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#1
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impedance matching
hi folks. i'll be installing a new amp in my car soon, and i'm not an
audiophile. my understanding is that i needn't worry wildly about impedance matching. i was hoping to wire three speakers in parallel (each crossed over, of course) for a reference impedance of 4/3, 1/(3*1/4). don't the loads at different frequencies vary by this much or more over the frequency spectrum? then i noticed that the specs for the amp i've been thinking about says "Min. Speaker Impedance 2 Ohm" (it doesn't give a maximum). ought i worry? alternately, perhaps i can wire the three speakers together with one dummy 4 ohm load as two pairs in parallel, joined in series? i think this would yield a total (reference) load of 4 ohms. forgive my ASCII schematic: -:-;- thanks, aaron. |
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#2
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impedance matching
Aaron Brick writes:
hi folks. i'll be installing a new amp in my car soon, and i'm not an audiophile. my understanding is that i needn't worry wildly about impedance matching. i was hoping to wire three speakers in parallel (each crossed over, of course) for a reference impedance of 4/3, 1/(3*1/4). don't the loads at different frequencies vary by this much or more over the frequency spectrum? then i noticed that the specs for the amp i've been thinking about says "Min. Speaker Impedance 2 Ohm" (it doesn't give a maximum). ought i worry? No. alternately, perhaps i can wire the three speakers together with one dummy 4 ohm load as two pairs in parallel, joined in series? i think this would yield a total (reference) load of 4 ohms. forgive my ASCII schematic: -:-;- Any time you start talking about adding dummy loads, you're wasting power and probably shopping for resistors that have such a high power dissipation capacity that they don't quite exist. You don't need to match impedances in solid state amplifiers. You just need to ensure that the load you hang off it doesn't go below the stated minimum of the amp. However, 3 speakers in parallel is usually goofy. You haven't indicated the impedance of each of the speakers, or indicated if the 3 are identical, which are both important pieces of data for determining whether the sonic result will be suitable. Best Regards, -- /"\ ASCII Ribbon Campaign | Todd H \ / | http://www.toddh.net/ X Promoting good netiquette | http://triplethreatband.com/ / \ http://www.toddh.net/netiquette/ | "4 lines suffice." |
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#4
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impedance matching
hi todd, thanks for such a detailed reply.
Hold on--I think I figured out your math and what you're attempting to do is parallel 3 4-ohm speakers off this amp. that was the idea. (i have two other channels to a sub.) Yes you SHOULD worry as 1.33 ohms is less than the 2ohm mininum your amp can safely drive without going either unstable or into thermal shutdown. isn't this figure terribly imprecise because it is only true at, what, 1khz? Now, you could parallel-series connect them as you mentioned and it would make it safe for the amplifier, and get equal power to all 4 speakers, but how that resistive load would affect the crossover points of the passive crossovers in the other 3 speakers is the primary concerns then. By adding that resistance, you're loading the passive crossovers, changing time constants and shifting crossover frequencies...which is going to have an audible effect I'd have to think. wow, i'd agree. perhaps mounting yet more speakers would save me.  Now, if you're talking about single drivers such as subwoofers, that's another matter. You could factor that resistance into your crossover point if you're doing passive low pass filtering, or likely ignore it if you're running an active crossover. gotcha. this is on the cheap so i'm only using high-pass passives (aka big caps). otherwise, i'd parallel the sub with the rear drivers - though that would probably leave it underpowered.... Even so, you're spending a lot of power in that big honkin resistor you'll have to buy. unpleasant, i agree. Better to get a stereo amp, run two speakers in parallel off one channel, one speaker off the other channel, and use some other means in the preamp to ratio the preamp levels to each channel appropriately to get equal power to all 3 speakers. perhaps you can advise me on getting a 6-channel amp. i'd really rather not have two amps, if it can be avoided. thanks much! aaron. |
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#5
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impedance matching
Aaron Brick writes:
hi todd, thanks for such a detailed reply. Hold on--I think I figured out your math and what you're attempting to do is parallel 3 4-ohm speakers off this amp. that was the idea. (i have two other channels to a sub.) Yes you SHOULD worry as 1.33 ohms is less than the 2ohm mininum your amp can safely drive without going either unstable or into thermal shutdown. isn't this figure terribly imprecise because it is only true at, what, 1khz? Nominal impedances are by their nature...nominal. They aren't necessarily at 1kHz -- the manufacturer makes a wild assed guess based on the impedance vs frequency curve. In fact, your 4ohm-nominal speakers may all have a frequency at which they are maybe 2.6ohms, and slap 3 of those in parallel, and boy yowsa will your amp be unhappy at that frequency. So, the "imprecision" of nominal impedances doesn't let you get away with overloading the amp. :-) Now, you could parallel-series connect them as you mentioned and it would make it safe for the amplifier, and get equal power to all 4 speakers, but how that resistive load would affect the crossover points of the passive crossovers in the other 3 speakers is the primary concerns then. By adding that resistance, you're loading the passive crossovers, changing time constants and shifting crossover frequencies...which is going to have an audible effect I'd have to think. wow, i'd agree. perhaps mounting yet more speakers would save me. True. 4 4-ohm drivers is much easier to deal with. You can series couple the pairs and run them in parallel, and end up with a nicely behaved 4ohm load that won't push the hell out of your amplifier and give you some dynamic headroom and thermal headroom. Damping factors on most amps really suck at 2ohms anyway even if the amp says it can handle it, so it'll sound better/tighter too. Now, if you're talking about single drivers such as subwoofers, that's another matter. You could factor that resistance into your crossover point if you're doing passive low pass filtering, or likely ignore it if you're running an active crossover. gotcha. this is on the cheap so i'm only using high-pass passives (aka big caps). otherwise, i'd parallel the sub with the rear drivers - though that would probably leave it underpowered.... So you're not running a choke inductor on the subs at all? If not, consider it or you might have some barky midrange. Even so, you're spending a lot of power in that big honkin resistor you'll have to buy. unpleasant, i agree. Better to get a stereo amp, run two speakers in parallel off one channel, one speaker off the other channel, and use some other means in the preamp to ratio the preamp levels to each channel appropriately to get equal power to all 3 speakers. perhaps you can advise me on getting a 6-channel amp. i'd really rather not have two amps, if it can be avoided. A 6channel amp would work...you could give each driver it's own channel I suppose. How the specifics of getting the sub signal to each of those channels would work, I'm not sure. I've been out of the autosound area for quite a while. :-\ Best Regards, -- /"\ ASCII Ribbon Campaign | Todd H \ / | http://www.toddh.net/ X Promoting good netiquette | http://triplethreatband.com/ / \ http://www.toddh.net/netiquette/ | "4 lines suffice." |
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