new to board... impedence question

[niget2002]

Hi all!

It's been a few years since I've done anything really electronic. I have built two speakers in the past, 1 MTM center and 1 MT. I used off the shelf parts from parts express (even pre-built crossovers). They sound good for what they are, but now that I'm moving into a house, I'll actually have a garage to help facilitate my fascination in speaker design.

I read something on another post that had me scratching my head... How does an impedance plot of a crossover/speaker show how it will effect an amp?

My first purchase after the move is going to be the speaker design 201 book.

If you can believe this... my degree is actually in EET, but I spent most of my undergrad focusing on embedded design and not mixed-signal/filters type of things (and my grades prove it). Now I'm just a lowly little IT guy with dreams of speaker building grandeur.

Currently my system is run off of a Yamaha xv750 receiver. My speakers are a thrown together group of old 3-way loudspeakers and book shelf speakers with a sony 12" sub (I don't even use the two speakers I already built). The goal is to start with a new sub and work my way around the room (center, then LR, then Rears). I really like the WMTW/WWMT setup I read about on one of the posts.

So, to repeat my question... How does an Impedance graph show how it will effect the amp pushing it? (it's amp friendliness)

Edit: I guess I should mention that my listening is about 90/10 HT/Music... and I'll usually only blast the stereo while cleaning the house.

[JonMarsh]

ideally, the impedance plot should show both magnitude and phase. That is, the "resistance/impedance" at each frequency, and the phase of the impedance. The less phase shift, the easier to drive. The higher the impedance, the easier to drive.

Most HT receivers will handle some phase shift, and "nominal" 8 ohm speakers, which means that they'll dip down to 6 ohms in the upper midbass/lower midrange, and be higher elsewhere, especially if there's more phase shift. Multiway speakers with paralleled woofers can have rather low impedance loads (say, 3 ohms), and will be better served by separates, or by recievers rated for 4 ohm loads (which is not commonplace).

The problem with phase shift (reactance) in the load is that the greatest current doesn't occur at the peak of the output voltage (which is when there's the least voltage drop across the output transistors), but may lead (capacitive) or lag (inductive) the voltage waveform, causing a combination of high voltage and high current to sometimes occur for the transistors. Which is not a good thing unless the amplifier is fairly overbuilt, like separates tend to buy in comparison to receivers.

[dobias]

I'm a rank neophyte at this crossover & impedance stuff. I'm collecting material, equipment & components to resurrect my Wharfedales into open baffles.
I was able to find additional Wharfedale tweeters & woofers to boost the full range 12CS-AL speakers I'm so fond of.
Originally, I was going to have a TFW arrangement but changed to trying TTFWW baffles.
Therein lies my problem. (After reading series-parallel wiring articles,) it appears that series connecting the woofers & series connecting the Tweeters, then parallel wiring them with the full range will not give any SPL benefits. Should I simplify to the TFW arrangement?
dobias

[Dennis H]

I'm a rank neophyte at this crossover & impedance stuff. I'm collecting material, equipment & components to resurrect my Wharfedales into open baffles.
I was able to find additional Wharfedale tweeters & woofers to boost the full range 12CS-AL speakers I'm so fond of.
Originally, I was going to have a TFW arrangement but changed to trying TTFWW baffles.
Therein lies my problem. (After reading series-parallel wiring articles,) it appears that series connecting the woofers & series connecting the Tweeters, then parallel wiring them with the full range will not give any SPL benefits. Should I simplify to the TFW arrangement?
dobias

Are you planning to design and build a crossover or just hook them up and run all the drivers full range? The latter won't work very well. Among other things, you'll blow the tweeters if you feed them a full range signal.

[dobias]

Wharfedale

The tweeters would have either a 2mF cap for the 2 in series or a 4mF if only 1 is used.
The woofers would have an 8 mH coil if 2 are in series or a 4mH if only 1 is used.
The full range wouldn't have any crossover applied.
Wharfedale's Briggs had intended the tweeter & woofer would augment the full range speaker at the low & high ends. He wanted to avoid crossovers as much as possible.
With a single tweeter & a single woofer in parallel with the full range, the impedence works out to 7.14 ohms.
I would like confirmation that adding an additional woofer & tweeter would be self defeating as to SPL because of the series/parallel requirement to keep a reasonable impedence.
dobias

[Dennis H]

I would like confirmation that adding an additional woofer & tweeter would be self defeating as to SPL because of the series/parallel requirement to keep a reasonable impedence.

If by SPL you mean sensitivity (how loud it gets with a 2.83 volt input), 1 or 2 in series is the same. 2 tweeters are almost never a good idea because of comb filtering. 2 woofers would have greater max SPL capability if you crank it but the fullrange would probably run out of gas first with no crossover so 1 woofer is probably enough for what you have proposed.

[dobias]

Series connections

Dennis H,
As I now understand it: at the same voltage, there is no difference in SPL output between one woofer or two woofers wired in series.
I was unaware of "comb" problems with dual tweeters(... I've gotta look that term up.)
Thanks for the response. I'm glad I didn't rush in with dual woofers & dual tweeters before I asked.
dobias