Impedence and Connecting external speakers in guitar amp...

[freddemillio]

The output transformer on a valve amp is expecting to see a certain load (ie impedance). If that load is not there then there will most likely be problems. If the current speaker is 4ohm impedance and you connect a lower impedance, you will most likely burn out the output transformer and that is often not something cheap to replace. Also, since the output transformer effects the overall tone a lot, changing this can change the way the amp sounds. That can either be a good or bad thing depending on the change, but it almost certainly will be an expense you'd rather avoid. Attaching a higher load is not as dangerous for the amp but it will change the sound and in the long run, it might not be the best thing for the output transformer.

So pay attention to your impedances, but also learn/understand how to add impedances in series and in parallel so that you are getting the right answers. There are differences between just changing one speaker for another and for wiring speakers together. If you do not have a grasp of how this works, google around for some sites on basic electronics and you should be able to pick up the info you need.

Tube amps can keep high (lethal) voltages for awhile after they are turned off, so if you are going to poke around inside the amp, do it safely. Before poking around, turn it off, physically disconnect it from the wall current, and let it sit for awhile. Google around for more precise info.

Changing speakers will make a change in sound and that can then make a change in perceived loudness. Changing speaker configurations will also change sound dispersion so you will cover more of the room (or not) and that in turn will effect perceived loudness. As has already been mentioned, speakers have different efficiencies and that will effect perceived and measured loudness. Speaker cabinet design will effect the tone and dispersion and again perceived loudness will change. All this adds together and so making predictions is not easy. The best thing is to test the change, as changing the cabinet/speakers will certainly change something, but it might not be a change that makes you happy.

By the way, 5 W is plenty loud for practicing alone or most recording situations. It might be a bit weak for group playing especially if you are trying to keep up with a rock drummer. But for practicing or recording I prefer 5W/tube or even better something like 3W.

[C00kie]

I've just looked up the manual of this Fender Champion 600 amp. It says:

SPEAKER — Connection for the internal speaker or a 4-ohm minimum external speaker cabinet.
[...]
SPEAKER: 4Ω, 6" Fender Special Design

Since 2 Ohm speaker cabinets are hard to find (probably it's two 4 Ohm speakers in parallel then) you should stick with proper 4 Ohm cabinets. Most Marshall stacks are 16 Ohms. The difference is too much. You'll probably damage the amp (it's most expensive part: the transformer) in the process.

[beboop]

Attaching a higher load is not as dangerous for the amp

Actually for tubes amps it's the other way round -- higher impedance loads are more dangerous.

Connecting a higher impedance load than specified to a transformer coupled tube amp is what fries output transformers -- the higher impedance the load on the transformer secondary, the higher the inductive kickback voltage across the transformer primary and output tubes. Too high, and the tubes and/or primary winding arc.

That's why the default no-speaker-plugged-in configuration for a tube guitar amp is a dead short across the secondary -- no, you don't want to run the amp that way, but it's less damaging than an open circuit.

Some guitar amps are designed to accomodate different loads -- Marshalls and Boogies with separate secondary taps, or old Fenders with an output transformer built to handle half the optimum matched impedance with no problem. But amps like that will be labeled with the rated load.

In the case of a Champ, it's a cheap amp, with a cheap output transformer (that just happens to sound great). It'd be a very good idea to hook it up to a cabinet with the rated impedance. Anything else is gonna be iffy, _especially_ a higher than rated impedance.

and yeah, it will sound great through a big cabinet

[Van_Mocen]

I'd guess Fender has already put in a darn efficient speaker in their amp. So using another speaker cabinet would alter the tone (more bass) but probably not be any louder.

Want louder? Get a frigging big amp like the big boys do.

that was just an example, i've got a fender hot rod deville.. is it enough for you ? you know what the say... the bigger the boy.. the smaller the d***.

[Van_Mocen]

anyway,that was helpful guys, thanx. but it was also a bit confusing. anything regarding impedance and some electronics in general has got to be my weak point, my real dumb side. eh eh

when you speak about 'driving a load' what does it mean ? the amp driving the speaker, is that it ?

and when it comes to connecting a cabinet to an amp, which one of the two (the amp or the speaker) can have the biggest impedance , if you know what i mean ? just to finish the subject, when people talk about matching impedance, is there actually a device or a way to do this ?

thanx very much !!

