Originally Posted by
linnmajik
OK, I'm glad I found this thread. I've gone to school for electrical engineering and have actually installed home theatre systems that sold for over $200,000. Let me begin by helping to reduce some confusion. I may repeat some things that were already said but this should be very helpful in summing up everything in one thread.
I don’t have time to write this perfectly so some parts may be point form.
This paragraph is my basic answer to the thread (I will expand and explain afterwards):
There is no problem with hooking up speakers with different ohm ratings as long as the total load impedance doesn’t drop below what the amplifier is designed for. The safest way to do the switching is to buy an impedance-matching speaker switcher. You can buy one that will allow the Sony to “see” 6 or 8 ohms - even when you are running both sets of speakers at the same time. I believe companies like Q.E.D. and Russound have these switchers.
Ohm’s law: As resistance (or in AC circuits - impedance) decreases, current increases.
As impedance drops, the circuit gets closer and closer to being shorted. This increases the current demands from the amplifier. Most amplifiers are current-limited and are not stable driving low impedance speakers (or a combination of speaker pairs that allow the amp to “see” a low impedance circuit) Since Sony mini systems have almost no current reserve, keeping the impedance high is much easier for the amplifier to drive.
Two sets of 8 ohm speakers switched on in a series circuit, the amplifier sees 16 ohms. Two sets of 8 ohm speakers switched on in parallel, the amp sees 4 ohms (current demand is 4 times that of the series circuit - but more voltage is required)
Ohm rating is nominal, impedance fluctuates with frequency. An 8 ohm speaker can be as low as 3 ohms at a certain frequency and some have been measured as high as 70 ohms at certain frequencies. At low ohms, the amplifier needs to swing large current, at high ohms, it needs to apply high voltage (to make up for the large voltage drop across 70 ohms)
Douggie,
Low impedence speakers are HARDER to drive than high impedence. Car speakers are 4 ohm because high impedence causes voltage drop and the power supply “rails” in a car amp operate at a lower voltage than a house amp. LOW ohm rating and low frequency is the most difficult - massive current demands and the amp has to control a larger moving mass with more back EMF. You are right about the specs of mass produced amps - it got even worse when home theatre came out - An amplifier that draws 150 watts from my wall outlet can’t put 110 watts out to each of my 5 speakers at the same time. Its all lies. And, in most situations, power has absolutely no bearing on performance. It all demands on how well the amp was made. I’ve heard great sounding 20 watt amps and terrible sounding 100 watt amps.
Jeff,
8 ohm speakers are not lower in volume than 6 ohm - their volume has to do with input power and efficiency, not their impedance. Current does not provide loudness, the product of current and voltage is power, which, combined with the speaker’s efficiency make it play at a certain volume.
If someone does have low impedance speakers, it is better if they have high efficiency.
A 90db speaker will require half the amount of power to get to the same volume as an 87db speaker. (A 90db speaker at 50 watts will play as loud as an 87db speaker at 100 Watts.)
A 4 ohm 90 db speaker can sometimes be as easy to drive as an 8 ohm 87db, but there are a lot of variables.
Egneg,
Your diagram is a parallel connection with no impedance matching, switching A + B and running at the same time, the Sony will see 3.4 ohms, "high-end" Sony ES amps can barely handle this.
Keep volume very, very low. (Which could be difficult outdoors)
I wish I was a better writer, but I can expand on everything later, I have to go.
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