A question of loading...

@atmasphere Ralph, thank you for contributing to this topic. What you posted was extremely informative and interesting. I certainly think that a lot of fellow a’philes can learn a lot from you.

In my case, i am still a little surprised as to why the new preamp which features a phono overload spec of 70mv rms for MC’s is so very different to my old model, I am guessing that this is high enough to not risk ringing with my particular cartridge, a Lyra Kleos that puts out 0.5mv. What is odd, and I am not quite understanding this part, is why with my older CAT phono stage, which did not use a SUT at all, that a load of 750 ohms was better sounding than straight in at 47Kohms, yet with my new model, this is definitely not the case. Presumably due to the SUT in my new CAT?? One thing, the CAT has a healthy output voltage at about 50 volts..

I still maintain-righty or wrongly-that two things are at play with loading. One can argue that they are interrelated. One is preventing ringing/overload and another is changing the magnetically induced behavior of the cantilever. 
My own misstatements about higher loading values being the same as higher loading is a common one and a common source of confusion. I think that the confusion stems from loading being nothing more than a resistor placed between the signal wires-left and right-and ground. A high value resistor such as 47K means less loading because the circuit is left relatively open. The cartridge/cantilever sees no electro-magnetic mechanical "constraint" because the resistor is to ground. Conversely, a low resistor value comes close to a short circuit/complete connection between signal and ground causing the cantilever to become electromechanically constrained. 
Ralph and others talk about the cartridge/cantilever having "to do more work" at higher loading. Ralph explains;

Of course if you use the loading resistor option, you are asking the cartridge to do more work. If you're using 100 ohms as a loading resistor, that's a couple of orders of magnitude more work than if the cartridge is driving 47K. This results in the cantilever being stiffer- the same as what happens with a raw woofer if you short it out (they are both based on the same principle of operation). This is not a matter of debate, if you feel the desire to do so, take it up with Mr. Ohm. Ohm's Law cannot be defeated and isn't open to interpretation. If there is more current flowing, it has to come from somewhere and that somewhere is the motion of the stylus in the groove. So the fact that the cantilever gets stiffer is not controversial. Anyone versed in the art knows this- Jonathan Carr and I discussed this issue at the Munich Show a few years ago.

I will freely confess that I thought the loose and flabby sound I heard with the 25 and 50 ohm settings on my Steelhead was the result of a LESS STIFF cantilever and not a too-stiff cantilever as Ralph explains above. It does make sense-once one understands the higher resistor value being to ground-that the lower resistor value approaches a short circuit and that the cantilever becomes stiffer and less damped, not less stiff and more damped. Damping is the absorption/dissipation of energy. You all know that. Why a less damped cantilever results in loose, flabby sound and a highly damped cantilever results in lifeless sound is still a tough concept for me to get my head around. I think I know what Ralph means when he speaks of the cartridge and cantilever having "to do more work" with higher loading by using his raw woofer analogy and imagining having to create an electric signal by applying more mechanical force-not less-pushing on a close to a short-circuited and electromagnetically stiffened woofer cone to generate a given amount of electrical output but still-this does little to explain why a less damped (stiffer) cantilever sounds livelier and why a highly damped (loose) cantilever sounds lifeless. 

Ralph-thank you for your very polite and informative response. I apologize for my strong words directed at you.

No worries- and no offense taken. But I appreciate your words.

What is odd, and I am not quite understanding this part, is why with my older CAT phono stage, which did not use a SUT at all, that a load of 750 ohms was better sounding than straight in at 47Kohms, yet with my new model, this is definitely not the case. Presumably due to the SUT in my new CAT??

A lot to unpack here....

Its a lot easier to build a tube input circuit that has much higher overload characteristics! In a tube circuit meant for LOMC, if you overload it with too much input voltage, the overload isn't occurring in the input stage- it happens further downstream. With solid state, the overload often occurs at or very near the input, often because the circuit uses feedback, and the input section might be outside the feedback loop- even if its only the base of a transistor. But semiconductors are far more likely to rectify RF energy too, since they are diodes at some point or another. Its this latter characteristic that makes them more pesky in this regard.

