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I made a wood frame for 2 speakers, intending to create my own 2x12 cabinet, but now i realised my speakers are 8Ohm and 4Ohm, they don't match, meaning i would get a weird value when matching them.

I thought about a series resistor with the 4Ohm to make it 8Ohm, then i can either wire them in series to make 16Ohm or parallel to make 4 Ohm.

What could possibly go wrong with this?

  • I know it must be a power resistor, i have one rated 200W (way more than any amp i will conenct there)
  • I am aware that the speaker connected to the resistor will probably have a volume loss compared to the other one.

What I'm asking is, can it damage the amp?

Thank you

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First of all, if you have a choice (and why wouldn't you?), avoid an impedance mismatch. So my advice would be to get one more speaker, either 4 or 8 Ohms to get an impedance which you can match. Then you don't need to worry, and if your amp ever dies on you, you won't have that nagging feeling that you should have known better ...

Having said that, and if you really want to use those two speakers, then I wouldn't use a resistor, because it's a shame to use amp power for heating your cabinet. I'd say it's probably most reasonable to connect the two speakers in series, which gives you 8+4 = 12 Ohms. Then you can either choose the 8 Ohms setting on your amp, or use the 16 Ohms setting, because almost all amps can handle a slight impedance mismatch. It depends on whether you have a tube or a solid state amp, and if you have a tube amp, it also depends on the model how sensitive it is to an impedance mismatch.

If you connect the speakers in parallel you'll get a resistance of 8*4/(8+4) = 2.67 Ohms, which is quite small, and which I wouldn't use, unless your amp has a 2 Ohms setting (I think I've seen this with some Mesa Boogies).

As a final note, it is important to realize that a speaker doesn't have a constant impedance anyway. If it is rated at 8 Ohms, then you will find frequencies at which its impedance is a bit smaller, and you'll find a wide range of frequencies where the impedance is considerably higher. That knowledge could be a motivation for not getting too neurotic about a small (!) mismatch.

  • Alright, that does it for me. I recognise that the inductive load that is the speaker is completely different from the resistive load that is the resistor. I just wanted to use what i have, but I guess I'll just get another speaker. The amp is an egnater tweaker 40. All tube, i reckon they are more sensitive and so this slight impedance mismatch can mean trouble, I won't risk it anyway, thank you – Antero Duarte Jan 29 '15 at 16:51
  • @AnteroDuarte: I think that's a good decision! – Matt L. Jan 29 '15 at 17:08
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Don't. "Impedance" means that for any sinoid voltage applied to the terminals, the effective current will not exceed the one derived by Ohm's law. But it does not make any claim that the impedance will be constant over frequency, or that the phase of the current will be the same as that of the voltage.

Amplifier standardize on the output voltage: how much current a speaker draws is its own choice.

As a consequence, the only thing you put in series with a speaker is another speaker of the same type. Or an audio crossover designed to work with this kind of speaker. Anything else, even a different speaker with the same impedance rating, is asking for trouble.

As an extreme example, piezo tweeters are a capacitive load designed to be hooked up directly with the voltage source. It is a pretty common mistake to hook them up with a branch of a frequency crossover designed for inductive tweeters. You'd think you'd be doing them a favor by giving them less range to work with (inductive tweeters will die pretty soon if you don't spare them the low frequencies), but you actually build a resonant circuit that way that excels at letting the actually rather robust piezo tweeters go up in smoke.

Of course, things can work even putting different stuff in series. But they are not likely to continue working when driven at the max of the spec, and the results may be worse than you thought for no discernible reason, both regarding frequency response and distortion as well as maximum specs.

  • I especially agree with the part about two radically different speakers driven in series will produce unpredictable sound. – Carl Witthoft Jan 29 '15 at 15:10

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