I have a slight update to the
"Universal" design I posted a while back. When I looked again at the max output impedance numbers they, seemed a little "off". As it turns out, I failed to account for the effect of the unbypassed cathode resistor in the numbers. The changes aren't really that big, but I thought I'd post them anyway just to keep myself honest. I also tweaked the value of the coupling cap. The 0.68µf value used originally might make the stage just a little too susceptible to blocking. So I lowered it to 0.47µf. This raises the low frequency cut off frequencies to just over 1Hz for all stages. Although you could probably go all the way to 0.1µf without any significant effect on the 20Hz roll off numbers.
Also, while I was at it, I calculated a reasonable value for a bypass cap that would work for all stages (it could even be switched in and out with a DPDT switch for both channels). It tends to push up the stage distortion just a bit. The highest value is for the 12AT7/12AZ7. It gets almost to 6% for a 1v peak input signal. However, the bypassed gain for the 12AT7/12AZ7 is about 33 so an input of less then 1/2 a volt would be more typical for this tube. This means that the "real world" distortion would be more like 3% (much more reasonable). This will definitely be the "warmest" of the 12Axxx series tubes in this circuit.
Here is the modified schematic:
Attachment:
%22Universal%22 Preamp Stage.jpg
And here are the calculated performance numbers:
Attachment:
Universal_performance.png
The circuit values are at the top, the performance for the unbypassed cathode stage for each tube are next, and the performance for the bypassed cathode stage for each tube are at the bottom.
As usual, questions and comments are more than welcome.
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