Input/output impedance question

[Kevsy71]

Please could anyone help me get my head around the theory of input vs output impedance?

[size=4][font=arial,helvetica,sans-serif]I've read that the output impedance of passive pickups (about 10k[color="#444444"]Ω, but varying with frequency) is best coupled to an input impedance of at least 1:10 ratio. In my case my p-bass plugs into a Boss TU-3 tuner with (I think, based on the TU-2) 1 mΩ input impedance / 1k[/color][/font][/size][color=#444444][font=arial, helvetica, sans-serif]Ω [/font][/color][color=#444444][font=arial, helvetica, sans-serif]output impedance, and then onto the head, and true enough the bass sounds 'fuller' than with my previous effects chain arrangement.[/font][/color]

[size=4][font=arial,helvetica,sans-serif][color=#444444]So my question...[/color][/font][/size]

[color="#444444"][font="arial, helvetica, sans-serif"]If the input impedance represents the load 'seen' by whatever is plugging into it, surely a higher input impedance means a higher load. Given that impedance is resistance (more or less), why does this allow more of the bass signal through - I would have thought it would have allowed less?[/font][/color]

[color="#444444"][font="arial, helvetica, sans-serif"]I'm guessing the answer has something to do with increased difference in electric potential, but any advice appreciated![/font][/color]


[color="#444444"][font="arial, helvetica, sans-serif"]Cheers[/font][/color]
[color="#444444"][font="arial, helvetica, sans-serif"]Kev[/font][/color]

[dincz]

Pickups are inductive so their impedance increases with frequency. An amp's input impedance is more-or-less constant and should ideally be much higher than the impedance of the pickup. The two together form a frequency-dependent voltage divider. If the amp's impedance is low you'll get serious drop-off in level as frequency increases.

You can hear this clearly if you plug a passive bass into e.g. a mixer's line input (typically only 20Kohm or so).

[flyfisher]

[quote name='Kevsy71' timestamp='1387213605' post='2309479']
[size=4][font=arial,helvetica,sans-serif][color=#444444]So my question...[/color][/font][/size]

[color=#444444][font=arial, helvetica, sans-serif]If the input impedance represents the load 'seen' by whatever is plugging into it, surely a higher input impedance means a higher load. Given that impedance is resistance (more or less), why does this allow more of the bass signal through - I would have thought it would have allowed less?[/font][/color]
[/quote]

It's the other way around - the higher the input impedance of the amp, the LOWER the load 'seen' by whatever is plugging into it.

You may be imagining the input impedance as a resistor in series with the amp input, but it's effectively across the input and ground. Thus, a low input impedance will 'pull down' the input signal down to ground, thus reducing its amplitude, more than a high input impedance.

Think of 'output impedance' as being a resistor in series with the output of the driving device and 'input impedance' as being a resistor between the input of the amp and ground. When the output device is plugged into the amp input the series resistor is connected to the input resistor, the other side of which is connected to ground. In other words, the output signal is connected to two series resistors then ground. The amp takes its input from between the two resistors, which effectively act as a potential divider.

I hope that helps. A drawing would be better but when I tried to draw a simple diagram the forum software seems to strip out all spaces and messes up any diagram.

Thinking in simple resistor terms can help with understanding the basic principles but, as Dincz rightly pointed out, there is inductance and capacitance to take into account in a real-world circuit, which makes the simple potential divider description frequency dependent.

[Kevsy71]

Flyfisher, Dincz - thanks very much for the detailed walkthrough, that explains it! Much appreciated