Cartridge Loading for a phono pre amp

atmasp, thank's but no.

🤣

@rauliruegas What he is referring to is the constant velocity aspect of the cartridge, which is part of the RIAA curve. The faster the stylus moves, the more voltage it puts out. So the RIAA curve rolls off against this characteristic. In fact its often referred to as a 'characteristic curve'.

While mechanical impact does occur as a result of electrical load- there is some back emf necessarily generated by the signal current that affects the mechanical motion, but a quick back of the envelope calculation using Lenz’s law and the 10uH cartridge suggests a 2 orders of magnitude difference between the generated signal and the back EMF for a 100 ohm load at 20kHz- certainly not enough to cause tracking issues .

This statement does not support Raul's assertion: it says something quite different!

How does this concept affect the transimpedance phono amps?

The input impedance is also known as 'virtual ground'. This is a point in any opamp circuit where the feedback is applied back to the input of the opamp. There is a resistor that also connects to that point, which is the input resistor, in series with the signal. The ratio of input resistor vs the feedback resistor sets the gain of the circuit; if the feedback is 10K and the input resistor is 1K the circuit has a gain of 10.

In a transimpedance amplifier, the cartridge itself is that series input resistor. Part of the reason for doing this is to reduce noise, since the Johnson noise of the resistor itself contributes to the noise floor. You can see that the gain of the circuit varies according to the impedance of the cartridge winding; as the impedance of that is decreased, the gain of the circuit goes up. Even with modern opamps care should be taken to keep this initial gain less than 20dB else you can get into distortion issues.

What is important to understand here is that the cartridge isn't actually driving ground, which is what zero Ohms would normally be! Its driving the relatively high input impedance of the amp amp, and is also interacting with the feedback signal. However the cartridge is remote- it is likely feeding a tonearm cable a meter long or so, so the RFI generation will still be present. So loading might still be required.

If the voltage drops as the loading is applied i won’t be linear power.

Since the minimum load specified is 10X the source impedance, we're not talking about a big issue here.

Raul is obfuscating.

His contention is that loading has no effect on the compliance of the cartridge. I showed that it does. 

Since the amount of energy directly affects how much force it takes to move any generator by the direct proportion of energy asked of it (for example if you ask 1 Amp of a generator and it takes X amount of energy to spin it, asking 10 Amps of the generator will make it 10X harder to spin).

I provided a link to this effect earlier.

Since a cartridge is also a generator, asking it to make 400X more energy will affect it in a similar way. The only variable is that the cantilever has a springiness that affects the outcome of the equation. But its a simple fact that the coil itself will be 400X harder to move (this being between 47K load vs 100 Ohm load per Raul's example).

Again, this principle is easy to demonstrate. Loudspeakers are moving coil, as they are mechanical transducers just as is a cartridge. If you try to move the woofer with your hand you'll notice it takes a small amount of effort. Now put a short across the speaker terminals and see how easy it is to move the woofer- you'll see its a lot stiffer. The same thing happens with a cartridge.

Raul is arguing for some sort of 'free energy' as best I can make out, since that has to be how it would work if loading did not affect the cartridge in this manner.

Occam's Razor sorts this out easily enough.

So you're not going to answer my question? That leads me to think you are simply trolling. Because there are internal losses, the cantilever will not be quite 470 times harder to move; we'll call it 400, assuming that its also designed to operate at 47K. 

This is based simply on this fact:

https://physics.stackexchange.com/questions/120005/why-do-electricity-generators-have-to-work-harder-for-higher-loads

Since the generator only converts mechanical power to electric power, the motor driving the generator shaft must supply at least 10 times more power in order for the generator to supply 10 times more power.

This is fundamental and inescapable.

You are correct on one thing, it was pretty easy.

