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Impedance
Old 3rd March 2014
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🎧 5 years
Impedance

I'm taking a couple of electronics classes right now and we've talked a lot about impedance in AC circuits. My question is, what does the impedance of a system (amplifiers, speakers, headphones, etc.) actually have to do with how the signal/audio is affected?

What I've gathered so far is that a higher impedance = higher equivalent resistance, which means a higher possible output voltage. Is this correct, or what have I missed?
Old 3rd March 2014
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Quote:
Originally Posted by matthollenberg ➡️
I'm taking a couple of electronics classes right now and we've talked a lot about impedance in AC circuits. My question is, what does the impedance of a system (amplifiers, speakers, headphones, etc.) actually have to do with how the signal/audio is affected?
Reactance...

Before you can understand impedance you need to first understand (1) inductive reactance, (2) capacitive reactance, and (3) pure resistance. The first two are vector opposites of each other and will completely cancel each other out if they are exactly equal. If they are unequal, they partially cancel, leaving either net capacitive reactance or inductive reactance, along with resistance in the circuit. Both forms of reactance are frequency dependent. The reactance formulas can be found in wiki and I suggest you memorize them because they will pop up over and over again in some form or another.

Impedance...

Impedance is the VECTOR SUM of the three components: inductive reactance, capacitive reactance, and pure resistance. Because both inductive and capacitive reactance vary with frequency, impedance can likewise vary with frequency if it has any reactive component to it. A more complete explanation is in order that could fill chapters or even a book, but that's it in a nutshell.

Quote:
What I've gathered so far is that a higher impedance = higher equivalent resistance, which means a higher possible output voltage. Is this correct, or what have I missed?
Yes, to some extent that's true. A very crude way of looking at it is in terms of power. The calculation for power is P= I*V, meaning current * voltage.

Lets say you have 1000 watts of power, but have two different possible scenarios for that power:

1. 1000 amps at 1 volt, or instead...
2. 1000 volts at 1 amp.

Both scenarios result in 1000 watts of power flowing in the circuit, but the first scenario is one of low impedance with respect to the second scenario, which is one of high impedance. The current that actually flows thru the circuit will be dependent on source as well as load impedance. In general, low impedance is associated with high current and low voltage, while high impedance is associated with high voltage and low current, relatively speaking.
Old 3rd March 2014
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matthollenberg's Avatar
 
🎧 5 years
Yeah I know Xc=1/2pifC and all of that stuff, but my question is how does this relate to audio? When I read that my headphones have a 54 ohm impedance, what is that telling me about the headphones themselves?
Old 3rd March 2014 | Show parent
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Originally Posted by matthollenberg ➡️
Yeah I know Xc=1/2pifC and all of that stuff, but my question is how does this relate to audio? When I read that my headphones have a 54 ohm impedance, what is that telling me about the headphones themselves?
You have two impedance matching situations:

1. Maximum voltage transfer.
2. Maximum power transfer.

In general, preamp stages are voltage amplifiers and need voltage coupling rather than power coupling. For voltage transfer to be maximized, a common rule of thumb is to use a 1:10 impedance ratio between source and load impedance. This applies to microphone circuits, for example, where the mic has, let's say, a 150 ohm output impedance and the preamp has a 1500 ohm input impedance.

On the other hand, for max power transfer, the rule is to instead use a 1:1 impedance ratio. Guitar amps work on this principle where the output impedance of the amp needs to match the speaker load impedance as close as possible if max power is to be be achieved.

Headphones are slightly different. If you have a 54 ohm set of phones it's a good idea not to have the source impedance any greater than 54 ohms, or else you will load down the source too much causing fidelity to suffer. But on the same hand, it's OK to have the source impedance lower than 54 ohms, meaning it will just transfer less power. Headphones fall under the power transfer scenario more or less, but with less of a concern for absolute max power transfer.

A solid state amplifier power output section has very a low output impedance, lower than the actual speaker connected to it. That is why SS amplifiers are capable of driving a variety of speaker load impedances, because the lowest speaker impedance that the manufacturer recommends is still greater than the output impedance of the amplifier which may be a fraction of an ohm. Cutting the speaker impedance in half under that scenario will cause twice the power to be drawn from the amplifier.
Old 3rd March 2014
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Okay, that makes sense. How do I know whether a situation needs voltage coupling or power coupling?
Old 3rd March 2014 | Show parent
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Originally Posted by matthollenberg ➡️
Okay, that makes sense. How do I know whether a situation needs voltage coupling or power coupling?
Voltage coupling is used anytime you need to amplify voltage such as in a preamp stage. A typical guitar amp, for example, may have three voltage amplifying preamp stages including the phase inverter section, and one power stage being the final output stage that drives the speaker. Voltage amplification is needed when you have very small signals such as from a mic or from guitar pickups that need to amplified into an even greater voltage. You usually need multiple stages of voltage amplification to get the signal voltage large enough to drive the final power stage. Only the final output stage to the speaker needs power coupling. Everything else relies on voltage coupling.

Power transfer, in general, is required anytime sound is generated through a transducer such as a speaker or set of headphones. Power stages may amplify voltage to some extent but really have the primary purpose of generating and transferring high amounts of power, meaning they generate substantial current as well as voltage.
Old 3rd March 2014
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Thank you so much!
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