Why does impedence matching matter for phono signals?

[rattlesnake]

I'll start with a question about impedence in a phono cartridge. If a mm cartridge is rated at 47,000 Ohms, is that the same as the unit's "output impedence," and is that also the impedence of the coils in the unit that generate the audio signal? Please correct me if any of this is wrong.

Given all this, why does it matter for a phono cartridge to have a similar impedence to the imput impedence of the phono preamp?

 

[sergeauckland]

There's no impedance matching for a phono cartridge, MM or MC. The input impedance is some 5-10x the output impedance of the cartridge.

Most MM cartridges are designed for an impedance of 47k, which is therefore what most phono stages provide.

If one wants to optimise an MM cartridge, then one needs to adjust both the capacitance and resistance, but that's unusual on many phono stages, if there's any adjustment provided it's capacitance only.

MC cartridges have the advantage of being pretty immune to loading, as long as it's 5-10x the coil resistance.

S

 

[AnalogSteph]

MM cartridges are severely inductive sources - imagine 750 ohms in series with 400 mH or so - and as such are quite touchy when it comes to load impedance (resistance and parallel capacitance), as impedance rises into the tens of kOhms in the treble region. The resulting overshoot in the response (RLC circuit) is commonly used to prop up the treble response which often is naturally drooping due to increasing losses.

 

[JP]

MC cartridges have the advantage of being pretty immune to loading, as long as it's 5-10x the coil resistance.

Even 1:1 or worse an LOMC is rather immune aside from output level.

 

[Angsty]

There is an argument to be made that MC phono impedance is not critical except to damp ultrasonic resonances - 100-150 ohms is typically sufficient. People do have preferences, though.

Post in thread 'Michael Fidler Spartan 20 Phono Stage Review'

Apr 7, 2025

Thanks for busting the "stylus harder to move" myth for me

Can you comment on MC loading in general. My experience is that loading at ~100 ohms works fine in most cases. My experience is also that changing the load impedance of an MC cart does little if anything to the FR even if taken to relative extremes.

From the manufacturers perspective: easy to implement with little electronics knowledge - you just need to switch in the components that amount to whatever you want to write on the switch face. It adds perceived value for little effort/allows the manufacturer to claim...

• Michael Fidler

• 

 

[DVDdoug]

This relates to normal moving magnet cartridges...

Capacitance is more of an issue than resistance. There is capacitance in the internal tonearm & turntable wiring, capacitance in the wires between the turntable & preamp, and the preamp usually some capacitance on it's input. Coax cable has a certain capacitance per-foot so occasionally the capacitance from the interconnects can be known and the input capacitance of the preamp is sometimes specified and sometimes adjustable-selectable, but the total capacitance (the sum) is usually unknown.

The cartridge manufacture usually specifies 47K and some pF capacitance and there is an optimum capacitance. It can be too high or too low.

The cartridge's inductance along with the capacitive load causes the opposite effect compared to "normal" resistive sources. Normally with a more-resistive source, "excess" capacitance causes high-frequency roll-off.* But the inductance and capacitance creates a resonant circuit that makes a boost, usually somewhere above 20kHz. As the capacitance increases, the resonant frequency comes-down toward the audio range and causes a boost in in the highs. (Longer cables can boost the highs!) ...I didn't know this in the analog days and I just assumed lower capacitance was better.

It's really awesome that we don't have these analog problems anymore!



* Capacitance with line-level signals is virtually never a problem because of the low source impedance. A long cable run between rooms or buildings MIGHT present a problem.