Some time ago I tried out a modification to my both my line and phono stage that Paul Hynes suggested- with superb results. I thought it was about time I shared it with you!
See the first diagram, which shows the circuit of a standard cathode connected triode stage- as used, for example, in Yeo’s 5687 preamp. Valve purists- and yes reader, I was one- tend to stick with this arrangement, though it’s far from perfect. Distortion increases with input level, and output impedance is high, unless you use a low rp valve (such as the 5687; but it’s still higher than we’d like); and susceptibility to power supply noise is high.
One obvious improvement is to replace the plate resistor with a constant current source (CCS). Some use a plate choke (the ultra purists) or a triode based active CCS (the purists), but neither approach is ideal. (For example the choke’s impedance falls at low frequencies, and must be carefully made to operate well in this testing environment- i.e. expensive; and the active; and it raises the output impedance a little).
Paul suggested a simple circuit using a couple of FETs- one a rather exotic high voltage IXYS device- to replace the plate resistor- see the second circuit. This performs two functions- it provides the triode with a constant current, and it lowers the output impedance of the stage enormously. In effect it “caresses” the triode into giving its best performance- hence the name, the Caresser!
As it’s Paul’s design, and as he’s now offering it as a small pcb- small enough in fact, to retrofitted to most pre-existing circuits- I don’t feel I can share any details of the circuit, except- changing one resistor on the board sets the current- Paul recommended 3mA minimum, and I’ve used up to 10mA. And too work properly, the circuit needs 50V or more across it. (Paul also offers a complete phono stage using this topology. I haven’t tried this myself, as I constructed my own using different triodes etc.) See-http://www.audiocircle.com/index.php?topic=86440.0
The caresser boards are the smallest upright pcbs, either side of the valve- the ones with the tree resistors above one another. (The other pair of boards are voltage regs for the HT supply; I’m still using choke filtered supplies in my amps).
The circuit has an additional advantage- see the third circuit. Replacing the cathode resistor and cap with a suitable forward biased diode (one or more 1N4148, or a suitable LED) provides fixed voltage bias too! The diodes are selected so their forward voltage drop equals the required bias, at the constant current provided by the Caresser (exactly the same arrangement which Monica originally use). And their “dynamic impedance” is very low, and so there’s no need for a cathode bypass cap! (The latter are, IMHO, at best a necessary evil, and best avoided if at all possible).
The sonic effects are staggeringly beneficial, without any downsides I can hear at all!
The choice of triode I leave to you. This circuit is a great leveller- it allows the valve to operate at its best. One thing to watch though. For optimum performance, use a triode needing no more than 2- 3 V bias. Any more than this, and the number of diodes required for bias will increase, and so will their dynamic impedance. (Though this may not matter in practice- I haven’t tried ). For triodes, mu (its voltage gain) rises as required bias falls, and thus suitable triodes will have a mu of 20-30. If the following power amp has high sensitivity, this will result in audible hiss. The triode I use has a mu of ~ 30 (the caresser CCS loading results in the stage having this gain too), and hiss from it is too high when used with e.g. Charlize. It’s fine with my valve power amp though (2 V sensitivity). (Note this isn’t a problem of the caresser circuitry- the same would apply when using any high gain pre amp with any high gain power amp. It’s not a problem when the caresser is used in a phono amp- high gain is required).