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Cathode Follower

Posted 14th May 2010 at 11:40 PM by Miles Prower

AC Coupled CF

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In this schemo, Rk is the normal cathode bias resistor. Rl represents the tail load in parallel with the load impedance. Rg is the control grid DC return. The CF gives excellent high frequency performance since Miller Effect is absent, and the Cgk sees very little current since the grid and cathode are always at nearly the same potential. This makes the Cgk effectively smaller than its static value. The main component of input capacitance will be the reverse transfer capacitance: Cgp. With small signal triodes, it is easy to present a Hi-Z, Lo-C load to the driving stage. This isn't just helpful at RF.

This is another circuit which has lately come under unjustified criticism within certain audiophile circles. Much of this is unjustified on the basis that the CF is a negative feedback circuit. This view that all NFB is all bad does have a basis in fact. It has been all too common to use NFB to cover up for poor open loop designs. If your open loop design is poor, just pour on the NFB to force "the numbers" to look good. Sure, you can sweep your mistakes under the carpet that way, but you will also sweep away much of the vitality of the music. However, this is a misuse and abuse of NFB. The blame properly belongs to these lazy designers who can't be bothered to correct their open loop designs.

Another big part of the problem lies with the nature of the cathode follower itself. Yes, it can present a low impedance source, but only to a high impedance load. There is a big difference between source impedance and load impedance. For example, a 6C4 small signal triode could be used to implement a cathode follower. If you implemented it thusly:

Vpp= 330Vdc
Vpkq= 140Vdc
Ipkq= 4.0mA
Rtail= 47K

r(p)= 10.5K
g(m)= 1.4mA/V

You could easily calculate a Zo= ~647R Given that Zo, you might think you could drive a set of 600R headphones with this cathode follower. However, you would be quite wrong about that. Your undistorted power output will be just under 10mW as you'll only be able to swing just 3.4Vp into that load. So what happened? When you connected a 600R load across the 47K tail, you killed most of your gain by making a nearly vertical loadline. Less open loop gain means less effective NFB. If it can not drive a 600R load connected to the plate it can not drive that load any better if you connect it to the cathode.

So what is it good for? This cathode follower would be ideal for isolating a grounded cathode gain stage from, let's say, a tone stack. The GC amp will have a very large Zo. You could incorporate that source impedance into the resistances of the tone stack, to be sure. However, tube characteristics vary considerably with the manufacturer, models within brands, with age. The output impedance probably won't stay put, and if it varies, the poles 'n' zeros of your tone stack will change with it. Isolating the tone stack with a cathode follower representing an insignificant portion of the tone stack resistances will prevent this. It will also allow for more resaonable values of resistance and capacitance in the implementation.

Cathode followers can also be used as active pull-up circuits to drive the control grids of audio finals. Even if you stay with Class A*1 operation, the input capacitances (Cgk + Cmiller + Cstray) are still going to require current to charge. If the current sourcing capability isn't there, then you will run into slew limiting at the higher audio frequencies. That sounds nasty. The CF can supply enough current to prevent this from happening, especially if you follow the "Rule of Five" from solid state practice: make the Q-Point current of your CF at least five times greater than your anticipated peak current.

Don't ever forget: the vacuum tube itself neither knows nor cares whether the load is connected between the plate and the positive rail, or if it's connected between the cathode and DC ground. It's always the same loadline, the same load resistance. You don't gain anything by trying to force the device into being something it will never be: a high current, low voltage device. A CF is not a magical power gain stage.

Attempting to use it otherwise will lead to degraded sonic performance. If you have a bad-sounding CF, blame the designer, not the topology.

As with any other audio subsystem, if the CF is designed properly, and used within its limitations, it is the most sonically transparent audio subsystem. If designed badly, it will sound bad. It's as simple as that.
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