The arm has an 11 gr effective mass, the cartridge is 9 grams; the compliance is rated 'Approx. 12x10-6cm/dyne at 100Hz' note the word 'Approx' (that is variable based on load). Now plug those values into the calculator:

http://korfaudio.com/calculator

You'll see for starters this cartridge is not a good choice for this arm. Now let's assume that the cartridge is loaded at 800 Ohms. By decreasing the load to 100 Ohms we ask slightly less than 8x the current out of the cartridge. This will increase the stiffness a bit, how much is difficult to know without more specific information about the cartridge, but as you see from the first link of this post it has to be significant. Let's assume that the load is a fractional bit of the compliance instead of dominating it. You can see what happens if the compliance is decreased by decrementing from 12 to 11, which is conservative; this puts the results outside of the green zone on both charts.

@rauliruegas I see you are refusing to answer my prior question to you. I've handed all the information you need to you on a silver platter. But the way you are reacting suggests you are not here for any other reason than to troll. If that is not the case, go ahead and answer my prior question. Here again for your convenience, are both of my prior questions.   If you are disinclined, I can only assume its because trolling is your goal rather than anything else:

Anyone with an elementary school education can work out the math here. Perhaps, knowing that, you can tell me where that current is coming from??  If you can answer that, a bonus question: what is the consequence of that current flow?

The cartridge.

@holmz Oh man, you're doing Raul's homework for him😄! I suspect you know how any generator will respond when asked to do say, 470 times more work? BTW you are correct that the output voltage isn't the same with the two different load resistances since the source impedance isn't zero. But since both loads (47K and 100 Ohms) are over 10x the source impedance, they will have little effect on the output voltage (which I simply ignored to make a point). No need to hang your head in shame, although I suspect that comment was facetious.

At any rate, the answer to Raul's latest query to me is inside the answer to the last question I posed to him.

50K ohms, all inputs. (Magnetic phono may have any value from 10 ohms to 100K ohms substituted. Also has provision to add input capacitance for matching certain magnetic cartridges.)

The preamp has a 49.9K resistor to ground, but has other elements in parallel. It measures out at 47K.

You know we can do this all day.

In the other side you did not gave us the answer to what you posted before:

 

" can easily get you outside of this target window ( ideal resonance frequency range. ) "

Sure I did. Whether you are able to understand what I posted is another matter.

Why ARC featured such function in their reference phono stage that serve no purpose? So they can charge more?🤔

We have a loading strip on our preamps too, and I advise customers with LOMC cartridges to not use it. But if you have a higher output cartridge, the loading strip can be quite handy! Because of their higher inductance, high output cartridges have that electrical peak at a lower frequency, which can be at the high end of the audio band or just outside of it. The winding of a higher output cartridge tends to have a lower Q value so loading can be helpful to reduce ringing. 

" The industry spec is 47K for all cartridges.  " where is the " official " announcement about, say by the AES that coul confirm it? where?

@rauliruegas 

Cripes. If one is grounded in audio history then this is obvious. ARC SP6: 47K phono input impedance; similarly H/K Citation 1, Marantz 7, Conrad Johnson PV12, etc., etc.

"  less able to trace high frequencies. " again: where are the white papers that can confirm it.

It has to be published to be real?? Since JCarr published something, you're not challenging his statement (despite the obviously recursive aspect of that argument...), you're employing a double standard. In order for the cantilever to not get stiffer a fundamental rule of physics (law of energy conservation https://energyeducation.ca/encyclopedia/Law_of_conservation_of_energy) has to be violated. IOW you would have free energy and the world's energy issues would be solved 😂 

So no papers needed- just a grounding in basic physics (high school level), in particular how alternators and generators work. A 'Sheesh!' is in order here.

That is ridiculous and with no facts or diagrams/equations that shows exactly that. I That " can easily get you outside of this target window ( ideal resonance frequency range. ) " is not proved here or in any other subject threads. Again: only IMAGINATION.

Ohm's Law and the power formulas are not a figment of someone's imagination 😁

The cartridge is being asked to do just under 2 1/2 orders of magnitude more work with the example you provided:

47,000/100 = 470

-meaning 470X more current is drawn from the cartridge; because 47K is so high a resistance value relative to the impedance of the cartridge winding, the output voltage is essentially unaffected so the current increase also represents the wattage increase).

