has balanced Zout of 100ohms. Your speakers are Zin of 10k ohms. That’s 100:1. You should be good.

    100 ohm Zout from a DAC is nice and low, and 10-20k Zin is pretty typical for an amp. If that combo doesn’t work with a passive preamp then it would be pretty darn limited it what it would work with.
     
    Last edited: Jun 27, 2019
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  8. GoodEnoughGear

    GoodEnoughGear Evil Dr. Shultz‎

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    Cool, just so long as I'm not off in the weeds on this thinking :).
     
  9. GoodEnoughGear

    GoodEnoughGear Evil Dr. Shultz‎

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    OK, in some consultation with GoldPoint it looks like I'm kinda on the iffy edge of the use case with my scenario with common ground connections impacting total capacitance. I could get more creative but the price for that starts getting to $1500+ USD and is a bit kludgy feeling to me.

    That said, Arn Roatcap was incredibly helpful, and though an enthusiastic salesman, he was forthright and didn't try to smooth any rough facts over to land the sale. I can highly recommend dealing with him.

    Then as it happens, I stumbled on a lightly used SPL 2Control at Guitar Center for 400 bucks which is a steal, so I'm gonna give that a whirl. It seems to get decent reviews and at that price should be nicely resell-able if it sucks or I get upgradeitits. It pretty much does exactly what I want barring the headphone outs which I don't need.
     
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  10. Baten

    Baten Friend

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    I can get a GOLDPOINT SA2X for a fair price. This one has two inputs. Was wondering, can I route one XLR source and one single ended source over RCA>XLR cable, and switch between sources over the goldpoint?

    As in, can I volume control a single ended source? Thanks for any tips. Would be an ideal solution to my current set-up for sure...
     
  11. GoodEnoughGear

    GoodEnoughGear Evil Dr. Shultz‎

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    I take a SE in to my SA2X-O into my Genelecs no problem. Attenuation works fine. Make sure your amp/monitors can handle SE over XLR.
     
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  12. Baten

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    Excellent, thank you :)
     

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