Powering Subwoofer

nunayafb

Member
2005-08-17 11:05 pm
a 4 ohm resistor is not going to hurt anything as long as it has a high enough power rating. The power rating should be about half of what the amp can deliver. Madisound's 25W Wire-wound resistors are cheap enough ($0.65) that I would get two 8 ohm resistors wire them in parallel to eliminate any risk overheating even with extensive high volume usage. This would allow the resistors to easily withstand the ~80W peak power from the amplifier when wired in series with a 4 ohm driver.
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
I was going to recommend "bridging" your amp so you can drive the sub from both channels, but that won't work here. The sub would either have to be 16 ohms, or your amp channels would have to be able to drive 2 ohm loads. That is not the case here.

Here's the link to the discussion, not that it is going to help you in this application:

http://www.diyaudio.com/forums/showthread.php?threadid=750

As long as this is just temporary, then a 4 ohm resistor is the way to go.
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
One more thing. The amp that is going to power your main speakers, not the sub-is that a stero amp or an AV amp?

The reason I am asking is that many AV receivers have a jack in back which carries a bass-only signal to a powered subwoofer. If that is the case, then of course you don't have to worry about a passive crossover for your sub.
 

nunayafb

Member
2005-08-17 11:05 pm
ocool_15 said:
Keep in mind adding a series R will change the frequency response and you will be wasting ~half what the amp is capable of.
If you are using a passive crossover it will also be affected.

How will it change the frequency response?

how can you waste amplifier power when it is rated at 8 ohms and it will be seeing 8 ohms, with a 4 ohm driver he'll be most likely fry the amp, that would be wasteful.
 
Hmmm......

Sticking a 4R resistor in series is a very bad idea, driver Qts will
approximately double, not good at all, boombox will be the result.

A stereo amplifier can be wired in parallel (not bridged). Parallel
the inputs and add to each output 0.22/0.33R series resistors
before they are paralleled together. The resistors will ensure
current sharing and take up any gain / DC offset issues.

Each channel will see an 8ohm load (with a 4ohm load).

:)/sreten.
 

ocool_15

Member
2004-11-26 3:15 am
sk
nunayafb said:


How will it change the frequency response?

how can you waste amplifier power when it is rated at 8 ohms and it will be seeing 8 ohms, with a 4 ohm driver he'll be most likely fry the amp, that would be wasteful.


If his amplifier is 40W(assuming per channel) and you have 2 resistors in series of the same resistance they will each disipate half the power. So the speaker would see 20W assuming a fixed impedence.
A speakers impedence varies with frequency. When its impedence is higher the speaker will have more voltage drop across it and therefore more output. Less output when the impedence is low. For the low frequencies note the impedence is also affected by the enclosure. The higher the qtc of a speaker in an enclosure the more the impedence will vary. The least error overall would likely be in a vented enclosure with a lower qts driver.

Look up Joule's Law and Ohm's Law for a better understanding of the reasoning.

Post #9 by sreten is a better solution
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
Using Martin J. King's cheapware, available at
www.quarter-wave.com

Here are the parameters for a 12 inch sub, Qtc = 0.41, in a 2.25 cu ft box tuned to 30 Hz. That is a 4 inch diamter, (2 inch radius) tube of 13 inches long. Note the Qts is 0.41 when we do not add a resistor.

Note that with Martin's software, Vad = Vas, and Qtd = Qts. It is just his individual method of labelling.
 

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kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
Now here is frequency response chart of this woofer, again with no resistor added. Note the sensitivity-this is with 2.8 Volts running through it, which with a 4 ohm speaker is the equivalent of 2 watts.

The red line is tha ported response, the dotted blue line is response of the same size box with no port in it.
 

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kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
Now here is the frequency response graph with the added 4 ohm resistor. Note how the sensitivity drops at 2.8 Volts running through the speaker, except aroudn 50 Hz or so. Part of that is lack of efficiency, part of it is the fact that now the speaker is only getting 1 watt of power at 2.8 Volts-the 4 ohm resistor is absorbing the other watt.
 

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Saturnus said:
If I understand it correctly, it's a stereo amp then why not just parallel the two channel (not bridging them). Buy a splitter cable, and feed each input with the same signal and connect plus to plus and minus to minus on the outputs.

are you sure we can parallel the 2 channel?
are you sure we can parallel the 2 channel?
any difference in 2 channel can result in problem.

ok on 2nd thought i think it will work, how about taking only 30W o/p instead of 40W to be on a safer side [:p]