Cartridge impedance loading question

The correct loading for the cartridge is 47K.

However, this may result in radio frequency bursts that might disturb the preamp. That is why it is equipped with loading options. The RFI (Radio Frequency Interference) is caused by the excitation of the RF circuit formed by the cartridge inductance and the capacitance of the interconnect cable. The loading detunes the circuit so the RF bursts cannot occur.

Generally speaking, the higher the load impedance the better, while at the same time being sufficient to detune the RF circuit such that no interference is heard (which will manifest as a brightness).

Another way to put this is that if tonality is unchanged, go with the highest impedance you can. This will allow more output from the cartridge.

As pointed out, the capacitance of the interconnect cable has a big influence and may actually be more important than the cartridge. That is why you can't have the same loading for all installations of the same cartridge.

If the cable capacitance is low, the resonance will be in the MHz region, in which case if the preamp does not care about RFI you simply won't hear a difference unless you load the cartridge so low that its output drops.

A LOMC cartridge without a core is going to have a lower inductance value as compared to one that does have a core. A core has a way of increasing the inductance, all other variables being equal.

As Jcarr points out in one of the links I provided, and IME, as you load the cartridge more and more the output goes down, which makes for more noise from the phono section. Generally speaking, the greater amount of noise the less low level detail due to the ear's masking principle.

In our preamps you really don't hear any difference with 500 ohms or 47K; our preamps are pretty immune to RFI. I've not been contesting that you don't hear an improvement- in fact I am quite sure you do. All I am saying (and all I have ever said in this thread) is that if so this indicates that your preamp has a problem with RFI. So in your case loading is important. But what works for you is not going to be the same for someone else with the same cartridge if they have a different preamp and tone arm cable. That was brought out in the links I provided.

Fleib, I recommend you do some design work and see if you still say that. You might also do a search on some of the comments by the Jonathan Carr (Jcarr) who is active on this forum.

If a circuit does not have RFI problems then loading can still have a beneficial affect.

Loading of LOMC cartridges has no effect whatsoever so far as the cartridge is concerned. There is a slight benefit to loading as low impedance terminations of cables reduces cable artifact, particularly with high capacitance cables. This is better realized if the cartridge is operating in the balanced mode (as it is a naturally balanced source) and the signal is carried through a balanced line. Then the cable will have no artifact at all. 600 ohms will be sufficient to eliminate the cable artifact.

Novel way of looking at a generator, or is it a transmitter? The oscillation or ringing occurs in the preamp because of the extraordinary amount of gain needed.

The cartridge/cable combo acts as both. It does not matter that the phono section has no bandwidth at the resonant frequency, which is usually several MHz. The gain has nothing to do with it.

The tiny voltage of a LOMC still has inductance, which combines with cable and preamp capacitance and can cause HF ringing, but is only a problem with extremely low output carts with high resistance/inductance. I think such occurrences are beyond the bandwidth of your phono stages.

I wonder if there is a semantic issue underpinning this conversation. In the above quote you are correct in almost every way except that there are no LOMC cartridges with high inductance... if it is assumed that by 'tiny voltage' you are referring to about 1.0mV or less. The inductance you thus refer to is the inductance in question, the frequencies are those in question, and no phono section I know of can go that high, but they don't have to- they only need to have an RF sensitivity and then you suddenly hear loading making a big difference.

Saying RFI is generated by the cart is misleading. It is not. There are other loading considerations with MC's, none of which are about noise or tonality. You assume the higher the load, the better. I beg to differ. Such things as imaging, stage, dynamics, detail, and focus can all be affected by load, while tonality is unchanged.

But here in your next paragraph you seem to contradict the earlier paragraph in the first sentence. I did not nor do I assume that the 'higher the load the better': I said the correct load and most designers design for 47K.

I agree that loading affects these things if your preamp has RFI problems! I don't know if you have ever heard what RFI can do to an audio circuit outside of a phono situation but the effect it can have on soundstage, background noise, detail and the like can be profound. As a result all I can assume from your reactions here is that the phono sections where you have tried different loading options are all having trouble with RFI; that is why you heard a difference! RFI has big effects on audio gear if not tamed.

I do however agree with your closing statement as loading has a lot to do with both the cartridge and the individual preamp, for the reason that the inductance of the cartridge and the resulting RFI affects different preamps differently. In our case since we got a handle in this some years back it has almost no effect which is good- its more plug and play. But we originally included the loading strip on our preamp (which is still there) because in the old days we heard differences with nearly every preamp we auditioned.

Here are some links that you may find interesting:

http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1281468389&openflup&18&4#18
and further down on the same thread:
http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1281468389&openflup&22&4#22
pay attention to post number 3 at this link:
http://www.whatsbestforum.com/showthread.php?15077-Cartridge-Loading-A-Misnomer

The larger you make an air coil, the lower its inductance.
https://en.wikipedia.org/wiki/Inductor

The noise that a phono section has is usually not a function of loading. There are a lot of factors that have to do with noise- it is an immense topic.

The 'rules' are simply the laws of physics- and are impossible to violate.

