If wiring in series raises ohms why not parallel?

[joojoo1234]

I see alot of threads around this... is it just an SPL versus difficulty to drive question?

[paulfx]

Parallel cuts it in half!

[454Casull]

Lower impedance = more current from amplifier

[Speedskater]

Many solid-state amplifiers have a sweet-spot in about the 4 to 8 Ohm range.
Some will go lower (a few much lower) and still operate properly.
But amplifier designers like Doug Self write that it's not a good idea to go low.

[joojoo1234]

Okay. I thought it remained the same and the SPL was just increased. I have alot to learn.

[5th element]

Take 1 driver. 1 voice coil, 1 cone, one amplifier. Driven with 2.83 volts rms the driver produces 88dB. As we are assuming the driver has a perfectly flat impedance, that is 8 ohms, the driver also draws 0.35 amps rms from the amplifier.

If you now take a second driver and wire it in series with the first you've got 2 voice coils, 2 cones and one amplifier. The drive level is exactly the same = 2.83 volts rms, . In series the impedance doubles to 16 ohms, 2.83 volts rms is still dropped across the combined voice coils. As we're dealing with 16 ohms now the current halves to 0.175 amps rms.

0.175 amps now flows through each voice coil meaning that each coil has 2.83/2 volts rms across it.

Lets look at this in terms of power.

P = V*I

In the first instance the driver sees P = 2.83*0.35 = 1 watt.

In the second instance each driver sees P = (2.83/2)* 0.175 = 1/4 of a watt. Both drivers together = 1/2 of a watt. So in going from 8 to 16 ohms the total electrical power has halved.

The method of scaling powers against one another =

10log * (P2/P1) = 10log * ( 0.5/1) = -3dB.

So in other words you lose 3dB of electrical power when you wire two drivers in series when compared to a single driver on its own. However this isn't the end of the story. As you've got two cones with the two drivers in series this also means you've got twice the radiating area or twice the acoustic power vs the single driver, the net effect is that you lose 3dB of electrical power, but you gain 3dB of acoustic power so in the end you're still at 88dB.

Now lets look at parallel. Two coils, two cones, one amplifier, but now the coils are wired in parallel with one another.

Each 8 ohm coil sees 2.83 volts RMS and as we saw for the first driver on it's own this = 0.35amps rms or 1 watt. As we've got two coils in parallel this time and because each coil has 2.83 volts rms across it we've now got a total of two watts of electrical power.

If we pop this into the power formula we now get.

10log * (2/1) = +3.

So we've doubled the electrical power and gained 3dB because of it. Now though as we've got two cones vs one we've also doubled the acoustic power, so we've gained another 3dB too, so we're up +6dB vs the single driver on its own.

At least I think I've got that right, it's been a while since I've thought about it.

[joojoo1234]

Yes. I see. Not to difficult and a great explanation. I have copied it for future reference as a reminder. So I keep the ohms for the "cones and domes" between 4 and 8 ohms and this and my SPL are what guide me in this decision. If I am running three drivers could I run two in series and one parallel and it still sound natural? Just curious. Also. Is it better for distortion reasons on the driver to choose series or parallel? Would distortion be a guiding factor?

Thanks 5th element!

[Akrambash]

I have a handful of 4 ohm mid woofers ,but my technics amp impedance range is between 8 and 16 ohms..I have read somewhere that connecting two drivers in series is not a good idea...how far is this true

[rrrremus]

Originally Posted by Akrambash

I have a handful of 4 ohm mid woofers ,but my technics amp impedance range is between 8 and 16 ohms..I have read somewhere that connecting two drivers in series is not a good idea...how far is this true

Series is the only safe way to go with that amplifier and those drivers.

[chris661]

Originally Posted by Akrambash

I have a handful of 4 ohm mid woofers ,but my technics amp impedance range is between 8 and 16 ohms..I have read somewhere that connecting two drivers in series is not a good idea...how far is this true

The only reason not to wire drivers in series is the back-EMF created when the coil moves in the magnetic gap. As the drivers are (I assume) the same, this won't be a problem. The only time when it's really a problem is when you're trying to do a series crossover, and the back EMF from the woofer drives the tweeter, giving increased tweeter excursion (therefore distortion) around the crossover point - it's attenuated below the crossover point.

Originally Posted by 5th element

At least I think I've got that right, it's been a while since I've thought about it.

Can't see anything wrong with it from here.

Chris