Proper Cartridge Loading Can Make a Big Difference

Discussion in 'Audio Hardware' started by Raylinds, Feb 6, 2018.

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  1. patient_ot

    patient_ot Senior Member

    Location:
    USA
    Gain is set at 42db, input is between 2mV to 10mV.

    Era Gold V MM Phono Preamp - HiFi System Components

    Eroica H spec:

    Sensitivity
    2.5 mV ± 1 dB, 1 kHz @ 5 cm/sec

    Goldring Eroica H

    If OP wants more "juice" I would suggest a higher output cart when the Eroica is done.
     
  2. Ralph Karsten

    Ralph Karsten Forum Resident

    Location:
    St. Paul MN
    High output cartridges respond directly to loading. Being an inductor, they can ring at audio frequencies. This in addition to the high frequency resonance that can be set up by the inductance of the cartridge and capacitance of the cable.

    MC cartridges are not affected by loading at all. However, the way the preamp reacts to the cartridge and its 'load' can be quite different. I see this issue asked about a lot, so I will now present a short article about this topic:

    The Myth of Cartridge Loading

    There is a persistent myth that you need to load your low output moving coil cartridge for optimal sound. Usually the way it goes is that its too bright if unloaded. So people are often asking ‘is 100 ohms a good value?’ While it is true that 100 ohms is likely the most common value used, its also true that ‘good’ is a word that is being misused here.

    The reason is the 'loading' has nothing to do with the cartridge and everything to do with the preamp's ability to deal with RFI.

    The correct term for the resistor is its a 'detuning' resistor. To understand why this is so, its helpful to understand how the cartridge interacts with the rest of your system.

    (The image referred to below cannot be rendered by this site)

    This is an idealized schematic of the cartridge installed in a tone arm. The cartridge has a signal output (Vc) which drives an impedance (inductance Lc and resistance Rc). The output of the cartridge goes through the tone arm wiring and interconnect cable to the preamp; this is capacitance Ca. The input impedance of the preamp is Ra; usually 47K.

    The inductance and capacitance together form a tuned resonant circuit called a ‘tank circuit’. A tank circuit is used to tune AM and FM radios and is resonant at only one frequency. Change the inductance or capacitance and the resonant frequency changes with it. Larger values of either will result in lower frequency.

    Since the DC resistance of the cartridge is quite low, we can ignore it; the resonant frequency of the tank circuit is:

    f = 1/2Pi x squre root of LxC

    A tank circuit on paper looks a lot like a bell curve but with a more pronounced peak [take a look at the images posted earlier on this thread]. If we plug a few typical cartridge and interconnect values into the above formula, we see that the peak with a low output moving coil might be a few 100Khz up to several MHz. Low capacitance tone arm interconnect cables put the peak higher and conversely higher inductance of the cartridge drives it lower.

    How this works is the overall bandwidth of the cartridge can be affected by the resonant frequency. In a high output moving magnet design, the inductance can be quite high and might only be at the upper end of the audio band or a few Khz higher. What happens is as frequency is increased, eventually you reach the resonant frequency and above that frequency the capacitance dominates (rolling everything off), essentially meaning the cartridge will have no output above that frequency. So you can see by significantly reducing the inductance you can get much higher frequency response. This is the advantage of low output moving coil cartridges. Essentially instead of being limited by the tank circuit as it often is with a moving magnet, the frequency response of a low output moving coil is limited by its mechanical response which means it can easily go well above 50KHz.

    While we can thus get vastly improved high frequency bandwidth from a low output moving coil cartridge, it comes at a price. A resonance is harmless if it doesn’t do anything. When a tank circuit is active, its activity is called ‘excitation’. The energy of the cartridge itself is enough to drive the tank circuit into excitation, at which point its no longer harmless. As a result, the act of playing a record causes the tank circuit to inject Radio Frequency Interference (RFI) into the input of the phono preamp.

    The peak can be a good 20-30dB higher than the signal itself; this can vary a bit depending on the input impedance of the phono section (Ra in the diagram above). If the preamp is unable to cope with this sort of RFI at its input, it won't sound right (bright). To solve this, the tank circuit can be detuned, relieving the preamp of the RFI so it can perform properly. This is what the detuning (“loading”) resistor is for. It is placed in parallel with Ra (the input resistance of the preamp), and a low enough value kills the resonance. This is why 100 ohms is a common value.

