J-ROB wrote:
The old rule of thumb of drive impedance 5-10X lower than load is a good one, but you have to look at the specific case to understand what the tradeoffs are.
e.g. When driving reactive loads, it may help to have some series impedance to reduce/eliminate ringing.
I have seen this with silver wire amorphous core 10k input transformers. Those suckers wanted to take off and fly. 100 ohms in series and they calmed right down.
Transducers, transformers, etc. are a more complex situation than driving a resistor.
I don't know if this is a problem with the autoformers in question. Probably not. I think if the drive Z is 20% or less of the specified impedance then you should be cool.
Note that the autoformer will step up any impedance connected to the secondary, it is a TRANSFORMER, so the actual load might be higher than the impedance on the label.
A 6SN7 cathode follower should drive the snot out of most audio inputs, I should think. Just fire it up and rock on with your bad self!
All good advice, thanks. But I don't own any autoformers (if your post was directed at ME, which I assumed since you mentioned the 6SN7 follower. My line stage is an LDR input (around 7kΩ) to a grounded cathode 12B4 with LEDs in the cathode for bias and a 10M45S in the plate circuit. That's it. Current source'd (cascode DN2540s) and shunt reg'd with gas tubes.
David M's line stage is a pair of parallel connected 20kΩ stereo P&G pots.