Input Output Impedence ?'s

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Input Output Impedence ?'s

Postby travsbluz » 24 May 2012, 20:39

I've read every now and then about the input/output impedence of a pedal. I guess I missed why this is possibly good or bad. So my questions are:

1: How do you know what the input or output impedence is?

2: Can you measure it on an a pedal you already have made somehow?

3: What does it affect?

4: Is there a good range for guitar or bass pedals?

Thanks for your time in answering these questions.
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Re: Input Output Impedence ?'s

Postby MullisMan » 30 May 2012, 19:46

This should help you out. Much better than writing a long reply post.
I want to build it all.

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Re: Input Output Impedence ?'s

Postby travsbluz » 30 May 2012, 20:13

Thanks for this. I thought I had seen something like that somewhere, but I just couldn't find it in all the web sites I've saved learning how to do all of this.

Thanks again,
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Re: Input Output Impedence ?'s

Postby Dan Moos » 10 Jun 2014, 06:36

When I first got into electronics, impedance was never properly explained, so here's the explanation I'd wish someone had given me. Hope it helps.

With audio electronics, we are sending a signal from stage to stage in our devices. For many good reasons, the signal is represented as a voltage. It could be represented as current, but that turns out to be problematic, so we use voltages to represent our signals for the most part. Obviously you can't have one without the other, but it is the voltage we care about in most audio situations.

But lets think of another way of sending a signal. Consider knocking on a door. Sounds simple enough, but lets mess with the "impedances" of our door knocking.

Normally, one uses a bit of energy to knock on a hard surface when they knock on a door, and the sound is clear.

Bur what if the door was soft like a mattress? The knocking would be almost inaudible. One might say the soft mattress presents a much lower impedance (resistance) than the hard wooden door. Instead of providing a stiff surface to make the the knocks transmit, the mattress instead sucks up all the energy of your knocking signal and very little of your "signal makes it to the next stage (the ears of the occupant)

Now lets go back to electronics. Lets say you have a guitar plugged into a pedal. The signal from your guitar is only going to be transmitted into the pedal properly if it encounters the electrical equivalent of a hard surface. So we want alot of resistance at the beginning of the pedal. When you throw capacitors and inductors into the mix, resistance becomes a slightly more complicated thing called "impedance", but the results are the same. So all our devices need a relatively high impedance at the input, or our signal will encounter the electrical equivalent of the mattress and be stifled.

But that's only half the story. Imagine again you are knocking on the door, but instead of swing your arm through air, you have to swing it through molasses. All your energy would be sapped out whilst you swung, and very little knocking would be transmitted by the time you hit the door. You are trying to send your signal through a high impedance.

In electronics, the equivalent would be some sort of resistance at the output of the device that sucked up or choked all the current on its way out. You want a low resistance path out of the device. No molasses!

If you know a bit about electronics, then you might realize that what we want is lots of current coming out of a device so that it hits a stiff resistance at the next device and forms a voltage, (our signal).

Also, if you know about voltage dividers, you will recognize that a low output impedance feeding a high input impedance is the same thing as a voltage divider that delivers a large percentage of the input voltage to the output.

One final note. The specific impedences aren't as important as the ratios. A good rule of thumb is that you want the input impedance of a device to be at least 10 times the output impedance of the device feeding it.

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Re: Input Output Impedence ?'s

Postby Lucifer » 10 Jun 2014, 09:40

I love the mattress analogy !
I wish I had heard it before I started teaching electronics to reluctant apprentices (now I know why I couldn’t transfer knowledge into their soft heads !)
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Re: Input Output Impedence ?'s

Postby DrNomis » 10 Jun 2014, 23:45

I think it's a good analogy too, there's a good reason why the input impedance needs to be much higher than the output impedance, this is so that the input does not load down whatever is being fed into it, this is pretty important in circuits which operate at high-impedances, such as Valve circuitry........ :thumbsup
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