Does a 4 ohm driver with a 4 ohm resistor become a 8 ohm driver ?

[maxime.levesque]

Most drivers come in 4 and 8 ohm version, is there an audible difference between a 4 ohm version that has a 4 ohm resistor (lets assume it's a high quality resistor), and the 8 ohm version of the same driver ?

 

[solderdude]

Yes, there will be an audible difference and also will appear that way to the amplifier.
You will lose 6dB, have a tonal change and poor damping factor (woofer).
Better to put 2x 4ohm driver in series when you only have 4 ohm and need a load of 8 ohm.

 

[voodooless]

Better to put 2x 4ohm driver in series.

Without any further context, I would not right out make that recommendation. Would be good to see what @maxime.levesque is exactly up to?

 

[charleski]

Make sure the resistor can handle the power. You’ll need one with a wattage rating that’s at least half your amp’s power. And of course all this power is now going into the resistor rather than the speaker, so it’ll be down by 3dB. I’d be surprised if there weren’t some audible difference, as your speaker is now seeing an amp with greatly increased output impedance. It might be better to use two resistors in an L-pad arrangement, but crossover design is one of those Dark Arts I’ve never really looked at properly.

 

[solderdude]

And of course all this power is now going into the resistor rather than the speaker, so it’ll be down by 3dB

down 6dB as it receives half the voltage (-6dB) and the current is halved as well (-6dB) so 1/4 power = -6dB

 

[tomtoo]

Its hard to imagine a use case for doing this?

 

[charleski]

down 6dB as it receives half the voltage (-6dB) and the current is halved as well (-6dB) so 1/4 power = -6dB

If you leave the voltage output from the amp unchanged then yes, doubling the load impedance will result in the amp producing half the power, and then half of that gets wasted. If you match power output in the two conditions then the speaker is 3dB down.

 

[egellings]

Nope!

 

[maxime.levesque]

Thank you all for the great answers, DIY communities are awesome, and this one in particular !


I was contemplating a violin amplifier concept, that would be based on a single channel amp feeding a pair of Faital 3FE22-3 divers
https://www.parts-express.com/FaitalPRO-3FE22-3-Neodymium-Professional-Woofer-16-Ohm-294-1102

But before ordering them, from what I understand, I could chose two 4 Ohm and wire them in series, or two 8 Ohms in paralel, and (from my understanding there is no difference between these two choices).


I was contemplating an enclosure design, where the two drivers have their own dedicated sealed enclosure that can be either attached together, or detached.

Detachability allows to change the placement: array mode, at a distance with angle etc,

And the detachability has another advantage for traveling: a single enclosure takes half the space in luggages !

When writing this thread I had an 8 Ohm amp in mind, and thought that in this case, there could be a problem while operating in single driver mode, if I go with 4 Ohm drivers.

But after reading the replies, I went back to read the specs of the amp I am planning to use: https://www.allo.com/sparky/volt-plus-amp.html

and saw that the secs mention the amp can drive "from 4 to 8 Ohms".

So I suppose I could chose either 4 or 8 Ohm drivers, and wire them accordingly, i.e. :


Choice A: 2 x 8 Ohm ->
dual driver mode: drivers are in parallel, the total load is 4 Ohm
single driver mode: the load is 8 Ohm

Choice B: 2 x 4 Ohm ->
dual driver mode: drivers are in series, the total load is 8 Ohm
single driver mode: the load is 4 Ohm


Now I am wondering which choice is better.

Is it better to have 4 Ohm in single driver more because when there's a single driver you want to be able to feed it a bit more power ?

Another choice, would be to take a two amps (of the same kind) : https://www.allo.com/sparky/volt-plus-d-amp.html

and only use a single amp while in single driver mode...

My knowledge is insufficient to reflect on which choice is best.



Note: there will be a preamp between the violin mic and the amp

 

[voodooless]

Several things here:

First off the amp. Usually for mobile designs, go for a lower impedance. Most modern amps work just fine with them (the one you choose as well), and it will give you more output power.

Next the drivers: I would not use two driver like this. It will give quite a lot of comb filtering and will sound uneaven. Better to go one size larger then with a single 4FE32-4.

Alternatively you could stay with the two drivers, and high-pass the second one. That will get rid of the comb filtering, and give a bit more output in the lower octaves. For that you can use two 8 Ohm models.

