Tjeu wrote:
One question: Why not employ a cathode-follower configuration at the very end; it would result in a much lower output impedance to the stages that will follow?
A just question. And one worthy of a good answer.
If you look at the thread
here on building a 4S with a CF output, you'll see that this is exactly what i did. The prime requirement in this other thread was for a buffered stage with low output impedance. The resultant circuit has Ro≈1.4kΩ. It's a good buffered preamp with low output impedance.
This project is a little bit different. The primary purpose here is tone control, which presents a few design challenges. The operation of a tone stack presents wildly varying impedances at the input and output based on frequency and tone control settings. So in general, it requires isolation from the upstream and downstream stages. It also presents a significant loss in signal (≈21dBv in this case) with tone controls at the nominal settings. As such it generally requires a gain stage to make up the lost signal. This situation leads to a three stage circuit: a gain stage, a tone control stage, and a buffer stage.
In addition, I made an arbitrary decision here about how much gain should be in the gain stage. I decided that for maximum flexibility, I would incorporate a 4S universal stage as the gain stage. This gives the user more options for overall gain without changing the circuit. With this decision made, the next question is to pick the topology. The choices here are (unless I wanted to a add a fourth stage, which I did not) "Buffer→Tone Stack→Gain" OR "Gain→Tone Stack→Buffer". Now a properly designed vacuum tube cathode follower has both a very high input impedance and a low output impedance. As such, it can go on either the input or the output without much problem. So the real question is what happens when I connect a 4S gain stage to the tone stack.
So, as you have indirectly pointed out above, the output impedance of the 4S stage is not so low as a CF and it varies from tube to tube. The 4S Universal output impedance is as low as 11.5kΩ when using a 12AU7 and as high as 38.7kΩ when using a 12AX7. So the first question is what happens when the gain stage is on the input to the tone stack. The answer is
it's not pretty. The following two plots show the response of the combined 4S preamp and tone stack when the 4S is on the input for both the bypassed and unbypassed cathode configurations.
Attachment:
Bypassed.png
Attachment:
Unbypassed.png
As you can see, there is significant variation in the upper frequencies when shifting between tubes, There is also significant variation with controls in the nominal settings. I was (and am) unwilling to settle for such performance in one of my designs. As such, I decided that the buffer should go on the input and the gain stage on the output. Compare these two plots with those I posted
above. You can see that the ones above are far better behaved.
Having the CF on the input also has some other advantages. Even with the volume at full, it is virtually impossible to drive the circuit in to overdrive. Combined attenuation prior to getting to the gain stage is about -22.4dBv. This means that even with the 12AX7 in the gain stage, it will require a full 15v peak to overdrive the circuit (29v peak with the 12AU7). Also the input impedance of the chain is driven entirely by the volume control (500kΩ).
One major disadvantage is of course, the output impedance is not quite a low as it could be. However, given the design constraints under which the design was done, I still think that this is a good solution. These output impedances are more than suitable for the vast majority of tube equipment out there.
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