Guitar Amp Impedence ~ Series or Parallel?!?

Oh, I also have a second question which i would be greatfull for an answer to. :) What is the wattage of the loudspeakers likely to be? I know the amp head is 15watts so would each speaker be 15watts?

iDevil

You have nothing to worry about, they are the speakers that come with this head. So obviously they are both 16 ohm speakers (very common with Marshall) and the two Speaker Outs on the head are wired parallel. So when you hook up both speakers (as is intended) you will have a total 8 ohms load. Marshall is not going to send you two speakers that instantly damage the head are they? That would not be a very smart business practice in my opinion.

But those markings are on there in case you use other speaker cabs with the amp. You could hook up any combination of speakers or cabs as long as you do not go below 8 ohms total impedence. Impedence is just resistance to current flow. If you go below 8 ohms, too much current will flow through the amp and it will overheat and possibly damage it very quickly.

You can go over 16 ohms total load, but the amp will not perform at full power. I always say it is like driving your car with your foot on the brake pedal. The high impedence of the speakers (resistance to current flow) is not allowing enough current to flow through the amp. If you go over 16 ohms you will probably notice the amp does not seem nearly as loud or powerful.

So your amp is designed to work best at 16 or 8 ohms. It will work best at 8 ohms.

This might confuse you but it is hard to explain how many watts each speaker will get. Actually, it is easy to explain with both speakers connected, then the amp will operate at the full 15 watts and the two speakers (because they are identical speakers) will share the watts evenly. So each speaker will get 7.5 watts.

Now, you would think with one speaker it would get 15 watts? Not necessarily. If that one speaker was an 8 ohm speaker, then the amp can operate at full potential and yes, it would get 15 watts.

But if you hook up just one of those 16 ohms speakers, now it is just like I described, like driving with your foot on the brake pedal. Usually, an amp will produce about 70% power at 16 ohms than it would produce at 8 ohms.

So 15 X .70 = 10.5 watts

I bet you will notice that the amp seems much louder with both speakers connected. It should for two reasons, you are pushing more air with two speakers, but also, the amp is operating at it's full potential of 15 watts.

Hope that helped, and have fun with your new amp! :D

Thankyou very much for your reply! it helped a lot. Yes, i wouldnt have thought they would have given me some dodgey speakers that would blow up straight away:P but i was thinking about if i was to get a new amp head, like the Zvex nano head.. i was just wondering the impedence of the speaker cabs so maybe if i was to eventually get a zvex head then they would work. Anyway, thanks again for the reply, very helpfull indeed :D

Oh, and a 3rd question: If i was to go out and buy an 8ohm cab, would i be correct in just connecting the one jack to either of the jacks on the back of the head? And also, if i was to get a 4x12" cab, would this 15 watt head power it without being damaged? (providing that obviously its a 8 or 16ohm, prefreably 8ohms for full power from the head)

iDevil

Oh, and a 3rd question: If i was to go out and buy an 8ohm cab, would i be correct in just connecting the one jack to either of the jacks on the back of the head? And also, if i was to get a 4x12" cab, would this 15 watt head power it without being damaged? (providing that obviously its a 8 or 16ohm, prefreably 8ohms for full power from the head)

Yes, if you buy an 8 ohm speaker (or cab), just connect it to either one of the Speaker Outs on the back of the head. Now, do not add any more speakers or cabs, that will take you below 8 ohms (most speaker cabs are wired parallel, but not all).

I am not an electronics guy, but I studied impedence years ago when building my PA system. But I like to think of it in terms people can understand. The reason you get lower impedence (or resistance) with multiple speakers is similar to water flowing through a pipe. More water can flow through a larger pipe. And the same with electricity, more electrons can flow through two speakers than one. Hope that makes sense.

The exception is series. In series, two 16 ohm speakers equals 32 ohms. So, you could actually find a cab with two 4 ohms speakers wired series for a total of 8 ohms and connect it to your head. There are cabs like that.

To answer your second question, the number of speakers makes no difference whatsoever as long as the total load is 8 ohms or above. Almost all of the old Marshall 4 X 12 cabs are rated 16 ohms, you could use one of these no problem, however at 16 ohms your amp will only put out about 10.5 watts. And each speaker would get only about 3 watts!

But you would be surprised how loud that is. I have a little Smokey amp that is the same size as a pack of cigarettes (that's why they call them Smokey amps). It is 9V battery powered and puts out 1 watt. But it has a real Speaker Out on it. I have connected it to a 2 X 12 cab (8 ohms) and you would be amazed how loud it is, loud enough to disturb the neighbors!

And the sensitivity of the speaker makes a big difference. Look up speaker specs and you will see specs like this:

98 dB @ 1W/1M

This means this speaker will produce 98 decibels volume with only one watt of power applied at one meters distance. Yep, just one watt. And that is loud.

And everytime you double the watts, you get approximately a 3 decibel increase in volume.

1 watt- 98 dB
2 watts- 101 dB
4 watts- 104 dB
8 watts- 107 dB
16 watts- 110 dB

So, one way to get lots of volume from your amp is to buy a speaker that is very "efficient" with high sensitivity rating.

On the downside, many times a speaker with very high sensitivity ratings sound harsh. Bass requires more power than high frequencies. So many times a speaker with very high sensitivity ratings does not produce much bass. They can be harsh sounding.

I know you didn't ask for all that, but it's good stuff to know.

Oh yeah, Welcome to Guitar Noise. Visit the forum, lots of friendly folks here.

What Wes said ... nice job Wes! 8)

Thankyou for your replys Wes, Much appreciated. I have learned a lot of interesting stuff from this. :)

You're welcome. :D

Hey, you should get two Marshall 4 X 12 cabs rated 16 ohms and hook them both up to that head. That will be a 8 ohms load, the amp will put out 15 watts, and each speaker will get not quite 2 watts each!

That would wail. :twisted:

Wes. I have the HUGHES & KETTNER Warp 7 cab, it's rated 4-8 ohms with two output sockets. Manual here for more info http://www.manualslib.com/manual/73349/Hughes-Warp7.html?page=8

Is this going to fry the marshall head unit mentioned in previous posts?