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In Reply to: )the unit sucked all the nuance out of my system( posted by Allen Wright on January 2, 2005 at 02:26:51:
>> I call basic CF's "Miss Piggy" and say they have NO place in an audio signal path. >>
Nonsense. Common sense dictates that if a resistor-loaded cathode follower is good enough for the input of an oscilloscope with a bandwidth of over 1MHz, then it's good enough for any audio application I can think of.
Take it from me, they sound awful. :) Seriously, whether it's the result of the CFs, biasing, feedback, etc., the nuance I lost wasn't my imagination. A different circuit is called for.
I have often looked at the specs of some of the Tek plugins and thought they would make a very cheap high gain preamp - instant phono amp, just add equalisation.
That wasn't much of a response Allen. I'm disappointed.
Anyway, my comments were also backed by my listening experiences which have proven to me that a cathode follower with sufficient voltage, bias, and load resistance is completely transparent.
So, yes, my ears are already open - thanks for the suggestion.
I just thought you might like to comment on the apparent disagreement you have with the designers of ultra-sensitive ultra-wideband test equipment. If "plain jane" cathode followers were bad for audio, they would surely be awful as the input to an oscilloscope, no?
You make a snide response to the "nuance" word and you expect a "good" answer?
> my comments were also backed by my listening experiences which have proven to me that a cathode follower with sufficient voltage, bias, and load resistance is completely transparent <
OK, that's your experience - that's a valid experience,but that others have a different experience is NOT nonsense - OK?
> you might like to comment on the apparent disagreement you have with the designers of ultra-sensitive ultra-wideband test equipment. If "plain jane" cathode followers were bad for audio, they would surely be awful as the input to an oscilloscope, no? <
First - what EXACT model of "ultra-sensitive ultra-wideband" scope uses a Miss Piggy CF at the input? And do subsequent models also - or do they enhance the CF with a CCS and/or bootstrap?
Second - a scope needs to have good flat bandwidth and good (but adjustable) sensitivity, as to make accurate measurements with it one always tries to fill the screen - as with any analog scaled instrumement. The human ear has a huge dynamic range - at least 100dB, which is VASTLY more than the visible resolution of any scope CRT. My HP 182C allows me (with x2 glasses) to just see a 0.1 div p-p sine wave (but have NO chance of accuarely measuring it) which is only 38dB with respect to the full screen of 8 divisions - and that's on it's large screen - a small screen scope (like a TEK465) will have an even lower visible "dynamic range"!
So extended dynamic range and very low distortion is NOT a vital requirement for scope electronics - as an inteligent user will always use the input atten to have the internal signal as close to full scale as possible.
The tube amp designers bible, Valley & Wallmann's "Vacuum Tube Amplifiers" goes into CF's in quite some detail, and focusses on how to make them linear WHEN DRIVING CURRENT into a load - and this is where I first read about the advantages of CCS loading and bootstrapping them. Then knowing what to look for - I found CCS loads on CF's all over the good HP & TEK tube scopes I looked at.
Good enough now?
> > The tube amp designers bible, Valley & Wallmann's "Vacuum Tube Amplifiers" goes into CF's in quite some detail, and focusses on how to make them linear WHEN DRIVING CURRENT into a load - and this is where I first read about the advantages of CCS loading and bootstrapping them. > >
Valley and Wallmann is great if you're designing a RADAR. Not so useful if you're trying to make a practical audio amplifier. Yes, I own it.
No one is arguing that feedback and current sources do not make cathode followers more linear. That's undoubtedly true. I was merely responding to your brash taunt that all non-ccs'd cathode followers are "miss piggies" and don't belong anywhere in audio. That's a silly statement.
And declaring that because several tube scopes use CF's with current sources, and thus they are the only correct way to use a CF in an audio application shows a lack of understanding of the fundemental differences between the two tasks, and the limitations inherent in audio to begin with (we won't even go into speaker distortion).
I find it amusing to note that you find a "plain jane" cathode follower repulsive, tweaking them for more gain, bandwidth, and linearity, and yet you seem to be fine with using output transformers!
Bleccch! Talk about limited...
I must side with Allen. EVERY CF I have built sounds better with CCS HANDS DOWN!
And if you haven't tried bootstrapping them -you have another treat coming, perhaps even bigger than the CCS. But the bootstrap has to be a tube...
I've owned two tube scopes in my life. The crappier one was an old Tek 502 with a plain-jane 12AT7 cathode follower. 500kHz on a good day, drifty, noisy, and not terribly linear. The far better one was an old Tek 545 with a W differential plug-in that used... a cathode follower with a current source load and a voltage bootstrap. Solid. The first scope ended up being stripped for parts, the second sold and probably still being used today.
notice how Joelt never answered my question of which great scope used a plain jane CF input?
Actually, the 502 used it as an intermediate stage, and used a pentode at the input. The W, a legitimately worthwhile preamp, used its follower as the input. And a VERY interesting follower it is.
Allen, you're not going to be convinced by me mentioning any particular scope, and you know that. One guy's anecdote that "I had a scope with a plain cathode follower in it, and it wasn't very good" is hardly proof of anything!
The undeniable fact is that even a lousy oscilloscope has to have a bandwidth and noise figure much better than even the best audio amp. And many scopes from many manufacturers used cathode follower inputs. This, to me, is proof enough that your statement that they don't belong anywhere in audio was sophomoric.
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