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2-channel passive mixer-independent volume control for each pickup

Discussion in 'Project Construction Technologies' started by bluebb, Feb 7, 2019.

  1. bluebb

    bluebb

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    Feb 7, 2019
    I thought that Alec_t in his diagram had supposed that I knew that the lug 1 must be welded on the metal box, and for this he did not put it.
    I know that it must be welded on the box but I asked for be sure 100%.
    I need that you tell me exactly everything, even where the gnd must be welded.
    Now, after only 6 hours, Fishman's support has answered me, the output impedance of the Fishman 501 T is 3.5kOhm.
    For the K & K pure preamp wr do not understand what they mean with 10kohms, it seems too much, but I downloaded it from the official website.
    At this point what do I do? I can not tell you the input impedence of the P.A.s, I can only tell you the output impedence of the Fishman 501T.
    It's not important for me that it's perfect, the important thing is that it is safe.
    If you did not tell me you I would have directly connected the two outputs on the endpin directly. :D Fortunately i wrote here.
    If for safety I put 2 resistors a bit higher for each preamp what can happen?
    What do I lose a bit of volume ??
    The important thing is that is safe.
    Thx
    regards
     
  2. Alec_t

    Alec_t

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    672
    Jul 7, 2015
    If the pots have metal cases then those should both be connected electrically (by soldering or with tags) to the circuit ground, which pin 1 of the log pot (NOT the lin pot) also connects to.
    A 10k blend pot will be safe, but with preamp output impedances of ~3.5k the blend adjustment range won't be large. A 25k pot would give a bigger range, but at the expense of reduced volume.
     
    bluebb likes this.
  3. bluebb

    bluebb

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    Feb 7, 2019
    Ok,I understand,
    And this pot is no needed to be adjustable everytime? One time for always?i can leave it inside the body of the guitar?
    How i will understand at which value i will set it?
    Somewhere in the middle?
    Thx
     
  4. bluebb

    bluebb

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    Feb 7, 2019
    P_20190212_224530.jpg
    is ok the wiring on the photo i attached?
     
  5. bluebb

    bluebb

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    Feb 7, 2019
    also the KKsound replied me,i told them that 10k is very high,maybe impossible :-D
    and their answer:

    10K ohms is certainly not impossible. The Pure Preamp uses a very simple potentiometer on the output to act as the volume control. This results in the output impedance being a variable amount from around 470 ohms to about 10K ohms depending on the volume control setting. This impedance also changes with frequency as there is capacitance in the circuit as well.

    Best regards,
     
    Last edited: Feb 12, 2019
  6. Audioguru

    Audioguru

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    535
    Sep 24, 2016
    A preamp with an output impedance that is very high at 3.5k and another preamp with a variable output impedance as high as 10k? VERY weird.
     
    bluebb likes this.
  7. bluebb

    bluebb

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    Feb 7, 2019
    K & K's technical support continues to say that the output impedence of the K & K pure preamp goes from 470ohms to 10kohms.
    While I found this page where it is said 3k ohms.

    https://www.gollihurmusic.com/product/1304-KANDK_SOUND_PURE_PREAMP.html

    From fishman they confirmed that for the Fishman 501T the output impedance idms 3.5kohms.

    I will put this lin pot of 10k and that's it!what you say?

    How i will understand at which position will be set the knob ot this lin pot of 10k?
    Thx
     
    Last edited: Feb 13, 2019
  8. Alec_t

    Alec_t

    2,546
    672
    Jul 7, 2015
    It's wherever the sound blend suits you best.
    Since the preamps each seem to include a level control pot, set those for maximum level (volume).
     
    bluebb likes this.
  9. bluebb

    bluebb

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    Feb 7, 2019
    For me is important that is safe.
    There is a knob position that can lead to some failure with a 10k pot?
    Or if I turn the knob to the maximum or minimum,and one preamp has zero ohms resistance ,does this can failure/burn one preamp?
     
