Matching stepup to phono stage

You have gotten both incorrect advice and some that is just confusing.

It is very common to change the load by adjusting the input impedance of the phono stage, less common to load the input of the transformer. That's why most phono stages have provisions to change the impedance.

Your transformer doesn't have an input impedance of 250 ohms. That is probably what it gives you when you load it with the standard 47K. Do you know the gain of the stepup?

The impedance is determined by the the input impedance of the phono stage divided by the square of the turns ratio. Just like it transforms the voltage by making it larger, it also transforms the input impedance of the phono stage making it appear to be smaller. So it's not a combination of the 250 and the 47K.

It is very common to change the input impedance of the phono stage to get the load you want on the cartridge. To figure it out you need the turns ratio, or the gain since the ratio can be calculated from it.

You need to choose the right step-up transformer to present the recommended loading of the cartridge AND simultaneously give you the correct amount of gain.

This isn't going to work in real life. Normally you choose the gain you need and then adjust the input impedance of the phono stage to get the loading you want.

The impedance presented to your cartridge will be the combination of the 250 ohms of the step-up and the 47Kohms of your phono stage.

If it did have a 250 ohm resistor at the input of the transformer, the load would be the 250 in parallel with transformed phono stage input impedance, but I doubt this is the case.

I used a K&K Audio step-up with a Hagerman Trumpet. The step-up had a 10:1 ratio. Using 47k ohm resistors in the step-up in parallel with the Trumpets 47k ohms yielded 940 ohms

That math doesn't work. Two 47Ks in parallel is 23.5K which divided by (10 times 10) gives you 235 ohms. To get 940 you would have to put the 47Ks in series.

Herman,
I kind of figured out what you are saying. The Stepup has 23db of gain. I believe that would calculate into a turns ratio of 13.7. Thus, 13.7 X 13.7 = 188. 47,000 divided by (13.7 X 13.7) = 250. Is this correct?

Thanks,
Rich

That's right, but 23 dB is actually a turns ratio of 14.1. That still gives you a load of 235 ohms, close enough to call it 250.

Gain in dB = 20 times log of the Turns Ratio
23dB = 20 log TR
1.15 = log TR
inv log 1.15 = 14.1

Herman, yes that is correct, in series. Sorry for the confusion. The spreadsheet I have does the calculations correctly.

Thom Mackris gave me the spreadhseet and I believe the Galibier Design website has a link where you can download it. It is quite useful.

Herman's math is correct, as is the chart on Bent Audio's page and the spreadsheet on Galibier's (Thom Mackris') site. My experience is similar to his also.

Patrick's advice is good in theory but difficult to apply in real life. As Herman says, it's normal to choose a stepup with the right amount of gain, then match impedance as needed.

I've had 5 or 6 stepups in my system. Results varied depending on the cartridge, but with my ZYX UNIverse the best results were obtained with Bent Audio Mu and the appropriate resistors on the secondary side. Other trannies were less satisfactory, whether with resistors or without.

There is a sort of cartridge/stepup synergy that transcends mere impedance, since I've also heard stepups other than the Mu sound better with other ZYX cartridges, even though their gain/impedance requirements were nominally the same as the UNIverse's. This was somewhat mysterious, thoroughly unpredictable from specs, and seems consistent with what Patrick reported.

WARNING: LOMC's playing through stepups are EXTREMELY sensitive to input impedance and resistor type. I had to pair resistors on the Mu's to fine tune the value to obtain optimum response. I then tested various brands/types of resistors to find the optimum one, and discovered that the values required changed ever so slightly compared with the cheap resistors I'd started with. In this application the tiniest changes are audible, and experimentation is the only way to find the right values for a particular cartridge in a particular system. If you're picky, it's unlikely that a single resistor will hit the optimum value on the head.

Best of all is no stepup transformer, but that's a different kettle of fish.

Doug