Phono Preamp and Cartridge selection

Further, with LOMCs you need to get into impedance matching. Unlike MMs and HOMCs, which are all designed to play into 47kOhms, every LOMC cartridge performs best with a different impedance at the phono inputs. The optimal impedance for any given LOMC is further impacted by all the downstream components in the system. This requires adjustability and/or custom matching of phono stage to LOMC.

This statement is incorrect! (and is a common myth): nearly all LOMC cartridges load perfectly into 47K which is the standard input impedance of nearly all phono preamps. The 'loading' issue only exists if the **phono section** has troubles with Radio Frequency Interference!

The reason for this is that the inductance of a LOMC cartridge is quite low- so it will not ring (distort) at audio frequencies. However, its inductance combined with the capacitance of the tone arm interconnect cable forms a tuned radio frequency circuit- that is energized by the cartridge itself.

If you find that the loading value of the cartridge is critical, it means that the phono preamp is having trouble with the RFI thus generated. The loading value, if set correctly, detunes the RF circuit.

RFI does strange things when it gets into audio circuits. Its best if the audio circuitry is immune to RFI. To this end, FWIW, transistor phono sections are more susceptible to RFI because the diode junctions within the semiconductors can easily rectify RFI. Special consideration has to be done on the part of the designer to avoid this problem, but you would be amazed at how many people simply don't know that RFI is at the root of the loading issue!

Count me among them. Very interesting, Atmasphere.

Ralph,

Even if that's true (I lack the electrical expertise to discuss the point), my personal LOMC listening indicates that loading at 47K is optimal only with a minority of phono stages... typically, higher end units at higher end prices.

In most cases, including some all tube + SUT setups, showing the cartridge a lower load consistently resulted in better balance between the frequency extremes.

Perhaps your statement is true of better designs (like yours, I trust)? One might suppose that some of the "many people [who] simply don't know that RFI is at the root of the loading issue" may be phono preamp designers!

Hello Doug, it is indeed true and I am not the only one to say so by any means. Talk to Stig at Lyra and he will tell you the same thing and has posted to that effect on this forum.

You are correct that a low impedance is what is needed to detune the RF circuit. Also, SUTs don't pass RFI very well at all, so they can help too. But I find that SUTs also impair detail, so I prefer to do without them if I can.

You are also correct that only a few phono sections don't need the loading (obviously ours are examples in that regard), the thing is, its easy and cheap to fix this problem but as you surmised most phono preamp manufacturers (and quite well-known at that) don't even understand what is causing the problem so they are not in a position to fix it. So the myth survives quite easily.

One note- high output MM cartridges are different in that they are very much affected by loading at audio frequencies! But oddly enough, you hardly ever hear about anyone doing it, which is really too bad since many MM can easily keep up with much more expensive MC cartridges once the loading issue is addressed.