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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. Audioguru

    Audioguru

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    Sep 24, 2016
    No, you cannot connect the outputs of the preamps together because then each one is short circuited. Instead you must use a mixer to mix or blend them together. A series resistor from each preamp output can be connected together to make a simple mixer but then you cannot adjust their amounts of blend.

    Why doesn't the input of your mixer or amplifier have a volume control for these mixed pickup preamps? If your mixer or amplifier input does not have a volume control then the extra volume control you are adding (it probably will not be 500k) must be the correct resistance for the unknown input resistance of the mixer or amplifier.
     
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  2. Alec_t

    Alec_t

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    Jul 7, 2015
    Without knowing the output impedances of the preamps and the input impedance of the power amp it's difficult to specify the optimum value of the pot(s).
    Assuming the preamps each have an output impedance < 1k and the power amp has an input impedance >500k, here's what I would try :-
    GuitarPots.PNG
    There's no guarantee it would be ideal for your setup.
    R1 and R2 protect the preamps from inadvertent short circuits.
    You will probably find pots cheaper from a general electronics parts supplier than from a guitar accessories specialist.
     
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  3. Audioguru

    Audioguru

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    Alec gave me a couple of ideas:
     

    Attached Files:

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  4. bluebb

    bluebb

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    Feb 7, 2019
    yes of course,the guitar amplifier have the volume controls,as you know,but for "live situation" every guitarist need for quick adjust the volume knob also on the guitar.
    For this guitar i will have about 70% of volume from the bridge piezo which is connect to the "K&K pure amp",and 30% of volume from undersaddle piezo which is connect to the "Fishman 501T".
    The preamps have volume control and tone controls,i will adjust all on them and i will have one volume pot for general volume.
    I know that electric guitars with magnetic pickups have pots of 250k or 500k.
    Amplifiers and PA will be different,i can't say their input impedence,but on my PA i have one channel to push for low-Z instruments,and if i need ,for some instruments which have low signal i have the gain control for each channel.
    Ok,i though that if for passive magnetic pickups is used a 500k or 250k,for output of the preamps i need 500k or more,and you say only 50k?
    Ok.
    Then i have to add this resistor of 1k fpr each outout in series,and the to pot.
    Alec_t this answer is for you to,i am on the smartphone and new on this site and i have no time to search how to tag you.
    Thx to both,and to all who wrote on my thread.
    Greetings from italy
     
  5. Audioguru

    Audioguru

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    Experts say that a magnetic guitar pickup should feed a preamp that has an input impedance of 1M or more, to make a peak at about 4kHz. When it feeds a lower resistance then the peak disappears and the high audio frequencies are muffled. That is why vacuum tube guitar amplifiers had an input resistance of 1M, and all pickups were magnetic.

    I have never seen or heard a piezo guitar pickup. The Fishman piezo pickup installation instructions show the parts that are needed.
     
    bluebb likes this.
  6. bluebb

    bluebb

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    Feb 7, 2019
    i found the output impedence of my preamps,3k for K&K pure preamp and 3.5k for fishman 501T presys.
    Does R1 and R2 remains 1k?
    And for the input impedance of P.A. or amplifier I'm going to use, there will be many different ones.
    How to calculate the value of the output volume pot?how manufacturers do if they do not know with which amp or P.A. their product will be used?
    How i can know if 50k is ok for me?
     
  7. Audioguru

    Audioguru

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    Sep 24, 2016
    Most preamps made with an opamp have an output resistance of 100 ohms or less so 1k mixing resistors are fine. Since your preamps have a higher output resistance then use a 10k linear blending pot instead of the 1k resistors as I showed. Then you can use a 50k log volume control.

    Amplifiers usually have a fairly high input resistance so a 50k volume control will be fine.
     
    bluebb likes this.
  8. bluebb

    bluebb

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    Feb 7, 2019
    why my fender stratocaster with magnetic passive pickups have 500k?
    there are two more pots of 250k in parallel with two capacitors if 47μF for the two tone knobs.
    But the volume is 500k.
     
  9. bluebb

    bluebb

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    Feb 7, 2019
    Ok,then i need one blending lin pot of10k,and 1 log pot of 50k.

    If the lugs are towards me, and the shaft is at the top, counting from left to right we will have lugs 1,2,3 in the row above and lugs 4,5,6 in the row below.
    I have to wire the one output to lugs 2 and the 2nd to lug 5,lugs 1 and 6 to gnd,and lugs 3 and 4 are outputs?
    And then how to adjust it?
    How to know?

    And this pot is no needed to be adjustable everytime? One time for always?i can leave it inside the body of the guitar?

    One more question,why the general volume pot of a electric guitar with passive magnetic pickups is 500k?and in this case of preamp is 50k?

    Thx!!
     
