Sowter 1990 SU 1:10 config: load impedance issue

[maarten]

Hi, as per the below image I installed Sowter 1990 in serial mode (so 1:10 configuration) on the right input channel.

Now, strangely, when I measure the load impedance (ground to right input center resistance), I get 10 ohms. Instead of the 475 ohms I would have expected.

I don't understand, what have I done wrong? Note the 4752F loading resistor I added across terminal 1-2, also note I connected A9 resistor (a 90R9F) to terminal 2.

This load seems too low, I need to get > 100 ohm.

I double checked the left hand channel which still sits at its expected 47.5k ohm level.

Any help? Thanks!

[maarten]

Actually I would expect (1/(1/(47.5k (amp))+1/(47.5k (loading resistor)))/(100^2 (sowter)) = 237.5 Ohm

[maarten]

The problem seems to be that across terminals 1/2 I measure a resistance of 1300 ohm, even though there is a 47.5k ohm load resistor across there.

Which means the secondary load resistor is actually only 1300 ohm instead of 47500 ohm.

Of course if across those terminals, there is a 1300 ohm load resistor instead of a 47500 ohm load resistor, you would get a 1/(1/1300 + 1/47500)/(10^2) total load impedance (47500 comes from the amp, the 10^2 comes from the step up), which is *exactly* the value I'm getting.

So why is there only 1300 ohm across those two terminals?

[maarten]

Hmm.. is my fallacy, and idiocy, that I'm measuring DC impedance across terminal 1 and 2 (and also DC impedance between ground and input jack), which also gets affected by the DC secondary resistance.

But, when I listen to music it is an AC impedance that counts? That seems to be a silly mistake I make then.. can I measure this in any way?

[Paul Birkeland]

The DC resistance of a signal transformer is only the impedance your cartridge sees at 0Hz (DC), so it's not a particularly useful measurement.

[Paul Joppa]

Impedance is measured with an impedance meter or bridge. Here's a link:

https://www.hioki.com/global/learning/how-to/u-impedance-meters.html