Set up for ZU103 cart and K&K SUT

I owned one of these and was a bit confused, too -- it's a quagmire for the uninitiated. But the ONE thing to read is right on Kevin's web site, http://www.kandkaudio.com/mccartsetup.html. I had to read it several times. I'm slow. The goal is to configure the step up transformer for (a) the desired amount of gain and (b) optimum cartridge loading. I looked up the specs for your cartridge an the recommended resistive load is 80-150 ohms.

Following the example in Kevin's article, with your step up the transformer set up for a gain factor of 8x, your cartridge would "see" a load impedance of approximately 734 ohms, which is arrived at by dividing the phono stage's input resistor setting (47,000) by the step up transformer's impedance ratio (64). To achieve optimum loading, you would need to add a parallel resistor in one of the values shown in the chart in Kevin's article. For a resistive load of 150 ohms, use a parallel resistor with a value of 15300 ohms. You can buy them at Radio Shack; they're inexpensive.

But: when you add a parallel resistor, it will reduce the effective gain from the transformer, significantly. My suggestion would be to try the step up as configured now, with an 8x gain factor, and use a 15300 ohm parallel resistor to bring the cartridge loading into the manufacturer's recommended range. Listen, and if you have enough gain, great. If not, then you'll need to reconfigure the transformer for a 16x gain factor. Hope this helps.

First Q: why use a stepup tranny at all? The GPCH has 60db of gain at its highest (unbalanced) setting, which should be adequate for a 103. You would experiment with its 100, 500 and 1K impedance settings to see which gives you the flattest response.

If you must use the K&K's for some reason, leave them at 18db of gain and set the GPCH for its lowest gain setting (42db on the unbalanced inputs). That will give you 61db of phono gain, which happens to be "perfect" for a 0.3mv cartridge, according to the KAB Preamp Gain Computer. :-)

Leave the GPCH's input impedance at 47K, that's what the K&K was designed for. The other impedance settings would be far too low when using a SUT.

This will leave your cartridge seeing an impedance of 47K/64 = 734 ohms. That may sound okay, given the 103's fairly high internal impedance of 40 (or 45?) ohms, but it is somewhat system dependent.

If this setup's impedance is off it's likely to be too high, which would over-emphasize the HF's. The good news is, too-high impedance can be adjusted. If it were too low you'd be out of luck and would have to change component(s).

Once the cartridge and trannies have settled in (give them 100+ hours), if the HF's seem excessive the fix is to insert resistors between the K&K and the GPCH. The optimum value would be very system dependent. Fine tuning impedance in a LOMC/SUT combination is very finicky, and in-system experimentation is the only way to do it. The resistors which yield the flattest response in your system would not do so in my system, or anyone else's. This is by far the most difficult and time-consuming aspect of using SUT's, and those who ignore it can never be sure they're getting the best performance.

Great post by Jbaxley. One minor correction/suggestion: if inserting resistors reduces gain too much, it would be easier and more effective to switch the GCPH to a higher gain setting than to open up the SUT's and change taps.

He can choose 42, 48, 54 or 60 db of gain from the GCPH without affecting the impedance structure. Changing the K&K's gain setting might require starting over with different resistors. No point going in circles!

I agree with both of the posts above. Doug's first point hits the nail on the head: why do you need a step up at all? I hadn't looked up the specs for your phono stage, but now I have. Your GPCH has several different gain settings and different cartridge load settings. The 100 ohm load setting is right in the middle of Zu's recommended range; just use that. And you can easily select the best gain level for your system. You have up to 66 db gain, which is enough for even a passive set up. Your phono stage eliminates any need for a step up transformer.

Hi matem
Thank you for your great feedback.

The GCPH is very flexible phono in the market and sufficient for Zu103. I bought the SUT because I have read a lot of articles (Stereophile) mentioned the superior sounds achived by using SUT togeher with Denon 103 based cartidges.

I did set up the GCPH (no SUT) with loading 47K and lowest gain 42db. I intend to compare between the set up with and without SUT to findout what will be the best for the ZU103. Most of my gears, cart, TT and phono are still in break in period, so it may need another 50 hours at least.

It seemed that the most confused part left for me is the way to opimize the loading by adding or replacing resistor. Does it mean the resistor in the GCPH will need to be replaced ? How did you reach the resistor with a value of 15300 ohms. I still could not find the chart you mentioned in Kevin article, but will explore more.

Also I was told that generally SUT is better matching with Tube Phono, and Head amp is far better with Solid state. Any ideas ?

Thank you very much for helping to improve my understanding.

regards

Victor



"To achieve optimum loading, you would need to add a parallel resistor in one of the values shown in the chart in Kevin's article. For a resistive load of 150 ohms, use a parallel resistor with a value of 15300 ohms. You can buy them at Radio Shack; they're inexpensive."