MOSFET cathode follower

[xtian]

See Aiken's page:

http://www.geofex.com/Article_Folders/m ... tfolly.htm

I was hoping to get some extra gain in my preamp by adding a cathode follower prior to the tone stack (as shown in attached image), but it didn't seem to help.

Would you expect the CF to add gain by allowing the tone stack to not be as lossy?

[tubeswell]

You need to do the AC-load sums for that schematic to work out whether adding a source follower will make any noticeable difference in bandwidth or not in your case.

Formula for gain (taking AC load into account) is:

[Ux(Ra||Rl)]/[(Ra||Rl)+ra]

where:

U = amplification factor of the tube
Ra = plate resistor
Rl = the rest of the load resistance (i.e. the load presented by the tone stack in combination with the grid load of the following stage in this case)
ra = plate resistance

The calculation of Rl will be complicated by the particular frequency roll-off/shelving presented by the tone stack. In your tone stack, you have a number of filters in parallel. The treble control is a variable high-pass filter. The middle and bass controls are parallel low-pass filters with differing roll-off points, that are all in parallel with the treble control. All of that is in parallel with the volume pot.

Remember also, that you are working off the input stage, which will produce a given amount of gain from the guitar pickup signal level. Then you are boosting the signal at the 2nd stage in any event. By the time the signal gets through the other gain stages in your amp, then seeing as how V1 is a conventional inverting triode, you might not notice too much difference between having the SF and not having the SF.

[xtian]

My informal observation agrees with your theory. Is there much different in the JTM45 circuit?

[pdf64]

Tsc allows the source impedance (of the stage preceding the tone stack) to be adjusted.
So just select the JTM45 stack values and flip the source impedance from ~2k to ~35k.
I've not checked but I doubt that it will make more than a few dB difference.

[WRC34]

xtian, were there any tonal differences/benefits by adding the cathode follower?

[xtian]

No significant changes.

[tictac]

followers, cathode or mosfet don't add gain, they lower impedance...

TT

[didit]

Gain is sum, more accurately mostly the product, of the whole. Mishmash of impedance can make lost what'd be desired. Anticipate this case with Aaron's evidence to back us up only a few dB measurable but barely audible, bit more at limits of tone controls.

Best .. Ian