 

[Trif]

Well... the comb filtering is beneficial. Look at how many guitar players use two 4x12 cabinets.

Remember, you're creating a musical instrument. We've skipped the high fidelity option of going straight to the board and have decided to compete with... other musicians? The audience? Whatever you're trying to amplify your sound over it's unlikely 3" speakers are going to be effective.

Unless you're a late-night, I-want-to-practice-with-all-my-pedals, electronic violinist.... in which case, carry on!

But for 'playing out', unless you have a full compliment of compression in your rack, you don't want hifi speakers. Violin strings can be plucked. So can phono needles, but you wouldn't do that, would you? (It'll let the smoke out of your speakers!) You need musical instrument speakers. Stiff surrounds to handle that pluck, heavy cones to stand up to the stiff, huge magnets with efficient voice coils to make all that mass and energy turn around and go the other way... and for low distortion, they need to have enough square area that they barely move (see above). If you're playing with a drummer, two 12" speakers are about right.

(and yes, you can get 4" or 5" MI speakers if you're making a practice amp.)

 

[maxime.levesque]

Well... the comb filtering is beneficial. Look at how many guitar players use two 4x12 cabinets.

Remember, you're creating a musical instrument. We've skipped the high fidelity option of going straight to the board and have decided to compete with... other musicians? The audience? Whatever you're trying to amplify your sound over it's unlikely 3" speakers are going to be effective.

Unless you're a late-night, I-want-to-practice-with-all-my-pedals, electronic violinist.... in which case, carry on!

But for 'playing out', unless you have a full compliment of compression in your rack, you don't want hifi speakers. Violin strings can be plucked. So can phono needles, but you wouldn't do that, would you? (It'll let the smoke out of your speakers!) You need musical instrument speakers. Stiff surrounds to handle that pluck, heavy cones to stand up to the stiff, huge magnets with efficient voice coils to make all that mass and energy turn around and go the other way... and for low distortion, they need to have enough square area that they barely move (see above). If you're playing with a drummer, two 12" speakers are about right.

(and yes, you can get 4" or 5" MI speakers if you're making a practice amp.)

Remember that a violin's lowest note is 196 hz, so the membrane will not be moving that much, even with a small driver.

Now I don't know exactly what diameter is too small, that's what I intend to find out with this project, perhaps 3" is too small, I'm thinking of perhaps ordering a 3 and a 4, just for trying out.

My goal is transparent sound, i.e., not interested in "musical distortion". Which is why I would avoir guitar drivers, as they often add texture to the sound.

 

[Trif]

Remember that a violin's lowest note is 196 hz, so the membrane will not be moving that much, even with a small driver.

If it's 3" in diameter it will move just over a millimeter @85dB.

Now I don't know exactly what diameter is too small, that's what I intend to find out with this project, perhaps 3" is too small, I'm thinking of perhaps ordering a 3 and a 4, just for trying out.

A 4" speaker will move half as far, effectively acting like two 3" speakers (but without the combfiltering, as voodooless noted). That will be better, but not much better, given the dynamic range of the violin. Again, I urge you to consider a speaker like:

https://www.parts-express.com/Eminence-Alpha-6C-6-Midrange-4-Ohm-290-399

See how the surround looks like a spring, and not a pillow? The large magnet will provide an even field throughout the gap and beyond. The spider's stiffness matches the surround's to keep things aligned. And, despite a 93dB sensitivity, I think your violin will drown it out.

 

[MusicNBeer]

I guess it would do that, but thete's absolutely no reason to ever do it. It would raise all the dips in the driver's impedance magnitude and screw up the frequency response. Plus you'd have a large part of the source amplifier's power being dissipated in the resistor. If you wanted to raise impedance, you'd put two identical woofers in series and design around that.

 

[Head_Unit]

I am wondering which choice is better.

There is really no such thing as "4 ohm" or "8 ohm" speakers. Those are just nominal numbers representing very complex curves (look at Stereophile speaker tests), and sometimes lied about.
- So as a loudspeaker engineer, I don't like series connections, because the power won't divide equally. Is it the end of the world? For casual use eh not really.
- A resistor in series with an entire fullrange driver is just [vomit], don't do that.
- So I'd put 2x8 ohm = 4 ohm nominal load, which the amp says it can handle, and don't crank the hell out of it nor expect miracles from tiny drivers.