8ohm vs. 16ohm

[The Dude]

So there was all this giberish about 16ohm actually being more efficient, I can't explain why it something to do with spec sheets. But I have a post in technical questions about using some Altec 511bs, I was looking for a driver to use for now, and was looking at selenium drivers I have the option for 8 or 16 ohm and on another forum some one said based off of the spec sheet the 16 ohm would be more efficient. Maybe this would help.

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=264-271

or

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=264-273

Had something to do with the voltage input being the same

thanks

duder

[Al Klappenberger]

Duder,

The efficiency will be the same for 8 or 16 Ohms. Which one to get depends on what impedance the network was designed for.

Power = voltage squared over impedance. (P = E * E / Z)

If Z is 16 ohms, the driver draws half the power and delivers half the output with a GIVEN VOLTAGE. The difference is 3 dB less output from the 16 Ohms driver than the 8 Ohm with a GIVEN VOLTAGE. The efficiency is the same.

Again: Get the driver with the impedance that matches the network it is to be used with. In the case of my networks, it makes no difference. You just move the autotransformer tap up 3 dB higher for the 16 Ohm driver than for the 8 Ohm driver.

Al K.

[The Dude]

Thanks Al, what if I am going to use a active crossover.

Nick

[Al Klappenberger]

If there are no passive filters between the driver and the amp, impedance makes no difference. BEWARE though that an amp with a turn-on thump can destroy a driver if it is connected directly.

Al K.

[The Dude]

Anything one can do for protection.

thanks again.

Nick

[The Dude]

Something like a blocking cap, or is there something better. If not is how does one go about choosing one, spec, and quality wise.

[The Dude]

I also wouldn't mind a 2 way crossover or somthing along the way if your or anyone else for that matter has a low cost one in mind.

[The Dude]

Ok last time I post before a response. But how do I determine were I want my crossover point.

[Nat Denkin]

If you are using active crossovers, within the capabilities of your drivers vary the crossover frequencies and listen to the sound. Stop when you like the results. I am tri-amping and not using either capacitors or fuses to protect my drivers, but rather, I am using Crown D-45 and D-75A amplifiers that are unlikely to fry my drivers. Here it makes a difference whether you are using 8 ohm or 16 ohm drivers because these solid state amplifiers deliver a lot less power into 16 ohms than 8 ohms. There is a slight popping sound when the amplifiers are turned off.

If you do want capacitor protection, you can choose a capacitance that with the driver impedence gives a cuttoff frequency an octave or two below your selected crossover frequency.

nat

[The Dude]

I am learning here, looks like the drivers I was looking at for my project don't go low enough. But I have some better Ideas now, thanks for the education Al, and thanks for the recommends Nat.

Nick

[The Dude]

Wait Nat are you also saying that I would less of a chance of blowing the 16 ohm drivers with a setup like yours or vice verse.

[Nat Denkin]

Wait Nat are you also saying that I would less of a chance of blowing the 16 ohm drivers with a setup like yours or vice verse.

Nick,

Let me answer your question by way of some examples. The specifications for the Crown D-75A include a single-cycle tone burst at 1 kiloHertz. At the same distortion level of 0.05%, the amplifier would deliver 30 watts at 16 ohms, 60 watts, at 8 ohms, and 120 watts at 4 ohms. The reason for this has to do with the design of the amplifier. The output voltage, Vout, is the same for loads between 4 and 16 ohms so that the output current, Iout, equals Vout/Zload where Zload would be the impedance of the driver at 1 kHz. Since the delivered power is the product of Vout and Iout, it is proportional to Vout^2/Zload. Amplifiers using negative feedback to provide a constant voltage for a given signal would behave this way and could be expected to deliver larger turn-on/turn-off transients to lower impedance loads.

An amplifier designed to deliver constant current would actually deliver more power into a higher impedance. However, since woofers generally have low impedance (4 ohms is typical) and most of the signal power goes to the woofer amplifer designers would rather control the voltage rather than the current.

Since vacuum tubes have very high output impedances, output transformers are used with different taps for different impedances. So a McIntosh MC275 is specified to deliver the same 75 watts into 4, 8, or 16 ohms.

There are other factors to consider. A power amplifier using symetric power supplies should have smaller on/off transients than an amplifier with a single rail.

If you hear a big thud or any nasty sounds when you turn on or turn off your amplifiers, you may need to protect your drivers by blocking the low frequency portion of the transients. I am in no hurry to put capacitors in front of my midrange and tweeters; however, if capacitors do not affect the sound enough for me to hear a difference, I may put some in just to be on the safe side.

nat