Re: 3 dB bandwidth




Pooh Bear wrote:
>
>Guy Macon wrote:
>>
>> Back to the topic, 3dB is also the amount of drop you get when you
>> have the output impedance and the input impedance matched, which was
>> common in the days of transformer-balanced audio lines.
>
>Nooooo ! That's a 6dB drop.

Yeeeeees! That's a 3dB drop. :)

Actually neither one of us should be saying "drop." If I had
specified power drop and you had specified voltage drop it
would have been clear that we are both right.

>Thank God we got rid of so-called 'matched impedance working'
>and went for 'voltage matching' in audio a long long time ago.

Try to make it work without transistors sometime. In the days
when we all used vacuum tubes/valves and transformers, getting the
maximum amount of power out of one transformer and into the next
transformer was important, thus the matching, and thus the 3dB
power drop.

When semiconductors and especially op amps hit, it became feasible
to have a low output impedance feeding a high input impedance
(and occasionally the reverse, with trans impedance amps). When
that hit, not only wasn't there a power drop, the power transfer
was annoyingly close to zero, and everyone started thinking in
terms of voltage rather than power. Once you start measuring
voltage in a Low Z to high Z line, the drop when you add a load
approaches the convenient number zero. Filters, however, still
needed a -(something)dB point, and the old -3dB stuck.

A common occurrence nowdays is for newly minted recording
engineers to think that adding together two close to identical
phased signals (think two microphones right next to each other
in front of the bass guitar amp) add up the same way as adding
together two random phased signals (think two microphones close
micing two violins playing in unison). Loads of fun watching
them trying to figure out why they don't add up the same way...

--
Guy Macon <http://www.guymacon.com/>




.

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