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Post by sparrowhawk (Bob) on May 10, 2008 5:15:40 GMT -7
I have a couple of questions on the fundamental operating principle of the Air Brake. Amp experts (including Dr. Z), feel free to chime in here.
First, this is my understanding of how the Air Brake works: If you look inside, there's a big 25 ohm resistor that is put in series with the speaker when you are on setting 1. There's another big resistor with taps installed to add progressively higher resistance in series to the first big resistor as you go from settings 2 to 4. For Bedroom setting, you get the full value of both resistors in series, plus the additional progressive resistance of the rheostat. (There's a cap across the rheostat, presumably to allow the high frequencies to pass.) All this resistance soaks up the power that would normally go to the speaker to be turned into "loud", so the power to the speaker is reduced, resulting in decreased sound output from the speaker.
Question 1: Is my understanding correct?
Now for the thing that's confusing me: If we're adding resistance in series with the speaker, doesn't this change the impedance that the output tranny "sees"? Does this present an issue to the output section of the amp (tranny, tubes...)? How can you change the effective speaker load by a factor of 4 to 10 and still keep the tone reasonably OK?
It just seems that by changing the load on the amp by so much, the amp and/or the tone will suffer. So far this hasn't stopped me from using the Air Brake on almost every gig and rehearsal, but I thought I'd ask. Any insight will be greatly appreciated.
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Post by sudsysul on May 15, 2008 10:39:12 GMT -7
The amp is not seeing the 24 or 25 ohms of resistance, but is in a safe impedence range regardless of the attenuation setting. That is probably why the tone is not affected by much, just as playing into a 16 or 4 ohm cab with an 8 ohm amp setting isn't dramatically different.
Perhaps others with a more technical response will chime in.
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