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Dissecting 20-year-old outdoor speakers (SpeakerCraft OE8 One), found this electronics board inside. I recognize the capacitors, but what are the big custom coils of copper wire, and what is their purpose within the circuit? The speaker enclosure is being fed an amplified audio signal only (no AC power), and splitting the signal to three separate speaker drivers (tweeter, mid-range, 8" woofer). Are these filters?

Electronics board with a few capacitors, and five separate coils of thick copper wire wrapped in cylinders of various sizes.

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2 Answers 2

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Are these filters?

Yes. Also called "cross-overs" in the audio industry. They split up the output voltage from the amplifier into separate frequency ranges to be converted to sound by separate speakers. This is necessary because it is very difficult to design & build one speaker cone to cover the entire audio range, 20Hz to 20kHz. Here is a typical schematic for a three-way cross-over:

enter image description here

Image source:
https://www.stereonet.com/forums/topic/301381-designing-a-3-way-speaker-crossover-and-cab-talk/

In the case of the three-way cross-over (as per the diagram above), the three speakers serve three different frequency ranges:

  • Low, typically from 20Hz up to ~300Hz;
  • Mid, typically from ~300Hz up to ~3kHz;
  • High, typically from ~3kHz up to ~20kHz.

The exact start and stop frequency for each speaker can vary over a wide range depending on the make/model of the system. Some reasons for this variance are technical (depending on the speaker drivers selected, the dimensions & materials of the speaker box, and the speaker box designers' design rules and preferences), while other reasons are commercial (prices, availability of materials and components).

The coils you see are called "inductors", they are represented on the schematic by curly coils. Sometimes these wire coils are wrapped around a core material to increase inductance value, some of the inductors in the photo seem to be like this.

EDIT: Why is the polarity of the MID range speaker inverted?
@fraxinus posed this question (paraphrasing):
why is the MID range speaker polarity reversed compared to the other two?

I suspect it is because the MID range filter is a band-pass filter, comprised of a high-pass (C2 & L2) followed by a low-pass (L3 & C3); whereas the other filters are either hi-pass for the tweeter (C1, L1) or low-pass for the woofer (L4, C4).

Out of curiosity, I simulated the schematic posted, using the values of Cs, Ls, and Rs (speakers) as listed. The mid-range filter has -3dB points at:

  • 455Hz (phase = +73 degrees, ie: leading),
  • 3.4kHz (phase = -73 degrees, ie: lagging).

Let's examine how the phase of the MID filter interacts with the other two filters:

  • The filter for the bass speaker starts at close to zero phase shift and goes to -90deg (lagging) at the cross-over F (455Hz), which clashes with the leading phase of the mid-range filter.
  • The filter for the tweeter is at zero phase shift at 20kHz, and goes to +90deg (leading) at the cross-over F (3.4kHz), which clashes with the lagging phase of the mid-range filter.

But if the MID speaker polarity is reversed, then in terms of sound waves, these phase shifts become reversed, so -107 degrees lagging at the low end, and +107 degrees leading at the high end; these phase shifts are now (relatively) in-phase with the sound waves produced by the other speakers.

If we assume the total sound of all three speakers combined can be modelled by summing the voltages across the three resistors, we find that reversing the polarity of the MID range speaker results in a much flatter (better) amplitude vs frequency graph.

Legend: Top chart (Purple trace): MID speaker polarity is reversed.
Second chart from top (red trace): MID speaker polarity NOT reversed.

enter image description here

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    \$\begingroup\$ An addendum: The core material mentioned in this answer will typically be a blackish ceramic called "ferrite", or sometimes laminated iron (not usually for anything operating higher than mains frequency, though). \$\endgroup\$
    – Hearth
    Commented May 6 at 0:02
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    \$\begingroup\$ @fraxinus Without reversal, the mid-range filter is leading at the low end (cross-over with woofer), and lagging at high end (cross-over with tweeter). These phases are opposite to the phase shifts of the other speakers, so the MID will tend to cancel the sound from other two speakers at these cross-over regions. With MID speaker polarity reversal, these phases swap polarity, so MID will then re-inforce the sound in the cross-over regions, resulting in better (flatter) dB vs F. \$\endgroup\$
    – Fabio Barone
    Commented May 7 at 5:05
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    \$\begingroup\$ @Karl The Simple answer is yes, the design of cross-over filters is non-trivial. Consider what is being done here: designing and building a set of passive filters to drive multiple speaker-cones drivers from one voltage source, to give the optimum audio response when they are combined into a single speaker system; optimum meaning having a frequency response that (a) is flat with regard to amplitude, and (b) is linear with regard to phaseshift (= constant time delay). And yes, the characteristics of the individual speaker cones is a crucial factor. \$\endgroup\$
    – Fabio Barone
    Commented May 8 at 1:55
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    \$\begingroup\$ The realisation of the passive design is another factor: building a passive cross-over from passive elements (L, C, Rs) is a whole other problem. How to realise an inductor whose L value is stable with temperature, age, and excitation? Not trivial. Which is why active cross-overs are worth considering: each speaker cone has its own dedicated power amplifiers, and the filtering is done at the low power signal level. Much more predictable results are obtained in this way. \$\endgroup\$
    – Fabio Barone
    Commented May 8 at 2:00
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    \$\begingroup\$ And the amplifiers can be designed to suit each driver, eg: sub-wooder amplifiers are very different to tweeter amps, for example. Also, the characteristics of the cross-over filter can be easily changed or even adapted by the user, for example, to better suit the acoustics of the listening room, or to adapt to replacing one of the speaker-cones. \$\endgroup\$
    – Fabio Barone
    Commented May 8 at 2:01
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Are these filters?

No, technically those components are inductors, but they way they are used here makes them a core (no pun intended) part of the filters discussed by others.

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