LA3A Input Impedance 150 Ohms!!

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anhphuong

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Joined
Jan 17, 2019
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What is the input impedance of an LA3A?

According to Cinemag, the B11178 is equivalent to their CM-2511 which is a 1:5/10 mic input transformer with 600R/150R:150K impedance. But the circuit loads the secondary with 39K which means the impedance on the primary is going to be more like 39K/150K*600 = 156 ohms. Actually with attenuation at it's maximum it could dip to as low as 108 ohms.

This can't be right. What am I missing?
 
> the circuit loads the secondary with 39K

Plan: http://www225.pair.com/audio/waltzingbear/Schematics/Urei/Urei_LA-3A.JPG

39K, in parallel with 68K+100K, and another 220K full-up.

Pencil 30K input.

The Cinemag print shows primary parallel, probably for MIC use nominal 150 Ohms.

The LA3A is clearly wired series, nominal 600 Ohms, 1:5 turns ratio.

1:5 turns is 1:25 impedance.

30K transformed down by 25 is 1,200 Ohms. 1,100 Ohms when Gain pot is full-up.

So it is mis-matched high. That's always fine. Especially going into a level-modifier which is going to fix your levels for you.

If you truly must see *exact* 600r (which you can never get *exact* through a transformer), put 1.2K in shunt with the input.

The alternative, not implied by LA3A plan, would be 1:10 connection and 300-275r input. Which would be fine for the network which used nominal 150r interface.
 

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