Bottlehead Forum
General Category => Technical topics => Topic started by: shane on October 07, 2018, 04:21:44 PM
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Hi, I'd like to ask if the AC impedance of the bottlehead shunt regulator is the Rp of the triode section, or 1/Gm of the triode section.
Thank you
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It's a bit more complicated than that, as the TL431 itself has gain that changes rather significantly with frequency. We also sometimes throw a bypass cap in to send more of whatever is riding on the HV regulated rail back into the regulator, and that also alters the AC impedance a bit.
IIRC, the AC impedance of something like the shunt regulator in the Extended Foreplay III without the extra bypass caps was a couple hundred ohms at 120Hz.
PJ could author a more thoughtful response on this topic but he's on vacation so I'll fill in as best as I can till the expert returns.
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Thanks for the reply. If it's a couple hundred ohms, I guess that the impedance cannot be equal to the Rp of the tube. Is it as simple as 1/Gm plus the AC impedance of the voltage reference in the cathode circuit?. Thanks, Shane
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No, it's primarily set by the transconductance of the chip, adjusted by the normalized voltage. In theory (and by the specs) that works out to around 35 ohms for a 350v regulated supply.
I don't recall making a measurement. In the Foreplay days we built several (6 IIRC) shunt regulators and did comparative listening to choose one.
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Thank you.