Speaker wiring, ohms, watts, spl..

Speaker wirings

Here's a pretty good article about Speaker Wire

It's explains in both laymans terms and tech terms, and contains cables laws and tables in dimentions vs length et al..
Less applicable for short guitar amp<->speaker cables; very good for PA stuff.

How many watts different speakers with different power handling capacity can handle:

Incorrect: ~~75w + 60w = 135w loadable~~
Correct : 75w + 60w = 120w loadable, because power will be distributed evenly across the two different drivers, meaning they would each recieve half of the 135 watts, i.e. the 60 watter would have to deal with 67.5 watts.

In other words, in a two-speaker setup, you can only apply twice as much as what the weakest link can handle. by the same token, in a four way arrangelment, it'll be four times that of the weakest link.

Further, this require that the drivers have the same impedance, i.e. the same Ohm's.
But.. you should never mix different Ohm'ed speakers - unless you know Ohm's law (see wikipedia) and understand voltage/current/power distribution

How many watts can guitar speakers really handle?

Just because a speaker is labelled i.e. 60 watts doesn't mean it can always handle the 60 watts.
The way we manhandle speakers with all kinds of distortion, EQ'ing and effects can change this.
In technical terms, applying our belowed distortions increases the energy in the signal, usually expressed as the RMS value, which will put more electrical load on the speakers: The voice coil simply gets hotter.

Also, how the cabinet is made, it's dimentions and hense volume, open baffle or sealed, presense of dampening material inside or not.. will all determine how the cones are loaded and how well they're controlled and dampened, and as such how much they physically flap back'n'forth, stressing the suspention, and eventually reach their damage excursion point.

Coming up with a calculation formula isn't straightforward.

There's a good reason a 4x12 stack is outfitted with four 50 watters for a mere 100 watt amp..
Instead of attempting lots of theory, simply make sure guitar and bass speakers are well oversized, especially if you're into heavy distortions et al..

Maching speakers to amps:

Speakers should generally be wired to match the speaker Ohm's (i.e. it's output impedance).

You can, however, always safely use speakers at a higher impedance than the amp impedance, i.e. a 16 Ohm speaker to an 8 Ohm amp.
Doing so will drop some power (theoretically half, though reality doesn't feel quite like it), and the damping characteristics between speaker and output transformer (in the case of a tube amp) will be different, possibly/maybe resulting a slightly different sound, due to different amount of cone 'ringing'.

In the case of a transistor or FET amp, this loss of damping doesn't matter, unless it's a pretty poorly designed amp (some self-proclaimed experts may argue otherwise, but you can overlook such comments).

If the amp has 8 Ohm impedance, four 8 Ohm speakers should be arranged as:
. two by two-in-series, and those two series sets wired in parallel
. two by two-in-parallel, and those two parallel sets wired in series

In the case of an older 15/16 Ohm amp, the same wirings as above can be used with four 15/16 Ohm speakers, or four 4 Ohm speakers can be wired in series.

For a 4 Ohm amp, four 15/16 Ohm speakers can be wired in parallel.

Deliberatly mis-maching speakers to amps:

You can keep a cab wired at i.e. 16 Ohm, and set the amp to any of 2/4/8 Ohm without damage.
As long as your cab has a higher impedance than the amp, all will be well.

Having the cab at higher impedance than the amp will cause a little or some power loss. This you may hardly notice.
However, this power loss can be used to allow cranking the amp a Bit higher up for having the right tone, when working on smaller venues.
The power loss with an 8 Ohm amp into a 16 Ohm cab will hardly be noticeable; a 2 Ohm amp into a 16 Ohm cab could drop the volume just enough to allow cranking the amp.

Running amps with no speakers connected:

In general, tubeamp should never be turned on without a speaker connected, as this can damage the output transformer.
Some tube amps have a build-in load resistor, which is there exactly to prevent this from happening. Check you amp specs or the schematics.

Solidstate amps can allmost always be operated without a load. A few solid state amps includes an output transformer, and could be damaged like with tubeamps.

Wiring speakers in an X pattern:

X pattern wirings (nope, I never did this) may sound like it makes sense when one set of speakers are mounted lower and a different set mounted top row.
It may be faintly possible that an X wiring might make them blend better, in a fashion, though I have a hard time understanding how the current flowing through them would know about the chosen wiring scheme..
X wiring four identical drivers doesn't make sense at all.

Watts, dB, sound pressure level (SPL)
SPL is shorthand for Sound Pressure Level.

Many gets a little shock when realizing how little power is really needed at home/bedroom levels - live is quite a different story..
So, let's examine how ~~many~~ few watts are really needed, at least for studio, rehearsal and club levels.

Let's start by assuming any guitar speakers will deliver at least 90dB SPL at one watt RMS.
Doubling the power doubles the SPL, which is a 3dB increase of the SPL.
So, at two watts RMS, we have 93dB. 4watts = 96dB. 8watts = 99dB. 16watts = 102dB. 32watts = 105dB. 64watts = 108dB. And so forth...

Using a more efficient speaker makes it more interesting. Let's consider a speaker delivering 93dB SPL at 1watt RMS.
1watt = 93dB. 2watts = 96dB. 4watts = 99dB. 8watts = 102dB. 16watts = 105dB. 32watts = 108dB.

In other words, the 93dB speaker plays twice as loud, requiring only half the power.
Many guitar speakers will easily deliver 90-94dB at one watt RMS, which is the reason a Tiny Terror plays so unexpectedly uncanny loud happy

Notice that the same holds true to all speaker systems, guitar/bass, PA and hifi alike.

As can be seen, initially not much effect is needed to reach a decent sound pressure level. It's when we need to play fairly loud that progressively more power is needed.
How many cute watts are suddenly needed depends not only on how loud we need to be, but very much on how efficient the speaker system is.

In the examples, I stopped at 108dB. Using speakers delivering 90dB@1watt, to reach 120dB, roughly 1000 watts would be needed.
If the speakers had been twice as effective, i.e. delivering 93dB@1watt, only 500 watts would be needed.

When Zilthy in here speaks of 1300 watts and 1800 watts to the subs, they're likely not using the whole power train, but rather is having some available for handling peaks and transients - with as little distortion as possible (for a live system).

So, if referring to the fairly common fairly compact 350-500 watt systems, if those employ decently efficient speakers, you'll likely be able to rock the house just fine, unless doing decently large venues.

Further, if starting with, say.. a 350 watt compact system, nothing prevents later using an additional crossover to limit this system to handle only frequencies above, say.. 100Hz, letting a dedicated sub handle the lows.
Doing so will allow cranking the volume on the compact system higher up without introducing more distortion and risking blowing the woofer, because that one now won't need to handle nearly the same physical excursions.

Driving speakers from a preamp

Q:
Will I hurt the speaker or the preamp by running the cabinet directly from the preamp outs?
The volume level is much more suitable and actually just loud enough...

A:
You shouldn't use any preamp for driving a speaker, unless the manual says it's designed to do so.
Preamps are build without driver/output stages suitable for driving a low-impedance speaker voicecoil (which is working within a fairly nifty magnet field, controlling the coil).
The preamp simply isn't build to source the needed current, and even if it does work, all chances are it's output suddenly fries.