Dexter Industries Raspberry Pi Speaker

General Discussions
1

I purchased a Dexter Industries Raspberry PI Speaker and it does not have a compatible plug for the Raspberry PI. PI = 4 conductor 3.5 mm, Speaker = 3 conductor 3.5 mm. This does not seem to be compatible.

2

It is compatible. The A+, B+, Pi 2, Pi 3 and Pi 4 all use the same 4 pole socket. (The fourth contact is for video.)

3

I understand that all Raspberry Pis use a 4 pole connector. Why does the speaker the speaker use a 3 pole connector?

4

Until a DI engineer responds to you: My guess is to mix the left and right audio channels for a mono speaker? (Ground, Left, Right)

Perhaps you could test by:

aplay /usr/share/sounds/alsa/Front_Center.wav
aplay /usr/share/sounds/alsa/Front_Left.wav
aplay /usr/share/sounds/alsa/Front_Right.wav

btw: getting your audio setup can be interesting also. This is how I test mine:
=== first see if audio output works the way you want (auto) ===
Test audio out:
aplay /usr/share/sounds/alsa/Front_Center.wav

===== If no sound: Set output device for desired audio output device (0=auto, 1=analog, 2=HDMI):
amixer (to see current setting)
sudo amixer cset numid=3 1 (set to output sound to 3.5mm jack)
(up arrow till) aplay /usr/share/sounds/alsa/Front_Center.wav

========== If audio output is too soft =======
if soft: alsamixer
s (select card)
downarrow to “0 bcm2835 ALSA”
8 (4=default loudness, 8=100%, 9=125%)
fn-F6 dn to USB Audio Dev
8 for 90% then up-arrow for 100%

      Escape (press Esc)

then make setting default:
sudo alsactl store 0

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5

The three poles (sleeve, tip and ring) are for:

Sleeve: Common return for both speakers, usually the speaker’s “-” terminal.

Tip: Right channel “+” terminal.

Ring:. Left channel “+” terminal.

The fourth pole, (a second ring contact), was used on pre Pi-4 boards for composite video. The Pi-4 abandoned composite video in favor of an additional HDMI port.

Note that there is no mixing done here. If you plug in something that isn’t stereo, you’ll only get one channel - usually the right channel - unless you configure the sound settings to down-mix to mono.

In the case of the Dexter speaker, I presume it’s only one speaker, right? If that’s the case, then they probably just wire both tip and ring together to create a mono-like experience.

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6

One additional caution here:

Many modern amplifiers use MOSFET pairs - one “N” type and one “P” type, wired in series between the positive and negative power rails, to provide the needed positive and negative power swing to drive the speakers. In some cases a set of transistor pairs, (similar to a “H-bridge”), are used to provide additional voltage and current swing across both ends of the speaker, providing increased power output.

Because these MOSFET devices are low-impedance devices - unlike vacuum tubes - no output transformer is needed to match the impedance of the output dewvices to the speaker.

In some cases, the MOSFET devices are wired directly to the speaker itself.

Because of this, it can be unwise to directly connect the left and right channels together, for if the right channel is driving high, and the left channel is driving low at a particular instant in time, the result is a direct short between the two power rails through two low-impedance transistors.

If the output transistors are not protected against over-current, (or are not isolated through electrolytic coupling capacitors), this can destroy the amplifier’s output stages.

Many modern amplifiers do protect against this, but it’s not universal and cannot be taken for granted.

A better way to do this is to use isolating resistors - a pair of 5kΩ resisters with one end attached to each of the two right-and-left channel leads, the other ends joined, and a small amplifier used to drive the one speaker you’re going to use. This value is not critical. Larger resistors provide better isolation, but reduce the output signal. Larger resistors are also needed on higher power amplifiers to reduce the current load.

For example:

  1. If the outputs are “single ended” (most headphone jacks are wired this way, with one wire for the right channel, one wire for the left channel, and a ground wire), connect a wire from one resistor to the right channel lead and another resistor of the same value to the left channel lead. Join the other ends together and amplify the signal between there and the ground lead.

  2. If the outputs are not single ended, (having separate positive and negative speaker wires for each channel that are not tied to ground), you need four resistors of the same value. Attach the two positive speaker wires to two resistors as noted before. Tie the two negative speaker wires to the other pair of resistors, joined at the center like the positive pair were. Attach the center of the negative pair to the amplifier’s negative input terminal, (usually ground), and the center of the positive pair to the positive input terminal.

  3. If, in either case, you get a lot of hum in the output, tie the circuit ground of the amplifier to the chassis of the device you’re mixing the signals from.

Many amplifiers provide a “line out” jack, or a headphone jack. If this is the case, use this jack for your signal rather than the actual speaker leads.

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