Bryanp,

I'm afraid there's a lot of confused information being posted here.

The resistor sockets on the secondary side of your BentAudio Mu's are not there simply to show your phono inputs a particular impedance. Nor is BentAudio assuming you're using an MC phono stage. If you were, you probably wouldn't need a transformer in the first place. Bent is assuming you're using a MM input with 47K ohms input impedance. The page you linked is correct.

1) Resistance on the secondary side of a transformer IS reflected back to the primary side. Your transformer's primary is connected with the cartridge coils in a closed AC circuit. The resistance reflected from the secondary is seen as impedance by the cartridge. Adjusting that is the purpose of the Mu's resistor sockets.

2) Impedance on any AC circuit does act as a frequency tone control, as Raul said, but in this application it also acts as impedance on the electro-mechanical motors in the cartridge. If circuit impedance is too high the cantilever will be under-damped and excessive HFs will result. (You may also get higher output, which is probably what you were hearing before you inserted the resistors.) If impedance is too low the cantilever will be over-damped. HF response and overall dynamics will suffer.

2) The formula "25 times coil resistance" is for MC's feeding a voltage gain stage. It has nothing to do with transformers. When playing through stepup trannies most MC's like to see a reflected impedance somewhere between 1x and 5x the coil resistance. IME low output ZYX's usually prefer something between 1.5x and 2x their 4 ohm coil resistance, ie, 6-8 ohms. (You're actually in this range now, stay with me.)

3) When using a tranny the impedance "seen" by the primary = the resistance on the secondary side divided by the SQUARE of the tranny's turns ratio. Your 26db trannies have a 1:20 turns ratio, so those 3.3K resistors are "seen" by the cartridge as an impedance of 3300/20^2 = 8.25 ohms.

4) Any resistor you place in the Mu's socket is in parallel with the resistance of your phono inputs. With a 47K ohm MM phono stage, the actual resistance on your secondary side can be calculated with the formula for resistors wired in parallel:

1/R = 1/ra + 1/rb...

R = total resistance on secondary side

ra = resistance value "a" (eg, phono inputs)

rb = resistance value "b" (eg, resistor in Mu socket)

For your present setup, the formula is:

1/R = 1/47K + 1/3.3K

R = 3083 ohms

Dividing this figure by 400 (the square of your turns ratio) gives us 7.71 ohms. That is the actual impedance your ZYX is presently seeing (not counting the trivial additional impedance of your phono cable and the Mu's wiring).

5) IME, MC's playing through a stepup are EXTREMELY sensitive to VERY small changes in reflected impedance. A change of 0.1 ohms will be more audible than a change of 25 ohms when using an MC gain stage. In one demo I took a ZYX UNIverse from dull to perfect to bright by going from 6.54 to 6.65 to 6.71 ohms of reflected impedance.

This sensitivity is both a blessing and a curse of course.

How do you make these tiny impedance adjustments? Simple. You put two resistors in each socket of the Mu. You can calculate the total impedance on the secondary side by extending the formula above to:

1/R = 1/ra + 1/rb + 1/rc

Divide R by 400 to get the impedance seen by the cartridge.

For my low output ZYX I'm current using a 680 ohm and a 27Kohm resistor in each socket of my (20db, 1:10) Mu's. That works out like this:

1/R = 1/47K + 1/680 + 1/27K

R = 654 ohms

654/10^2 = 6.54 ohms

In my system with this cartrige, 6.50 ohms is too low, 6.62 ohms is too high. That's how touchy this is. I'd suggest buying some cheap resistors and experimenting. IME the same cartridge will need slightly different values in different systems.

Unfortunately, asking the manufacturer will get you nowhere. Few MC makers offer more than general impedance guidelines, and even fewer offer any guidelines at all when using a tranny. How could they? The turns ratios of trannies vary all over the lot, and as you can see from the formula above that has a major impact on the impedance actually seen by the cartridge.

Hope this helps more than confuses,

Doug