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FDP Forum / Miscellaneous and Non-Fender Topics / I was asked to explain impedance matching (amp OT & speaker)

Peegoo
Contributing Member
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Lauren's

a tractor
Oct 4th, 2019 07:28 PM   Edit   Profile  

Discussing speaker swaps with a pal today, he asked me why it's so important to use a speaker with the proper impedance rating.

We all know it's good practice to do this--chiefly because it increases the efficiency of the amplifier (more volume for your watts, basically).

More importantly, it allows the output transformer (OT) to run without overheating caused by standing waves/reflections in the AC current through the OT created by the impedance mismatch.

I was explaining this, and then remembered a 16mm film we watched in school when I was a wee Peegoo. It was one of many produced by Bell Telephone Labs. The Bell films were *the best* aspect of elementary school as far as I was concerned as a kid. Science was the shiz for me, and every other subject was boring. But I digress.

I found that film on Youtube. It's like a time machine!

But if you'd like a really good review of wave theory, presented by a real genius, click below and be amazed. If you're not 100% up on wave theory, you will have a few "aha!" moments, I guarantee.

Dr. Shive made this stuff easy enough to understand for my 6th-grade peanut brain, because he uses a wonderful visual tool of his own invention. I built one for myself when I was in the 7th grade. It was not nearly as good as the one in the film :o(

Like all the Bell films, this is a good one. It's from 1959.

LeftyMeister
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Buckeye Country, USA

Tone is in the lingers
Oct 4th, 2019 08:07 PM   Edit   Profile  

It's too late for me to watch it tonight but I will tomorrow. Thanks for sharing it!

Impedance is the total opposition to current in an AC circuit. It's resistance + reactance. Per Ohm's Law, if resistance increases and voltage remains constant, current decreases. The opposite is true if resistance decreases. So, using an improper ohm value for a speaker will cause the amp to either over-work or under-work, neither of which is good for the amp.

(This message was last edited by LeftyMeister at 10:56 PM, Oct 4th, 2019)

Peegoo
Contributing Member
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Lauren's

a tractor
Oct 4th, 2019 08:46 PM   Edit   Profile  

The other aspect of impedance is that it's frequency dependent: as different notes and harmonics are amplified and sent through the OT and speaker, the impedance dynamically varies.

However, these swings in impedance are extremely rapid, and the OT and voice coil being matched is what makes them work efficiently together.

LeftyMeister
Contributing Member
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Buckeye Country, USA

Tone is in the lingers
Oct 4th, 2019 08:55 PM   Edit   Profile  

Good point!

FDP Forum / Miscellaneous and Non-Fender Topics / I was asked to explain impedance matching (amp OT & speaker)




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