jacoviii Posted January 22, 2020 Share Posted January 22, 2020 OK so I have been AVR shopping and basing power output as 8ohm load for the RF7 ii. I got a idea to check the 7's. So with the plates on the binding posts I get 3.4 ohms. With the plates off I get 4ohm on the HI and 3.4ohm on the lows. What am I missing. They say 8ohm compatible in the specs. Sorry car audio guy and Bi-amp & inline cross overs are not my norm. Quote Link to comment Share on other sites More sharing options...
MechEngVic Posted January 22, 2020 Share Posted January 22, 2020 What you are measuring with your multimeter is the DC resistance of the speaker's circuit. The 8ohm impedance that is refereed to in the speaker's specs is the AC impedence. And it's even get a bit more complicated than that. The 8ohm rating is a sort of average. During the speaker's use, AC impedence can be lower and higher than 8ohms. If a speaker says it's an 8ohm speaker I would trust it. 1 Quote Link to comment Share on other sites More sharing options...
HiFi Heaven Posted January 22, 2020 Share Posted January 22, 2020 Easy to inflate speaker sensitivity (dB/W) +3dB by stating output at 2.83 Volts, calling them 8Ω speakers, plopping in 4Ω woofers. My Forte' II's, Quartets, and KG4's all measure under 4Ω DCR. Passive radiators don't double the AC impedance to 8 Ω - ever! Quote Link to comment Share on other sites More sharing options...
jacoviii Posted January 22, 2020 Author Share Posted January 22, 2020 I understand that a driver's ohm changes as it plays. but if at a rest if they are 3.4 ohms that is it. Most 2,4,8 ohm measured at the driver do not read 2 ohm (1.8) 4 ohm (3.4). and so on. If the RF7ii read 3.4 ohm I would take it that when I'm comparing AVR or separates amps. It would be best to look at the 4 ohm. This is important to me due to not all amp scale power output the same from 8 to 4 ohm. So if amp A has a good 8 ohm watt output (135w @8 & 160w @4) but amp B has (125w@ 8 & 180w @4). This is why knowing is so important. Quote Link to comment Share on other sites More sharing options...
MechEngVic Posted January 22, 2020 Share Posted January 22, 2020 41 minutes ago, jacoviii said: I understand that a driver's ohm changes as it plays. but if at a rest if they are 3.4 ohms that is it. Most 2,4,8 ohm measured at the driver do not read 2 ohm (1.8) 4 ohm (3.4). and so on. If the RF7ii read 3.4 ohm I would take it that when I'm comparing AVR or separates amps. It would be best to look at the 4 ohm. This is important to me due to not all amp scale power output the same from 8 to 4 ohm. So if amp A has a good 8 ohm watt output (135w @8 & 160w @4) but amp B has (125w@ 8 & 180w @4). This is why knowing is so important. When you measure at the speaker's posts, you're measuring the driver and its crossover. Quote Link to comment Share on other sites More sharing options...
pzannucci Posted January 22, 2020 Share Posted January 22, 2020 4 hours ago, jacoviii said: I understand that a driver's ohm changes as it plays. but if at a rest if they are 3.4 ohms that is it. Most 2,4,8 ohm measured at the driver do not read 2 ohm (1.8) 4 ohm (3.4). and so on. If the RF7ii read 3.4 ohm I would take it that when I'm comparing AVR or separates amps. It would be best to look at the 4 ohm. This is important to me due to not all amp scale power output the same from 8 to 4 ohm. So if amp A has a good 8 ohm watt output (135w @8 & 160w @4) but amp B has (125w@ 8 & 180w @4). This is why knowing is so important. DC resistance is not a good reflection of a speaker playing music. You have coils and other components in the mix that vary in "resistance" impedance with the frequency that is put through them, not 0hz DC. With that said, your DC test is likely not far off the low spot in the impedance curve. My rule of thumb is if a speaker has multiple woofers, it is likely going to drop in impedance under 4 ohms in the bass / mid bass area though not necessarily. I always pick the most robust amplifier to drive them (think lots of current), one that is spec'd to drive 4 ohm speakers and doesn't usually have a switch to allow use with < 8ohm speakers. BTW, these speakers are "compatible" with 8 ohms since much of the frequency range they are above 6-8 ohms though if you want the best out of them, feed them well. 1 Quote Link to comment Share on other sites More sharing options...
glens Posted January 22, 2020 Share Posted January 22, 2020 Impedance and resistance are (most) only ever the same in a DC circuit. Sticking a multimeter on the binding posts satisfies that condition since the meter's (almost invariably, unless you've spent near $1k on the instrument and set it accordingly) using DC to derive its measurement. All you did was perform a "go / no go" check and it has little bearing on what the variable impedance vs. frequency ends up being. To measure impedance over frequency as in the posted chart requires a more elaborate setup. If the AVR of choice is "4 ohm stable", you're surely good to go no matter what. Actually, I'm surprised at your reading on the HF terminals. It should read very much higher, if at all... Quote Link to comment Share on other sites More sharing options...
jacoviii Posted January 22, 2020 Author Share Posted January 22, 2020 10 hours ago, glens said: Impedance and resistance are (most) only ever the same in a DC circuit. Sticking a multimeter on the binding posts satisfies that condition since the meter's (almost invariably, unless you've spent near $1k on the instrument and set it accordingly) using DC to derive its measurement. All you did was perform a "go / no go" check and it has little bearing on what the variable impedance vs. frequency ends up being. To measure impedance over frequency as in the posted chart requires a more elaborate setup. If the AVR of choice is "4 ohm stable", you're surely good to go no matter what. Actually, I'm surprised at your reading on the HF terminals. It should read very much higher, if at all... I did re-check the HF and it slowly went way up. Quote Link to comment Share on other sites More sharing options...
pzannucci Posted January 23, 2020 Share Posted January 23, 2020 13 hours ago, jacoviii said: I did re-check the HF and it slowly went way up. Choke'd then cap charged, opening the circuit. Quote Link to comment Share on other sites More sharing options...
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