# Adding resistor to make 4 ohm speaker 8 ohm



## GTI-DNA (Nov 4, 2010)

As the title states;

If I have a 4 ohm speaker but would like the amp to see 8 ohms, could I just add a 4 ohm resistor in series?

If so, which 4 ohm resistor would I use?

Thanks
D


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## chad (Jun 30, 2005)

it's a better idea just to purchase an 8 ohm speaker. All you are doing is making heat with a resistor..


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## S3T (Sep 21, 2008)

1. Half of output power will be lost on resistor. Therefore, if you wish to use 100W amplifier (which is rated 100w into 8ohm), get yourself 50W resistor. Amplifiers rated 100w into 4ohm will deliver 50w in 8ohm. Half of that power will be lost in resistor (25w) and other half will drive the speaker. 25w... Is it enough for you?
2. The total Q (Qts) of the driver+resistor will increase - giving you better response in free air (door) with low-Qt home drivers. Bass quality/definition may suffer.


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## ChrisB (Jul 3, 2008)

If you are going to go through all that trouble, may as well go one step further and DIY a Zobel network. Read here for more information: Passive Crossover Network Design


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## tbassuva (Sep 14, 2009)

That would drop your damping factor to 1, so don't expect too much quality [edit: if this is a subwoofer].


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## S3T (Sep 21, 2008)

Well, tube amps/amps without feedback (class A etc) don't have much damping factor, but have pretty nice sound...


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## tbassuva (Sep 14, 2009)

S3T said:


> Well, tube amps/amps without feedback (class A etc) don't have much damping factor, but have pretty nice sound...


It depends.... I [perhaps wrongly] assumed the thread was about subs. If it's not, my comment is not particularily accurate. Here is more detail:

Tube amps typically have a wamer sound. Some people prefer this sound, but I would say that applies to midrange and treble rather than to bass. Damping factor is most important in the bass frequencies because the cone moving mass is high and the suspensions don't provide sufficient mechanical damping [thus electrical damping is useful]. 

Lower feedback levels also have lower damping factor as one of the consequences, but that's not the goal of using no/low feedback. Instead, the idea is to generate less higher-order harmonic distortion, which the ear is typically more sensitive to. Again, this may not be as relevant for subwoofers.

Class A vs B,D etc describes the biasing of the semiconductor devices, but doesn't really determine damping factor. That said, being 'purists', class A amp designers are probably less likely to use as much feedback as others.


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## djtsmith007 (Sep 21, 2010)

First off adding a resistor wont work, why? Because you wont find a high wattage resistor to be in parallel with that sub. The sub coil is pretty much a large resistor. High wattage resistors can handle high heat, which are done through the actual size of the resistor. BTW im a electrical engineer


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## tbassuva (Sep 14, 2009)

djtsmith007 said:


> First off adding a resistor wont work, why? Because you wont find a high wattage resistor to be in parallel with that sub. The sub coil is pretty much a large resistor. High wattage resistors can handle high heat, which are done through the actual size of the resistor. BTW im a electrical engineer


Not to rain on your parade, but the resistor would be in series, and I would have no problem finding a resistor able to dissipate the heat, one example.
Also, the sub coil is just that, a coil. It creates back-emf, which lowers the amount of heat dissipated substantially. 

That said, I agree it's not a good idea!


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## djtsmith007 (Sep 21, 2010)

tbassuva said:


> Not to rain on your parade, but the resistor would be in series, and I would have no problem finding a resistor able to dissipate the heat, one example.
> Also, the sub coil is just that, a coil. It creates back-emf, which lowers the amount of heat dissipated substantially.
> 
> That said, I agree it's not a good idea!


Guess i need to stop reading and working at the same time! haha. Series would work but yes not a great idea. 

Although a coil of wirewound, around a core DOES create a resistor which is ideal for heat dissipation. 

Just get another sub?


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## S3T (Sep 21, 2008)

Hey guys, who said "subs"? 
Well, term "speaker" could relate to box with drivers and crossovers too. In this case it will cause different abnormalities with crossover frequencies. 


BTW, series resistor could do pretty nice things on cheap midranges, as long as you stay away off their Fs.
Impedance rise introduced in driver's motor without shorting rings will cause high-frequency roll-off in regular case, but will somehow correct itself, as the ratio between coil resistance and resistor will change, and coil will see more power on higher frequencies = less hi-frequency roll-off apparent in cheap midranges.


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## tbassuva (Sep 14, 2009)

Yep, we definitely need clarification on the 'speaker', though at this point GTI-DNA may not need any more input!


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## Wicks (Apr 8, 2011)

The more efficient option would be an impedance matching transformer between the amp and the speaker.
Although you'd have to find one with the right frequency response and one that could handle the voltage and current transferred to the speaker...
If we're talking subs then it'd probably be cheaper and easier in the long run to just go 8 ohm.


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## Rob M (Nov 25, 2008)

What are you asking of the amp? If you take the amp's 8 ohm rating, you can derate it to see what safe 4 ohm load it can drive. An amp that can drive an 8 ohm speaker at 150W can drive a 4 ohm speaker at 75W.

The maximum current the amplifier is designed for can be figured from its output power (P1) and designed load impedance (Z1):

I=sqrt(P1 / Z1)

Now, for a given load impedance (Z2), we can find the maximum voltage (V2) that the amplifier can output into this load before exceeding it's designed maximum current output:

V2=I*Z2

And from this, we can find the power (P2):

P2=V2*I=I^2*Z2=sqrt(P1/Z1)^2*Z2=P1/Z1*Z2

Simply put, the maximum power an amp can safely put into a lower-impedance load is the original power times the new impedance over the old impedance:

P2 = P1 * Z2/Z1

So if your amp can put out 150W into an 8 ohm load, it can safely put 75W into a 4 ohm load, but can be overdriven by a too-high input signal.

A mid or high amp won't be driven this hard, but a sub amp may very well be.


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## req (Aug 4, 2007)

id say just get another of the same speaker, and run the system at 8 ohms that way, or buy a different speaker.

not the answer you were looking for, but adding a resistor, or a passive crossover, or an impedance matching transformer is ridiculous and complicated.

sorry


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## Navy Chief (Jun 14, 2010)

What about adding an Aura Bass Shaker, they are 4 ohm. Could add some interesting tactile feel so a small system and keeps you from just turning that additional power into just heat. It's rated at 50 watts rms/100 watts max and is only 50 bucks, sounds perfect to me.

Aura AST-2B-4 Pro Bass Shaker


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## Mr. D' (Dec 3, 2019)

THANK YOU!

Everyone's input has been eye opening. It IS a sub-woofer. I got the idea from a friend who coupled two Satori 9.5 inch spkrs in series. They are 4 ohm. He designed wooden a case and found a great crossover network. They rock!

You have given me much to think about as I re-work my idea. But I do have to say the 50 watt/ 4 ohm resistor did hold up and produced a good response in the spkr cabinets I found. I knew it couldn't be the end of story. That's why I tried this conversation board. 

Always enjoy seeing what you guys come up with. So thanks again!!


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## miniSQ (Aug 4, 2009)

Buy a second sub and wire that in series.


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## Barnaby (Aug 25, 2009)

Mr. D' said:


> THANK YOU!
> 
> Everyone's input has been eye opening. It IS a sub-woofer. I got the idea from a friend who coupled two Satori 9.5 inch spkrs in series. They are 4 ohm. He designed wooden a case and found a great crossover network. They rock!
> 
> ...


Have you been contemplating this for eight years? I would have lost interest within a few hours....


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