airbrake build

[catalin gramada]

Hello all
I just finished an airbrake project intended to use for severe attenuation for a non MV amp. Did not found a 150ohm reostat, used 100ohm instead. Now, the dc resistance readings in "bedroom mode" (with 8omm speaker conected) shows between 13.5 - 20.5 ohm. and will be a litle higher in real world due to raising ac impedance of speaker. After everything I've read the directions are to match the amp output with the speaker. My question is why?? this values fit more to use the 16 ohm output of the amp into this range even for 8 ohm speaker. What's the clue? Thanks

[Littlewyan]

The Airbrake by design doesn't match the impedance perfectly to the amp's output. It increases the impedance to try and preserve the tone at all attenuation levels. Reason being a speaker isn't a constant impedance device, i.e. an 8Ohm Speaker won't measure as 8Ohm at all frequencies, it will change. So don't worry its perfectly normal.

I can explain more later but I have to go to work now .

One thing I will say before I go, the Airbrake was clearly designed to work best with an 8Ohm speaker. 4Ohm or 16Ohm speakers tend to not work as well through it.

[catalin gramada]

Speaker won't measure as 8Ohm at all frequencies, it will change. So don't worry its perfectly normal.

You're right and I allready said that. speaker impedance will change-but just in one way: up. So, that means more reactance in parallel with 25ohm shunt resistor rising the attenuator impedance more. What I did not understood is why to run a 20-23 ohm load into a 8 ohm output instead of 16ohm output,please?

[catalin gramada]

pretty similar versions:
komet
[img733]http://i66.tinypic.com/dh6bdt.jpg[/img]
and lite
[img749]http://i66.tinypic.com/34p064o.jpg[/img]

[Littlewyan]

Speaker impedance goes up and down depending on the frequency. Hence why when you measure an 8Ohm speaker with a multimeter it will measure a bit lower at perhaps 6 Ohms .

The attenuator presents a higher load to try and emulate the speaker better at high attenuation levels.

I've attached a txt document with an article written by the creator himself explaining how it works.