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Stupid ohms question


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-Okay, so I'm about to get my first PA system put together. The speakers I am getting are said to be 8 ohm speakers 750 watt handling, The poweramp I am getting is 200W stereo @ 8 ohms. So, I was wondering about a few things.

#1. When speakers are said to be 8 ohm speakers -do they mean 8 ohms only, or are they bragging about them being able to do the 8 ohm thing but can also do the 4 ohm thing?

#2. Does an 8 ohm program usually provide clearer, louder and better general sound than 4 ohms or not?

#3. 200W into 750 watt cabinets... will I be underpowering these speakers?

 

Thank you for your help:confused: ;)

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1) Speakers provide the impedence in the circuit. If it's says they're 8 ohms, that's a measure of the resistance to current flow they provide. It varies slightly with frequency.

 

2) Most amplifiers produce more power the lower the impedence of the speakers, so given the same amp, 4 ohm speakers will allow the amp to generate more power. Whether they're louder or cleaner depends on the speakers and particularly how efficient they are. Amplifiers require a certain minimum impedence to keep from burning themselves up - usually 2 or 4 ohms.

 

3) No way to tell if you're underpowering. Depends how efficient the speakers are and how loud you want to get. Unless you drive the amp so loud it's clipping badly, you won't hurt anything.

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Originally posted by SilleeSpyder

-Okay, so I'm about to get my first PA system put together. The speakers I am getting are said to be 8 ohm speakers 750 watt handling, The poweramp I am getting is 200W stereo @ 8 ohms. So, I was wondering about a few things.

#1. When speakers are said to be 8 ohm speakers -do they mean 8 ohms only, or are they bragging about them being able to do the 8 ohm thing but can also do the 4 ohm thing?

#2. Does an 8 ohm program usually provide clearer, louder and better general sound than 4 ohms or not?

#3. 200W into 750 watt cabinets... will I be underpowering these speakers?


Thank you for your help:confused:
;)

8 ohms is a constant. That is how much resistance they provide to the amp. As for #2,the impedence of the speaker has nothing to do with how clear it sounds. #3,what are the speakers,and what type of rating is the 750 watts? Just throwing a number out means very little.

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Alright, alright, these are the speakers I ordered that you guys suggested to me earlier. They are inexpensive, but you guys also said in other threads that they were actually a good deal and that this series is actually halfway decent so pleeeeease dont give me too hard of a time about it. They are the NADY THS 2012 speaker cabs -dual 12s. I needed something for less than $200 that would cut me (singer) through a metal band with solid state half stacks, not tube, though we are still pretty loud.

 

Oh yeah -and on question #1... Will I have the option of switching to 4 ohms -or does it automatically put me on 8 ohms? or if I can would it be a good or bad idea? Also -why do they make speakers 8 ohms if you can get more power through 4 ohms -is it a safety or reliability issue?

 

Even though I would really really love to get a QSC 1850HD, I aint got the cash, so I am getting the $200 NADY XA-900 Pro Stereo power amp. I looked around and it actually got good reviews. I guess I'll just look into the future for the QSC, but seriously -I need to get something started here.

 

Okay one more thing. Do the cabs usully have outputs on them where you can connect more speakers cabs such as subs when you might need them? or will I have to use all the outs available on my mixing board?

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Originally posted by SilleeSpyder

Oh yeah -and on question #1... Will I have the option of switching to 4 ohms -or does it automatically put me on 8 ohms? or if I can would it be a good or bad idea? Also -why do they make speakers 8 ohms if you can get more power through 4 ohms -is it a safety or reliability issue?


Okay one more thing. Do the cabs usully have outputs on them where you can connect more speakers cabs such as subs when you might need them? or will I have to use all the outs available on my mixing board?

 

 

Not all amps will drive 4 ohm loads, although most will. Most cabs will allow daisy chaining, but you halve the impedence when you do it. 2 8 ohm cabinets show a 4 ohm load to the amp, which is another reason for 8 ohm cabinets. If you daisy-chained 4 ohm cabinets, you'd run the risk of hurting your amp.

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Originally posted by SilleeSpyder

Alright, alright, these are the speakers I ordered that you guys suggested to me earlier. They are inexpensive, but you guys also said in other threads that they were actually a good deal and that this series is actually halfway decent so pleeeeease dont give me too hard of a time about it. They are the NADY THS 2012 speaker cabs -dual 12s. I needed something for less than $200 that would cut me (singer) through a metal band with solid state half stacks, not tube, though we are still pretty loud.


Oh yeah -and on question #1... Will I have the option of switching to 4 ohms -or does it automatically put me on 8 ohms? or if I can would it be a good or bad idea? Also -why do they make speakers 8 ohms if you can get more power through 4 ohms -is it a safety or reliability issue?


