Speaker Wiring and Ohms Questions

[Telewanger]

I have a few questions concerning ohms and wiring.

I went to several speaker manufacturer's websites. It says, at 100 watts 4 Ohms puts out 43% more electrical current than the 8 Ohms. At 4 ohms the amp is brighter, burns hotter due to lower resistance, and will burn tubes out much quicker.

I have a Laney 50 watt tube amp with a 70 watt 12" x 8 ohm speaker inside. There is another jack for an extension speaker. I have an Eminence 70 watt 12" x 4 ohm speaker box that I thought about connecting to the Laney. Total ohms will be 12 ohms. I have a choice on the amp of selecting 4, 8, or 16 ohms. Of course right now it is set on 8 ohms, since the Laney has an 8 ohm speaker in it.

Question 1:

If the total ohms will be 12 ohms, can this be done, and if so, what do I set the ohm switch to on the amp? The output jacks are in parallel on the amp.



I ordered a Bugera V55HD amp head. It is on it's way. It also has a 4, 8, and 16 selector switch on the head. I have a Marshall cab with 2 X 12" 70 watt Fane speakers and both are 8 ohms. If I hook them up series they are 16 ohms, and parallel will be 4 ohms.

Question 2:

What is better? Some people say parallel is brighter and cleaner, but makes the tubes and transformer work harder and wears the tubes out faster. They say series wiring is darker sounding, but tubes will last longer?

Any help will be great!

Thanks!

 

[Ch0jin]

Question 1.
Danger! Plugging a 4 Ohm speaker in parallel with an 8 Ohm speaker will result in a speaker impedance of only 2.67 Ohms. Even with your output set to 4 Ohms this is a tube melting situation and best avoided.

If you really want to use both speakers, you'll need to modify the wiring to connect the speakers in series which would give you 12 Ohms.

If you do this, then setting the amp to either 8 Ohms or 16 Ohms will work, however you'll have a bit of a drop in output power due to the impedance mismatch. Amplifiers that use output transformers, like all tube amps, work best when the output impedance matches the speaker impedance.

At 4 ohms the amp is brighter.....

.

Is that something you read, or are you saying your own amp sounds better at 4 Ohms? My personal opinion is that mismatching impedance to gain some brightness is definitely the wrong way to go about things.

Question 2.
In my opinion Parallel is better for a 2 speaker setup and it has nothing to do with the sound, which as you say, everyone one the internet seems to have an opinion about. It's so subjective the only way to find out which you prefer is to try both and not be too surprised if you don't hear any difference at all, especially as both speakers are identical.

I chose Parallel for the simple reason that if (worst case scenario) a speaker blows while your rocking out at full noise, at least you'll still have an 8 Ohm load for the amp. Wide open tube amps tend to go all burny-smelly-smokey with no speaker load, which is what would happen in the same situation with a series setup.

As for series/parallel wiring changing the life of tubes, I very much doubt it makes any difference.

 

[mainestratman]

Ch0.. if he soldered a 4ohm resistor in series on the extension speaker (between the wire and (+) terminal on the speaker), would that make it an 8ohm speaker, then when used in parallel with the Laney make a 4ohm circuit?

Resistance is futile.

 

[Telewanger]

Thanks for you help!

I read a bunch of posts on the subject last night. I will just buy 2 each 12" 16 ohm speakers in a few months and be done with the mismatched speaker problem on the Laney amp.

Question about new amp:

I have a new tube amp head on the way from Musicians Friend. Bugera V55HD. I have 2 each 12" Fane 8 ohm speakers in a cab. The speakers are 70 watts each. Parallel ohms wired together will be 4 ohms. My new head will handle 4, 8, or 16 ohms. So with my current speakers, I will have to run the head at 4 ohms.

Is there any benefit of running a tube amp at a certain ohm setting? I'm not talking about matching the speakers, I mean, would someone ever run their tube amp at a certain ohm for saving tube life, transformer life, tone, cleaner sound, etc?

 

[Ch0jin]

Ch0.. if he soldered a 4ohm resistor in series on the extension speaker (between the wire and (+) terminal on the speaker), would that make it an 8ohm speaker, then when used in parallel with the Laney make a 4ohm circuit?

Resistance is futile.

Not exactly.

You see when we say a resistor measures 8 Ohms we are talking about its "Resistance" meaning a material's opposition to the flow of electric current, either AC or DC.

When we say a speaker is 8 Ohms we are talking about its "Impedance" which is a bit more complicated as it is specifically the measure of the opposition to the flow of an alternating current or "AC" for short.

This can get a bit complex...

