The effect of coil impedence on frequency response

[rattlesnake]

If coils are used in speaker crossovers as low-pass filters, why don't the coils in phono preamps and in speaker motors also limit audio bandwidth? Is it because their inductance is too low to make a difference? Does impedence matching (between the cartridge and the phono preamp input) affect whether the phono cartridge coils start filtering out high frequencies?

 

[AnalogSteph]

If coils are used in speaker crossovers as low-pass filters, why don't the coils in phono preamps and in speaker motors also limit audio bandwidth?

But they do!
Le 0.8 mH

VISATON BG 20 - 8 Ohm - 8″ Full-range 8Ω

Technical parameters and comparison of the VISATON BG20-8Ohm 8″ Full-range 8Ω · Resonance frequency 38Hz · Power handling 70W · Sensitivity 92dB · Xmax 2mm

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Le 2.2 mH

VISATON W 200 S - 8 Ohm - 8″ Woofer 8Ω

Technical parameters and comparison of the VISATON W200S-8Ohm 8″ Woofer 8Ω · Resonance frequency 30Hz · Power handling 115W · Sensitivity 88dB · Xmax 3.5mm

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(Responses shown are calculated from TSPs and will be modified by diaphragm breakup and whatnot)

Does impedence matching (between the cartridge and the phono preamp input) affect whether the phono cartridge coils start filtering out high frequencies?

Absolutely. The cartridge is basically a series R-L (and very much so inductive in the case of MMs - think roughly 750 ohms + 450 mH), and cable + amplifier input impedance is a parallel R||C, so you get an RLC lowpass response. This tends to be overlaid with losses in the cartridge coil increasing with frequency.

You can reduce the input R if you cannot get treble peaking under control with a given input capacitance, but assuming that you have tuned the response maximally flat, the more capacitance the earlier the treble dropoff will occur - as you would expect. Conversely, if you manage to keep capacitance low, you can afford increasing input resistance which extends bandwidth and slightly reduces noise along the way. Generally about 100-150 kOhms tends to be the practical limit. You'd have to go to extreme lengths like incorporating a tiny buffer circuit at the headshell (much like an active guitar pickup), and then the whole shebang becomes cartridge type specific. Swappable headshells with electronics for MM or MC carts... yeah, maybe.

 

[rattlesnake]

Fascinating - thank you!!

 

[DVDdoug]

the more capacitance the earlier the treble dropoff will occur - as you would expect.

That's what I used to assume... But the tuned resonance is usually a bit above 20kHz, creating a boost. Higher capacitance brings the resonance DOWN toward the audio range creating a BOOST in high-audio frequencies. That means there is an optimum capacitance and it shouldn't be too high or too low. I sort-of wish I would have known that in the analog days... (This is with a regular moving magnet cartridge.)

With speakers, the specs & measurements are with "constant voltage", so we don't care what happens to the voice coil impedance as long as it doesn't drop too-low to the point where the amplifier voltage doesn't hold-up (or go into current limiting or thermal limiting or burn-out which also means the voltage isn't holding-up).

The voice coil inductance may also affect how the crossover needs to be designed.

 

[AnalogSteph]

That's what I used to assume... But the tuned resonance is usually a bit above 20kHz, creating a boost. Higher capacitance brings the resonance DOWN toward the audio range creating a BOOST in high-audio frequencies.

Correct. That's the treble peaking I was referring to. When talking about bandwidth, I meant the eventual dropoff that follows above that. Maybe I should have put more emphasis on the fact that

assuming that you have tuned the response maximally flat

is a pretty big if. It means that parallel R is always assumed to be just the right value to obtain said maximally flat response. Level playing field and stuff.

In practice a bit of >10 kHz peaking is often accepted in favor of higher bandwidth.

 

[wwenze]

As with anything science and audio (and pseudoscience) related

- You learn about an effect e.g. moon gravity can affect your weight

- But don't forget to learn the maths to calculate the extent of that effect