The inevitable speaker question...

[armstrom]

I'm sure this has been brought up countless times on message boards such as this but a quick search of the history here didn't seem to yield much so I'll go ahead and ask.

with a solid state power amp, how do you decide what impedance speaker to use? I know all the simple electrical stuff that the same power amp will output a lower wattage through a higher impedance speaker (basic ohm's law, same voltage swing through higher impedance = lower wattage) and that you have to make sure not to exceed the maximum current capacity of the power amp... but how about the speakers themselves? Is there any tonal advantage to choosing the 8Ohm version of a particular speaker over the 4Ohm? (or vice versa).  How about loudness? Most manufacturers only give the SPL for one impedance, even if the same speaker is offered in other impedances...

So, here's where the rubber meets the road. I'm trying to choose an affordable(ish) speaker to use with my LM3886 power amp and I'm not sure if I should go with a 4ohm or 8ohm. I plan to keep the rail voltages at a reasonable level so the current draw should be fine regardless of which choice I make.

Obviously, the 4ohm will mean the amp is "putting out more watts" but will that make the 4ohm speaker significantly louder than the 8ohm assuming all else stays the same? How about the tone? I know this is highly subjective and varies from speaker to speaker, but is there a general guideline regarding the sound quality of 4ohm vs 8ohm drivers? One of the speakers I'm considering is the Jensen Mod 12-70 http://www.jensenvintage.com/mod12-70.htm which is available in 4/8/16 ohm configurations.

I know this is probably a vague question but I'm a bit lost. I know any of the choices will "work" with my amp (not going to blow anything up) but what configuration will extract the most from the amp (highest output while still maintaining good tone).

As always, thanks in advance for any and all advice you can give!
-Matt

[darwindeathcat]

I'm curious to know the answer to this question too, as I think I'm in about the same spot as Armstong there (by the way, Armstong, are you the same Armstrong who has been posting on the valvecaster and the Prof Tweed over on the DIY Stompboxes forum? If so, I read over there that you are building a Harp amp. So am I! It seems that we are trying to build almost the exact same set up!)

Another thing I'm curious about is wattage and parallel wiring of speakers. Do they act like resistors where wireing them in parallel increases the overal wattage capability whilst decreasing the overall resistance? ie. if you wire two 8ohm 15watt speakers in parallel, does the amp "see" them now as on 4ohm 30watt speaker?

[darwindeathcat]

sorry, meant "Armstrom", not "Armstrong"!

[armstrom]

not a problem, my name is Matt Armstrong, hence the ArmstroM (Unix style user name, can't seem to break myself of it).

I can answer your question about wiring in parallel vs. series. Yes, speakers behave the same as resistors. You can really think of them as "dynamic" resistors. The resistance varies as the speaker motor moves at different frequencies so the values you see quoted are really just nominal values. Impedance factors in static resistance of the coil as well as its reactance.

So, wiring two 8Ohm speakers in parallel will "look like" a 4 ohm speaker to your amp while wiring in series will "look like" 16 ohms.

As for the amp I'm building, it's pretty much done. All that's left to do is apply the decal to the control panel. You can hear some clips in the last two posts of this thread: http://www.ssguitar.com/index.php?topic=899.0

Hopefully we'll get some feedback on the process of choosing a speaker impedance though
-Matt

[darwindeathcat]

oh cool, I'm a *nix man myself!  :tu: I'll def check out the clips when I get home (can't do that sort of stuff at work!)
About the speaker wiring, I was already aware of the effects to impedance from Parallel and Series wiring of speakers, I was asking more about the effects to power rating. I know that if you have a need for, say, a 2kohm resistor that will dissipate 1/2 watt of power (say the current limiting resitor for an LED between your power rails), but all you got is 1/4 watt resistors, you can wire two 4kohm 1/4 watt resistors in parallel for the equivalent. My question is: does it work the same way with speakers. For example if you have an amp that will push 30 watts into 4ohms, and you have two 15watt 8ohm speakers, will their sum power rating in parallel now be 30watts or will it still be 15watts. On the surface, it seems to me that they ought to now be able to handle 30watts, since if they are in parallel, they should each be taking exactly half the current presneted to them (assuming that the hookup wire, etc. leading to each is identical in length and resistance). If you wire speakers in series, will the power rating change? I don;t think it should since they will both see the same amount of current as if they were wired independantly. I am basiclly seeking confirmation of these thoughts from people who are more sure in their knowledge than me!

Cheers,

Isaac

[Enzo]

Power shares, yes.   Series or parallel, doesn;t matter, they still share.  FOur 25w speakers wires series/parallel in a 4x12 cab will make a 100 watt cab.  Two 15 watt speakers wired together will take 30 watts.

If you have two different impedance speakers, they will not share equally.  That is if you place a 4 ohm and an 8 ohm speaker parallel in a cab together, the 4 ohm speaker will take 2/3 of the power applied.  But mostly let's deal with groups of similar speakers.

You can ignore the cab wiring length and resistance.  The length doesn;t matter, electricity travels a foot of wire in only a nanosecond.  The resistance is going to be a small fraction of an ohm.  100 foot cables is one thing, but inside a cab, wire resistance is small.

[Jack1962]

Generally the higher the impedance the better the sound quality. It all depends on how many speaks your going to use and how you plan on wiring them in(parallel or series) myself I usually buy the 8 ohm speaks , they are easier for me to cofigure.

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