BEGINNER!! Need help understanding Crossover/Impedance/Wattage

[sdbarnold]

I just built my very first speaker with the little knowledge I gathered from YouTube and research on different sites, but I'm still having trouble answering a few questions in regards to what power amp to use, and how the crossover's I purchased affect wattage and impedance.


I built mostly based on design, and that I just wanted to give it a try, so if my design and or choices for hardware are way off, that's why (and also why I'm writing this, so I can learn for next time).


Consists of .....


2 2-way speakers (w/3" Peerless PLS 75F25AL04 [8 ohm] and 4" Dayton Audio PC105-8 [8 ohm])


Peerless
8 ohm, 15wRMS/30wMAX, 100Hz-20kHz
Dayton Audio
8 ohm, 40wRMS/80wMAX, 80Hz-15kHz


Crossovers are DROK 2-way
150W, 4-8 ohms, 12db/oct on high and low frequency, 3200 Hz split
Amazon.com: DROK 150W Bass Treble Dual Channel Audio Frequency Divider Distributer, 4-8 Ohm 2-Way Crossover Filter for Subwoofer Hifi Speaker: Car Electronics

Amp is DROK 2 channel 50W digital amplifier
Amazon.com: DROK 50W+50W Dual Channel Stereo Amplifier, DC 9-24V 12V Mini Bluetooth 4.0 Audio Amplifier Car Home, HIFI Audio Receiver Amp Ampli Module with US-type Power Supply Adapter, Built-in EMI Filter: Home Audio & Theater


Here are my questions .....


Power - How is wattage calculated going through a crossover? Are the speakers split between the two channels going through the crossover? (meaning is the 15W and 40W added up = 55W then split in two to roughly 27W?) Or is the power I need the total 55W at what impedance? Also, what does the 150W rating on my crossover mean?


Impedance - How does the crossover affect impedance? I have two 8 ohm speakers, so does that make the impedance going through the crossover go down to 4, and therefore 4 ohm load needed from my amp?


I know, you're thinking this info is easily found on the internet, but for some reason every search I entered did not answer these questions. A little help is greatly appreciated!!! Thanks in advance from a new DIY Audio member.


-sdbarnold

[Gnobuddy]

Originally Posted by sdbarnold

Power - How is wattage calculated going through a crossover?

Wattage (power) is usually measured with a sine wave test signal, at one single frequency. The amount of power delivered to the speakers is determined by the amplifier and speaker. An ideal crossover has no losses, and will have no effect on power delivered to the speaker - as long as the sine wave is within the frequency range that the crossover lets through itself.

Real crossovers have some small amount of power loss, because of the resistance of the inductor. So they will slightly lower the amount of power delivered to the speaker.

Things are different if you are talking about music, rather than a sine test tone. If music is played through the speaker, then the crossover divides up some frequencies to send to the woofer, and some to send to the tweeter. If you measure average power, usually most of the power ends up going to the woofer, and very little to the tweeter. But there may be very short bursts of higher power levels to the tweeter.

Since most of the power goes to the woofer, you can think of the system as still having only 40W of power handling capability.

I should add that the "40 W" number may be optimistic - some manufacturers inflate their power ratings to make the advertisement look more tempting. I have seen "25 W" speakers that would burn up or fall apart if you fed them even 10 watts.

Originally Posted by sdbarnold

Are the speakers split between the two channels going through the crossover? (meaning is the 15W and 40W added up = 55W then split in two to roughly 27W?)

15W and 40W are not the actual power delivered to the speaker - they are only a manufacturers specification for the maximum amount of power that speaker can handle without damage.

The actual amount of power delivered depends on the amplifier (how many volts is it putting out?), and speaker impedance. That may be only 0.1 watt, or it may be 1 watt, or it may be 10 watts, or anything in between.

An analogy: if your car has a 15 gallon gas tank, that doesn't mean you have 15 gallons of gas in it. You may only have 5!

Originally Posted by sdbarnold

Or is the power I need the total 55W at what impedance?

Forget all about that 55W, it is confusing the heck out of you.

The power you need is whatever is enough to play music at the loudness you want.

The speaker power handling needs to be able to cope with this. Hopefully, you are satisfied with the volume level before you get to 40 watts.

Originally Posted by sdbarnold

Also, what does the 150W rating on my crossover mean?

A small amount of power is wasted in the resistance of the coils in the crossover network. This power heats up the coils, and also creates a magnetic field in the iron core (you can see the core in the Amazon.com photos you linked to.)

The 150W rating of the crossover means that the coil will survive the amount of heat, and magnetic field, that will be created if you pump 150 watts of music through that crossover, from amplifier to speaker.

Originally Posted by sdbarnold

Impedance - How does the crossover affect impedance? I have two 8 ohm speakers, so does that make the impedance going through the crossover go down to 4, and therefore 4 ohm load needed from my amp?

First, I'm going to give you the super-simplified version: the crossover has no effect on impedance. If you have two 8 ohm speakers, your combined (crossover + woofer + tweeter) is still an 8 ohm system.

In reality, the question you asked is a big can of worms to open. The crossover network you bought from Amazon will work perfectly if loudspeakers were resistors, but they are not. Real speakers have big peaks and dips in their own impedance at different frequencies, and this "confuses" the crossover network, which ends up presenting even bigger peaks and dips in impedance to the amplifier. Real tweeters and woofers also have very different characteristics from each other (dispersion at the crossover frequency, sensitivity, time response), and a good crossover network has to take all that into account.

