Added an SUT...not sure I understood this

JCarr makes the exact point I am ultimately trying to get to when he says the below.

 Also, IV phono stages sound qualitatively different to voltage amplification phono stages., and part the reason is that undoubtedly the cartridge is forced to operate into a zero-ohm load (or some other values that is quite close to the cartridge's internal resistance).

I don't necessarily agree with his use of the term "forced" but that is for another day.  He attributes  "part of the reason" for the difference  in sound to the load the cartridge sees and I fully agree with that thought. Since this branch of the  topic is about the load the cartridge sees and not how that load is obtained, discussing the difference between current and voltage amps is not at issue.  It just so happens that a current amp gives an extreme example of "non-traditional" loading.

 A cartridge is a two terminal device that sees a two terminal load downstream.  It doesn't know or care what form of amplification that load takes, it just does what the load tells it to do.  

dave

Yes, JCarr sort of contradicts himself a bit and ends up being half right.  The reason a cartridge sounds different under a heavy load is because the low load impedance suppresses output voltage. But a preamp that is uninterested in output voltage would not be affected by this phenomenon.  In the case of the IV preamp, the low input impedance (not quite the same as load impedance) results in high current flow through the preamp.  It does for the IV preamp what a high load & input impedance does for a traditional preamp--maximizes the electrical signal that the preamp is using.

Changes in sonic character between the two approaches may be based on inherent differences in the architectures, or based on differences between the way the cartridge's--let's call it "frequency response"--as expressed by its current output varies from its "frequency response" as expressed by its voltage output, but those changes cannot be attributable to the effect of loading on voltage output since the IV preamp effectively ignores that effect.

@woofhaven1992 

 The reason a cartridge sounds different under a heavy load is because the low load impedance suppresses output voltage.

so add more gain.  I'm not trying to be cheeky here but this becomes a problem related to what happens downstream of the cartridge and not with the load the cartridge sees.  If you load a cartridge severely and do not have a capable low noise good sounding stage after it things will indeed go south.... this is not a problem with the cartridge load per se but with the behavior of what follows.  

In the case of the IV preamp, the low input impedance (not quite the same as load impedance)

how can the two be separated form each other?  from the cartridge POV, how can 3Ω be different than 3Ω?  I am speaking primarily about the effects of the load on the electromechanical behavior of the cartridge and what impact that may have on the sound.

 but those changes cannot be attributable to the effect of loading on voltage output since the IV preamp effectively ignores that effect.

Sure the IV preamp can ignore that its low input Z effectively reduces the output voltage to 0.  My question is how can the cartridge ignore that it is seeing a load that is possible a fraction of its internal impedance?

Lets get back to the simple question I asked....  If going well below a cartridge manufacturers load is not recommended, how can we reconcile the use of transimpedance amplification or in your case a load that is 1.7X the cartridge internal impedance?

It is my belief that when specified by the cartridge manufacturer the load value range would relate to the behavior of their cartridge and not in response to the unknown capabilities of what follows.  In any case... Like you I feel that number is only a suggestion and people should feel free to use whatever load sounds best to them.

dave

This is a second take on woofhaven’s post, nothing to do with what Intactaudio posted above, or only tangentially. Woof, you seem to ignore the fact that none of the available current driven phono stages actually presents a "perfect" zero impedance load to the cartridge. Each of the different units has a finite input impedance that is above zero and usually less than 10 ohms. Case in point is my BMC MCCI that I bought recently to experiment with current drive in my home system. (Otherwise, I would have to depend upon others’ opinions, which I don’t find helpful to do.) The MCCI has an input Z stated by the maker to be <3 ohms. I guess that’s why Intactaudio uses that value in his thinking process. In other words, none of these devices is "perfect" when it comes to current drive, because they really cannot be perfect; zero ohms is a short circuit to ground. My experience with two or three very good LOMC cartridges driving the MCCI is that you cannot predict in advance what you’re going to get out of the phono stage in terms of either gain or tonal balance. I’ve had to think of the cartridge and the MCCI as a single device where a major determinant of the output SQ and db is taking place at the interface between the cartridge and the MCCI I/V stage. The cartridge internal impedance, its voltage output (because that affects current output), and probably its inductance along with the ~3ohm input Z and the current amplifying devices of the MCCi would seem to be the important variables. So, there’s no free lunch, and there is nothing inherently superior about current drive vs voltage drive, at least so far in my experience. This is not to say that the results cannot be superb.

the industry standard of 47kΩ was adopted from the MM cartridge world and applied to the MC realm because is first and foremost "does no harm".... until you throw a SUT into the mix that is.  Once a SUT is added the load the cartridge can see can get vanishingly low to the point where you actually start losing gain as you increase turns ratio.

Most SUTs are meant to drive a 47,000 Ohm load since they are also built to work with a specific cartridge. Generic SUTs (like Jensens) might also be built to drive 47K although with some additional loading to allow it to express the turns ratio correctly with different cartridges (which have different source impedances). Some Jensen transformers are meant to drive lower impedances but IIRC that's still about 6.8KOhms.

Lets get back to the simple question I asked....  If going well below a cartridge manufacturers load is not recommended, how can we reconcile the use of transimpedance amplification or in your case a load that is 1.7X the cartridge internal impedance?

A transimpedance input is a special beast. It requires an opamp and when you set up an opamp circuit with feedback, you get something called a 'virtual ground' where the input signal meets the feedback signal. Its not actually ground ('virtual' means 'almost or nearly as described, but not completely or according to strict definition.'). This is why "virtually 100%" is not in fact 100%...

Anyway, in a transimpedance amp the cartridge is substituted for the input resistor. The significance here is that if the virtual ground were actually ground, the signal going thru the input resistor (or coming from the cartridge) would be snubbed at the virtual ground- the signal would stop dead. But it doesn't!

The lesson here is that virtual ground isn't in fact ground. Thus while its impedance might appear to be quite low, it does not function as if its impedance is actually that low. IOW it does not have all the qualities of an actual ground!

Its confusing yes, but in actuality the cartridge isn't driving an impedance that is at or near ground (which would be 0 Ohms or very nearly that). Its actually driving something quite a lot higher.

The gain of an opamp circuit is defined by the input resistor vs the feedback resistor. For example if the input is 100 Ohms and the feedback is 1000 Ohms, the gain will be 10. Since the cartridge is a variable source impedance from cartridge to cartridge, the gain of the circuit will vary from cartridge to cartridge too- more gain with a lower source impedance, less gain with a higher source impedance. This means the output will not vary that much as you change out the cartridge, but it also means that the circuit inherently has limits (due to the amount of feedback applied) and won't work with every cartridge.