Anyone with an elementary school education can work out the math here. Perhaps, knowing that, you can tell me where that current is coming from??  If you can answer that, a bonus question: what is the consequence of that current flow?

This is such simple math I don't see why a paper needs to be written about it, but maybe even though its basic, some people simply haven't thought it thru. Or didn't do so well in math.

Please do answer the questions.

@rauliruegas To be clear, at no point have I stated that loading affects the frequency response of a LOMC cartridge. Your statements to the contrary are false. Please cease and desist. When you engage in such statements and then debunk them, you are engaging in a logical fallacy known as a strawman. Logical fallacies are inherently false.

How does one safely send a square wave through a cartridge? Just curious as I have no intention of trying it.

Apparently PS Audio did their homework.

I suspect if you put too much energy through the cartridge you might be able to damage the magnetic field generated by the magnet structure, so your reticence is wise. I used a square wave generator set to a very low output. I put the cartridge in series with a nominal resistance so I could read the square wave across the resistor- essentially if the coil of the cartridge were a problem for the square wave, it would be readily visible. The signal was low enough (I have to admit I was a bit concerned about damage to the cartridge when I did this the first time) that the cartridge was unaffected. This meant the scope was at a very high gain setting.

Note that a load to the cartridge would be a resistance in parallel with the cartridge, not in series. Anyway, I found that the cartridge could pass a square wave at any frequency- even 20KHz, with no ringing at all.

This surprised me at the time, since the reason for doing this was to create a box that would allow you to plug in the cartridge and easily determine the ideal load (this was about 35 years ago). When I saw that the cartridge could pass a perfect square wave at any audio frequency, I realized that the loading was not affecting its frequency response so something else was afoot.

One of the side issues associated with this topic is that of ticks and pops. If the phono section has good RFI immunity and if it also has good high frequency overload margins, that peak at 1 or 2MHz won't overload the input of the phono and you'll get less ticks and pops. I had this graphically demonstrated to me when an employee complained of a noisy LP he had bought. His LP played fine on the shop system so he brought in his preamp and there were the ticks and pops, sounding for all the world as if they were on the LP surface.

How that works is the energy of the cartridge can set off the electrical resonance, even though that peak is well outside of the audio band. This phenomena is known as 'excitation' in the radio world. Once the peak has gone into excitation, it puts out a signal at its resonant frequency which is applied to the input of the phono section. Since its a good 20-30dB higher than the cartridge signal, it can overload some phono sections if they have not taken this phenomena into account.  I suspect such phono sections are a lot more common than we like to think- that preamp I mentioned earlier was a Toshiba and Toshiba is a supplier of semiconductors so you would expect good engineering... this has led me to think this problem was an epidemic back in the 1970s and 80s. 

My question is how significant that affect the cantilever to be stiffer and introducing possible tracking issues, OR its effect is negligible?

Your target for mechanical resonance is between 7-12Hz. The mechanical resonance is a product of the mass of the cartridge in the arm vs the compliance of the cantilever of the cartridge. Changing the load from 47K to 100 Ohms can easily get you outside of this target window- and that can cause tracking problems.

So, all those very well regarded designers are wrong?

IF they have ignored the significance of the inductance of the cartridge in parallel with the capacitance of the tonearm cable in their design

THEN yes, they blew it.

If you doubt this I recommend a simple test, which is to run a square wave through the cartridge itself and observe the results on an oscilloscope. If the square wave rings, then loading will affect it at audio frequencies. If you do this with any LOMC cartridge you'll see that the inductance of the cartridge is so low that a square wave at any audio frequency will look perfect on the oscilloscope. So its obvious then that the loading isn't affecting the tonality of the cartridge. Something else is.

So if the loading does not affect the cartridge at audio frequencies, why would it affect how the cartridge sounds- because in some preamps that is quite audible. The answer is simple- the preamp itself is reacting to the RFI generated by the cartridge (hence the emphasis on 'some'). I've explained this a good number of times, I've dropped links to Jim Hagerman's site http://hagtech.com/loading... If the preamp is designed with the RFI present at the input of the phono section, the loading resistor will make little difference- it won't tone down the high end nor will there be a need; it will sound fine with a 47K load.

