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Old 23rd February 2012, 04:02 PM   #1
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Default Impedance bridging

If I'm connecting a preamp to my tube amp, I want the preamp output impedance to be at least 10x times the input impedance of my amp? Is that correct?
Is the greater the difference the better?
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Old 23rd February 2012, 04:08 PM   #2
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It must be the load impedance (the amp) greater than the source impedance (The "pre"), in you case, the most of the output voltage from the preamp will be lost in its internal impedance. In the better case, the impedances must be equal, so the best POWER transfer between them will be true.
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Old 23rd February 2012, 04:11 PM   #3
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I think you mean 10x times less the input impedance of your amp.
And yes, the greater the better.

By the way, this holds for everything in audio systems. Loudspeakers etc...no impedance matching is used here !
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Last edited by the_manta; 23rd February 2012 at 04:12 PM. Reason: typo
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Old 23rd February 2012, 04:26 PM   #4
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Right, sorry had that backwards!

Thanks guys.
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Old 23rd February 2012, 04:27 PM   #5
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Don´t worry, boy!
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Old 23rd February 2012, 05:38 PM   #6
DF96 is offline DF96  England
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Quote:
In the better case, the impedances must be equal, so the best POWER transfer between them will be true.
No, you definitely don't want that. The aim here is minimum signal damage, not maximum power transfer.
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Old 23rd February 2012, 06:06 PM   #7
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Quote:
Originally Posted by rntlee View Post
If I'm connecting a preamp to my tube amp, I want the preamp output impedance to be at least 10x times the input impedance of my amp? Is that correct?
Is the greater the difference the better?
Not necessary. The more non-linear and frequency dependent are output and input impedances, the higher mismatch you need. But when they are linear and frequency independent in the frequency band it is the best to have them of equal impedances.

Loosing power on impedance mismatch you have to amplify it back adding more distortions. But sometimes it is easier to make more linear gain than more linear input/output resistances, especially when you don't know what kind of signal source will be used with your power amp, or what kind of load will be used with your preamp.
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Old 24th February 2012, 11:01 AM   #8
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Originally Posted by Wavebourn View Post
Not necessary. The more non-linear and frequency dependent are output and input impedances, the higher mismatch you need. But when they are linear and frequency independent in the frequency band it is the best to have them of equal impedances.

Loosing power on impedance mismatch you have to amplify it back adding more distortions. But sometimes it is easier to make more linear gain than more linear input/output resistances, especially when you don't know what kind of signal source will be used with your power amp, or what kind of load will be used with your preamp.
I agree. I have the impedances as matched as I can. Particularly, in speaker systems impedance matching IS definitively important.
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Old 24th February 2012, 11:24 AM   #9
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No
A speaker is not a linear resistance. It even produces negative resistance due to induction voltage from the voice coil movements.
With impedance matching you get worse damping of the speaker und more distortion.

Impedance matching is only useful in RF circuits and antennas.
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Old 24th February 2012, 11:28 AM   #10
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There may be some confusion here about "impedance matching" and the difference between maximum efficiency and maximum power transfer. Impedance matching for a speaker means a DF of 1, and boomy bass unless the speaker was specifically designed for this unusual situation. People often loosely talk about impedance matching, when what they really mean is "using the correct impedance" - usually a significant mismatch.

Impedance matching for a line level connection would require unusually low amounts of stray capacitance or low and linear impedances at both ends. As Wavebourn says, generally easier to go for even lower impedances at source and high at load, then you don't have to worry about impedance linearity so much.
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