First off, the reason speakers are not rated using "resistance" is because they are actually little electronic circuits.

"What?" I hear you cry, "But there's nothing side them but a coil of copper wire and a magnet right?"

Right, but when you make a coil out of a conductor, like copper wire, it exhibits some new electrical properties. Namely "Capacitance", meaning it can store an electrical charge, and the big one, "Inductance", which is why a speaker works as it does. Inductance is the effect whereby EMF (Electro Motive Force) is generated by passing AC through a coil. Simply, when you feed music (AC) into a speaker coil, it creates a force that pumps the cone in and out making the sound you hear.

Now all thats relevant how? Glad you asked. Well the important thing here is that capacitance, and inductance are both AC properties and have different effects based on the frequency used in the circuit. In a speaker we roll up the Capacitance and Inductance values and call it "Reactance". Obviously as it's made up of two AC characteristics, Reactance is also variable depending on the frequency of the AC.

Now to get us back to where we were, if you take Reactance and our old mate Resistance (both measured in Ohms as it happens) and mix them together using maths you come up with.......Yup, Impedance. Ta Da!

So, what I've tried to illustrate is that whilst a resistor and a speaker may both be rated at 8 ohms, they way they behave when you pump AC (the signal from your amp) is going to be quite different.

Because I made that preface so long, I'll leave the conclusion short as I can

1. You -can- actually do what you suggested and it shouldn't hurt a tube amp.

2. This is because the DC "resistance" of an 8 Ohm speaker is usually quite a bit less than 8 Ohms (5 or 6 Ohms or so usually) so subbing in an 8 Ohm resistor shouldn't load the amp excessively from a purely resistive point of view.

3. It may, or may not effect your tone. Chances are the louder you play, the more effect it'll have, probably rolling off your high frequencies if I were to make a guess.

4. You'll need a really, really, big resistor! If you match it to the power rating of the actual speaker your talking a 70W resistor. (I just had a look at Ted Webers site and he sells 2 Ohm 50W resistors for dummy loads, so I'd use two of those in series if I had to do this)

5. Wasted power. This is the main issue with banging a resistor in series with the speaker. Whilst your speaker takes all that amp power and turns it into sound, the resistor is just going to turn it into heat. So if its a 50W amp, you'll be burning 25W of that as heat with only 25W left to make screaming solo's.

Anyway, enough theory, my head hurts

 

[Ch0jin]

Is there any benefit of running a tube amp at a certain ohm setting? I'm not talking about matching the speakers, I mean, would someone ever run their tube amp at a certain ohm for saving tube life, transformer life, tone, cleaner sound, etc?

In theory, no. If designed well, an amp -should- sound the same and live for just as long running a matched cabinet at either 4,8 or 16 Ohms.

The theory being that the primary winding of the amps Output Transformer stay the same regardless of the secondary windings used (4/8/16) and being that your power tubes feed that primary winding, they should always be under the same kind of load IF you have matched your speakers to your amp.

However.

I've said before that mismatching your Tube amp and speakers is bad because no matter which way you go (4 into 8, 16 into 4 etc) you WILL lose power, you WILL put strain on your OT and Tubes. and you WILL effect the frequency response of the rig.

That's all true, but I think if we scale that back we can see why some people can hear a difference between say running the same amp at 4 into 4 and 16 into 16.

Lets say the OT isn't that well made, and due to sloppy tolerances the 4 Ohm tap is spot on, the 8 Ohm is more like 7 and the 16 is, I dunno, say 18.

It might all be within manufacturers tolerance, and the amp will work just fine on all settings, but for the sake of argument, you have a 4 Ohm cab and a 16 Ohm cab (Identical speakers, cabinet and wiring of course).

It's possible that because of the slight mismatch between the amps 18 Ohm output and the cabs 16 Ohm speakers, a careful listener would say "The 4 Ohm combination sounds more powerful and a little brighter".

As I said though, in theory, as long as you match your cab to your amp, it should all sound the same regardless of the impedance selected.

Then again, some people reckon they can hear the difference between cloth covered wire and PVC covered wire...............

 

[mainestratman]

@Ch0: :beer: :beer: :beer: :beer: :beer: :beer:

I'm almost sorry I asked, but feel a lot smarter for it.

 

[Telewanger]

Thanks!

I really appreciate it!!!

 

[Plank_Spanker]

ChO once again seals the deal with a home run and closes the thread with the good word! :beer:

 

[tunghaichuan]

ChO once again seals the deal with a home run and closes the thread with the good word! :beer:

+1. We're lucky to have him posting here. His response was lucid and detailed without being overly technical.

