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In Reply to: RE: What are your readings? posted by Ralph on June 15, 2012 at 08:29:06
Ralph, I believe the bass transformer has enough self-inductance to take itself out of the circuit pretty quickly above 2kHz. It cannot have much effect at all at 5kHz and above. My data suggest this, if I compare the AU90 transformer alone to the [bass + AU90] impedance readings at above 2kHz. With a 1.5mH inductor in front of it, the bass transformer alone gives 60 ohms impedance at 2kHz. Anyway, I am pretty sure Chuck (Throwback) is using an inductor, which is at least 1.5mH in value or higher.
Follow Ups:
Actually, no: no inductor. It came out clean when I removed the rest of the stuff--resistors, Brilliance Control, capacitors.
But I have to assume that it still represents a load at 20KHz. Have you done any measurements of just the bass transformer at that frequency?
The reason I ask is if there is self inductance, then its impedance should be going sky high if its not influencing the impedance at 20KHz. But all I have ever seen of ESLs is that impedance goes down as frequency increases. That really suggests to me that while the transformer may not be making any output, its still loading the amp and for no good purpose unless you like heat. Am I way off the mark??
You wrote, "The reason I ask is if there is self inductance, then its impedance should be going sky high if its not influencing the impedance at 20KHz. But all I have ever seen of ESLs is that impedance goes down as frequency increases. That really suggests to me that while the transformer may not be making any output, its still loading the amp and for no good purpose unless you like heat. Am I way off the mark??"
Here's the way I think of it, right or wrong: (1) Yes, I think the impedance of the speaker as seen via the bass transformer alone would by "sky high" at 20kHz, but no, I did not measure the bass tranformer alone (with no treble transformer in parallel) above 2kHz, because it was going straight up to the sky already at that frequency. I did it only from 20Hz to 2kHz in log intervals. Did someone else put out some data to the contrary? (2) I think the reason the speaker has a low impedance at very high frequencies is by far mostly to do with the speaker itself and its capacitative nature. So, when the speaker is seen by the amplifier, almost entirely reflected via the treble transformer at high frequencies, the impedance will be very low. The effect of the transformer should be a constant, related to the turns ratio, at all audio frequencies. I think R West himself recently wrote that the speaker is ~1600pF capacitor. (Can't remember, was some number between 1600 and 1900. These capacitances seem too low to me; I thought K Covi calculated the M1 as a 2uF capacitor.) Figure the impedance of a 1600pF capacitor at 20kHz and then divide that by the square of the turns ratio of either the SL toroid (75:1) or the AU90 (90:1). Gosh, I think the result would not correspond to reality, too high Z.
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