Even though I would really really love to get a QSC 1850HD, I aint got the cash, so I am getting the $200 NADY XA-900 Pro Stereo power amp. I looked around and it actually got good reviews. I guess I'll just look into the future for the QSC, but seriously -I need to get something started here.


Okay one more thing. Do the cabs usully have outputs on them where you can connect more speakers cabs such as subs when you might need them? or will I have to use all the outs available on my mixing board?

OK,I check and those cabs are 8 ohms. Most dual woofer cabs are 4 ohms. Anyway,that is a constant. You can't change a speaker's impedence,other than by wiring multiple speakers differently. As for why? If a single speaker cab puts an amp at its lowest allowable impedence,there is no room for running multiples on the amp. Each time you daisy-chain or hook a second speaker to the amp,you cut the impedence that the amp sees. BTW,the 750 watt rating on those speakers is peak program(whatever that means exactly)in case you didn't notice. I would say 350-500 watts RMS would be about ideal for them. Did you order those speakers yet? I'm one of the people that said they should be ok. But if you haven't,I have a much better deal for you. MTS is closing out all of their BULLFROG line of speakers at great prices. They have all Eminence components(woofers and high drivers) and are built very well. I just got a pair of the PR1510 models for about $420 w/shipping. You might want to check them out. http://www.mtsproducts.com/pgs/proaudio.html http://www.mtsproducts.com/imgs/hq/pr1510.jpg If you've already ordered the Nadys,they should work ok. This is just one of those special deals that came up. I would also think twice about that amp. The same money will get you a better quality used amp on ebay or elsewhere. But if it is the Nady amp or no amp at all,it'll work. And if it holds together,it will probably sound decent. I started out with way less in my day.

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Thanks Tbonehead, those speakers are extremely tempting but they weigh 115 lbs and are big boxes. If I had a club or something I think this would be an easy decision to make but I've gotta move my stuff around. How good are these bullfrogs, and do they come with casters or anything... either way they are still big and heavy. -oh yeah aslo, how did you purchase them -was it from the same website you showed me?

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Originally posted by SilleeSpyder

Thanks Tbonehead, those speakers are extremely tempting but they weigh 115 lbs and are big boxes. If I had a club or something I think this would be an easy decision to make but I've gotta move my stuff around. How good are these bullfrogs, and do they come with casters or anything... either way they are still big and heavy. -oh yeah aslo, how did you purchase them -was it from the same website you showed me?

He has been putting them on ebay recently. That's how I got mine. But he will let me buy more at the same price If I want more. Otherwise,there is an e-mail link on the home page of the website. Look for the one that says sales(Scott Mount is the salesperson.) They have Eminence Delta woofers and Eminence high frequency drivers. Very good drivers for the money. I guess if you have to do everything yourself,they might be a bit heavy. But with a cart,they aren't hard to handle at all. They don't seem that heavy.

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  • 6 years later...
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Hi, below I am pasting a correspondence I had with someone selling resistors. Basically, my amp is too loud and I don't want to have to spend $150 for an attenuator if I can solder a resistor into the speaker cable cheapy and easily. I'm pretty clueless though and help is much appreciated. I don't understand the most basic stuff about ohms! My email is funtimesman@live.com. Thanks for any help!

 

Dear me,

 

To do this you need to keep the impedance the same while sending part of the power into the resistors.

 

It sound like your 4-12 cabinet uses pairs of speakers wired togehter into two 8 ohm loads. Those two loads can be run seperately for a mono 8 ohm load per side (one input option) or driven togehter wired in series for a 16 ohm load total. (two 8 ohm loads together to make 16 ohms total, thus the "16 total")

 

If this sounds right let me know. I'm going just on what you've told me. You didn't say which channel you were using to drive these speakers but I assume it to be the 8 ohm channel driving all 4 12"s at 16 ohms. If so that will mean that you could use 4 of the 4 ohm resistors in series to make an 80 watt 16 ohm resistor. You could then wire that resistor parrallel to the 4-12 speakers to acheive an 8 ohm load that would drop the volume by 50%.

 

This is dependant on the final impedance of the 4-12 cabinet. The best way to check this is with a digital multimeter. Many housholds have them or if you can't find someone with one they are about $20 for a cheap but effective one at any department store. Set it on the ohms or omega symbol and with the speakers unplugged from the amp check the two contacts of the plug and see what they read. Your reading will be the resistance of the coils not the impedance so it will probably not be an even number like 16 or 8 but more likely 14.7 (for 16 ohms) or 7.2 (for 8 ohms) or some such thing but it will tell you what you need to know. Check this out before you buy anything to make sure you are getting the right parts and let me know what you find out. If it's different than I'm thinking let me know and I'll see what else can be done. Thanks.

