First LDC build with OPIC.43

[madronan]

Many thanks to @rogs for sharing the OPIC.43 design. I was looking a bias supply I could build on protoboard and decided to build the whole thing.

I paired it with a JLI TSC-1 and I think the result is fantastic. I'm using it as an overhead for drums.

I previously retrofitted 5 of these BM700 style bodies and Jules' OPA Alice boards to use on toms. These ZRAMO mics came with 25mm capsules (I swapped in JLI 2555 for a couple of them). I used 220pF caps to pad the output to better work with my audio interface.

I compared the output level of OPIC/TSC-1 with the electret mics and found the output at the lowest voltage to be right in between the electret mic with and without the 220pF cap. I'm using it as an overhead so I don't need quite the same level of padding.

Is this the best way to pad the output? Can you pad LDCs with a cap in the same way as an electret?

 

[rogs]

Adding a capacitive pad should work OK? (although I've never actually tried it!)

With the circuitry of the OPIC.43 voltage multiplier available, there is a simpler way of reducing the sensitivity of the mic....

The senstivity of the mic is directly related to the polarisation voltage across the capsule. Reducing that value by half will decrease the sensitivity by c.6dB.... Divide by 3 and it's c.10dB, and so on...

Assuming you have your mulitplier voltage output set at c.75v - measured at the junction of D6 cathode and R5 - then the actual voltage across the capsule is going to be about 65v ( the OPA1641 'half rail' output DC voltage is c.10v).
That's a typical value for a K.67 style capsule like the TSC-1.

If however you were to disconnect R5 from D6, and connect it instead to the junction of D2/D3, then the sensitivity of the capsule would be reduced by c.10dB.
(The multiplier DC voltage at that point is around 29v...... Subtract the OPA bias voltage of c.10v gives you c.19v across the capsule )

Connect R5 to the junction of D1 and D2 and the sensitivity would be reduced by c. 20dB.

So no extra components to be added to the circuitry. Simply using what's already on the stripboard to best advantage!