  10. Alec_t

    Alec_t

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    Jul 7, 2015
    If the pickups are magnetic and connected directly to the pot then 500k or more is required to avoid loss of the higher frequencies. If they are fed to the pot via a preamp then 500k would work, but so would a lower impedance.
    I'm no musician, but won't piezo pickups on an electric guitar sense a lot of unwanted noise from knocks, fingering, clothing rustle etc, or is that the intention?
     
    Last edited: Feb 12, 2019
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  11. Alec_t

    Alec_t

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    That sounds like a dual-ganged pot. Why use that? The 10k pot and the 50k pot would not be ganged. Presumably you want both accessible for individual adjustments during a performance?
     
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  12. bluebb

    bluebb

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    Feb 7, 2019
    i just asked why the electric guitar has 500k and for the output of a preamp is enough 50k.

    I don't want to add a piezo pickup to my electric guitar,i want to add another piezo with his preamp to my acoustic guitar.

    My acoustic guitar from factory has one undersaddle piezo with an onboard preamp from Fishman,i want to add a piezo sensor for the bridge of the guitar position with his preamp from K&K,called "K&K pure preamp",and the transducer is called "K&K pure mini".

    With the pots on the preamps i will make the blend with my preferences with the different colours of sound you can get in different position on the wood of the guitar,and then connect this two outputs to the general volume for quick adjust or solos during the live gigs.

    Is very rare but there are el.guitars with both,piezo and magnetic pickups,not to use together,with the piezo you get an acoustic sound but very far from a real ac.guitar.
    Thx
     
  13. bluebb

    bluebb

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    Feb 7, 2019
    for me must be accessible during the gig only the general volume,how to connect the blending pot of 10k?like i wrote uspide?
    Thx
     
  14. Alec_t

    Alec_t

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    Jul 7, 2015
    Here's how they would be connected.
    Now that we know the output impedances of the preamps, the 1k resistors aren't needed.
    PotConnections.PNG
     
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  15. Alec_t

    Alec_t

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    Jul 7, 2015
    Perhaps these frequency response traces will help to explain.
    ImpedanceEffect.PNG
    The yellow and blue solid line traces for a magnetic pickup respectively show how amplitude drops away from 1kHz onward with a 50k load but from 10kHz onward for a 500k load. This is because the pickup has a high inductance whose impedance increases as the frequeny increases. This impedance forms a potential divider with the load impedance.
    The white solid line trace shows that a preamp with a 3k output impedance and a 50k load has a flat frequency response up to over 100kHz.
     
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  16. Audioguru

    Audioguru

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    Sep 24, 2016
    You have preamps designed for piezo pickups. They can easily drive the 50k or higher input of an audio amplifier.
    The input of many guitar amplifiers is 1M or more to be fed from a magnetic pickup. With a volume control and tone control on the guitar then the total resistance fed by the pickup is 100k to 500k.
    The frequency response is affected by the total resistance fed by the pickup.

    Here is a preamp designed for a magnetic pickup. It says that 1M is the minimum resistance.
    Here is the frequency response of a magnetic pickup with a peak at 4kHz when the resistance is high.
     

    Attached Files:

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  17. bluebb

    bluebb

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    Feb 7, 2019
    ah ok,now i understand,i don't have to put a blending pot,i have to use a pot as a blender.
    The pots doesn't need to have the lug 1 to his body?
     
  18. bluebb

    bluebb

    49
    1
    Feb 7, 2019
    Now is all more clear!
    i made a best researches about the two preamps,i wrote to you wrong output impedance.
    The "K&K pure preamp" has 10kohm,and for the "Fishman 501 T" i didn't found nothing.
    I sent a mail to the fishman support.
    I attached the files to this my replay.
    Is a problem if i don't know the output imp. for the Fishman?
    The pot remains of 10k now?
    Thx
     

    Attached Files:

  19. bluebb

    bluebb

    49
    1
    Feb 7, 2019
    very very clear!
    Finally i understand this:)
    I made a best researches about the two preamps,i wrote to you wrong output impedance.
    The "K&K pure preamp" has 10kohm,and for the "Fishman 501 T" i didn't found nothing.
    I sent a mail to the fishman support.
    I attached the files to this my replay.
    Is a problem if i don't know the output imp. for the Fishman?
    The pot remains of 10k now?
    Thx
     

    Attached Files:

  20. Audioguru

    Audioguru

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    Sep 24, 2016
    If the metal case of a pot picks up electrical hum then it will feed the hum into the circuit. But if the metal case is connected to the circuit ground then there will be no hum produced by the pot.

    Since the output impedance of the preamp is much higher than all other preamps then the output level will be less and maybe distortion will be increased with the pot values we show.
    I think the spec's for the K&K preamp means its output can feed 10k.
     
    bluebb likes this.
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