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I'm in the process modernizing a 1960s Magnavox Astrosonic stereo console. I'm wanting to keep the speakers as they look to be in great shape. It's got two 12 inch woofers (581206-1) that test (via DC multimeter) at 4.1ohms so I assume the nominal impedance is 4ohms. It's also got two horns (580069-1) that test at 13ohms so I'm assuming they are 16ohms nominal.

I've also got a receiver / amp with these specs:

  • MAX Power Output: 200 Watt @ 4 Ohm
  • RMS Power Output: 100 Watt x 2 @ 8 Ohm
  • Tone Control: Bass, f=100Hz +/-8dB
  • Frequency Response: 20Hz - 20kHz
  • Frequency Adjustment: +/-10dB
  • Signal to Noise Ratio: >71dB
  • T.H.D.: <0.1%

I know I could just put a capacitor in front of the tweeter and wire them in parallel. Would that be safe with my amp and/or sound decent or should I get a proper crossover? If I need a proper crossover what should I buy. The speakers having different impedances is throwing me off.

Clearly, I don't know what I'm doing.

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    \$\begingroup\$ Impedance != resistance. In most cases, its not even close. \$\endgroup\$
    – JRE
    Commented Apr 4, 2018 at 15:45

2 Answers 2

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I'm wanting to keep the speakers as they look to be in great shape.

I don't think they are. The paper and plastic of the membranes becomes brittle after 50 years. One boom and they are gone.

While you could use a passive crossover, this isn't state of the art any more (was it ever?) for horn tweeters – because of the impedance mismatch, you wouldn't get a decent volume out of them, and the capacitor makes it even worse. They should get their own amplifier with matching impedance.

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  • \$\begingroup\$ So your recommendation would be to purchase individual amps and adjust treble/base to filter the highs and lows. Even then you think the speakers may not be in great shape. That doesn't sound like a wise use of money. Given that, I'm inclined to just to buy a cheapish pair of bookshelf speakers and hide them in the console. \$\endgroup\$
    – Joseph Yancey
    Commented Apr 4, 2018 at 17:05
  • \$\begingroup\$ No, you should check the speaker case. That one may be worth saving if it's made from thick chip tray and old reliable glue. Try find speakers which match the holes you have. Modern tweeters are also available as 8Ω impedance, check if you find one which you could mount on the horn you have. Then you could use a simple passive crossover with an 8Ω bass speaker. \$\endgroup\$
    – Janka
    Commented Apr 4, 2018 at 17:44
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You would still run into issues. This is not as simple as 2 quick answers are painting it. There are several ways, you could go with active crossovers that would require separate amps for each speaker set(Bass, midrange and treble(horn)). You need to read up on passive crossovers, the different slope types as well. You will end up with phasing and timing issues if just pay no mind to the rolloff slope vs the natural rolloff of the speaker.

Speaker sensitivity plays a huge role here too. That is why they had a 16 ohm speaker, way less draw, and a 4 ohm woofer, being the woofer is much less sensitive so needs more power to drive them to the same volume as a more sensitive speaker. On some crossovers, you will see attenuation in the circuit meaning you can raise the volume that is going to the woofer or lower going to tweeter by -2,-4,-6 so on so forth.

An Awesome 15 inch woofer is going to 95db or so. A horn is going to much higher. Match speakers carefully with natural rolloff vs the crossover. Less filter is more, but 1st order is not enough in a horn set-up. Unless your name is Paul Klipsh, trust me, you arent making a first order crossover work with modern speakers.

Hope this helps, points you in the right direction.

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