2

Modern speakers have permanent magnets and are called PM speakers. Permanent magnet materials technology has improved a lot over the last 80 years. Around WW2, electromagnet (EM) speakers were common because permanent magnets were not as good as they are today.

The old school EM speaker had the usual voice coil and a wound electromagnet where the permanent magnet is today. The electromagnet used a lot of power - .5 to 10 watts being typical. This DC power burn was often more than the audio output of the radio.

Mains radios had electromagnets wound from many turns of very thin fragile copper wire giving a coil DC resistance of about 1000 to 2000 ohms. These coils would fail open circuit meaning that you won't see many of these today.

Car radios would also use EM speakers with low resistance EM coils to match to the 6V battery. The car radio EM speakers were more reliable despite the high temps encountered in a car.

The audio amplifier output power of these old radios was often lousy by modern standards, like 3 watts. The old radios with EM speakers sounded loud despite the low audio power.

Are EM speakers more efficient in terms of dB per watt than PM speakers?

If a PM speaker is installed in place of an existing EM speaker that has died, how much more audio power is needed for the same sonic result?

JRE
  • 71,321
  • 10
  • 107
  • 188
Autistic
  • 14,616
  • 2
  • 28
  • 65
  • 1
    They may be better in terms of dB per watt applied to the voice coil if the field generated by the field coil is stronger than the permanent magnet would produce, but the field coil is a static power consumption, so overall efficiency of the system would still be lowered by that, especially at low outputs. – Phil G Feb 22 '19 at 21:14
  • On the radios with EM speakers that I have encountered the EM power Exceeds The voicecoil power .If a PM speaker is installed in place of an existing EM speaker that has died how much more audio power is needed for the same sonic result? – Autistic Feb 22 '19 at 21:39
  • 1
    If you copy over your text into your favorite word processor, can you see it underlines every single period? This is because space comes AFTER period. – winny Feb 22 '19 at 22:36
  • 2
    In the old radios, the huge EM for the speaker was often part of the DC_smoothing filters of the high-voltage plate supply. – analogsystemsrf Feb 23 '19 at 04:47

2 Answers2

7

Permanent magnet materials were expensive, and not all that permanent, back in the 1930s, capable of losing their magnetic strength under shock or other abuse, or even heat - which was common in a valve radio. This made the electromagnetic field generator a cheaper and more reliable option.

But that's not the real reason they were so ubiquitous.

The HT power supply for the radio (about 250-350V at 50-100mA) needed low pass filtering after rectification, to eliminate mains hum (ripple voltage). And large value high voltage electrolytic capacitors were expensive.

The solution was a choke - a large inductor in the 10 to 100 Henry range, forming a low pass Pi filter between 2 relatively low value (10 to 30 uF) capacitors.

And the most economical place to wind that choke was ... the loudspeaker's field coil, where the DC component of the rectified current also provided the speaker's magnetic field.

So, far from being wasteful, the EM loudspeaker could do 2 jobs at once.

(Oh, and don't underestimate 2 or 3 watts in a nice large 8 or 10 inch paper cone loudspeaker. They still are plenty loud enough)

To answer the question in the comment : ( add it to the Q please!) because it sounds like "how do I maintain such a radio?" is the real point of the question:

On the radios with EM speakers that I have encountered the EM power Exceeds The voicecoil power .If a PM speaker is installed in place of an existing EM speaker that has died how much more audio power is needed for the same sonic result?

if you replace the speaker with a PM driver with the same impedance, cone size AND WEIGHT and suspension compliance, you'll get a similar result.

Cheap paper cones might be a better match than high end hi-fi exotic cone materials where frequency response and controlling the breakup modes is more important than acoustic efficiency.

As a good first step, look for the same cone size, and then the look for a high "acoustic efficiency" in the datasheet. You should find values up to 90dB/watt (@ 1 metre) but don't worry about the last 2 dB or so.

  • And don't overestimate the difference between listening to a loudspeaker convincingly turning 300 W into sound and the same listening system with a 3 W signal. – greybeard Jan 19 '23 at 09:20
1

An old AM radio had no bass sounds and had no high frequency sounds so they made the speaker shriek noises at a high sensitivity.

Audioguru
  • 1,218
  • 6
  • 5
  • 1
    This may be more related to the audio bandwidth of the AM channel and the other effects caused by modulation/demodulation, etc. Also, it is not an answer to the question as far as I can see. I am fearing you may receive some down-votes because of that. – user57037 Feb 23 '19 at 01:42
  • If by "old" you mean 1970s transistor radios with 2 or 3 inch speakers. But this certainly doesn't apply to 1930s radios with speaker cones bigger than most modern stereos. –  Feb 23 '19 at 13:03