Two stage Cathode follower

[bob orbell]

As some of you know, I, for the last 5 months been using my pre configured to a CF with active load, all is well here, I have now come up with idea of using a CF, followed by the volume pot, and then another CF, I have never seen any papers on the idea, what do you guy's think, it should work, but dose it make sense. BOB

[John Caswell]

Hi Bob,
Well the first question is why?
However it does make sense in a way as you will be feeding your signals into a fixed high impedance, adjusting gain, then exiting at low impedance to the following stages. Depending on the type of CF you use you may need isolating C's between the V/C and the two stages, but that is not problematic
Obviously you will be losing some gain say 0.9 x 0.9 = 0 .81, but if you have plenty of signal to start with what the heck.
Go for it.

John

[andrew ivimey]

1. what's the point of a CF?
2. Surely, whatever you have as a front end the volume control changes the parameters so much as to lose the benefits of a CF?
3. I've never seen it done before (CFs are deffo final stage in my book) so why not try it.

[bob orbell]

Hi John / Andrew, exactly what you said John, signal in to a very high impedance, and low output impedance, I do not want any more signal gain, in fact I need to lose a bit, to that end, I would like to try this with a shunt pot, so the input signal sees a constant high impedance, well at least you don't think Im mad, I think I will give it a go, thank's. BOB

[pre65]

What's wrong with being mad ?

Bob, it will either work, or not. Have fun trying it out.

[bikerhifinut]

My Pennorth Bob.

You'll be adding a small amount of extra noise into the circuit, probably not significant.
I don't know what your choice of valves are, I was thinking why not try something like an ECC83 as first valve which will reduce the current draw significantly over something like an ECC82, you aren't needing to drive a length of connecting wire.
I have seen a buffer stage (that's my perception of this first CF stage) used prior to a baxandall tone control to present a constant source impedance to the TC network. So it's not unique. And a subsequent buffer after the tone control which kind of has a parallel with what you are proposing.r

However:
I really can't see what advantage you'd get as at some point in any circuit using a variable potential divider you will have a variable resistance presented to the incoming signal. Perhaps because you intend to use a shunt pot there will be a potential gain by this. Some if not most of you know that I am not a fan of the shunt pot mod. I have tried it and gone back to using a decent quality potentiometer in the conventional way. Horses for courses.
I suppose if you wanted or needed to use a very low value of volume pot, less than 10k, then a buffer stage will also be beneficial.
Or if your source is high resistance, for example straight off the anode of a valve stage and you haven't any alternative.
Most modern sources in my limited experience have pretty low output resistance of less than 1k and would have no problems being fed into a pot as low as 10k, given that as short an interconnect as practically feasible is used.

I do like using a CF buffer stage though and currently the broskie ACF circuit is very nicely driving a 20 foot! length of standard screened cable to my power amps situated close to the speakers. I'm looking forward to eventually constructing a version of the WD transformer preamp when we get the house licked into shape as I hope that will be even better at driving a long interconnect to the power amp. I'm mulling over a circuit mod to reduce the gain to more or less unity overall without adding extra attenuation in the signal chain.

It will be interesting to find out what you discover Bob, you may be on to something.

A

[bob orbell]

Hi Andy and all, we are very busy decorating the three bedrooms, two almost done, so the projects are next in line, hope to start about the end of November and will post my findings. BOB

[bikerhifinut]

Originally Posted by bob orbell

Hi Andy and all, we are very busy decorating the three bedrooms, two almost done, so the projects are next in line, hope to start about the end of November and will post my findings. BOB

Good luck Bob,
So far we've done a bit of tarting up in the lounge and front bedroom/"mancave" of this oldish dormer bungalow, plus with the help of a local plumber for the tricky bits I have refurbed the upstairs bathroom with a fancy bath with a door in the side for mrs nuts use.
at the end of the month it gets really busy when the workers come to rebuild the conservatory from the bricks up with a fancy warmroof and special glass to turn it into a usable room (dining and sitting area), then its a new kitchen in what was the old dining room plus disabled friendly wetroom next to it. The old kitchen is to be made into a guest bedroom but with the view over to the pennines across the valley I think we might use it ourselves!
I reckon Xmas before I get solder slinging again. And next year we dig up the patio............................. wonder what we'll find under the paving flags haha

I never want to move again............................