    Like any solution, the detuning resistor comes at a cost. It forces the cartridge to do more work and that energy has to come from somewhere (else a new branch of physics is created :D). Since the energy of the cartridge comes from the stylus/cantilever motion interacting with the magnets in the cartridge, the end result is that the cantilever becomes stiffer and less able to trace high frequencies. You are far better off if your phono section can deal with the RFI.

    Ticks and Pops

    Phono preamp stability and overload margin as a topic does not get discussed very much online. But ticks and pops do, and is one of the main reasons digital audio exists. What most people don’t realize is that many ticks and pops are actually caused by the phono section and not the LP!

    At this point (since I’ve been involved in a few discussions about this) I like to relate the experience I had back in the late 1980s that led me to this understanding. A friend and I purchased two copies of a UHQR pressings of ‘Tea For the Tillerman’ from a record dealer. Even at that time they were pricey, so my friend Tom called me the next day a bit disappointed as his copy was noisy. Lots of ticks and pops. My copy was silent and upon hearing that, Tom wanted to bring his copy over. Being a UHQR in nice condition, it too was silent on my system. Since we had the same cartridge, amplifier and speakers I suggested he bring his preamp over since it obviously wasn’t the LP itself. He did so and as soon as we plugged it into my system there were his ticks and pops.

    His preamp was an inexpensive solid state Japanese design; mine was a tube preamp I had modified. That was the moment I realized that a good phono section is a circuit that is more than enough gain, proper EQ and low noise. It also has to resist RFI at its input, have good internal stability and high overload margins.

    Remember that 20-30dB peak I mentioned earlier? Decibels are a means of dealing with the ear’s logarithmic response when we only have linear circuits to record and play back audio. Apparently that sort of peak can cause the preamp to misbehave. Many preamps are on the verge of instability as their designs do not include a simple device known as a ‘stopping’ resistor. This is simply a resistor at the input of any active device like a tube or MOSFET that prevents that device from oscillating. Any active device has a capacitance present at its input; in conjunction with a resistor in series with the signal the two together can cause a high frequency rolloff. This is known as Miller Effect. What can happen if there is no stopping resistor is there is always a bit of inductance present on any wire or circuit board trace; this can set up a tank circuit which can resonate. Stopping resistors prevent the oscillation via Miller Effect.

    Because some preamps lack stopping resistors they can also be slightly unstable. Between this and poor overload margins in the preamp, RFI injected by the tank circuit can cause ticks and pops. Its not all on the LP surface!

    Many phono preamp designers simply don't realize that a good phono section has to have good overload margin, be resistant to RFI and otherwise be stable, so phono preamps that make ticks and pops tend to be more common than those that don’t (and FWIW feedback or passive EQ has nothing to do with it). As best I’ve been able to make out, its harder to design a solid state preamp that lacks ticks and pops than it is using tubes.

    If the preamp itself has options for loading its a good bet the designer hasn't thought this through- they may have noticed that the preamp sounds different with different input “loading” resistors but have only assumed that its due to the cartridge itself without really sorting out what’s going on.
     
  3. DaveyF

    DaveyF Forum Resident

    Location:
    La Jolla, Calif
    I think cartridge loading is ultra important with a low output MC. It took me considerable experimentation to get the correct loading with my Lyra. Ended up at 750 ohms with a custom Vishay resistor into the loading area of my CAT preamp. The difference between even 500 ohms and 750 ohms was considerable...better imaging precision, better bass control, more extended highs and slightly less noise. The cartridge type does seem to come into play here, I have utilized Benz's and other MC's that are not quite as demanding as this new design Lyra. Which, BTW, is one of the main reasons that a lot of folks seem to have strong opinions about the Lyra sound...because I believe that a lot of dealers just plug the cartridge in a forget about the loading...and the specific load required. With a lot of cartridges you can get away with this, not so IMO with the newer Lyra line.
     
