Newbie question about speakers

[zener]

I know how speakers' impedance in series and parallel. But how about the wattage? Does it add up when you connect two speakers either in series or in parallel?

I'm planning to make a 20W bridged amp (TDA2003). The problem is I only have two 6" 15w speakers. I really want to use these two speakers that I have if possible.

I already have an alternative if the bridged amp is not possible due to the speakers wattage. I'm just going to make two separate 10w amp, each with a 15w speaker. I would put the two amps in one cab. I'm not just so sure how am I going to make one guitar input feeding both amp at the same time? Is it as simple as a splitter jack-plug?

Also, any tip on building a wooden cab for this amp I'm planning to build?

Thanks for any help

[Ge_Whiz]

As I recall, bridging two ten watt amplifiers (and remember, you need to arrange for the two to receive out-of-phase inputs) actually quadruples the available power, so you would have the equivalent of a 40W amp (but it won't sound four times as loud as a 10W - or even twice as loud). Normally, you can allow that the power is split between the speakers, but you must allow some leeway. In this case, you'd be overloading them.

I'd personally build the two 10 W amps in the same box, one speaker each - much more versatile. Then you can couple the inputs (best to use a simple buffer stage) or use them separately for sonic effect. But others may disagree...

[smoguzbenjamin]

If you use more than one speaker but they're all the same impedance then the poewr is sortof distributed evenly. So check out the power of your amp and buy speakers accordingly. However, try leaving about 10 to 20W of room just in case the amp's underrated. I have a 10W amp by Ibanez and it came stock with a 30W speaker. But that's only true when the speakers are the same impedance. But I read somewhere that using speakers of different impedances isn't a good idea anyway

[zener]

OK, perhaps I'll just go with the two amps in a box. My plan is to hook up this dual opamp preamp/EQ


I will add a vol control after the EQ and if necessary, a volume recovery stage.

So, I'll just take the output of the preamp/EQ and connect it at the respective input of the two TDA2003 amps. So, the two amps will have one preamp/EQ and the same ground, right?

I've just thought about this. What if the two power amps will have their own preamp/EQ? Then, I would just have dual gang pot for all the controls. Is that possible? Does it make any sense? What do you think?

I'll be powering this amp with just one power supply. To make sure there will be no ground loop, I'll just hook the V+ and power ground to just one of the two amps and only the V+ for the other. Isn't that right?

Thanks a lot

[spongebob]

If you want to get in the 20-30 watt region, why not use a single TDA2030/40/50? Drawback is that it would require a bipolar power supply, but part count will be much lower, I soldered all the resistors and caps directly to the chip pins and mounted it on an old cpu heatsink. Works perfectly, I can make a picture of it later...

[zener]

I know that I can always go for something bigger, more powerful. But of course, as always, it will cost me a lot to buy another chip and another speaker :? .

I have two TDA2003 and two 15w speakers that were been lying around for a month. The speakers were just salvaged from the junkyard. I just want to make the most out of the things that I already have at the least possible cost. :wink:

[spongebob]

I have two TDA2003 and two 15w speakers that were been lying around for a month. The speakers were just salvaged from the junkyard. I just want to make the most out of the things that I already have at the least possible cost. :wink:

Sure, if you already have the parts together, use them!

Here's a schematic from the datasheet:


The schematic shows a 4 ohm speaker load, so your two speakers in parallel should be fine (they are 8 ohm each?). If each one handles 15 watt, I think it is safe to put out 20 watt into both of them.

[zener]

(they are 8 ohm each?) If each speaker handles 15 watt, I think it is safe to put out 20 watt into both of them.

Yeah, they are of 8ohm. Actually, I've already seen that schem and I knew that putting two similar speakers in parallel would reduce the impedance by half. That lead to this thread :wink:  

Are you sure that the bridged amp wouldn't blow up my speakers, as if the wattage of the two speakers are added to provide 30W, therefore more than the rated power of the bridged amp which is 18 - 20w?  

