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In Reply to: RE: output impedance posted by Lew on July 03, 2015 at 07:59:30
The 27 will have a high output impedance if used with a plate resistor. For a linestage I would either use a cathode follower or, much better, a CCS for the 27 plate load with a mu-follower output.
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The 27 isn't a great choice for a mu follower. If you look at the datasheets, there is no insulation rating for the cathode to filament interface, and in my experience this insulation is only good for 10-20 volts.
I was talking about a single 27 with a cascode CCS on the plate, but with the output coupling cap connected not to the plate directly but rather to the "mu output" of the CCS. In other words, connected to the source terminal of the "lower" transistor in the cascode.
Or would it be defeating the sonic advantages of the Type 27? I ask as someone who has not tried anything like what you suggest. I would sooner use a CCS and take the output off the plate, but that would still beg for a buffer stage.
Lew, I have never done a fair A/B comparison. I have used the mu connection when I needed a low output Z, and it sounded very good. I am confident it sounds better than adding a cathode follower. But in a situation where the output Z is fine even with a plate connection, I don't know whether the mu output would sound better or not as good.
I have had really good results using a CF with a CCS on the cathode side. Better yet with Allen Wright's "Super Linear Cathode Follower". In my amplifiers, I went through a progression where I listened to a plain CF as a driver stage for several years, then added the CCS for a few years, then upgraded to SLCF. The speakers and upstream gear did not change, essentially, so I feel very confident that the improvements were due solely to these modifications. Difference between no CCS and CCS was greater than between CCS and SLCF. I guess if I did a mu-follower, I would use a dual section triode rather than transistors.
Interesting. The only time I tried a CF as a driver was my first 300B SE amp where I started with a 6SL7 direct coupled to a 6BL7 CF. It sounded pretty decent even with resistor loading on the cathode. Unfortunately I never tried a CCS on the CF driver. And since it was the first input/driver I tried for the 300B, I felt obliged to go on and try a lot of others and never went back to the first version.
On preamps with a CF the CCS made a major improvement compared to resistor loading.
Amen to that.
Lew,
My co worker, and good audio friend, used to get rid of cathode follower stages, from Audio Research SP3 A-1s, etc, and thought, into a tube amp, with short cables, the deleted stage sounded markedly superior !!
There are commercial products he told me about, it MAY have a C-J preamp series, where the only difference in the later model was an addition of a cathode follower stage, and most everyone prefers the early "simple " edition of the preamp, he tells me. We are referencing to the used market, and posted comments, after several decades of both versions' existence.
I can get the model numbers from him, if you wanna know.
Jeff Medwin
I and Salectric are both agreeing with you, that a "plain Jane" CF, at least all the ones I've heard, does impart an unpleasant coloration that is most evident when you "fix" it, at least a little bit, with a CCS. You can fix it even more, IMO, by adding a constant voltage source on the anode side of the CF, while keeping the CCS. That's the "SLCF" described by Allen Wright. If the interface requires lowering the output Z, a CF with a CCS or an SLCF is pretty darn transparent. However, I would not argue that avoiding the need for a CF is not better than any CF or variant thereof, when possible. Often that is one advantage of an "all-in-one" preamplifier, with phono stage on the same chassis as the linestage. In that case, one can usually dispense with any CF or other output buffer on the phono output.
Well posted !!! Xln't. Thanks Lew.
Jeff Medwin
The thing about using the mu output.....
It puts the power supply back into the signal path.
Tre'
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