Matching stepup to phono stage

Richard,
By empirical testing (aka lots of swapping) we ended up with Riken carbon resistors as the best sounding in our copper Mu/ZYX UNIverse/c-j PV11 setup. Second best, and cheaper for honing in on optimal values, were Kiwame carbon comps.

Every metal film resistor we tried produced audible skin effects, ie, inadequate/no impedance to very high frequencies. There was a step-like effect. Lowering resistor values attenuated upper mids but the real highs remained uneffected. Very artificially bright sounding even at values that were clearly too low for the rest of the spectrum.

Interestingly, the optimal values for one resistor were often not *quite* optimal for another resistor, even of a similar type (but different brand). The sensitivity of LOMC's to secondary-side resistor loading with a tranny is unbelievable. We always had to pair resistors to find appropriate values that hit the sweet spot. No single resistor value was ever "perfect".

I don't have lengthy experience with enough different tranny/phono stage combo's to elaborate on Jhendrixfan's points, but what he describes makes sense from the little I've heard. There certainly is no "one fits all" tranny, and resistor-swapping alone will not make it so (though it's better than NOT resistor swapping).

Best of all IME is a really high quality gain stage, but those are a lot more costly than a tranny, or even a boxful of trannies.

Doug

There's an old adage - "No (actual) resistor is as good as no resistor".
Resistor loading on the MC Step-up secondary is OK but I caution folks about using a parallel resistance of lower than 47K. 47K in parallel with 47K, from a strictly resistive standpoint, halves the current into the phono preamp input. The high input impedance of a tube phono stage is "swamped" by the 47K in the resistive sense (DC) but not in the impedance (AC) sense. So IF you need lower than 23K phono input resistance it is better to go in and change the preamp "swamping" resistor than to put lower than 47K values in parallel.
As far as Step-up transformer VOLTAGE gain (a passive device produces no power gain) is concerned it is the main reason why some combinations of preamps, step-ups, and cartridges simply do not "marry" or work well together. However when they DO, it sounds fantastic! OTOH, head amps usually offer much more flexibility but usually at the expense of sound quality. It truly is a trade-off and there is no "one size fits all" transformer.
Lastly, be aware that what you are searching for is the cartridge output voltage and output impedance characteristics (AC parameters)as a function of frequency. Cartridge output voltage is generally specified at only one point (1000 HZ) and the true output impedance is never provided. BUT if you had these specs, you would be able to attempt to calculate what type of SU xfmr is needed. Without them, it can only be derived empirically.

Doug/All,

Any thoughts on best resistor type (and wattage) for loading the Bent? I've played around with various 1/8 and 1/4 watt carbon comp, metal-film, ceramic-chip types, etc, yet I tend to keep coming back to plain old 1/4 watt carbon film types - somehow, they sound smoother in the long run. Regards,

-Richard

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

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,
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

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.

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.

Sorry, I didn't mean to talk down Bent Audio - John Chapman is a great guy who makes fantastic products. I just don't agree that it works to add resistors to step-ups to gain the desired load - from my experience with 8 or 9 different step-ups in various configurations with different carts and phonostages, it's always more desirable soundwise to get the load right by choosing the right step-up.

For a general discussion of loading, with formulae and online calculators, see Hagerman's page on the subject:

http://www.hagtech.com/loading.html

(Note: Hagerman also recommends varying the load with a resistor at the phonostage side - I've only done this once with a step-up, back when I had a Rogue 99, so I have less experience than with adding a resistor before the step-up. I'd still guess it's advantageous to simply choose the right step-up for your specific cartridge - keep it simple.)

Actually Bent Audio has a great page and chart explaining this.

http://bentaudio.com/parts/tx103loadhow.html

Actually Bent Audio has a great page and chart explaining this.

http://bentaudio.com/parts/tx103loadhow.html

Rhbblb1 - if you send me an email offline I will send you a spreadsheet that I used that will do the calculations based on different step-up ratios in combination with the resistor values of the step-up and phono stage.

As an example. 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.

"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."

OK, but how do they interact. How can the "final" impedance be calculated?

Patrick

Why are you not recommending Bent Audio.

Keep the CJ loading at 47K.

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. 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.

Adding resistive loading to inductive loads, pace Bent Audio, is not recommended.

Patrick