Help with volume pedal electronics

Discussion in 'General Electronics Chat' started by match999, Mar 15, 2016.

  1. match999

    Thread Starter New Member

    Mar 15, 2016
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    Hi,

    I'm busy with modifying a pedal.
    I build a dual 100K b taper pot in it.

    1 half will control my effect unit.
    The other half of it I want to use as volume for a sampleplayer

    The range reads between 30K and 90K because of the limited sweep of the pedal.

    I have a few questions.

    Since the sampleplayer is stereo, how to make a circuit so that the 1 pot will control volume of both channels.
    How to make it so that it can go from silent to max volume.

    BTW. I want the volume sort of min/max (pedal up/pedal down), it's not necessary to have a perfect audio controlling.

    Very basic drawing [​IMG]

    A B
    o o
    o o*******
    o o

    A is controlling the effect pedal, B would be for volume.
     
  2. Marcus2012

    Member

    Feb 22, 2015
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    Hi :) You can get stereo potentiometers for this very application that have two tapers and as it is a volume pot logarithmic is probably best so 0-30K won't be very loud at all.

    Dual/Stereo potentiometers
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Since you already have a dual pot in the pedal, I'm not sure how another dual pot would work. The all-electronic way uses the dingle pot in the pedal to produce a variable voltage. That is used to control two VCA's (Voltage-Controlled Amplifiers). These amplifiers turn the stereo audio signals up and down. These are not beginner-level circuits, and they probably will need two power supplies, like +12V and -12V. Is that something you can handle?

    ak
     
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  4. Marcus2012

    Member

    Feb 22, 2015
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    The second pot would be mechanically bound to the first in the pedal arrangement with a small, thin gear over the splines of the pots. I've done this with single turn close linear pots but I haven't tried it with 2 stereos that are linear and logarithmic. Is the volume log btw?

    EDIT I'm envisioning a suspended pedal here btw and the first pot would be at the pivot point of the pedal armature, but if it's a floor mounted this might not be be practical. But I was thinking a mechanical solution would be more robust than amalgamating two complicated circuits into one pot.
     
    Last edited: Mar 16, 2016
  5. match999

    Thread Starter New Member

    Mar 15, 2016
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    thank you both for your answer.
    The mechanical thing I didn't think about it yet, but that will be not practical in this pedal as you stated.

    About the VCA: that should be te solution but maybe a bit too extreme as I read. I'm an experienced beginner (made some little circuits in my life) but in the whole thing it will be too much I guess.

    But that made me think of another solution. Basically I want the signal on(pedal down) or off (pedal up)
    Is there a circuit, which simply switches the L and R signal on and off, triggered by the pot.
    And it doesn't have to be exactly at the end of the pedal sweeps.
    So let's say under 60K it's off, above it's on.

    When I'm playing the guitar I just want to kick in some sound at some point, and later turn it off.
    (btw a simple switch is not the solution, since controlling a pedal is more natural to do in the music)
    Thanks in advance
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Where do you envisage the volume-switching occurring? At the input, output or some intermediate point of the sample-player?
    Is the sample-player a standalone sound system or something which feeds your main amp?
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Yes, there are lots of ways to turn two different positions of a pot into on/of states, and all of them are easier than matched, tracking VCA design.

    Assume that you need 1-2 cubic inches and a little 5V or 12V power. Where will the circuit reside, how is it powered, etc? It is the external factors that will narrow down the design.

    Also, full-on / full-off, or just louder / softer?

    ak
     
  8. match999

    Thread Starter New Member

    Mar 15, 2016
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    Ok, interesting thoughts! Thanks!

    It's an Akai Sample player: http://www.akaipro.com/product/mpx8
    The setup will be my pedalboard at front stage. With the effect processor, and pedals.
    From there there goes cables to my rackunit at the back of the stage where also the Sample Player resides.
    There's 220V there.
    So can put an extra cable from the pedalboard to rackunit.

    In the rackunit there's more space of course than 1-2 cubic inches.
    I want to control the output volume. before it goes to FOH. So it's standalone.
    Just switching full-on/full-off.

    Let's forget the VCA, and go for a nice circuit with on/off
    Awaiting your very interesting insights!
    Grtz M
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The User Manual for the sample-player shows it has 1/4" outputs for monitor speakers or amplifier, but doesn't say what the output level is or indicate whether the output is AC-coupled or DC-coupled. Do you have any info on that?
    If your extra circuit is to be inserted there, then care would have to be taken to avoid audible clicks as the player cuts in and out.
     
  10. match999

    Thread Starter New Member

    Mar 15, 2016
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    Have asked Akai, will come back later!
     
