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Volume settings for unpowered speakers

If a computer needs external sound, the usual advice is to connect active (powered, amplified) loudspeakers, which These disadvantages can be worked around by finding a pair of passive (unpowered) speakers designed to connect to the 3.5mm jack, but:

Passive Hi-Fi speakers instead

If you find a pair of passive Hi-Fi type speakers (larger than typical computer speakers but still small by Hi-Fi standards), you may wish to try connecting these for a better frequency response (especially in the bass) and probably better performance at volume. But check the characteristics.

(You no longer have to worry about "magnetic shielding" around computer speakers unless you still use an old CRT or FDD---magnetic hard disks have their own shielding, and damaging them takes something stronger than a speaker magnet. But I wouldn't keep a speaker right on top of a hard disk.)

Typical onboard sound circuits are rated to supply a headphones socket with up to 2 watts before the onboard amplifier starts distorting due to saturation. This is is typically at 1.4V RMS (=2V peak, =4V peak-to-peak), although the Raspberry Pi reportedly uses the lower "line level" standard of 1V RMS and I don't know what its current-limiters are set to.

I'm not sure if the "2 watts" figure for PCs/Macs is total or per channel---assume 'total' to be safe, so 1 watt per channel---but I expect it won't include the power dissipated in the amplifier itself, as this is known from its own impedance regardless of the load's, so if things are behaving as specified we just have to set V2/Z < 1 i.e. Z > 2 to ensure no saturation. In the unlikely event that your speakers are under 2 ohms, you'll have to compensate by reducing the maximum volume so that the peak voltage (normally 2) does not exceed the ohm rating (1 watt per channel does make the maths easier).

Actual listening loudness in decibels = SPL + 3*log2(V2/Z) - 6*log2(distance in metres), where SPL (sensitivity) can be 84 for low-end and 90+ for high-end speakers. If the only thing you want is loudness then it's probably not worth investing in speakers with higher SPL: a 16-ohm set with SPL=90 would give the same 3dB increase as would connecting two of the 8-ohm SPL=84 sets in parallel over a Y-splitter (presenting a 4-ohm impedance to the PC, which is still above its lower limit of 2), and this latter option is almost certainly cheaper. But the "good" speakers might have better frequency response.

Hi-Fi type speakers are not typically fitted with 3.5mm jacks, so, unless you are able to make up your own connector, you'll need something like:

RCA connectors are coloured red for the right-hand speaker, and a small Philips screwdriver is needed for securing the screws in an RCA-wire connector---"Blu-Tack" may be useful to hold in the wires if the screws are too stiff to turn on a low-quality connector (school physics experiments on the electrical conductivity of "Blu-Tack" showed it's variable across different batches, but at room temperatures and low voltages it should provide high enough resistance to count as only a small reduction in load impedance if a speaker's terminals both touch the same lump of tack, as long as the stripped parts of the wires don't touch each other directly), but you might find a Blu-Tack'd connector is easily disturbed by vibrations leading to a loss of volume, so it's better to order two pairs of connectors so you have spares. The red wire connects to the + terminal, but many speakers will work either way.

You shouldn't have to worry about a maximum volume level if the Hi-Fi speakers' impedance is 8+ and they are rated to be able to take a current of 5 watts or above, although of course you may still wish to keep it down when the neighbours are at home! (If you are in an upstairs flat, then try to avoid putting loudspeakers on the floor.)

Disclaimer: the above notes are provided in the hope that they are useful, but what you do with your sound is at your own risk---I will not take legal responsibility for damage to equipment, hearing, community etc.


All material © Silas S. Brown unless otherwise stated.
Blu-Tack is a trademark of Bostik SA.
Mac is a trademark of Apple Inc.
Raspberry Pi is a trademark of the Raspberry Pi Foundation.
Any other trademarks I mentioned without realising are trademarks of their respective holders.