Thanks

[spongebob]

Are you sure that the bridged amp wouldn't blow up my speakers...

Not 100% sure, but if it's 18 watt into a 4 ohm load, and your 4 ohm load consists of two parallel 8 ohm loads then each speaker should dissipate 9 watt, right? At least that's my understanding, but I'm no expert at all...

I would just try it, if you are careful with the volume knob there shouldn't be any danger :lol:

You could always lower the supply voltage, as this drops the power output (power = voltage*voltage / resistance)

[Ge_Whiz]

If you built two pre-amp modules, you can use the system as 'stereo' - okay, not much separation, but useful. If you have only one pre-amp, I would recommend that you split the output through two 10k resistors before feeding into the two power stages, to keep the power amp inputs separated. It shouldn't affect the volume much.

[Mark Hammer]

The rating of speakers for wattage is partially a statement about how much heat they can tolerate before the voice coil burns up.  The voice coil itself is essentially like a wire-wound resistor.  In the same way that resistors can only take so much current before the heat resulting from passing too much current destroys them, voice coils can only take *so much current* and that partly accounts for their wattage rating.  You CAN use thicker wire to wind the voice coil and so increase tolerance of more current without heat damage, but that also increases the mass of the coil, which of course necessitates either a more powerful or more efficient magnet structure.

Of course, where voice coils and resistors differ big time is that resistors just sit there, where voice coils are in motion.  That motion, in turn, can result in heat stemming from friction, on top of whatever heat occurs simply by passing lots of current through skinny wire.  Top notch speakers are designed to provide as purely piston-like straight a movement as possible, so that there is no lateral wiggle or rubbing of the voice coil against the sides of the magnet structure.  Some speakers also use fluid in that gap to help drain off heat.  Cheap speakers don't spend as much effort to make the magnet gap efficient.  The voice coil may rub less against the sides, but that's because the sides are farther away, which, in turn makes the speaker less efficient.

Part of the heat build-up stems from the ability of voice coils to actively dissipate the heat that is acquired via friction and current.  I mentioned fluid cooling and you will sometimes see larger speakers with a kind of "finned crown" around the magnet assembly specifically to dump heat.  

So, what does this all have to do with your question?

Think of speakers as being like heatsinks.  When the same amount of output current from the amplifier is divided up amongst multiple speakers (providing the same impedance load) each voice coil is subject to less heat.  The wattage rating of the individual speakers provides an approximate indication of how much current-related heat the whole speaker assembly can take.  So, if you have one 4-ohm speaker rated at 30W, replacing that with two 8-ohm 30W speakers in parallel provides reasonable insurance against heat damage from a 50W amp run full tilt.

I say "reasonable" because speakers are subject to damage from non-heat factors too.  That cone can only move so far forwards and backwards, and extended excursions at very high speeds can strain both the voice-coil structure and the cone.

Note that use of multiple speakers that result in *different* loads than the original can change things.  So, an amp may be able to deliver 5W into 8R.  If that load is changed to 16R by means of two speakers in series, the output wattage will drop, usually by half.  If the load is changed to a pair of 8R speakers in parallel, yielding a 4R load, the output power will increase (often doubled).  So, it is not enough to simply add up the wattage ratings of the individual speakers and compare them against the spec'd output of the amp into a "standard" load.  One needs to ask what the output would be for that load.  It may make the speakers under-rated, OR overrated, with respect to heat tolerance, etc., compared to what you originally thought.

[ejbasses]

Hey man

Paralleling the 2 speakers you have should be okay. That would be good enough to handle 20 watts of power. Cheap practice amps use almost exact same design your using even the preamp.

For the cabinet, its gona be a small amp so 1/2 plywood will be enough. separate the amp from the cabinet itself and shield it.

you can put the speakers horizontally side by side. Also using an open back configuration will let you have more volume. Closed back cabs eat a lot more power but have better bass response.

Just remember to make the cabinet solid and take the time to make it look good. Youl have a better time playing through a sweet looking DIY amp.

Dont forget to post pics!!