  11. Alec_t

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    Some further thoughts:
    Won't it sound rather strange if you suddenly switch the volume up mid-sample? Surely you want to hear the attack part/intro?
    For that, wouldn't you need to send a MIDI message to trigger the sample play?
    Your pedal won't have any control over sample selection. How will that be done? Or do you have just one sample being looped continuously?
     
  12. match999

    Thread Starter New Member

    Mar 15, 2016
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    I have modified the sample so that I can use it in the intro, or I play continous loop samples.
    1 good example is "Stories" of Therapy?
    In the intro there's sample and guitar. I have to create silence with the pedal where I play the guitar.

    And the Akai MPX8 isn't capable of receiving MIDI messages :(
     
  13. Alec_t

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    Are you sure? This is what the web-site ad states:
    "MPX8 also has USB MIDI plus standard MIDI inputs and outputs. Control everything from music software on your laptop to outboard MIDI gear. The included USB cable and 1/8"-to-MIDI adapter make it easy to connect. - See more at: http://www.akaipro.com/product/mpx8#sthash.cspE4Pl4.dpuf"
     
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  14. match999

    Thread Starter New Member

    Mar 15, 2016
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    Answer from Akai: With regards to your previous message, the MPX8 utilises line level outputs. This output is AC coupled.

    And yes you're right. I've asked them a while ago is there's MIDI switching on the volume, but that wasn't the case.
    Confused that. There's still MIDI messaging, so I could start/stop samples with my effect board.

    Still needs the volume controlled by the pedal tho.....
     
  15. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Here's a possible candidate circuit, using the pedal pot to control a reed relay to route the sample-player signal to your mixer/amp. I would have preferred to use a solid-state switch instead of the reed relay, but in simulations the latter provided cleaner switching and less noticeable audio clicks/transients/distortion than when using e.g. FETs. Perhaps someone else could suggest an all solid-state solution.
    SamplePlayerGate.PNG

    Edit: Circuit modified slightly to reduce standby current.
     
    Last edited: Mar 21, 2016
  16. match999

    Thread Starter New Member

    Mar 15, 2016
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    Thank you very much!
    That's so nice on a forum like this. People can translate Q in circuits, 1 thing I'm totally incapable of.
    This is something I can build, and it's simple but effective!
    I will wait for others to chime in for a SS solution, but for no this is nr1! :)

    1Q: What does the Sim only tell me? I never seen such a drawing....
     
  17. Alec_t

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    Not part of the actual circuit you'd build. It's for the LTspice simulation. Pedal movement of the pot is simulated by use of a triangle-wave voltage source V3 and a voltage-controlled pot model.
    Circuit operation: When the Pedal pot wiper voltage goes above a reference voltage set by the trimmer, Q2 turns on and hence so does Q3, energising the relay. Any other low-coil-current relay could substitute for the reed relay.
     
  18. Alec_t

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    Post #15 circuit edited.
     
  19. match999

    Thread Starter New Member

    Mar 15, 2016
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    I have posted this same question some tome ago on a music-electrinics forum. There' wasn't somebody with a circuit, so that's why I started it hree, with more succes.
    But today someone chimed in there and said the following:

    Switching a stereo signal on/off from a single pot could be done. The problem you'll come across is that the idea is straightforward, but the circuit to do this is more complex.

    When you switch a signal off, the signal source and signal destination expect to see different things. Your sampler would be OK running into an open-circuit, or load resistance, but may not be tolerant of a straight short across the outputs. At least, not for any length of time. Equally, your amp input would hum badly if you just had an open circuit. A shorted input is fine for the amp.

    Signal switchers that mute usually take a dual approach - known as series/shunt. You have a switching element (FET, LDR, CMOS switch etc) in series with the input, and one across the output. So the input side goes open and the output goes short.

    Using the pot as a switching threshold would require a comparator circuit and the output of this used to control the switching. Again, not entirely simple.


    How to include the above in the circuit?
    I assume more than a basic relay and connect it to the ground, when switched?
    Thanks!
     
  20. Alec_t

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    The long-tail pair of transistors, Q1/Q2, is a comparator.
    Since you've confirmed that the sampler output is a line-level one, the implication is that it has a low output impedance which would happily drive a ~10k resistive load.
    So, I'd just connect a ~10k resistor between the circuit output and ground. That way:-
    (a) the sampler output would see either an open circuit (relay de-energised) or 10k to ground (relay energised) and
    (b) the amp input would see either 10k to ground (relay de-energised), or the sampler output impedance in parallel with 10k (relay energised).

    Now that a stereo signal has been mentioned, a DPST or DPDT relay could switch both channels. However, it would draw about double the coil current. It might be cheaper to use two SPST relays, which seem more widely available.
     
    Last edited: Mar 21, 2016
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