Steven

- the_e

 

 

 

 

 

From: me

 

Dear the_e

 

hi I have a 100 watt amp head that I would like to drop the volume by about %50. would this be good for soldering into the hot conductor cable in a speaker cable (ie, not the ground conductor cable) in order to make a guitar amplifier that is too loud at even its lowest volume less loud, ie less volume, while at the same time allowing the guitarist to get the tone achieved by cranking the amp up in volume or distortion? would i just cut my speaker cable (the one from the amp's speaker out to the speaker cabinents input) and solder in something like this resistor? is there impedance concerns? i have a fender performer 1000 head with a 412 speaker cabinent. it is a 100 watt amp. it has two outputs, one allows for a single 4 ohm load, the other for a single 8 ohm load...the 412, stereo ready speaker cabinent has two input jacks and says use the left jack for a single 8 ohm mono load. it says 16 total above and in between these two jacks. not sure whatleft input jack is.

 

 

 

 

...should point out my amp is apparently mostly a solid state amp, some weird hybrid of solid state and apparently a tube or valve preamp for one or both of the distortion channels. I thought I read that one should not use an attenuator with a solid state amp? Is this true? Aren't I basically making a cheap attenuator? I recently soldered in a

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You may need one of these

 

*edit*

Spend the money. You won't rerget it. You will if you jack up your rig with out the knowledge to attempt what you think you want to attempt.

 

Don't forget:

If you don't have a lot of experience blowing up cheap gear, you will GAIN alot of experience blowing up cheap gear.

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Alright, alright, these are the speakers I ordered that you guys suggested to me earlier. They are inexpensive, but you guys also said in other threads that they were actually a good deal and that this series is actually halfway decent so pleeeeease dont give me too hard of a time about it. They are the NADY THS 2012 speaker cabs -dual 12s. I needed something for less than $200 that would cut me (singer) through a metal band with solid state half stacks, not tube, though we are still pretty loud.

 

 

Don't count me as one of "you guys". I would not have made that recommendation.

 

Really, pass on the amp and get a QSC GX-3. No comparison.

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Even years ago, SOMEONE should have informed the Funtimesman that the kind of resistors required have to be very high wattage types (I'd spec 200 watt or better to be on the safe side). These things are big and will get HOT when dissapating the energy that isn't going to the speakers, so mounting them safely requires a bit of engineering as well.

 

I worked in an audio repair shop and we made a dummy load to "cook" power amplifiers. It was a big board (like that concrete/drywall board) of 50W resistors series paralleled so that we could simulate different loads depending on the amps needs by where we patched into it. It would get really hot!

 

At least they weren't trying to change the load on a tube output transformer (I've seen them melt down when loaded wrong (all the wire varnish oozes out and it smells really bad)).

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Isn't that amp head solid state? If it is, and you perform the parallel resistor mod as mentioned in the email, you won't be making the amp quieter - you'll just be wasting a lot of power through some resistors, creating a potential fire hazard, and based on how you do the work and how safe your resistor-cable-creation is you'll also probably be creating an electrocution hazard. If you're not properly trained to perform electrical work, don't perform electrical work. There's plenty of power in a guitar amp to kill you or burn a house or club down to the ground if you do the wrong thing.

 

Please note that the rest of my post is meant to be for discussion purposes only, in an effort to continue a discussion that I think is interesting. It is NOT meant to support any work by anyone. Please don't try to wire up some resistors to your speakers unless you know what you're doing.

 

Correct me if I'm wrong (and I VERY EASILY could be - amplifier design is not my field), but solid state guitar amps are usually voltage sources, basically - to make it quieter you need to put a resistance in series with the load. Tube guitar amps are usually current sources. To make one of those quieter you'd need to put a resistance in parallel with the load (and then make sure you match the impedance of the new load with the proper amp output). That's why you should make sure you have a load on the output of a tube amp before turning it on. If it tries to output a current with no load, it can cause a voltage spike that can damage the amp because it's trying to push a current through an open circuit. With a solid state amp you're generally dealing with a constant voltage and then the current depends on the impedance of the load, so trying to play through a solid state amp with no speakers connected won't hurt anything - it just puts voltage at the output terminals and enjoys not having to work very hard.

 

But like I said, this isn't my field, so I could be wrong. But it seems to me that a power attenuator for a tube amp and a power attenuator for a solid-state amp would need to be designed differently.

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You are correct.

 

Also, the primary reason for the damage to tube amps under no load conditions is most commonly due to clipping the output which causes the output transformer to ring due to the LC tank circuit formed that stores and releases energy (partial cause), as well the flyback voltage that's created when the amp clips into the inductive load and the output stage's feedback circuit causes the output impedance to shift much higher under clipping so the flyback voltage can exceed the voltage rating of the output transformer as well as the tubes and various wiring/pcb traces.

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Thanks. :) I know just enough about amplification to know that I've really no idea what I'm talking about once you move beyond the basic parts of the circuit. It's a field about which I should educate myself, if only to gain a better understanding of the music equipment I own and use every day.

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