what tradeoffs by lowering driver impedanse with resistor?

[elviukai]

hi, anybody played with lowering impedanse of woofers with resitor?
I manly use series xover. my current project have 10ohm drivers acoustical impedanse and final speaker impedanse measurement do not look very good to amp with which it will be using( 30w@8ohms , 110w@2ohms) from 1 to 20khz it rises from 9 to 25ohms. using of woofers impedanse corection RC is not solution, i need to lower whole woofer or even speaker impedanse . to unwind voice coild is also not good idea.
does anybody had expierence with placing resistor in paralel at amps input 12ohms or so? does it has negative side? what wattage resitor should be?

what about placing resitor only across woofer terminals?

thanks in advance

[myhrrhleine]

Hi,

Putting a resistor across the voice coil 'works'
It's not usually done because it's just wasted power.

[elviukai]

what do you mean waste of power? and how much power? power throught resistor? if my amp has 30w@8ohm and 60w@4ohm is it waste of power ? I think that woofer get more power . if there only woofer I just left it 8-10ohms but unfortunaltely its integrated in prett complex design whee I must push acoustical impedanse to 4-5ohms..

[kelticwizard]

Myhrrhleine is correct.

Suppose you have a woofer that is getting 12.5 watts of power at 10 Volts from the amp. Say it is at 1,000 Hz, though the principle will apply to any frequency.

You amp is putting out 12.5 watts of power.

Now, you take an 8 ohm resistor and put it across the terminals of your woofer. You now have a 4 ohm speaker.

At 10 Volts, your amp is putting out 25 watts-but 12.5 watts goes to the woofer, while the other 12.5 watts goes to the resistor, which does you no good at all in raising the SPL.

There is nothing you can do with resistors that can get more power to a speaker.

[elviukai]

it will not be delivered 50%-50% because speaker rdc is 6,3ohms and resitor 12ohms. so I guess 75/25? well thats sad anyway.. should I dismount woofer and unwind voice coil to 4-4.5ohms?

[Bazukaz]

Hi,
You shouldn't dismount the speaker.If you unwind part of voice coil , the maximum power dissipation of the driver will decrease. You may damage the driver as well.

regards,
Lukas.

[elviukai]

unwinding from 6.3 to 4.5-5ohms should not be the problem I think? . I have never burned woofers. BTW I always wonder how manufacturer makes diferent impedanse woofers. I dismounted 2.3ohms scan speak and its voice coil winded the same lenght as 5.6ohms.

greets from Kaunas
R

[Bazukaz]

Hi,
The diameter of higher resistance voice coil's wire should be lower if the length is the same.That's another reason why you shouldnt unwind voice coil - the thinner wire may not handle the increased current. Sound compression(the increase of voice coil's resistance due to heating) is another problem with drivers.If you unwind some wire , the coil will heat more , and therefore the compression will be higher.

Regards,
Lukas.

[AndrewT]

Hi,
if you go back to the very simple example of 8r resistor in parallel to 8ohm speaker.

The amp can push 12.5W into 8r, but into 8r//8ohm it will not push 25W because it cannot hold the 10Vrms of drive voltage into the lower impedance.

A very good ClassAB amp could manage 2.5db extra, a good amp about 2db extra, a mediocre amp about 1.5db extra and a poor amp about 1db extra.

Let's assume your amp is good and manages 2db extra into half the load impedance. This equates to 19.8W into 4ohms.
for an equal division into both loads then the power absorbed by the speaker has gone down from 12.5W to 9.9W.

But the story gets worse.
The speaker designer has assumed nearly constant voltage drive across the frequency spectrum to give a reasonable frequency response. If a constant resistance is placed across the speaker (or output) terminals then the effect at low speaker reactance is similar to the above, but when the speaker has a very high reactance, at resonance and again at higher frequencies, the voltage available to ensure adequate output is seriously reduced by the parallel resistor. Some designers even use this method of adjust their crossovers to compensate for response anomalies.

Now consider your voice coil. It has a fixed number of turns IN THE MAGNETIC GAP, with a few turns extended roughly equally beyond the magnetic gap. The Xmax is the length of the turns beyond the gap on each side (approximately). A sub relies on Xmax to produce SPL.

If you reduce the turns on each end of the voice coil you are reducing Xmax and reducing SPL. If you remove turns from one end of the voice coil you will end up with asymetric Xmax and maybe zero Xmax in one direction.
Reducing the turns on the voice coil will not reduce the turns in the magnetic gap unless taken to extremes, but it does reduce the Re and so changes the Q of the speaker. You end up with a lower Q and a lower Xmax. A consequence of lower Q is a reduced lower bass output. You have lost bass SPL twice over.

All that earlier stuff about thick and thin wire being chosen for each driver is correct as well.

A very poor proposal to put it mildly. If I were to express my views about your B...np...r I would end up Texased or worse. This is why education is compulsory upto the age of 16 in the UK and why mathematics is not optional.

[myhrrhleine]

Originally posted by elviukai
what do you mean waste of power? and how much power? power throught resistor? if my amp has 30w@8ohm and 60w@4ohm is it waste of power ? I think that woofer get more power . if there only woofer I just left it 8-10ohms but unfortunaltely its integrated in prett complex design whee I must push acoustical impedanse to 4-5ohms..

Hello,

if my amp has 30w@8ohm and 60w@4ohm is it waste of power ?

No new power will go to the speaker itself.
The speaker will receive the same power it would without the resistor.
Additional power will simply go to the added resistor.
How much? V^2/R is the formula.

Also, the speaker impedance will vary with frequency. It's not a constant.

Messing with the voice coil won't help either.
It'll drop the ohms, but the speaker gets power to the air determined by current in the magnet gap multiplied by wire length.
More current but less length doesn't help.


Fianally, your amp's power is a maximum output rating for a given distortion.
Average power to your speaker will be 10-30 times less (or even less). You'll be averaging 1 watt or less.