J.Carr ( Lyra cartridge designer and phono stage Connoissour. ) said/posted:

 

" Phono cartridges are floating sources rather than balanced..."

If JCarr really said this, then he isn't aware of how balanced operation works. A dynamic microphone is a balanced source because its floating. An Ampex 351 tape machine has a balanced output because the secondary winding of its output transformer is floating. My Neumann U67 microphones are a balanced source because their output transformers have a floating output.

Atma-Sphere was the first company anywhere in the world to offer balanced line components for home stereo use. The equipment supports the balanced standard, known as AES48.

In a balanced connection, ground is ignored- its not part of the audio signal. This is why in a phono connection with RCAs, you have that weird ground wire that no other single-ended source seems to need. This is because you have a balanced source that is being fed to a single-ended input, and you have to do something with the ground, which otherwise isn't connected to the audio signal, but is its shield nevertheless- that being the tonearm tube of course.

More on this topic: http://www.atma-sphere.com/en/resource-why-balanced.html

Note in this article about halfway down the phrase

Both sides of the signal connection have an equal impedance to ground and may be floating

Latter on appeared the active high gain phono stages to drive LOMC in exclusive way with no facilities to handle MM/MI cartridges, even those LOMC phono stages cames ( even some today ) with impedance load different values choices coming from 100 ohms to 1k ( more or less. ) where no one came with the option to choose 47k.

This statement is misleading. It is correct that some of these phono sections that had/have switchable input impedance did/do not feature the standard 47K, probably on account of poor RFI management at the input of said phono sections. That does not change the simple fact that 47K is indeed the industry specs for cartridge loading.

@rauliruegas  Apparently you've yet to sort out that I've never stated that loading affects frequency response of LOMC cartridges. Please cease and desist from claiming otherwise.

47K is the standard loading value for all cartridges, which is why phono preamps have a 47K input impedance. The reason many LOMC cartridge manufacturers might state a lower value (like 100 Ohms) is that they have no idea how competent the phono section will be to which their cartridge will be connected. Obviously it won't sound right if its sensitive to RFI from the cartridge and the loading detunes that resonant peak to which JCarr, Jim Hagerman and I have referred. So a nominal value of 100 Ohms is specified since there are so many phono sections that do have problems in this regard.

In this context, is there any relationship between cartridge bandwidth and cantilever stiffness?

The stiffness is a function of the load impedance. Obviously it will be more supple if loaded at 47KOhms as opposed to 100Ohms!

This says nothing about the bandwidth of the cartridge itself, which would be unaffected by the load. However, the stylus has mass and to trace high frequencies must be low enough that its mass is easily moved by the modulations in the groove. Adding stiffness to the stylus support isn't going to help- at some high frequency (which may well be outside the audio band) the stylus will no longer be able to keep up. This does not take much to sort out.

Any modern cutter has bandwidth well outside the audio band. We use a Westerex 3D cutterhead, which was designed in the 1960s and it has bandwidth to 42KHz, at which point its bandwidth is intentionally limited by the electronics.

We can cut a 30KHz tone on a lacquer and play it back on some pretty conventional LP equipment- for that we use a Grado Gold mounted to an older Technics SL1200, the idea being that any cut we make should be playable on that machine. Its got no worries playing a 30KHz tone, but if we start loading the cartridge we can see the output level drop (with some distortion/noise apparently added), whereas at lower frequencies its still perfectly flat (once equalized).

Empirically speaking its easy to deduce that the load is affecting the ability of the stylus to trace the groove, which is why we see distortion as essentially the stylus is mistracking. 

Even though there is no microphone that has bandwidth that high, it seems a good idea to keep the stylus as planted in the groove as it possibly can be. So IMO its important that the phono section be immune to the RFI generated by a LOMC cartridge so a 47KOhm load be used to allow the cartridge to track to the best of its ability.

