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TK's 6SN7 non-inverting cathode follower preamp: Z out ?

50.126.50.3
Posted on June 6, 2017 at 11:11:06
deathtube 667






I was playing around with this non-inverting cathode follower preamp in LTSpice.

At lower Hz's, the Z out seems to have a bit of a slope going up to 3.6K ohms @ 20Hz.

Raising up the output cap value from 2.2u to 47uf seems to flatten out the Z curve at lower frequencies a lot, but doesn't seem to influence gain or THD%.

Would it be OK to flatten out the Z curve with a bigger uF output cap or should I just keep to the schematic values?

dt 667

 

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Line Stage Output Z, posted on June 6, 2017 at 12:42:12
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 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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