Impedance Selector Switch Values

[kimothebeatmaker]

Hi, Im swapping the transformer out of my preamp and the Secondary Load is different.


So im trying to determine the value resistors I need to achieve the following impedance on the mic input.
Transformer is a CMMI-3.5APC by Cinemag
18K Ohm Secondary Load

Impedance switch position 1 - 1800
Impedance switch position 2 - 1200
Impedance switch position 3 - 600
Impedance switch position 4 - 300
Impedance switch position 5 - 150

Does anyone know the formula?

 

[Bo Deadly]

That tranfo is 150:1k8 and usually people want the mic load to be ~1k2. So 1k2 / 150 = 8 * 1k8 = 14k4. So a 15k resistor is a fairly common value. But two 33k, which are more common, in parallel would be 16k5 which is ok. 18k seems high to me. From the graphs it looks like it's going to have a 2dB bump at 100kHz which is hardly cause for concern but you really don't need that kind of bandwidth for a mic pre either so you might add a zobel to knock that peak down to 0 and at a lower frequency. But its such a small transformer I wouldn't fuss too much over such details.

 

[kimothebeatmaker]

so should I do 15k instead of 18k?
So what would the impedance switch resistor values be?

 

[john12ax7]

Impedances go by the square of the turns ratio, in this case 3.5.

So if you want the mic to see

1800:
1800 * (3.5)^2 = 22050 secondary load

1200:
1200 * (3.5)^2 = 14700

And so on

 

[kimothebeatmaker]

I think you may have gave me exactly what I was looking for! I can now use this formula for future projects as well. Thank you for educating me!