Question about Dynamic Microphone Electrics

[murphyslabrat]

If it's a dynamic microphone element, and does not receive a bias-input, does it need to have a completed circuit? What I imagine happening is that the voltage generated by stimulation would serve as the input, and the return would be the traditional deal.

Also, if the above assumption is true, assuming that it has a contact wired to hot and return, would it matter which side the return was hooked up to? If not, would changing that swap the polarity of the signal?

 

[bobsmith1492]

What microphone is it? Most have an active amp built-in, so you need to supply voltage with the correct polarity.

 

[murphyslabrat]

It's not a microphone. What I was planning on doing, is taking a cheap dynamic speaker element, and placing it behind the regular speaker in a set of isodynamic headphones.

I was wanting to do this to create a slight reverb effect, to simulate the effect of a good soundstage.

I figured I'd ask this here, before I go slicing and soldering.

 

[bobsmith1492]

Ok, gotcha. You could do it two ways:
1. Capacitively couple the speaker straight into your preamp (ground->speaker->capacitor->preamp in), with polarity not mattering other than its effect on your reverb phase
2. Bias the speaker with a resistor divider like a traditional microphone (V+ -> resistor -> speaker -> ground, connecting the preamp input to the junction between the resistor and the speaker)

I think option 1 would be the best bet for using a small speaker as an amp. It will generate some voltage like you said that should serve as your audio signal. The low impedance of the speaker makes option 2 impractical.

The polarity of the signal will flip with the speaker's polarity; you'd have to try both ways to see which works best with your reverb algorithm.

You should also check that the input to your preamp has some significant input impedance. A speaker might generate high voltages with a super-low load, like a FET op-amp input. You could add a resistor and optionally a bidirectional zener or TVS across the speaker.

 

[Paperdoc]

In a true "dynamic microphone" no bias voltages are used. Such a device basically couples a mechanical receiver of vibrations to a coil of wire placed in a fixed magnetic field. In fact, you're correct to view a simple speaker with voice coil, cone and permanent magnet as a Dynamic Microphone. Movement of the coil in the magnetic field induces current flow in the coil that is passed through a very low-impedance amplifier input circuit. You can appreciate that the signal power is low, so the amp needs low input impedance, high gain and VERY low noise.

 

[canis]

OP are you trying to use the element as a speaker?

 

[murphyslabrat]

Ok, this is for a pair of headphones. I had assumed I had communicated that clearly enough, but apparently not. To remove confusion, what I want to do is this:

Take a pair of headphones, and stick a cheap dynamic element from another pair of headphones, and wire that as a parallel input beside the incoming audio signal.

It would not be amped, and the signal would not be modified in any other way. I think Paperdoc got closest to answering my question.

 

[Paperdoc]

OP, if you do exactly as you say - that is, wire in parallel two dynamic elements inside an earphone - you really will not get the second one to pick up the sound waves created by the first and feed them back as some sort of echo. What will actually happen is that the two simply will act as identical devices to convert electric current into sound waves. Your audio amp output circuit will see two identical devices in parallel and drive them both identically, with the small problem that the two in parallel have only HALF the impedance of the single unit the amp was expecting.

Wiring them in matched phase would mean the sound wave output from each would be in synchronization and would reinforce each other. Wiring one of them the opposite way would have the phases exactly opposite and reduce the total sound wave energy output. Same principle as when you use two identical speakers in one cabinet and run them in parallel (with appropriate impedance matching). You have to ensure the two speakers' phases are matched for reinforcement, not canceling of sound.

 

[murphyslabrat]

which is why I asked about the return. If the only wire connected to it is from the return on the "mic" wired to the live feed on the primary speaker element, then would it behave in the way I want?

 

[Paperdoc]

If you mean you want to connect ONLY one lead on the second element ("the mic") to one of the speaker's leads, and leave the "mic's" other lead not connected, then that "mic" will do nothing at all. For any circuit element to do anything, it must have a full circuit path for current to flow through it.

 

[murphyslabrat]

Which is, again, why I'm asking this question. Would the "input" from the electrical current generated by the coil's oscillation through the magnetic field be sufficient to provide the effect of a completed circuit: electrical current arriving, and then exiting, being grounded via the primary transducer?

So, will that not happen?

 

[bobsmith1492]

Which is, again, why I'm asking this question. Would the "input" from the electrical current generated by the coil's oscillation through the magnetic field be sufficient to provide the effect of a completed circuit: electrical current arriving, and then exiting, being grounded via the primary transducer?

So, will that not happen?

Not without two wires connected to it... current has to flow in a loop, there's no way around it (static electricity notwithstanding).

 

[murphyslabrat]

Okay, I feel my question has been adequately answered. You guys are saying that even considering inductance, this wouldn't work. I appreciate your help.