Line to mic level

[peter purpose]

Gentlemen,
A friend needs a line to mic level gizmo and I'd like to know if this would be a suitable solution.

Cheers
peter

 

[pstamler]

It'll work, but it won't be optimum. The 150R load resistor will overdamp the transformer, probably curtailing the high-frequency response.

Two better solutions:

1) Change the resistor across the primary to 634R, which provides a 600 ohm source impedance for the transformer. Assuming preamp input impedance is 1.5k (typical), add the following network across the secondary of the transformer:
a) 7.32k across the secondary (shunt)
b) Two matched 249R resistors in series with the two ends of the secondary. The other ends connect to the output XLR, and also to:
c) 174R across the output jack

That gives the transformer 600 ohm source and load impedances, which it will like, and shows the mike preamp a 150 ohm source impedance, which (if transformer coupled) it will like. Noise performance might suffer, though.

Or:

2) Get a stepdown transformer designed for DI use.

I'd recommend the latter. Jensen sells nice ones. Sescom sells more plebeian but still decent ones. Shure's (contained in adapters -- get the one with 1/4"F - XLR M connectors, and get an RCA-F - 1/4"M adapter) are also decent. Avoid the ones from Radio Shack.

Peace,
Paul

 

[NewYorkDave]

...Or, use a 10K:150 stepdown transformer, such as the inexpensive Edcor WSM10K/150. Terminate the secondary with a 150-ohm resistor. If 18dB attenuation isn't enough, you can always add regular balanced-U "mic pads" after the transformer, or omit the terminating resistor and use a 150-ohm H-pad of the appropriate value.

 

[peter purpose]

Marvellous stuff gents.... thank you very much indeed.
:thumb:

peter