Is this ok, or am I hurting my amp?

[MacroFilkins]

I have a pair of 8ohm 2-way floor-standing speakers and a pair of 8ohm orb audio mod 1 speakers. Originally, I used these speakers for two different systems, but one day after returning home from a week long vacation, I discovered that one of my roommates had thrown a party and the dip s**t had blown out the tweeters in the floor-standing speakers.
Ever since then I've been using them in combination with my orbs wired parallel. I disconnected the tweeters inside of the floor-standing speakers since they don't work anyway, but now I'm wondering if that has changed the impedance of the floor-standing speakers. At first, I just assumed that the two pairs of speakers wired in parallel would be a 4ohm load.
I removed the 8" drivers in the floor-standing speakers and saw that they are 4ohm drivers. By my calculation, running the two sets of speakers in parallel would be around 2.67 ohms.
Does the crossover in the floor-standing speakers change the impedance, and if so, what would the new impedance be?
I should mention that the two sets of speakers sound surprisingly nice together. the crossover point on the 8" driver is at 2khz (not sure if that has changed after disconnecting the tweeters) and the orbs have no crossover, so there is a bit of a roll-off on the high end and all of the mid range is filled by both speakers from about 200hz to 2khz.
I'm expecting a number of you to tell me that this setup is bogus, and I agree with you, but I would like to get opinions on this, especially regarding the impedance of the setup.
Thanks!
-newb

[speaker dave]

The impedance of your speaker isn't constant. It changes with frequency. The rated impedance is usually the approximate value in the middle of the woofer's range and maybe in the middle of the tweeter's range as well. At low frequencies your two 8 ohm systems will parallel to approximately 4 ohms.

Now for the blown tweeter its a little hard to say what the effect is on impedance is without seeing the whole network. If it were a simple network such as a first order series capacitor then the blown tweeter would just mean that impedance drifts high as you go up the frequency scale. The whole network is out of circuit because the blown tweeter is open circuit and breaks the loop. In that case your two systems in parallel will still be the 8 ohms at high frequencies of the one working system.

Now the difficulty comes if you have a second order network. Removing the tweeter (either physically or just by it blowing) will turn the tweeter network into a series resonant circuit. This means it will have a deep notch in impedance at what used to be the crossover frequency. It won't necessarily blow the amplifier but it wouldn't be a good thing.

If you can follow this, and can trace out the network to answer what kind of network you have, you would be better off removing all of the tweeter's network, assuming it is 2nd order or higher.

By the way, did I mention that your setup is bogus? (But if you like it thats fine!)

David S.

[MCPete]

If the impedance of the woofer of your floor-standing system is 4 Ohm, then it would be very unlikely that the impedance of the floor-standing system is 8 Ohm.

If the woofer of the floor-stander is 4 Ohm and your amp is like most solid state discrete or IC amps, then your current set-up is probably presenting too low of an impedance to the amp. That is, commonly manufacturers recommend not less than a 4 Ohm load per channel. Whether or not that will in practice cause a problem is dependent on how far you advance the volume control of the amp. If you don't hear distortion and overheating of the amp is not apparent, then you might conclude that your amp can handle the low impedance at the sound level that you have been using. The condition of lowest load impedance occurs when both the woofer of the floor-standing system and the orb system are in a common pass-band range, perhaps below 1 kHz.

Your combined system would be contrary to accepted practices in that (1) most likely it produces a boost of the midrange frequencies, and (2) the driver of the Orb most likely can't be positioned close enough to the woofer of the floor-stander to effect something approaching flat frequency response through the crossover region.

Possibly your combined system has a good sound quality because the non-flat response of your system (emphasis of the midrange) is a remedy for what your listening room does to the reproduction. -Just a thought.

-Pete

[MacroFilkins]

Thanks for the info guys.
The room I'm using this "bogus" system in is a very bogus and unpredictable shape as far as frequency response goes. The living room is combined with the dining room, entry way, stairway, and kitchen making lots of corners within the room for bass to accumulate and cause a ruckus. Also, this entire area is hard wood floors with one rug in front of the stereo and mostly wood furnishings. Maybe that's why the roll-off in the higher and lower frequencies sounds better than a flat response. I have a total of nine corners in my listening room. This is probably (by luck) why this set up sounds good to me.
The frequency response that the two sets of speakers have in common is roughly 200hz to about 2khz. If I sweep a sine wave from 0hz to 20khz, from most listening positions, it has very few peaks no more than 4-6db or so in the low end, and a peak of about 4db around 2khz. This measurement is only by ear combined with an eq for reference (which i don't use for listening).
At any rate, I'm mostly concerned with the load on my amplifier. I have the schematics and manual for it (Technics su-v7) so I can confirm that it will handle a 4ohm load per channel without problems.
The two-way floor-standing speakers are a pair of inexpensive Athena Technologies Audition series speakers.
Also, I never turn up the amplifier past 40-50%, so maybe the low impedance is ok?
Thanks again!

[MacroFilkins]

To MCpete: Failed to mention that the orb speaker sits on top of the floor-standing speakers within about 6" of the woofer. This, combined with the room, probably helps with the apparent flatness of the system.

[lduarte1973]

why dont you replace the tweeters with silkdomes ?