Split CF Impedance

[GainFreak]

Hi,

I'm not very much into theory so here's my question:
If we assume that a 12AX7 cathode follower has an output impedande of roughly 1k then what is the impedance of a split cathode resistor CF like the one in SLO100 for example?
I'm asking this because usually if you take the signal right after the volume pot it copuld reach 30V which is too much for FX out. Signal amplitude must be "cut" somewhere and this is one of the ways to do it but what happens with the output impedance then?

[Steve Conner]

The output impedance of that circuit is roughly equal to the two cathode resistors in parallel, but adding the tube's own cathode impedance to the top one.

Usually the bottom one is much smaller, so it's reasonable to say that the output impedance is about equal to the bottom one. In the example of the SLO100 FX send, you have 2.2k and 100k, and the output impedance works out at 2.15k.

[GainFreak]

Thanks for the quick reply.
If we assume say a 50k Send level pot after the coupling cap then the low cut frequency will be 1/2pi*Ccoupl*Rpot, right?
In this case the low cut frequency will be at ~32Hz which is way below of what we need? Why then in most such situations much bigger value caps like 680nF or 1uF are used?

[Merlinb]

Why then in most such situations much bigger value caps like 680nF or 1uF are used?

Because with a line-out it is desirable to keep the output impedance as low as possible all the way down to very low freqs', for maximum rejection of environmental noise.

[Russ]

As a side question, what would be the gain or gain reduction of such a split resistor cathode follower stage?

[GainFreak]

Because with a line-out it is desirable to keep the output impedance as low as possible all the way down to very low freqs', for maximum rejection of environmental noise.

Isn't 32Hz low enough considering the ~80Hz of the lowest guitar string?

As a side question, what would be the gain or gain reduction of such a split resistor cathode follower stage?

I guess it will work as a regular voltage divider?

[Merlinb]

Isn't 32Hz low enough considering the ~80Hz of the lowest guitar string?

Nope. If it is 3dB down at 32Hz then the reactance of that cap is going to be pretty high at 50/60Hz, where mains freq' radiation exists. Ideally you want to keep the reactance negligible down to 1Hz. This is a shortcoming of the SLO100 for example, which has an output impedance of 2.2kΩ at mid’ frequencies, but this more than triples at mains frequency and below, due to a peculiarly small output coupling capacitor