 

[duhvoodooman]

+1. We're lucky to have him posting here. His response was lucid and detailed without being overly technical.

Another +1! I know I learned a couple of new things from his last two posts! :AOK :applause

 

[Ch0jin]

Awww shucks

Just trying to help out :AOK

 

[kiteman]

Interesting post and I'm smarter now but still a question.

Set the amp on 16 ohms and hook up the 16 ohms cab. Play.

Set the amp on 4 ohms and hook up the 4 ohms cab. Play.

Now, did one played louder than the other?

 

[Ch0jin]

Do you mean "In theory" or are you asking Telewanger for the results?

In theory, no, they should sound the same, but if some one has tried it out by rewiring their cab and comparing the results I'd love to hear how it went!

 

[kiteman]

Reason I asked is when I see the SS amps with different wattages at diferent ohms in their specs.

Wonder if it's the same with tubes.

 

[tunghaichuan]

Reason I asked is when I see the SS amps with different wattages at diferent ohms in their specs.

Wonder if it's the same with tubes.

Tubes are high impedance, high voltage, low current devices. To interface tubes with speaker(s) the amp needs an output transformer. So the power output should be the same no matter what impedance the amp is set to, as long as the speaker load matches.

Transistors are low impedance, low voltage, high current devices. They usually connect directly to the speaker, no transformer needed. So the higher the impedance, the less power out.

 

[Ch0jin]

Well spotted Kiteman and I'm glad you asked.

I know I covered it in another insanely long post somewhere here, but it's important so I'm happy to answer it again

The short answer is, mismatching impedance with a TUBE amp is not recommended. They are designed to be impedance matched to their speakers. Mismatching impedance takes the combination outside it's intended operating parameters and may result in less power and altered frequency response.

Now SS and Tube amps are VERY different electronically and thats why impedance mismatching with a SS amp is a whole different kettle of fish.

As you've seen, a SS amp will supply more and more power the lower you go with speaker impedance, right down to the lowest impedance the amp is rated at.

In practice I used to suffer from this phenomena all the time when I was playing bass.

I have a 210W SS Peavey bass head that I could run into either a single 4 Ohm 15" box or a 16 Ohm 4x12 box. The 16 Ohm quad sounded nicer, but if I wanted volume I'd have to use the 4 Ohm box to get all the amps power to the ground.

It's quite complex to explain WHY SS amps operate over a wide range of impedance and tube amps don't, but the basics are;

An audio amp of any type should have a very low output impedance. This ensures the most efficient transfer of power from amp to speaker.

Transistors have a very low output impedance and can directly drive speakers because of this. (you can find hi-fi amps with output impedance less than .1 Ohms)

Tubes have a relatively high Output Impedance. A pair of 6550's in a PP Pentode mode amp would have an output Impedance of like 10 Thousand Ohms! WAY too high to drive a speaker! That's why all tube amps, even the little bitty sub 1W ones, have an output transformer.

I'm a bit sketchy on the theory so I'll have to do some reading, but it's accurate to say that because you are using an output transformer, rather than running directly from your transistors, a mismatch of impedance in a tube amp will result in less power and a modification of your "tone".

Does that help?

 

[Ch0jin]

LOL.

Tung snipered in there and beat me with a way better answer

Maybe you can help me out :help

I'm struggling to explain simply why the power transfer ability of a transformer falls off with an impedance mismatch and a transistor does not.

Is it as simple as the the fact that the output Z of a transistor is already so low the impedance of the speaker is negligible at audio frequencies?

I know damping has to come into it somewhere too.

I also have clunking around in my head something about the output Z of the OT being frequency dependent and something about transformers not really having impedance, but an impedance ratio. Meaning that the output Z would be actually also be dependent on the output Z of the tubes used...

OK let me ask a direct question to make it simple.

Why don't we use an OT with a secondary impedance < 1 Ohm, and if we did, would we start to see it behave like a SS amp with regard to power increasing with speaker impedance reduction?

(as an aside, i'm sitting here beating myself up for not knowing this. I really should remember this stuff from uni....)

 

[kiteman]

You guys covered quite well. I understand it better. Thanks.

 

[mainestratman]

I'm loving this (and other electric theory/science) threads. I worked on electronics in the military, but I'll be dammed if I can remember ANY of it... I would really love to get back into it and finally have a wall of stomp boxes. Or just one REALLY big one. :-D

 

[Ch0jin]

Thanks for reading it Kiteman.

