Newbie introduction, quick impedance question...

[Apfelsauce]

Hi! I've been skimming the boards for a while, but this is my first post. I'm not new to speaker building, but my knowledge is slim, and everything I've ever done, speaker-wise, has been on paper... I sound like a luddite, but I'm really not! In any case, I look forward to being (a small) part of this community, and will be documenting my current project (an ultra-mini tower, like the Totem Arro, with tiny SB Acoustics mid-woofers and Vifa silk tweeters) as it comes along.

This question pertains to that project. I'm not any kind of engineer - I have a graduate degree, but not in EE! - and thus most of my speaker building forays have all been flying blind. But that's the fun of it for me, so I won't be buying LEAP anytime soon. My question is this: my drivers are both 4 ohm nominal impedance. Is there anything simple I can add to the crossover to make the circuit look to the amplifier like each individual driver load is 8 ohms? ie, if I just throw a 4 ohm resistor in series, probably ahead of the crossover (does it matter?), will the load "seen" by the amplifier be 8 ohms? My amp will drive 4 ohm loads fine but these drivers, in parallel as they will be once all built up, represent a 2 ohm load...right?

Sorry about such a trivial question on such a (generally) advanced forum, but I hope someone can answer my question in layman's terms! Thanks in advance!

[firechief]

sounds like you are talking about 2ea. 4 ohm drivers per channel. Do I have that right?
If the drivers are connected in series they will present an 8 ohm load to the amp.

[mrfeedback]

Most amplifiers are not rated to drive 2 ohms load impedance.
You are correct, two 4 ohm drivers in parallel will present 2 ohm load.
Run the 4 ohm drivers in series and try it out - there are all sorts of arguments that this will destroy damping but don't worry about that, just try it - Bose have done this since forever !.
Adding a series 2 ohm resistor to the parallel combination will work also, but is not the preferred option.

Eric.

[Apfelsauce]

Originally Posted by mrfeedback

Run the 4 ohm drivers in series and try it out - there are all sorts of arguments that this will destroy damping but don't worry about that, just try it - Bose have done this since forever !.
Adding a series 2 ohm resistor to the parallel combination will work also, but is not the preferred option.

Forgive my ignorance, but to drive them in series I have to redesign my crossover, right? Do I need a fairly nontraditional crossover?

What are the potential drawbacks of adding a series resistor to the circuit to bring up the nominal impedance? Will it colour the sound? Thanks!

[wakibaki]

Tell us what the drivers are and where you intend to cross them over. Sounds like you have a crossover. Details?

w

[Apfelsauce]

Originally Posted by wakibaki

Tell us what the drivers are and where you intend to cross them over. Sounds like you have a crossover. Details?

Yeah, I already have a crossover. ...or all the parts for one, anyway. The drivers are SB Acoustics SB12NRXF25-4 and Vifa DX25SG0504. Currently my plan is to cross them over 2nd order L-R at 2650 Hz (not for any reason than because the inductor and capacitor values worked out better than 2500 or 2750 Hz, etc.). I designed a bit of an L-pad for the tweeter (to accommodate its higher sensitivity) and impedance EQ circuit for the woofer. I've done all this on paper, or I'd provide a fancy computerized schematic! I can provide the exact values if it's any assistance...

Thanks again for your help, and I'm already looking forward to more input...just hoping I don't find a reason to start from scratch and have to buy new kit!

[Apfelsauce]

Oh, I should add - I would like to keep them bi-wireable. Thus a series resistor, depending on its drawbacks, is a little more attractive to me than wiring the drivers in series (which I assume prevents bi-wiring?).

[wintermute]

Hi Apfelsauce, the main problem with adding series resistance is that it will reduce the sensitivity, basically converting energy to heat in the resistor. What you loose in the resistor will be made up for in the fact there are two drivers. Depending on the amount of resistance you may end up with more less or basically the same spl as you would with a single driver.

If you wire in series you will get no spl benefit either (basically the same as a single 4 ohm speaker), but your amp will be happy.

With regards to the crossover, provided you measure the impedance of the drivers wired in series and use that for calculating the correct crossover components it really should not be any different to if you have two drivers in parallel... it is the impedance presented to the crossover that matters, not whether there is one or two drivers in series or parallel.

Modelling in unibox (or another program that allows the addition of series resistance) should show what adding series resistance will do to the performance. You should be able to compare two drivers in series, to two drivers in parallel with series resistance added.

edit: with respect to bi-wiring (if I understand correctly) you are basically running separate speaker runs from the amp to the inputs of the low pass crossover and the high pass crossover, so it should make no difference whether your woofers are wired in series or parallel as they are after the low pass xover.

Tony.

[wintermute]

I should clarify. With respect to the higher spl when in parallel, this is only due to the fact that the amp can deliver more current (for the same voltage) into the lower impedance, and can therefore drive the speakers harder (ie more watts) two speakers in series or two speakers in parallel driven with the same number of watts will deliver the same spl...

Reflecting on that, then what I said before is not correct. running in parallel with series resistance added will almost certainly result in lower spl than running the two in series (for the same power input), if I'm not completely confusing things

edit: I'm confused, just dug out my loudspeaker design cookbook and dickason says 2 drivers in parallel = +3db and 2 drivers in series = -3db compared to a single driver.. I need to re-aquaint myself with the basics...

Tony.

[Apfelsauce]

Thanks for all the information, but I think I was being unclear: this isn't an MTM design, I'm just using a single 4 ohm woofer, and single 4 ohm tweeter. But even keeping them "bi-wireable," I still want to have them such that both woofer and tweeter can be plugged into the same set of (single) speaker terminals. Does that qualify as a woofer and tweeter "wired in parallel"? Now I'm confused! (Hence my confusion about wiring both woofer and tweeter in series... I know, or think, that sort of thing is possible, but I don't think I'm interested. I definitely understand the attraction or necessity of wiring two MTM woofers in series.)

Basically I want to build my crossovers with separate inputs and outputs (two neg., two pos.) for tweeter and woofer, and then have separate (i.e., four) terminals. But for the time being, I have only one (stereo) amplifier, and will use, for example, just the right channel output to drive both woofer and tweeter in the right speaker. (OK, now I'm sounding like a totally virgin freshie audio guy... I'm not, really. This is actually the second pair of speakers I've built like this, and the first have been going strong for ten years now. I'm just not clear on the basics, maybe!) But driving both a 4 ohm tweeter and 4 ohm woofer from the same output, disregarding minor impedance increases from the internal wiring and crossover components themselves, I'm actually presenting the amplifier with a 2 ohm load in that channel, correct? I think this has been my real question all along. If the answer is yes, a 2 ohm load results from (1) 4 ohm woofer and (1) 4 ohm tweeter, then I'm trying to figure out what my options are.

Unfortunately (or fortunately, maybe...) I don't have any measuring equipment, and at this point don't really want to invest in any. That might be setting me up for a sub-par result, and I'm OK with that. (I kind of appreciate the guesswork, I guess, and don't want to get obsessive - not that there's anything wrong with that!) I just don't want to overheat my amplifier by driving an insufficient load. Thanks again for all your erudition!