    Heckto35 and Catcher10 like this.
  4. patient_ot

    patient_ot Senior Member

    Location:
    USA
    I know of one SS circuit designer that figured it out - that person uses an active design. I assume there are others but yes, it's something that rarely discussed. When you start pushing a phono preamp with hot cartridges and hot records then you care about it more. The phono preamps I've used that were worse re: overload margin and tick and pops were also solid state designs, but passive EQ rather than active EQ designs. One was a passive design from a world famous designer that has designed and made SS as well as tube designs. His SS design fell apart when you started pushing it.
     
  5. Ralph Karsten

    Ralph Karsten Forum Resident

    Location:
    St. Paul MN
    Certainly! Nelson Pass appears to be one- one way to tell is load the cartridge at 100 ohms then try it with 47K stock loading. If the preamp sounds the same then whoever built it had it sorted.
     
  6. Blair G.

    Blair G. Senior Member

    Location:
    Delta, BC, Canada
    All this math is fascinating, and confusing.
    My cartridge specs say 220pf capacitance.
    Available setting on my phono stage are 100pf , 330pf and 430pf
    Which of those three should I be using?
     
  7. patient_ot

    patient_ot Senior Member

    Location:
    USA
    100pf. Your tonearm cabling will add another 100pf approximately.
     
  8. draden1

    draden1 Forum Resident

    Location:
    Des Moines, IA
    Whichever one sounds best. Seriously, I’m not being snarky. Try all of them and see which one you like.
     
    Nascimento Brasil likes this.
  9. Blair G.

    Blair G. Senior Member

    Location:
    Delta, BC, Canada
    Ah, never thought about the impact of the cable.
    Plus draden 1’s advice obviously makes sense, use my ears as the gauge, though I don’t alway trust them lol
     
    Nascimento Brasil likes this.
  10. soundQman

    soundQman Senior Member

    Location:
    Arlington, VA, USA
    Well if you had an oscilloscope and a test record with frequency sweep tones, you could display the cartridge's response and pick the flattest or best one. I wish I had the equipment and setup now to do that. My dad used to have a scope when he was alive.
     
  11. Davey

    Davey NP: Jōetsu Shore ~ Underworld (2022)

    Location:
    SF Bay Area, USA
    Pretty easy with a test record and a cheap audio interface (A/D with USB) to your computer (or sometimes you can even go direct from preamp outputs to analog inputs on the computer with adaptor cable). If not used for critical listening, you can often get something for under $50 that will allow you to do audio capture to computer via USB, and then you can analyze the data with other software tools.

    Of course, most of the free software tools are somewhat limited, but plenty of people here that could offer some suggestions on good ways to go.
     
    soundQman and patient_ot like this.
  12. Blair G.

    Blair G. Senior Member

    Location:
    Delta, BC, Canada
    Interesting idea.
    Have access to an oscilloscope (not that I know how to use it), a record with sweep tones might be tougher
     
  13. patient_ot

    patient_ot Senior Member

    Location:
    USA

    A record with an accurate frequency sweep is a must. Without it you cannot do the FR test. The record I use for this purpose is the CBS STR100.

    For around $30 you can buy an audio interface that is accurate enough to use the record to create FR graphs. Will it create audiophile grade needledrops? No, but it is good enough for basic measurement taking. ARTA will even allow you to calibrate it, or a similar product.
     
    soundQman likes this.
  14. vo_obgynmd

    vo_obgynmd Forum Resident

    Location:
    Issaquah, WA
    I have a Linn Krystal cartridge on my Arkuate level Linn Sondek LP12. Linn says that the load resistance for this MC cartridge is betwee 50 -200 ohms. My phono stage (Chord Huei) has a variable impedance selection range from 100 - 3700 ohms for MC cartridges. I was wondering, what would be the best impedance selection on the Chird Huei phono stage for this cartridge? 100 ohm? 470 ohm? Or something higher?
     
  15. Ralph Karsten

    Ralph Karsten Forum Resident

    Location:
    St. Paul MN
    Always use the start out with the highest value. As the loading value is reduced, so it compliance of the cartridge. Its compliance is measured with a 47K load, so this can affect the mechanical resonance of the pickup system (arm and cartridge).
     
    vo_obgynmd likes this.
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