SUTs are another matter altogether! I suspect Ken got tired of people calling about noisy tubes and SUTs are a way to get around that. For tubes to be really quiet in the front end of a phono section, they have to be at the top of their game. As they lose transconductance with age, the noise goes up. You have to keep the tubes active even when the preamp is being used playing CDs, so the tubes are going downhill all the time. By installing an SUT, you can easily quadruple the usable life of the input tubes.


But that comes at a price! SUTs have to be properly loaded to prevent ringing (distortion) and the proper load varies from cartridge to cartridge, since transformers **transform** impedance. So if you have a 10 ohm cartridge, the output of the SUT will be an impedance much higher than if you have a 5 ohm cartridge. So the load it needs will be different too. If the load is insufficient (too high impedance) the transformer will ring, which is to say some very high amplitude harmonics will appear at its output. This makes them very tricky to use! I find that even with them set up right, you lose a bit of detail (bandwidth at this signal level shouldn't be an issue)- that's why I've really stayed away from transformers in the audio path.

It does make sense-once one understands the higher resistor value being to ground-that the lower resistor value approaches a short circuit and that the cantilever becomes stiffer and less damped, not less stiff and more damped. Damping is the absorption/dissipation of energy.

Just to be clear- if you load the cartridge more, the cantilever will be more damped in addition to being stiffer. This can affect tracking if you get the resulting mechanical resonance outside of the 7-12Hz window.

I still maintain-righty or wrongly-that two things are at play with loading. One can argue that they are interrelated. One is preventing ringing/overload and another is changing the magnetically induced behavior of the cantilever.

We need to be really specific about what is happening here. Many years ago I had this idea about building a little box that would sort out what the ideal loading value was for a LOMC cartridge. This might have been about 30-35 years ago... At any rate, what I found was that **the cartridge itself does not ring at audio frequencies**. You can pass a 10KHz square wave through it and it will look exactly like a 10KHz square wave at either end of the cartridge. Quite simply the inductance is so low that its inconsequential at audio frequencies. It can't ring (and on this point, MM cartridge most definitely **can**, so loading with them as affecting things at or very near audio frequencies). BUT- it can have effects at much higher frequencies as I described earlier. (The result of my research in this regard was that I would not be able to make such a box, since ringing wasn't the issue.)

BTW, if there is some question about what the load should be, @lewm 's rule of thumb of being 10X higher than that of the cartridge itself is a pretty good one. Such a value will detune the radio frequency issues and won't affect the output level of the cartridge.


There are more than just two things going on with loading- and they are very much interrelated as you say. Two are caused by the cantilever getting stiffer- it can affect how the cartridge tracks and its arguable that being less supple, is less able to trace higher frequencies. So that's two things. But the loading affects the preamp too; by eliminating the RFI at the phono input, it can make the preamp less bright (a common result of RFI in audio circuits) and possibly less ticks and pops if the phono section has poor high frequency overload margins.

@atmasphere  Once again, very interesting and informative..thank you.

Ken uses a SUT that doesn't really have a lot of boost because I think he believes that as the step up increases, the loss is SQ is too great. What i am still guessing at is that the SUT allows for a better synergy with my Lyra as to what it wants to see as a load than the prior 750ohm resistor that I was using. The Lyra has a fairly healthy output for a MC at 0.5 mv and I am sure this is not putting much stress on the SUT. It was not putting too much stress on the phono stage before on my old non-SUT unit. Now the advantage of using an SUT has aways been to me that I could entertain ultra low output cartridges without too much tube noise, by ultra low, I am saying below 0.3mv...which there are quite a few very good ones out there.I wonder if that would be an option with this new CAT model, somehow, I doubt it???

Ken uses a SUT that doesn't really have a lot of boost because I think he believes that as the step up increases, the loss is SQ is too great.

That's been our experience too. The more stepup you use the less bandwidth you get.


We regularly run cartridges that are 0.2mV. If you have an SUT you should be able to get in that same range quite easily. The only thing you would have to work on is the loading of the SUT, with which Ken might be able to help you. We use Jensen Transformers when we use an SUT; they have published a pdf file of all the loading values for a great deal of different cartridges including Lyras.


Keep in mind that 0.25mV is 6dB less than 0.5mV; that means you'll have to run the volume control up a few notches to get the same sound pressure. I don't think you'll have any problems doing that :)