Bypassing an op-amp

Discussion in 'The Projects Forum' started by GRNDPNDR, Mar 26, 2012.

  1. GRNDPNDR

    Thread Starter Member

    Mar 1, 2012
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    This should be simple but I can't really think of how to do it.

    I want to modify this portion of the circuit such that when there is no power to the op-amp (when the power is turned off to the whole unit) there will be a direct connection between pin 3 and pin 1.

    This way the original signal can still pass through without amplification, but when the circuit is turned on this path should close allowing the signal to be sent through the op-amp instead.

    It's just so I don't have to keep it running 24/7 nor do I have to unplug it when it's off.
     
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  2. wayneh

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    Sep 9, 2010
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    You want this to happen automatically, without a switch? It'd be easy to just add a switch.
     
  3. GRNDPNDR

    Thread Starter Member

    Mar 1, 2012
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    Yeah automatically.

    A switch would be simple but this whole circuit is 2xLM324. This is just one of EIGHT amplifiers.

    Not only would I need to flip 8 switches, it would add quite a bit of size to the overall box which will already have 16 TRS plugs, and 8 potentiometers. Also, should the battery die it would stop the signal from getting through, since this is for a drum kit that would be bad.

    Could a reed relay be used in this situation maybe?
     
  4. wayneh

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    Sep 9, 2010
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    I'm at a loss. You need the function of a NC relay between 1 & 3, so that without power you get signal pass thru. When power is available, the relay opens and the op-amp takes over. Simple enough in theory.

    But since you need 8 of them, it becomes a but impractical, IMHO. Maybe you can find suitable relays. I can't think of an alternative. That just says more about me than your project! ;)
     
  5. tracecom

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    Apr 16, 2010
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  6. GRNDPNDR

    Thread Starter Member

    Mar 1, 2012
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    That's what I was thinking, but they seem really hard to find and really drive the cost up.

    I've been searching for a suitable solid state relay though I just haven't really been able to locate one.

    Wouldn't there be a way to do this with a transistor?
     
  7. wayneh

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    Sep 9, 2010
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    Just thinking out loud here. What if you had backup power of some sort - I'm thinking 9v battery - to ONLY power the switches? You could use a comparator to detect power loss and close all the circuits if power is lost. Then each "switch" could just be a MOSFET that turns on (closes, shorting 1 & 3) when power loss is detected.
     
  8. GRNDPNDR

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    Mar 1, 2012
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    Trying to keep it simple, there is already two 9V batteries to provide the dual polarity power supply for the op-amps.

    I'm also seeing some weird things, namely the voltage that exists with no batteries in the simulation. With caps across the rails the voltages were showing huge, sometimes in the kV range, and without caps it seems to sit at around 1.373 V on the supply rails.

    I know it's just a simulation, but I'm thinking if it's acting like that then maybe there is a wiring problem that should be fixed before I actually build this thing.
     
  9. tracecom

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    How about a ULN2803A?

    ETA: Never mind; it's a signal you want to switch, not power.
     
  10. GRNDPNDR

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    Mar 1, 2012
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    It is essentially power. It's an AC signal from a piezo transducer. the drum brain decodes everything.
     
  11. Ron H

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    Apr 14, 2005
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    Is the signal straight out of the transducer, or does it have some sort of buffer, like a voltage follower?
    A transducer has no DC conductivity, so the LM324 won't work as shown unless there is an unshown amplifier or resistor to ground.
    The reason I'm asking is I have an idea that involves lots of transistors, but I need more info on the nature of the source.
     
  12. GRNDPNDR

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    Mar 1, 2012
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    It's the same piezo I was talking about in one of my posts before....so nothing in between. it goes from piezo to the module that decodes it.

    This circuit will go in between.

    HOWEVER I used an LM324 today and tested it with the drum pads and it worked fine.

    One single op-amp out of 4 on the chip though (8 in total) worked. I don't know how it would work when all connected like this.

    Where would I place a resistor to ground?
     
  13. Ron H

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    Does the op amp have dual (positive and negative) supplies?
     
  14. GRNDPNDR

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    Mar 1, 2012
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    Yes, I'm using 2x9V batteries.

    Heres what I'm really curious about though. The virtual ground I've created, as I understand this is virtual because it's half-way between the two rails.

    Is there a way to achieve a "real" 0V ground in a situation like this, would I need it?

    Or will the virtual ground function just as well?
     
  15. Ron H

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    If you stack two batteries, + to -, the node where they connect to each other will be your ground. That's not a virtual ground.
     
  16. GRNDPNDR

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    Ok, I must have mis-understood what I was reading. it had an op-amp with a dual supply but used both a virtual ground and a real ground.

    Anyway that aside, How can I go about bypassing this then.

    I think a small reed relay would work correct?
     
  17. Ron H

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    The problem you might have is loading from the input and output of the unpowered op amp.
    What does the op amp's output connect to?
     
    Last edited: Mar 27, 2012
  18. GRNDPNDR

    Thread Starter Member

    Mar 1, 2012
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    The output connects to a plug on the drum brain.

    Normally its

    Drum Pad -> drum brain.

    This will go

    Drum pad -> boost circuit -> drum brain.

    They connect with TRS plugs I believe (1/4" phono plugs?)

    I want to bypass the op-amp when the power is off because I may not always want to have a boost applied, nor do I want to unplug it each time (min. 16 plugs) in the case that I choose not to use it.

    So what exactly do you mean by loading from the input/output? is that like unbalancing the rails?


    I would prefer to use smaller components such as transistors if possible, the reed relays are a little larger than I would and I don't have the simulation models of the ones I would use so it makes it a little difficult to try and plan for it.

    I'm simulating so heavily only because I don't have the time or money to keep messing with a physical circuit that at this point probably would have been burned out multiple times.
     
  19. Ron H

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    If all you do is put a relay or some other type of switch in parallel with the op amp, the input and output of the unpowered op amp wind up in parallel with your drum pad, which is a high impedance device, and can't tolerate much loading. The input and output impedances of an unpowered op amp are unspecified, hard to predict, and hard to simulate accurately, due to the nature of the op amp spice model.
    Do you have a datasheet, schematic, or anything else that will tell me what the drum brain's input impedance is? How long is the cable between this preamp circuit and the drum brain? How about between the drum pad and the preamp?

    I have a circuit that, in simulation anyway, disconnects the op amp input and output while bypassing it when power is off. The problem is, it requires 5 small transistors, plus a resistor and small capacitor, per stage. Four of these transistors might not be needed, depending on the answers to the questions I asked in the previous paragraph.
     
  20. GRNDPNDR

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    The company will not provide any information to me at all. I could possibly measure the input impedence could I not?

    I believe the cable length is 6' per drum pad, but I really am hoping to shorten this even if I have to splice the tips myself. The cables are FAR long than I really need.

    since this circuit will go inline with that, it would be about 3' on either side, so still 6' of cable or less if I shorten them.

    I'm certain these would be low impedence cables as they are audio quality, albeit probably low end.

    5 transistors per op-amp would be insane, that would be 40 transistors. I don't even know how I'd begin to arrange that on a DIY PCB, and I don't have a rework station to do surface mount........however my school does ;) but I hate SMT, too freakin small.

    EDIT

    I should add, the reason I want automatic bypassing is due to the batteries. should the batteries die while i'm using the drum set I would like to be able to keep playing (without missing a beat....ba dum ching)

    I'm considering using a 7809 and a 7908 to regulate the voltage a bit better but I'm not sure if that's fiesable at the moment.
     
    Last edited: Mar 28, 2012
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