Stomp Box Buffers and Impedence

[leesoonghotmail.com]

Dear Craig,

 

I would like to find out exactly what a "buffer" is and does in your typical guitar stomp box.

 

Some have said that buffers basically turns a high impedence signal into a low one, and that as a side effect, boosts the upper band octaves.

 

To my understanding, most guitar amps have been designed to take in a high impedence signal from guitar. If your guitar signal is being processed by these stomp boxes with buffers in them, effectively turning your signal into low impedence, wouldn't that cause an impedence mismatch once the signal reaches the input of your amp?

 

Please help me out of this confusion.

 

Thank you in advance.

[Anderton]

Originally posted by leesoong@hotmail.com:

Dear Craig,

 

I would like to find out exactly what a "buffer" is and does in your typical guitar stomp box.

 

Some have said that buffers basically turns a high impedence signal into a low one, and that as a side effect, boosts the upper band octaves.

 

To my understanding, most guitar amps have been designed to take in a high impedence signal from guitar. If your guitar signal is being processed by these stomp boxes with buffers in them, effectively turning your signal into low impedence, wouldn't that cause an impedence mismatch once the signal reaches the input of your amp?

 

Please help me out of this confusion.

 

Thank you in advance.

 

 

Your understanding is mostly right of what a buffer does: transform the high output impedance of a pickup into a low impedance.

 

Mismatch with a high impedance amp is not an issue, because you don't want equal impedances unless you are trying to transfer the maximum amount of power. With guitars, we're trying to transfer the maximum amount of voltage. Therefore, you want an output impedance to feed something at least 10X higher.

 

This becomes more important with guitar, because a passive pickup's output impedance increases with frequency. For example, at 10KHz the output impedance (which would be, say 10k at 100 Hz), might be closer to 100 kHz! Observing our 10X rule, this would need to feed a 1 Megohm input impedance in order not to be loaded down.

 

With a short cord and a high Z amp, you don't really need a buffer. But if you use a long cable, then cable capacitance becomes an issue, as capacitance has more effect in a high impedance circuit. A low impedance line will also be less subject to noise pickup and other problems.

 

Where a buffer board is essential is feeding a device with a relatively low input impedance, like a mixer or recorder. The buffer preserves level and frequency response.

 

If any of this remains confusing, post again and we'll zero in on the details.