I guess both of us could have just said "No" and saved you a lot of time

@Mainestratman

Yeah I have an electronics background too, civvy though. I've spent a number of years in sales and marketing of late though so diving back into science makes me feel good. I like understanding the theory as well as understanding marketing. It's like simultaneously wielding light and dark magic

 

[tunghaichuan]

To be honest, I don't know. But it sounds like you are on the right path. The OT must be the limiting factor. It seems to me that tube amps do not like to operate into low secondary impedances. The lowest I can think of is a Super Reverb which uses four 8-ohm speakers in parallel for a 2 ohm load. The the SR is one of the few. Many amps have a 4 ohm tap. Some amps only have an 8 ohm tap. And some only have a 16 ohm tap.

I would bet this book would have some of the answers:

Electronic Transformers and Circuits, Reuben Lee, 1955, 349 pages

Available on this site:

http://www.pmillett.com/tecnical_books_online.htm

I used to own a hard copy of that book, but stupidly sold it

LOL.

Tung snipered in there and beat me with a way better answer

Maybe you can help me out :help

I'm struggling to explain simply why the power transfer ability of a transformer falls off with an impedance mismatch and a transistor does not.

Is it as simple as the the fact that the output Z of a transistor is already so low the impedance of the speaker is negligible at audio frequencies?

I know damping has to come into it somewhere too.

I also have clunking around in my head something about the output Z of the OT being frequency dependent and something about transformers not really having impedance, but an impedance ratio. Meaning that the output Z would be actually also be dependent on the output Z of the tubes used...

OK let me ask a direct question to make it simple.

Why don't we use an OT with a secondary impedance < 1 Ohm, and if we did, would we start to see it behave like a SS amp with regard to power increasing with speaker impedance reduction?

(as an aside, i'm sitting here beating myself up for not knowing this. I really should remember this stuff from uni....)

 

[kiteman]

I like this stuff too and none of you are not wasting your time with me so I appreciate it. I took a course in basic electronics so I understood the solid states.

Now it's tubes and I'm a sponge. I also took interest in the windsor thread because i have one.

Now let's jam. :AOK

 

[mainestratman]

Another speaker question that's kind of related...

If I'm sending 1200 watts of sound down my speaker cable into two speakers, does that split the wattage between the speakers, or do they each have the full 1200 watts? Does one speaker bear the "brunt" of the wattage and leave fewer watts to trickle down to the other speaker? Does it matter if they are in parallel or serial?

 

[kiteman]

Another speaker question that's kind of related...

If I'm sending 1200 watts of sound down my speaker cable into two speakers, does that split the wattage between the speakers, or do they each have the full 1200 watts? Does one speaker bear the "brunt" of the wattage and leave fewer watts to trickle down to the other speaker? Does it matter if they are in parallel or serial?

I think both speakers will share the load, 600 watts each. Parallel or serial wirings will determine the ohmage of the speakers. Two 8 ohm speakers wired in series will give you 16 ohms. In parallel, 4 ohms.

 

[mainestratman]

Thanks Kiteman. I know about the load variances eon serial and parallel wiring, and thought I'd remembered somewhere that the speakers would divide the load, but unless it has to do with pre-hospital emergency care, it's not in my brain.. lol

 

[deeaa]

One thing few people seem to take into consideration - and which is less of a problem with guitar amps admittedly than long-travel woofers on stereo and bass systems - is that the impedance of a speaker is NOT constant but changes all the time, and the louder it's played the more it changes.

I.e. if you have a speaker with a nominal impedance of 8 ohms, and you push it hard...what happens, is the coil is pushed almost out of the magnet at the peak of the push of the cone, which drops the resistance of the coil quite dramatically, even down to 2 ohms or so.

This is the main reason cheap power amps blow up good speakers.

I have a pair of quite long-throw B&W speakers, and their impedance varies apparently a LOT when pushed. If I run them with a cheapo, say 100W amp, that is rated at 100W@ohms, that cheapo amp may be able to produce, say, 2 watts into 2 ohms. Which means at the end of the travel the speaker is getting too little power; the amp goes into producing square wave and distorts like a mother, and trying to comply the speaker vibrates so that it burns the coil.

It's VERY hard to blow a speaker with too much proper power - I've run 100W amps into 50W speakers for years and years no problem - since if the power is clean and there's no square wave from an amp clipping due to lack of power, your ears will tell you to lower the volume way sooner than you blow a speaker.

BUT with a cheapo amp it's all too easy to blow a speaker due to impedance drop/inability to put out power and square wave production hence.

FWIW, I run my 36W Ceriatone into 16ohm 4x12" and it's FRIGGIN loud, but that is in my understanding more of a speaker efficiency issue than due to impedances used.