[C00kie]

Some basic electronic knowledge is what you need to understand it indeed.

First let's look at some wiring schemes of speakers.

If you wire speakers in series, then the impedance in whole will be the sum of the impedances of each speaker.

So put two identical speakers in series, and the impedance will double.

If you put two speakers in parallel, then the total impedance can be calculated by summing the reciprocals and take the reciprocal of that.

If you put two identical speakers (or cabinets) in series, then the total impedance will be half that of one speaker.

The amp here is rated 4 Ohms. If you want to connect two speakers, then you can use two 8 Ohms in parallel, or two 2 Ohms in series. Both schemes total to 4 Ohms.

Consider the amp as being a simple power supply. It delivers a potential (voltage) and current (amperage). By definition the wattage is V * A.

Consider the speaker as being a simple resistor. You can swap impedance for resistance for simplicity. By definition resistance is V / A.

With these two formulas and given the wattage and impedance you can work out that V = 4.5 and A = 1.1 (or more precise, which isn't nescessary: V = 4.472136 and A = 1.118034) This is the situation the amp is designed for. Keep it that way, and it will live happily ever after.

So what happens when you change the speaker to one of 2 Ohms? Assume the transformer will keep the voltage the same (4.5 V) while the amperage doubles to 2.2 A and also the wattage will consequently double to 10 W. Fender says the amp will work OK in this situation. If you lower the impedance further there will be a point at which it will break.

And what if you connect a 8 Ohms speaker? Voltage remains the same (that's how an ideal transformer should behave) while the amperage drops to 0.6. At the other end of the transformer there's a current produced by the tube that's not consumed. The tube and/or transformer doesn't like that, and will get hot (the energy's gonna go somewhere!?) and wear or break.

This is what I make of it. I know there are some holes in my reasoning. Let the people that know it all fill them.

I hope you now understand that the impedance of a tube amp and connected speaker cabinet have to match to ensure a long working life.

[C00kie]

Just thought of one more thing: is there a device that changes impedance?
Yes. It's called a transformer.

The ideal transformer keeps the wattage the same (if it doesn't get warm) while changing the voltage and amperage. It's quite hard to find a transformer that can handle say 50 W and has a primary impedance of 4 Ohm and secondary of 16 Ohm. Custom job probably...

In the tube amp there is already a transformer. The primary coil accepts a voltage coming from the tube in the order of 100 to 300 volts with a low amperage (so impedance is relatively high) and the secondary coil delivers a lower voltage with higher amperage (so impedance is very low, matching the speaker)

Some amps have an impedance switch at the back. That switches between different outputs of the transformer: different numbers of windings on the secondary coil of the transformer, giving different voltages and thus different impedances.

[Van_Mocen]

thanx a lot, that was really helpfull, cheers !

[Van_Mocen]

i just have one last question...

we've talked about connecting different speakers.. but lets say I have a marshall TOP whose impedance is switchable between 2-4 ohms and i wanna connect it to a 8ohm marshall cabinet ? what type of connection are we talking about in here ? or is this one of the cases where i need one of them impedence matching transformers ? thanx once again, damn my slow comprehension sorry guys lol

[freddemillio]

The short answer is that there is no way to safely, reliably connect the 2 or 4 ohm amp to the 8 ohm speaker cabinet. You need a matching transformer in between the two or you need to rewire the speaker cabinet (this might not be possible, you did not give any details about the speaker cabinet so it's tough to say if this is really an option) to 2 or 4 ohms. Or you would have to wire in an 8 ohm/100 W resistor (that's a pretty big resistor, it is not like one of the little ones you find on a circuit board) in parallel with the cabinet so that the effective impedance looks like 4 ohms. I am not aware of any simple (ie, plug and play without any tweaking), secure way to do what you suggest.

[tranceglobal]

Some basic electronic knowledge is what you need to understand it indeed.

Those are nice inductors. If you're going to teach them at least use the diagrams with resistances as loads.

Cheers,

D.

[Van_Mocen]

thanx a lot once again

[C00kie]

Those are nice inductors. If you're going to teach them at least use the diagrams with resistances as loads.

Inductors have more resemblance with speaker coils than resistors. That's why I used them. I could have used the schematics with resistors instead ofcourse:


Happy now?