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In Reply to: RE: What are your readings? posted by Lew on June 14, 2012 at 08:24:10
Lew,
I sent you a message with these readings and my whole Odyssey to date, but apparently you never received it. I'll try again.
Yesterday, I had a metrologist friend over to make impedance measurements. Configuration of back-plate: no resistors, no capacitor, no inductor, no brilliance control, midrange control +3. The results are below:
Impedance in Ohms
Freq (Hz) AU Black/Pos spkr term AU Black/Neg spkr term
(per instructions)
40 4 9.8
50 5 12
80 8.8 5.9
100 12 4.3
200 23 2.5
400 24 2.2
500 24 2.3
800 20 2.4
1,000 17 2.6
2,000 7.4 4.5
4,000 3.4 4.8
5,000 2.7 3.5
8,000 1.9 1.6
10,000 1.6 1.2
16,000 1.3 .58
20,000 1.3 .46
In-phase hookup shows a more benign load (higher impedance) for the amplifier; but even then, the impedance at the higher frequencies is awfully low for any amp, but especially an OTL.
I found out about the tube problem after I did these measurements.
Regards,
Chuck
Follow Ups:
I cannot make out what you actually got with what hook-up at 40 and 50Hz, and at 100Hz I got a much higher number, assuming you saw 8.8 ohms (see SLOG), but above 100Hz, your readings with the "AU black pos terminal", meaning I hope that the doublet of screw terminals on one side of the AU primary were connected to plus, are very much consonant with mine as posted on SLOG. (I only took readings at 20, 50, 100, 200, 300, 400, 500, 1000, 2000, 5000, 10K.)
The right-most row of readings are no doubt the result of wiring the treble transformer 180 degrees out of phase with respect to the bass transformer. Mine were very similar; don't use that hook-up.
Right on all counts. I'm sure now that I have it hooked up correctly.
This is the point where I am wondering if the Bass transformer is rolled off by a choke. The readings look very much like the bass transformer is active above the crossover frequency.
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.
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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