Quote:
Originally Posted by nzoomed
Quote:
Originally Posted by Jez1234
EF86 or better still EF184 in pentode form giving around x 400 gain followed by a cathode follower then wrap the feedback around the Baxandall network.
There are many ways of making a pre amp and hundreds of schematics on the web.
There will be plenty of designs from back in the day which you could just remove the phono section from. Beware of some old designs being intended to drive >100K load though! Keep the cathode follower.
If you didn't want tone controls I'd have linked to a design of my own intended for easy DIY build.
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That sort of sounds similar to what Robert has suggested in post #14 except his is not a baxandall feedback loop, I may be tempted to go with something like that and source some EF184 tubes perhaps.
How easy is it to implement a bypass switch on a baxandall design? Ignoring the rest of my schematic on the first post, would that way of inserting a switch work? Will the volume level change significantly when switchong on or off the tone stack?
Quote:
Originally Posted by Robert Gribnau
I put this together (my suggestion in post #9 but with a cathode-follower after the tone controls).
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Thanks for that, this looks similar to what I was looking at to begin with, but then I got onto the baxandall unity gain design and thought that may have been a better option, but if i can make this work with a baxandall layout, ill probably go with this.
Quote:
Originally Posted by Jez1234
FWIW after seeing reports of super linearity from EF86 when triode strapped I tried it....
It's not true! I tried several samples, tried different methods of triode strapping and experimented with load line etc and distortion was disappointing.
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Thats interesting to hear, I have read from others experiences that if you feed negative feedback into a triode strapped EF86, that it improves linearity immensely.
Does that mean that an ef86 as a pentode has poor linearity? |
I had a look at your first link and right down the page is Fig 11.1 which is what I was suggesting. Check out how tone defeat is done in usual practice and it should work fine here also.
6AU6 is used as the pentode in that schematic but many types will do the job, ideally with changes to resistor values for optimum operation.
Negative feedback (NFB) improves linearity of any device at the expense of gain.
Pentodes are intrinsically less linear than triodes as triodes have "built in" NFB which results in their much lower gain and lower anode resistance.
Alternatively the high gain of pentodes can be used to give high loop gain (high NFB) for excellent overall linearity.