  10. Alec_t

    Alec_t

    2,546
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    Jul 7, 2015
    Any setting of the blend pot would be safe.
    I'm thinking that IF each preamp has a level setting control built in, and you don't want to change the blend setting during a gig, then the blend pot is redundant and you could simply mix the two preamp outputs as shown in Audioguru's third circuit in post #23. Perhaps the two fixed resistors in that circuit could be increased to 4k7 Ohms to be extra safe.
     
    bluebb likes this.
  11. bluebb

    bluebb

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    Feb 7, 2019
    I prefer the solution with two resistors,I will make the blend with the volume controls of the two preamps.
    So, ascertained that the two preamps have an output impedence of 3.5k the fishman, and from 470 to 10k the K & K, put two resistors of 4.7k for me should be fine.
    If I lose a bit of volume it is not a problem, I increase the gain on the active mixer of P.A.
    The important thing is that with 4.7k for each preamp will be safe to avoid failure.
     
  12. BobK

    BobK

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    Jan 5, 2010
    I don’t think the K & K guy knows what he is talking about. If the output is from an op amp driving a 10K pot, the impedance would be low at both ends and 5K in the middle.

    Bob
     
    bluebb likes this.
  13. bluebb

    bluebb

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    Feb 7, 2019
  14. Audioguru

    Audioguru

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    Sep 24, 2016
    If the output impedance of each preamp is crazy at 3.5k and you add un-needed 4.7k resistors in series with them and connect them to a 10k blend pot then you will have a reduced output level and not much blending action.

    I suspect the preamp designs are old school using germanium transistors, not modern opamps.
    Replace the old preamps with simple circuits using audio opamps. Then their input impedance and gain can be as high as you want and their output impedance can be low to drive a 10k blend pot with lots of blending action.
     
    bluebb likes this.
  15. bluebb

    bluebb

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    Feb 7, 2019
    The preamp fishman is factory installed in the body of the acoustic guitar,and phisically perfect, I can not replace it, and is designed for the piezo fishman, and only the piezo costs 100 euros.
    The same goes for the piezo K & K with his preamp.
    I need only a general volume for these two summed signals.
    The amount of volume for each piezo I do with their preamp's control. P_20190213_172937.jpg
    I had just designed this project, and now I read your answer
     
  16. Alec_t

    Alec_t

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    Jul 7, 2015
    Give the post #55 arrangement a try. Use screened connecting cable to minimise noise pick-up.
     
    bluebb likes this.
  17. bluebb

    bluebb

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    Feb 7, 2019
    Sure, but I was asking here to be sure if this thing can be done or not?
    And if yes, how.
    Now I can proceed,
    I would have already tried but I have to order the second preamp, the K & K.
    Now I have the two piezos and the Fishman preamp.

    I was thinking,especially
    1.to have a box less in the body of the guitar 2.not to have the problems of having to mix two outputs of two preamps connected in parallel
    3.to spend less money
    4.avoid laborious changes on the K & K, which is also heavy,and with it I would make only one hole on the body for the volume pot and the thin wood of the guitar body would suffer (i need at least 2 pots).

    to buy this:

    https://schatten-pickups.myshopify....series-miniature-preamps/products/micro-pre-2

    And make this circuit:
    Screenshot_2019-02-13-23-21-47.jpg


    which is calculated for it,and connect the two piezo pickups on it.

    The output impedence is 3.5k, but then together with the 3 pots I do not know what the output impedance would be.
    But i think that two preamps in series is less complicated.

    The question is, if I put the output of this micropre 2 at minimum at the input of my fishman preamp should it work?
    The fishman is calculated to receive a voltage from the piezo of μV, but putting this buffer to a minimum would be too high for Fishman?
    Thx


    Thank you
     
  18. Audioguru

    Audioguru

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    Sep 24, 2016
    The volume controls for the pickup preamps are connected backwards. They short circuit the preamp output and also the other preamp output to reduce their levels instead of using each volume control as a separate voltage divider. The master volume control is wired correctly as a voltage divider.

    I am surprised to see that the micro-pre device has absolutely no audio spec's like distortion, frequency response and noise.
     
    bluebb likes this.
  19. Audioguru

    Audioguru

    2,408
    535
    Sep 24, 2016
    Looking again, it looks like guitar preamp outputs have a high output resistance so that their volume control can be connected backwards. When one preamp volume control is turned down to short its output level then the other preamp output level is not reduced. I call it Mickey Mouse mixing.
     
    bluebb likes this.
  20. bluebb

    bluebb

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    Feb 7, 2019
    So is it possible to connect this little preamp in the chain first and then my Fishman, but is it just a matter of correcting the connections?
    I downloaded the diagram I put here from the official website schtten.
    It would be nice if you put the right diagram here.
    Thx
     
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