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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: TK's 6SN7 non-inverting cathode follower preamp: Z out ? posted by deathtube 667 on June 6, 2017 at 11:11:06:
This is a problem with any tube-type line stage, not only followers. It isn't as apparent with common cathode designs, because their output Z is usually fairly high, even at 1kHz.
There are only two solutions I know of for flattening the output Z of a cathode follower. One is to make the cap larger. That's what most designers do. The other is to apply a negative voltage under the cathode resistor (or choke), then direct-couple the cathode to the output. This requires a servo circuit to apply negative bias to the grid, in order to keep the cathode at exactly zero volts DC. It also requires adding a small blocking cap at the grid in exchange for the larger output cap. Whether it's beneficial depends on the impedance of the loads being driven. A 100K resistive load can normally utilize a relatively small output coupling cap, so eliminating it probably has little or no advantage.
I thought of this technique and designed a circuit in SPICE while recently working on choke-loaded followers. The next day, I discovered that Sy at SYclotron Audio had done the same thing several years earlier. Oh well...
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- Line Stage Output Z - Triode_Kingdom 12:42:12 06/06/17 (0)
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