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Author Topic: Speaker Question - Concerning Ohms  (Read 2046 times)

kendallhadden

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Speaker Question - Concerning Ohms
« on: September 15, 2014, 08:49:15 am »

I tried to search this topic, but can't seem to find anything or i'm not typing the correct words for a thread to come up.

I have a Danley TH115 that is 8ohms.  Can a resistor be put in line to speaker to make the amp think that it is a 4 ohm speaker?  I would think this is possible, but I need someone that's knows more than me to say it can be done (or can't be done). 

Thanks,
Kendall
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Frederik RosenkjŠr

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Re: Speaker Question - Concerning Ohms
« Reply #1 on: September 15, 2014, 08:53:59 am »

I'm very curious what you would gain from that?

Well, you could put an 8 ohm resistor in parallel with the cabinet, and thereby get a 4 ohm load in total (it wouldn't be exactly the same as a complex impedance of a speaker, but it would still be a "4 ohm load").

The result would be that you'd draw more power from the amp, but the extra power drawn, would dissipate in the resistor, so you'd gain nothing but heat. And in practice your amp might not be able to deliver exactly twice the power, so you could very well lose a little output.

So:

Would it be possible to make the speaker look like a four ohm load: yes.
Would it be possible to do this in a way that you'd gain extra power from the amp and get louder: no. But warmer, yes :)
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TJ (Tom) Cornish

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Re: Speaker Question - Concerning Ohms
« Reply #2 on: September 15, 2014, 08:59:44 am »

I tried to search this topic, but can't seem to find anything or i'm not typing the correct words for a thread to come up.

I have a Danley TH115 that is 8ohms.  Can a resistor be put in line to speaker to make the amp think that it is a 4 ohm speaker?  I would think this is possible, but I need someone that's knows more than me to say it can be done (or can't be done). 

Thanks,
Kendall
As Frederik said, adding an 8Ω resistor will make the load at the amp approximately 4Ω, however now half of the power goes to the sub, and half goes to the resistor in the form of heat. 

The only practical way to do this is to add another 8Ω TH115 - now your load is 4Ω, and you get more output, albeit with somewhat more cost and space than a power resistor.  :)
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kendallhadden

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Re: Speaker Question - Concerning Ohms
« Reply #3 on: September 15, 2014, 09:31:31 am »

Thanks guys. 
The reason I asked is that I have been looking at picking up a Crest ProLite 7.5.  Normally I run my PLX3602 bridged mono with one amp per TH115.  At 8 ohms this would send approximately 2500 watts to each sub.  The Crest ProLite 7.5 is rated in stereo at 1425 watts per channel at 8 ohms and 2450 watts per channel at 4 ohms.  I was thinking I might be able eliminate one amp and just run both subs off of one Prolite 7.5 if I could get the TH115 down to 4 ohms.  I know I still can run both off one ProLite 7.5, but to me the TH115s need some wattage to get the goody out of them.  Thanks again for your expertise.  Sounds like I just need to go ahead and purchase some TH118 4ohm boxes and be done with it haha!
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TJ (Tom) Cornish

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Re: Speaker Question - Concerning Ohms
« Reply #4 on: September 15, 2014, 10:17:40 am »

Thanks guys. 
The reason I asked is that I have been looking at picking up a Crest ProLite 7.5.  Normally I run my PLX3602 bridged mono with one amp per TH115.  At 8 ohms this would send approximately 2500 watts to each sub.  The Crest ProLite 7.5 is rated in stereo at 1425 watts per channel at 8 ohms and 2450 watts per channel at 4 ohms.  I was thinking I might be able eliminate one amp and just run both subs off of one Prolite 7.5 if I could get the TH115 down to 4 ohms.  I know I still can run both off one ProLite 7.5, but to me the TH115s need some wattage to get the goody out of them.  Thanks again for your expertise.  Sounds like I just need to go ahead and purchase some TH118 4ohm boxes and be done with it haha!
According to the Danley spec sheets the 4Ω TH118 is 2dB more sensitive than the 4Ω TH115, and can handle more power, as well as better low-frequency extension.  Doing very fast (and potentially error-prone) napkin math, a pair of 4Ω TH118 would be 5dB louder on the ProLite than your current pair of 8Ω TH115.
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Ivan Beaver

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Re: Speaker Question - Concerning Ohms
« Reply #5 on: September 15, 2014, 01:20:07 pm »

I tried to search this topic, but can't seem to find anything or i'm not typing the correct words for a thread to come up.

I have a Danley TH115 that is 8ohms.  Can a resistor be put in line to speaker to make the amp think that it is a 4 ohm speaker?  I would think this is possible, but I need someone that's knows more than me to say it can be done (or can't be done). 

Thanks,
Kendall
You need to think in terms of VOLTAGE-NOT wattage.

Let's say you added a HIGH POWER 8 ohm resistor in parallel with the TH115.

The voltage across both units would be the same (assuming a flat impedance-which no loudspeaker is) so the power to each one would be the same.

Now if you remove the resistor-the voltage (and power stays the same) so there is no increase in power to the cabinet.

There is NO advantage to adding a resistor-but there are SEVERAL DISADVANTAGES to doing this.

For 2 equal impedance loads the power is equally split between them.  Since the amp can deliver more power into an 8 ohm load than a 4 ohm load (split between 2 loads) you will actually REDUCE the output power. available to the 8 ohm load.

Next the amp will be working harder-which means more heat- more current draw-less power supply reserve for peaks and so forth.

You can simply replace the 8 ohm driver with a 4 ohm driver and get a little bit more output, but not enough to worry about-in my opinion (I think it is less than 2dB).

The big concern would be if you add additional TH115s and what the impedance is.

The standard impedance is 4 ohms now (it was 8 ohms originally) , but you can specify an 8 ohm driver if you want when ordering them.
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!

kendallhadden

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Re: Speaker Question - Concerning Ohms
« Reply #6 on: September 15, 2014, 01:29:15 pm »

Thanks Ivan.  I appreciate your input.  Lesson for today has been learned. 
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Re: Speaker Question - Concerning Ohms
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