input transformer , a slave to impedence at the cost of bandwidth/OL margin

[Tubetec]

There was a slavish notion back in the old days about 600 ohms , if you need to be able to energise a twin unscreened pair over hundreds or thousands of yards maybe its appropriate , but over your standard 10meters or 33 feet of tube mic cable, a cathode follower can happily deliver a single ended signal with low enough impedence that the cable capacitance is irrelevant to several hundred khz , the B&K cathode follower mic is a great example .

 

[gyraf]

..aah yes, but work that setup close to a radio transmitter or the like (mobile-phone or wifi anyone?) and you get all sorts of crap into your circuit..

/Jakob E.

 

[Tubetec]

The trick with the B&K mic is that the cathode resistor is at the power supply end of the cable so the feedback of the cathode follower encompasses the entire loop back to the power supply . Ive tested a few variants on the theme with regular large membrane capsules and I didnt have any issue with radio interference , sametime I didnt try subjecting it deliberately to for instance a mobile phone in close proximity which gets into a lot of audio circuits.
Many mics such as the Sony C37a use a cathode follower then a stepdown transformer housed in the power supply , Ive tried an unbalanced feed to the grid (1meg) of a preamplifier from the cathode follower mic output and it works great.

I'm not trying to say transformers dont have their place , but a cathode follower mic can be made to do things much more simply if we give up the idea of a 600 ohm balanced line output .

 

[5v333]

Didnt we give up the 600r thing many decades ago?

 

[5v333]

Sorry if my reply was a bit sour...