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Old 12th August 2011, 02:47 PM   #1
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Default High and low impedance output preamps

I saw some preamps are having extreme output impedance: -

Herron Audio VTSP-3A Output impedance 100 ohms
Herron Audio: VTSP-3A Preamplifier

Melody Pure Black 101 Output impedance 250K
East Street Audio. Melody Pure Black 101 (Formerly P1688 Signature)

Why there is such an extreme in design?
What will happen when connecting to a power amp that has a different input impedance on the opposite side of the spectrum?
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Old 12th August 2011, 03:27 PM   #2
bigwill is online now bigwill  United Kingdom
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I think somebody put the input impedance there by mistake on the second one, to be honest
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Old 12th August 2011, 03:34 PM   #3
DF96 is offline DF96  England
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No extreme in design, just some confusion. The person who wrote the Herron text knows what output impedance is, as 100 ohms sounds like a reasonable figure for a preamp. Whoever wrote the Melody text was confusing output impedance and load impedance. Having said that, 250k is quite a high load impedance so it suggests that the Melody preamp has quite a high output impedance too so may be a poor design.

Whenever you see a preamp 'output impedance' of more than a few k it either really means 'minimum recommended load impedance' or very poor design. The former is more likely, although the presence of the confusion may imply the latter too.

As a very rough guide, you want an output impedance which is at least a hundred times smaller than the load impedance. That is, a severe mismatch. Matching is good for RF and video, but usually a bad idea for audio.
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