All I was saying is that if you load a cartridge (like any other source), the result *can be* reduced output which will mean that the noise floor is higher.

The MP-1 has loading strips mounted on the rear panel by the phono input. It might be one of the easier preamps to set cartridge loading.

Pkoegz, its a good idea to try the loading at least. If it improves the sound (reduces brightness without affecting speed or bandwidth) then it tells you that despite low noise, the preamp is sensitive to Radio Frequency Interference (RFI).

This interference is generated by the pickup itself, as explained previously. If you hear little or no difference, then its no worries- the designer of the preamp has done his homework.

There is no "correct" loading. Assuming you're going straight in, why don't you start at 100 ohms as suggested by Shelter and adjust it to taste?

I take it from this an other comments that my post was not read through completely! So, to reiterate in other words:

The correct loading for any cartridge will be such that it does not ring at audio frequencies- IOW if you run a squarewave through it, a squarewave will come out with no ringing.

With any LOMC cartridge ever made, this will be the case at 47K. So why do we hear differences? It has not so much to do with the cartridge and **everything** to do with the phono preamp. If the preamp is sensitive to Radio Frequency Interference, then loading the cartridge will affect tonality as it detunes the RF circuit formed thus:

The RFI (Radio Frequency Interference) is caused by the excitation of the RF circuit formed by the cartridge inductance and the capacitance of the interconnect cable. The loading detunes the circuit so the RF bursts cannot occur.

-as I mentioned earlier. IOW, if loading is audible in your system, your phono section is sensitive to RFI.

Please note that the correct loading value for such a preamp cannot be predetermined since the cable is part of the RF circuit and its capacitance will affect the value. If the manufacturer of the cartridge recommends a particular value, it simply means that *their* reference preamp has troubles with RF and nothing more. But you will find that most manufacturers recommend 47K as it is correct.

The answer has to do with how the phono stage reacts to RFI as I have pointed out.

Designers also have to be pragmatic about what sort of input impedance is needed to control noise! If it were to be high as you ask about, controlling noise becomes a major problem in a high gain circuit! So 47K was derived as a reasonable compromise decades ago- it allows low noise and a moderate, easy load for almost any phono cartridge.

Ringing at audio frequencies might be a your criteria for loading an MP-1, but there are certainly other considerations for the rest of us, and I would think for MP-1 users as well. That is the sound, irrespective of RFI.

To be clear, 'Ringing at audio frequencies' is no more a consideration for our preamp as any other, as such only occurs with high output moving magnet cartridges. For them, the inductance is much higher and so loading is important.

Regarding the sound, it cannot be considered 'irrespective of RFI': in plain and simple terms if the circuit has RFI problems **the sound will not be right if RFI is present!!** If such is the case then loading will have a beneficial effect.

In more detail: if the circuit is sensitive to RFI, due to poor layout, lack of input stopping resistors, RFI filters, bad grounding (IOW anything that can cause RFI sensitivity), etc., the result will be that RFI will adversely affect the sound. The RFI is generated by the pickup and cable itself: it is not a matter of shielding from radio stations.

Until this fact is understood loading will remain mysterious and a matter of debate.

Ringing and squarewaves is not just cute, it is part of the toolkit one uses to do circuit analysis. It might interest you to know that many years ago I tried to make a box that one could simply plug in the phono cables from the arm and the box would sort out what the right loading was. It was during experimentation to gather data that I saw that LOMC cartridges don't ring at any frequency anywhere near the audio range- in fact even with a 100KHz squarewave to 'ring' them, they still just pass the squarewave perfectly. Once I understood this fact, I was then able to make changes to our preamp design to make it more RFI resistant at its input, which had the effect of making the low impedance loading resistors nearly irrelevant and also improved the sound at the same time- that latter bit of course is what we are all after.

So loading is not about LOMC cartridges and it is entirely and 100% about good circuit design in the phono preamp.

^^ this is how prevalent the cartridge loading mythology is!

Here is an explanation of why this cartridge may not have sounded bright in Lew's system:

http://www.whatsbestforum.com/showthread.php?14815-Best-phono-stage&p=307116&viewfull=1#post307116

Capacitive loading will make a difference- its the same as using a more capacitive cable, which is to say its probably something you want to avoid as it will lower the resonant frequency of the RF circuit.

A capacitor can be quite useful in loading MM cartridges however, but this thread so far has been about LOMC cartridges.

If you're saying resistance load makes no difference in the absence of noise, you're wrong.

Pretty sure I didn't say anything like that. But to address the comment, loading can indeed affect noise because if the preamp is susceptible to RFI, eliminating the RFI very frequently will reduce noise it can cause.

Again, if you find that loading is improving your noise floor, its likely that your preamp has problems with RFI.

And not to repeat myself too much but also again, if the load is too severe (too low a value) it can have the effect of degrading your noise floor on account of the fact that it is reducing the output from the cartridge.

Now this means that loading can be a bit tricky if the preamp has RFI problems because it is not predictable what the right loading value will be on account of the interconnect cable. Things become a lot easier when the preamp resists RFI- then all you do is keep the cable capacitance down and life is good.