I was trying to measure the output impedance of a DIY preamp for the past couple of days without any luck. Each time, I got to the same result of around 3K3 ohm, which is unbelievably low.
This is the circuit I’m working with:
I’m using the book “Designing Valve Preamps,” by Merlin Blencowe as a reference. What I have tried:
Connecting a resistor to LTSPICE circuit and finding its value at 50 % output gave the output impedance around 3250 Ω.
The author mentioned that theoretically if you were to feed 1 V signal into the output, the ratio between the forced voltage and the current drawn will give you the output impedance. This gave me again around 3K3 Ω.
I have replaced the potentiometers by fixed value resistances, and simplified the circuit as shown below:
Then, I have used Merlin’s Thevenin equivalent for the gain stage (page 31), I tried both for bypassed and unbypassed capacitors, which resulted in:
Now, coming to basic circuit analysis, I have tried 2 ways:
a) set all capacitors as short circuits
b) treat the capacitors as complex impedances.
Both of those ways resulted in a total impedance of around 3K3 Ω.
This is impossibly low since It means the signal transmitted almost ideally to the amp. However, when connecting it to a high Z input and attaching a cathode follower in between, the sound seems to have more “breathing space”.
Was I on the right track? What have I done wrong? If not, then how do I calculate the output impedance of a preamp?
Any help is greatly appreciated.