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Post by iggs on Dec 18, 2007 19:04:47 GMT -7
So I measured the AB at 0 attenuation and it measures same as the cab ... cool, as expected.
Then I started going through the clicks and the measurement just kept going up and up where at the bedroom setting it was around a whooping 32 ohms???
I guess that's expected because of the added resistance inside but isn't that putting a huge strain on the amp? I mean ... it is 4 times the load the amp is expecting to see.
I could be wrong but in contrast, I thought the THD Hot Plate always put out the marked impedance regardless of the attenuation level.
Sound aside ... isn't that "safer" for the amp???
p.s. ... I just realized the AB only goes to -30dB vs. Hot Plate which goes to infinity on the rheostat ... kind of a bummer. I still like the sound of the AB better though.
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Post by benttop (Steve) on Dec 18, 2007 21:52:15 GMT -7
I'm not sure you can use a DC resistance measurement to determine much of anything. I mean, and 8 ohm speaker is only 8 ohms at one frequency, but usually measures around 6 ohms DC. So what does that DC reading really mean in the context of the full audio spectrum of AC the speaker is reproducing? I've never even attempted to understand the arithmetic surrounding all of this....
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Post by dixiechicken on Dec 19, 2007 7:15:05 GMT -7
I'm sure you know this but - it is really only about Pythagoras. The impedance varies with frequency. (lets call it Z ) The impedance have one resistive component that DOES NOT vary with freguency. (lets call it R) It has a 90 degree phase shifted inductive component that does vary o lot with varying frequency. (lets call it X) The Z (squared) = R(squared) + X(squared) Calculate the squareroot out of the out of the right hand of the equation and you get Z. Example lets say att 1000Hz: R = 8 Ohms ( 8 squared is 84 ) X = 28 Ohms ( 28 squared is 784 ) Z = the square root out of (64 + 784) Z = 29.1204 With complex wave forms like a chord from a guitar you have maybe six base frequncies + lots of harmonics so things get very complex. Cheers: Dixiechicken!
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Post by iggs on Dec 19, 2007 7:17:03 GMT -7
I'm not sure you can use a DC resistance measurement to determine much of anything. I mean, and 8 ohm speaker is only 8 ohms at one frequency, but usually measures around 6 ohms DC. So what does that DC reading really mean in the context of the full audio spectrum of AC the speaker is reproducing? I've never even attempted to understand the arithmetic surrounding all of this.... Correct ... I know that the impedance is a reactive measurement and it changes with frequency and can at times measure up to 10x the "static" amount. I was just wondering about the passive resistive load difference when Airbrake is used. From what I can tell, it is still "mismatching" the load by up to 4x. Edit: because the whole purpose of matching the load (and the necessity of output transformer in tube amps) is to ensure that the output tubes are "seeing" the correct load.
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Post by benttop (Steve) on Dec 19, 2007 8:59:20 GMT -7
iggs, have you tried this same dc measurement on a Hot Plate? I'm curious. In fact, it would be interesting to see if the various Hot Plates (4, 8, 16 ohm) react differently, and how they react compared to an Airbrake. I still don't know what it all means though...
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Post by iggs on Dec 20, 2007 20:55:06 GMT -7
iggs, have you tried this same dc measurement on a Hot Plate? I'm curious. In fact, it would be interesting to see if the various Hot Plates (4, 8, 16 ohm) react differently, and how they react compared to an Airbrake. I still don't know what it all means though... Just measured it and The Hot Plate impedance stays the same through all the attenuation levels and matches the speaker. The one I have now is 8 ohms and I had the 16 ohm one which I'm pretty sure I measured once and it was the same as well. I don't really know what that "means" either but I assume that's one of the reasons (if not "the" reason) for the warning regarding using Airbrake with vintage amps where the manual says it's safer to mismatch the impedance in order to keep the transformer "happy".
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Post by iggs on Dec 21, 2007 3:35:43 GMT -7
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