Mixing impedance

YYCPolk

I was shown a pair of amazing sounding speakers. 3 way speakers with after market woofers because the original ones were not up to par. The woofers were 4 ohm while the original speakers were rated at 6 ohm.
Is that ok to do long term? They were driven by a vintage amp so likely able to handle 4ohm.
And curious, is the woofer, being lower impedance, drawing more juice from the amp giving it better sound? Or does the amp provide the same power to all?
The speakers in question, Vivid Tx995. Cheap department store brand. I was shocked how good they sounded.

mhardy6647

So, here's the thing.
Well, there are a couple of things.
The nominal impedance of a driver is just that -- nominal.
The actual impedance of any driver (with very, very few exceptions, such as some planar loudspeakers) varies with frequency.



This is a fairly typical impedance curve for a woofer. This one would have a nominal impedance of about 5 or 6 ohms based on that graph.

Changing the woofer's impedance will alter its behavior (frequency-dependent acoustic output and phase) with respect to the crossover network, which was (i.e., should have been!) designed for the original woofer. This could put the driver and/or the amplifier in jeopardy*
There is also the matter of the sensitivity (acoustic output of the replacement driver per unit of input electrical power).
There is also the matter of the electrical and acoustic properties of the original vs. the replacement driver: typically described by a set of measured values called Thiele/Small parameters.

https://sound-au.com/tsp.htm

If you like it, and if your amplifier doesn't mind (which it may*)... knock your socks off.

________
* you'll find out, eventually, if you're asking too much of it.
Older amplifiers were generally not designed for the higher current requirement presented by lower-impedance loads (loudspeakers), simply because older loudspeakers were generally higher-impedance devices than loudspeakers have gotten to be in the last couple of decades.

YYCPolk

Thanks for all that detail. So is the issue strictly the amp's ability to handle 4ohm speakers? Does the 3 way consisting of 4 and 6 ohm drivers matter much?
And to take it in another direction, how different would it be if the woofer was 8 ohm? Would it sound weak? I take it it would at least be safe for the amp.

mhardy6647

The fundamental issue is that it is necessary either to do a like for like swap of new woofer for old (same Thiele/Small parameters and sensitivity) or redesign and rebuild the crossover to accommodate the new woofer.

AFTER that decision is made, the latter route would also require establishing that the amplfier to be used with the re-engineered loudspeaker is safely capable of operation with it (them) under the conditions in which it will be used.