Minimus 7w : 4 ohm replacement speakers / crossovers

[davstev]

Hi all,

I have a nice pair of Minimus 7w’s that weren’t working, and I also happened upon a nice pair of replacement speakers for cheap. The replacements are a perfect fit and look great too.

The new woofers are 4 ohm instead of the original 8 ohm, for which the existing crossovers were designed. Currently the sole cap in the crossover (which I think is for the tweeter) is at 4.7uF/50v. As I understand it, the original 8 ohm woofer had no capacitor in its path.

For two reasons, I need to redo the crossovers: firstly, because the non-functionality of the speakers as I found them may have been due to a faulty crossover, but also because I plan to put a 4 ohm speaker in.

Can anyone advise what an upgraded or revised crossover for this 4 ohm woofer setup would be? Is there a simple modification that I could do to the existing crossover, or build a new one from scratch? Upgraded crossover? I am inexperienced in figuring such electronic things out but good at putting it all together in the end. Feeling a bit lost at the moment. Thanks for any guidance.

(Pics below show the new woofer in place, and the original crossover.)

 

[Lavane]

That coil is for the woofer. You may need to that change that also? You will need to figure out the crossover point for the original crossover and then get a coil and cap that will give the same using a 4 ohm driver. Hopefully someone will chime in who understands the voodoo of crossovers.

 

[Stuart Pedaso]

Minimus-7 is one of those speakers that has a 2nd order filter on the tweeter while the woofer runs free.


As such, the crossover has no influence on the woofer and couldn't have contributed to its demise.

Whether or not a crossover mod for the 4 Ohm woofer is needed depends on exactly what performance issues you want to address. It's very likely the replacement will perform differently than a stock woofer. But just the fact that it's 4 Ohms doesn't necessarily mean it needs additional crossover work. Of note, the Optimus Pro X7, a later offshoot of Minimus-7, has a similar tweeter with the same crossover, but uses a 6 Ohm woofer that also runs free.

 

[swechsler]

It's very likely the replacement will perform differently than a stock woofer. But just the fact that it's 4 Ohms doesn't necessarily mean it needs additional crossover work.

The Minimus 7 was built to a price, and running the woofer wide open was one example of that. It wouldn't hurt to just install the 4Ω woofer just to see how it sounds.

You might also want to read this thread (warning: it's very long) and see if there are any mods you'd want to consider.

 

[Lavane]

The picture of the crossover the OP posted has a.coil.for the woofer, replacing the 8 ohm woofer with a 4 ohm woofer probably wont sound right.

 

[swechsler]

The picture of the crossover the OP posted has a.coil.for the woofer, replacing the 8 ohm woofer with a 4 ohm woofer probably wont sound right.

No, the coil is used with the cap for a second order high-pass crossover for the tweeter (see response #3). It's hard to see in the OP's picture, and I can't find the picture I saw before (probably in the thread I linked above), but in a better pic you can see that both sides of the woofer are wired directly to the input terminals.

 

[davstev]

Thanks for the input, guys. I gather from your input, that I can at least try the 4 ohm woofer directly connected to the terminals without worrying too much about causing damage (at normal volumes).

 

[davstev]

The picture of the crossover the OP posted has a.coil.for the woofer, replacing the 8 ohm woofer with a 4 ohm woofer probably wont sound right.

So, if the 8 ohm woofer is wired directly to the terminals, would there be any risk involved in wiring the 4 ohm woofer directly as well? Is bad sound the only possible result? And if it sounds bad, would introducing a capacitor or coil be of help for the sound? If so, what's the best way to arrive at the cap value needed? Lots of questions, I know!

 

[Lavane]

No, the coil is used with the cap for a second order high-pass crossover for the tweeter (see response #3). It's hard to see in the OP's picture, and I can't find the picture I saw before (probably in the thread I linked above), but in a better pic you can see that both sides of the woofer are wired directly to the input terminals.

Ah, ok, I see now.

 

[Stuart Pedaso]

So, if the 8 ohm woofer is wired directly to the terminals, would there be any risk involved in wiring the 4 ohm woofer directly as well? Is bad sound the only possible result?

Most risk is possibly to the amplifier, if using one that can't handle 4 Ohm impedance loads. Other than that, it's only at greater risk than the stock woofer if it's power handling capacity is less and you throw too many Watts at it.

And if it sounds bad, would introducing a capacitor or coil be of help for the sound? If so, what's the best way to arrive at the cap value needed? Lots of questions, I know!

There are multiple aspects of speaker performance that can contribute to "bad sound" in different ways. Steps to solving these issues involve: identifying specifically what's bad about it, figuring out the cause and then designing a fix.

The stock Minimus-7 has its own demons which, though recognized early on, multiple efforts spanning decades never completely tamed. The thread swechsler linked to in post #4 chronicles that victory finally achieved here at AK. It involved lots of study, measurement, testing and skills contributed by quite talented members.

It's always worth exploring improvements that modding might bring. But one should expect the solution may not be trivial.

 

[kevintomb]

The stock minimus 7 woofer is neither bad nor good. It is okay. It is used in a tiny box.
It runs free and that helps the price be a bit lower and allows the box to be as tiny as it is.

With a good crossover, (a cap and coil on woofer ALSO) you end up having barely any room in the tiny box.

Another woofer may sound better in some regards and worse in others.
4 ohms does not really matter.
The tiny box is sort of the main issue.

I have owned similar speakers, with 4" woofers, that were in larger boxes, and designed to work in larger boxes, that were vented that were FAR better bass wise than the minimus 7, but that is the main appeal, how small they are I guess.

 

[davstev]

The stock minimus 7 woofer is neither bad nor good. It is okay. It is used in a tiny box.
It runs free and that helps the price be a bit lower and allows the box to be as tiny as it is.

With a good crossover, (a cap and coil on woofer ALSO) you end up having barely any room in the tiny box.

Another woofer may sound better in some regards and worse in others.
4 ohms does not really matter.
The tiny box is sort of the main issue.

I have owned similar speakers, with 4" woofers, that were in larger boxes, and designed to work in larger boxes, that were vented that were FAR better bass wise than the minimus 7, but that is the main appeal, how small they are I guess.

I agree. In fact, because my 4 ohm woofer extends a couple of centimetres deeper into the cabinet, it now touches the crossover. I think my 7w (wood) might even be a bit shallower than the metal cabinet Minimus 7.

Rats.