Real crossover networks in good loudspeaker systems are far more complex than the ones you can buy from Amazon or Parts Express. Designing them is part engineering, and part art. It is a very complicated thing to do correctly, and needs a lot of knowledge, a lot of expensive equipment, and an even more expensive anechoic chamber in which to do the measurements. Because of this, the vast majority of DIY loudspeakers don't get crossovers right.

But the wonderful thing about loudspeakers is that, however bad the crossover, enclosure, or drivers, some sort of sound will come out of them. This is enough to make many DIY speaker builders happy!

-Gnobuddy

[sdbarnold]

Gnobuddy! Thanks so much for the detailed response. I'm still at work so just glanced at this, but man, I'll have some reading to do tonight. Really appreciate the help.


-sdbanold

[Gnobuddy]

I just saw that the "50 W" amplifier comes with a 12 volt power supply. And you are using 8 ohm speakers.

That means the maximum power this amplifier can supply (before distorting) is about 5 watts RMS per channel. (Not fifty, but only five watts.)

I hope you are okay with that, and were not actually expecting 50 watts. Tthis is a very common problem, Amazon and other similar vendors are full of amplifier ads that promise five or ten times as much power as they actually deliver.

I have a similar amp and small speakers I use for my wife's computer. It runs on the same 12V, and the speakers are the same 8 ohms, so it also delivers about 5 watts RMS per channel. It's plenty for computer speakers, but it will not shake the plaster off your ceiling, if that's what you were hoping for.

-Gnobuddy

[Sonce]

Power: No splitting, max allowable power to your loudspeaker is 40 Wrms (same as the "woofer").
150 W is the maximum amplifier power which can be connected to the crossover, IF the woofer can handle 150 W.
Impedance: total impedance of your 2-way loudspeaker is 8 ohm (same as the "woofer") and that is the load to the amp.

Important point: your amp max power with 12 Vdc supply is 15 W at 8-ohm load if bridged, or 4 W if SE.
Use a 24 Vdc supply for about 35 W amp output power at 8-ohms, if bridged.

Edit: I didn't see that Gnobuddy already posted a very detailed answer.

[sdbarnold]

Oh man, I didn't even think about that. So if I want more wattage, up the power supply to the amp, check. Thanks again fellas!








-sdbarnold

[sdbarnold]

BTW *Sonce and *Gnobuddy, how are you coming up with your calculations for the voltage of my power supply, and the wattage it supplies to the speakers?? I'm doing some searching online and a couple calculators I found are not coming up with the numbers you guys did. -sdbarnold

[Old'n'Cranky]

Please.
Stop saying 'wattage'.
It's 'Power'.
Watt (or Watts - Plural) is the unit of measure.

Same goes for 'amperage'.
Amperes (or Amps) is the unit of measurement.
Another measurement of Power.

Ohms law will help you understand.

To me those butchered words are like someone asking how much milkerage can be squeezed out of a cow.

And increasing the voltage and current to the amplifier will most likely do no more than destroy it.

More for you to research.
The logarithmic relationship between power and DeciBells.

[eriksquires]

Here is a link to a blog posting which may help you understanding how the crossover and driver impedance interacts:

A Speaker Maker's Journey: Crossover Basics - Impedance

There are simulation files available, so if you want to play with a completed design, you can do so here:

The LM-1 DIY Speaker is Available

Think of this as a free experimenting and learning lab.

Best,

E

[Gnobuddy]

Originally Posted by sdbarnold

...how are you coming up with your calculations for the voltage of my power supply, and the wattage it supplies to the speakers?

Power from an amp is measured with a sine-wave signal. If we know the biggest (sine) voltage the amp can put out, we can calculate the power.

(You might already know that the strength of a sine wave can be expressed in three different ways: peak value, peak-to-peak value, and RMS value.)

The type of amp you have is most likely four amplifiers packaged in one IC. Two of them are used for the left channel, two more for the right channel. Each pair in one channel are wired in what is called bridge mode.

In bridge mode, the *peak* voltage an amplifier can deliver to the speaker is equal to the power supply voltage, less any losses in the output transistors. A reasonable estimate is that there will be 1.5 volts of loss at the positive peak, and another 1.5 volts at the negative peak; so 3 volts altogether.

Okay, so with a 12V supply, and bridge-mode amplifier, it can deliver up to 9 volts peak (that's 12V - 3V).

The proper way to describe the power of an amplifier is the RMS power.

If you have 9 volts peak, and apply that to an 8 ohm speaker, the *peak* power is 9 x 9 /8, or just over 10 watts. The RMS power is exactly half of that, or about 5 watts RMS. This is the number I quoted in my previous post.

It is possible that the amp you have does not in fact operate in bridge mode. If that is the case, then the peak-to-peak (not peak) maximum output voltage will be about 9 volts.

That means the peak voltage is now only 4.5 volts. Half the voltage translates to one quarter of the power. That is about 1.25 watts RMS, instead of the 5 watts you'd get in bridge mode.

Originally Posted by sdbarnold

a couple calculators I found are not coming up with the numbers you guys did.

The two of us did not come up with the same numbers, either. People often forget to allow for the voltage losses in the output devices; if you plug 12V into the formula (instead of 9 V), you will get a wrong answer, a huge overestimate. In this case, you get the (wrong) answer of 9 watts RMS in bridge mode, 2.25 watts if not in bridge mode.

If you post links to the calculators you tried, I may be able to figure out what they are doing wrong.

By the way, it's true that the mathematics predicts that more power supply voltage will get you more power - but we don't know if the electronics in that amp can actually handle more voltage, or whether it will just burn up instead. Did you get any kind of manufacturer data sheet (that lists allowable supply voltages) with the amp?

-Gnobuddy