Bottlehead Forum
Bottlehead Kits => Eros Phono => Topic started by: maarten on November 25, 2024, 12:16:53 PM
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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!
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Actually I would expect (1/(1/(47.5k (amp))+1/(47.5k (loading resistor)))/(100^2 (sowter)) = 237.5 Ohm
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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?
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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?
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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.
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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