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In Reply to: RE: Transformer Inductance posted by Triode_Kingdom on October 13, 2016 at 11:58:39
Paul and hennfarm, let's say the primary is only 10H. I don't know the typical DCR, but I'll check this evening after work. For now, I'll assume 500 Ohms. If I drive a signal across that inductor using a generator with 100 Ohms output Z (similar to the followers I have in mind), 20 Hz response is down only 0.26 dB relative to 1 kHz. If the inductor is 20H and 1k Ohm, 20 Hz is down only 0.13 dB. This isn't as good as 50H or larger, but I'm still thinking the concept might be viable.
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Buy Chinese. Bury freedom.
Follow Ups:
At 20Hz, 10H is a reactance of 1250 ohms. A signal of 100vRMS would cause an AC current of 80mARMS, or peaks of +/-112mA. The choke must be able to take that current in addition to the quiescent DC without saturating, if you are to avoid added distortion.
"The choke must be able to take that current in addition to the quiescent DC "
Paul, I think these consumer-grade transformers are capable of much more than that. They're designed to work with a SE pentode like the 6BQ5 or 6V6 drawing 60-70mA at idle. In this service, the primary winding would be subjected to at least 160V RMS to produce 5W at the secondary. My application is much less demanding in terms of voltage and current, probably only whatever is present at the cathode of a 7044 or 6BX7 direct coupled to an output grid.
What really interests me about all this is the increase in performance of the winding when used in a low power, low-Z circuit. I believe both the high and low ends will be extended, relative to the original application. Incidentally, I picked out a transformer this evening, and the primary DCR measures 450 Ohms. I'll test it for inductance this weekend.
Honestly, I'm excited about the prospect of using these to choke load CFs. I've been looking for a less expensive alternative for some time. :)
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Buy Chinese. Bury freedom.
OK, I was able to measure the primary of these 5K:8 transformers at three points -
20 mADC: 20H
40 mADC: 14.5H
60 mADC: 10H
I'll have to wait for an actual CF project to test frequency response, but I expect it to be smooth and extended in such a low impedance circuit. Great, it looks as though these will work well with nearly any CF driver!
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Buy Chinese. Bury freedom.
Keep in mind that low impedance output impedance is a small-signal value. A cathode follower can deliver the most power into about the same impedance as a plate-loaded stage. Available undistorted output swing will be less as load impedance drops.
Yes, I understand the load shouldn't be a lower value than if it were located at the anode. I'm making use of the lower Z (and reduced DCV) at the cathode solely to permit direct coupling. Eliminating the coupling cap is primarily for the purpose of ensuring near immediate recovery from output stage overload.
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Buy Chinese. Bury freedom.
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