BTW, if an SUT is employed, you can side step this issue a bit as most SUTs lack the bandwidth to pass a 1MHz or even 200KHz noise source which can be really helpful to many phono sections! IMO this might be why SUTs have a certain following.

You follow with your tale at Munich but in at least 3 different threads in different internet forums where you, JC and I participated about and where you posted your same " tale " he never confirmed that agree in that " less able to track HF ".

You were not in Munich, plain and simple.

JCarr did not address the topic of high frequency traceability one way or the other. I recommend you set up an LP mastering lathe as I have and you can sort all this out easily enough.

If you are saying that by making the cantilever stiffer that its ability to trace a smaller and smaller waveform in the groove, requiring the stylus to move faster and faster against that increased stiffness, and somehow its ability to move that fast is unaffected, be my guest. But on a very basic level, the idea has no merit.

Raul's handwaving aside, I agree 100% with what JCarr has stated, quoted by Raul during his handwaving nonsense above.

Loading does not affect the bandwidth of the cartridge, nor is that something I stated. Raul is employing a strawman argument (a logical fallacy, by definition false) by attempting to put words in my mouth. 

If you ask JCarr directly, he will agree to the fact that loading the cartridge at a lower resistance will stiffen the cartridge cantilever (something quite different from changing bandwidth...). He and I met and spoke about this topic at a Munich audio show 6 years ago; he looked me up at our room, this was after an active thread on cartridge loading on the What's Best forum. 

What I have maintained is that the additional stiffness may decrease the ability of the cartridge to trace high frequencies, not that it would reduce the bandwidth of the cartridge!

If loading had no effect on the cantilever stiffness, then free energy would be the result. At least so far, free energy has not been recognized by the world at large- it remains the stuff of conspiracy theories. It would be quite odd that phono cartridges would somehow be exempt from the laws of physics 😂 Put another way you can safely ignore Raul's rant.

Always start with 47K to see if the preamp works with RFI at its input. If yes (the preamp is unperturbed by RFI), this will get you the best sonic performance. If it sounds better with lower resistance loads, then the preamp does not handle RFI well.

@holmz 

Back EMF?? No. Any cartridge is a generator of power. When the load on any generator is reduced in resistance, more current can flow. This energy has to come from somewhere (otherwise a new branch of physics would be created...) and it comes in this case from the motion of the stylus in the groove- the cantilever becomes harder to move.

You can easily demonstrate this principle for yourself. A loudspeaker operates on the same principle of a moving coil in a magnetic field. If you have a woofer with nothing connected to it you'll find it easy to move the cone with your hand. If you then place a short across the terminals of the speaker you'll find that the cone is a lot harder to move.

What the load resistor does is three things: it detunes the resonant peak I mentioned earlier, thus eliminating the RFI and it causes the cantilever to be stiffer, possibly reducing the ability to trace high frequencies and certainly affecting the mechanical resonance of the cartridge and arm combination.

So you solve a problem for a phono section with a design bug, in exchange for introducing possible tracking issues. Not a good set of options IMO.

@judsauce The industry spec is 47K for all cartridges. When you load the cartridge at a lower resistance you are asking it to do more work (produce more power to drive that load). Even though its a tiny amount of power, it will make the cantilever stiffer and less able to trace high frequencies. It can and does affect the interaction between the arm and cartridge (effective mass and mechanical resonance).

The loading is not for the benefit of the cartridge! The cartridge generates Radio Frequency Interference (RFI) because its inductance is in parallel with the capacitance of the tonearm interconnect cable. This forms a resonant peak which can be activated by the energy of the cartridge itself. That activation is called 'excitation' in radio parlance.

That RFI can be 1 or 2MHz and is injected directly into the input of the preamp via the interconnect cable. If the preamp isn't OK with that, it won't sound right.

If the designer was aware of this fact, 47K will sound fine because the preamp will be unperturbed. Otherwise you will have to use the loading and deal with all that comes with that.

So start with 47K. If the designer knows what he's doing, that will be the position that sounds best.