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autoformer question

2uberoller

2uberoller

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I have my B & W 804 S speakers attached to the mc252 at the 8 ohm setting. What would be the effect on the sound if I were to swithch to the 4 or 2 ohm settings ? Can you damage the amp. or speakers by doing this ? I would assume if I decrease the impedance I would be increasing the current , due to ohms law at a given volume setting ? Since the speaker is not constantly at 8 ohms but rather changes at different frequencies, will the change in autoformer position help accentuate certain tones ie increase base for example ?
 
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Everything will blow up, time will stop, and the world will reverse rotation.

Sorry, Sunday morning humor. You can drop it down to the 4 ohm tap and increase 3db. The Amp will be fine & so will the speakers.
 
I did that with a pair of Paradigm S8's -- couldn't hear much difference, but as mentioned above, it's harmless to try.
But, with the autoformers, the same 250 watts/channel regardless of impedance means no extra 3db which is the case with lesser amps...
 
entdoc said:
... frequencies, will the change in autoformer position help accentuate certain tones ie increase base for example ?
Click to expand...

It does make a difference, but only you can tell if it's the right difference for your system/speakers or if you can hear any difference at all. Playing with the transformer tap (that's the technical term for a connection point that allows you to select how many turns of wire winding you get) is something most audio nuts do :D. Most normal people read the settings on the speakers, match it, and start listening to music.

Guess what category I am in :D.

Alberto
 
I've heard its good to run B&W off the 4 ohm tap with Mcintosh. I'm going to do my 802D's that way when all my system gets in. Going to Mcintosh for the first time. Let me know if you try this and if you can tell any difference.

beerdraft
_______________
Classe dcp202...Mcintosh MT10....
waiting on the rest, 2 Mc2301's,,,C500T,,,B&W802D,,,,all Shunyata power and cables
 
I am by no means an expert, but I have done this recently and heard significant results. While experimenting with bi-amp vs mono bridge, I used all the taps on each amp and the result was dramatic. In the mono-bridge, placing the speaker cables on the 8 Ohm taps resulted in brighter sound. Placing them on the 2 Ohm taps smoothed everything out dramatically, but it lacked a bit of impact. The 4 Ohm taps were ideal for my Ren 90's. If you change the taps, you'll want to play the same sample of music and try to keep the volume the same to start. You'll notice that the highs are smoother on the 2 Ohm taps, but you may have to increase the volume setting a touch to compensate for the softer sound. I can tell you that the Ren 90's sing very nicely on the 4 Ohm taps, which in reality represents 2 Ohms in bridged mode. I've only hit the power guard once and the speakers (rated at 400 W) are just fine, even after getting an 800 W "cup check." Clean power shouldn't do in the speakers, if it's done within reason. I won't be playing with a pair of MC2KWs any time soon.
 
entdoc said:
I would assume if I decrease the impedance I would be increasing the current , due to ohms law at a given volume setting ? Since the speaker is not constantly at 8 ohms but rather changes at different frequencies, will the change in autoformer position help accentuate certain tones ie increase base for example ?
Click to expand...

Actually the reverse is true (more precisely you're thinking "backwards"). The impedance of your speakers is fixed by design/construction. With the autoformers the only change will be indicated power. Ideally, you want to use the taps that match the actual load so that indicated will equal actual power. My particular situation: My RF-83s are advertised as 8 ohm speakers. I toasted two Yamaha amps (M-2 and M-4) that were operating well below rated indicated power trying to drive them. I bought an MC-252 and hooked up to the 8 ohm taps. It would shutdown on thermal overload at sustained 40 watts indicated. It was loud for sure, but indicated only 40 W/ch. Since they are NOT 8 ohms, actual power was MUCH higher (high enough to overheat the amp). Changed to the 4 ohm taps. Now I have to drive it reallly hard (relying on the meters) to overheat it, but still can. This means I should probably be using the 2 ohm taps. Changing taps will halve, then quarter the indicated power. The real power consumed doesn't change for the same volume level. After talking with McIntosh engineers and Klipsch reps, Klipsch repaired my smoked amps at no cost to me. They still market them as 8 ohm speakers as far as I know. I have three amps that disagree. Klipsch blamed the Yammies/wiring but chose not to argue with the Mac's opinion. Is that clear as mud?
 
The power on the macintosh amp is 200 watts/ channel at 2, 4 or 8 ohms. Current I =volts (E)/ resistance ohms (R) or another way to look at it is power (watts)= I squared x R. In other words if you decrease the resistance tap on the amp and power stays the same then current increases.
 
entdoc said:
The power on the macintosh amp is 200 watts/ channel at 2, 4 or 8 ohms. Current I =volts (E)/ resistance ohms (R) or another way to look at it is power (watts)= I squared x R. In other words if you decrease the resistance tap on the amp and power stays the same then current increases.
Click to expand...

Yes and mostly no. You cannot change the actual impedance of the load by changing taps. The power is constant and indicated correctly only when they're matched, i.e. 2 ohm speakers on the 2 ohm taps, 4 ohm speakers on the 4 ohm taps, etc. As an illustration lets say you have a 4 ohm load on the 4 ohm taps (matched, 200W indicated = 200W actual power). Swap the same load to the 2 ohm taps (mismatched, 200W indicated = 100W actual power). Swap the same load to the 8 ohm taps (mismatched, 200W indicated = 400W actual power).
 
According to ron-c - at least for the 501 - the meters measure the actual power produced even if the speakers don't match the tap that's driving them. I don't know if that's also true for the 252 or not. Maybe he'll chime in.
ron-c said:
The meters on your MC501 are true watt-meters that read power based on both amperage and voltage. If one of your speakers is lower impedance than the other that channel will read higher.
Many forget that the meters are a tool to monitor your system. Simply adjusting them to read the same kind of defeats the purpose.

Thanks,
Ron-C
Click to expand...
To clarify what MudPuppy was saying (I think), if you connect an 8Ω speaker to the 4Ω tap and leave everything else the same, you'll get half the power, not double, because the voltage at the 4Ω tap should be about 71% the voltage at the 8Ω tap (actually half the square root of 2), 71% the voltage across that unchanged speaker impedance will cause 71% as much current to flow through it. Power is voltage times current and .71 * .71 = about .50.
 
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ByteWrangler said:
According to ron-c - at least for the 501 - the meters measure the actual power produced even if the speakers don't match the tap that's driving them. I don't know if that's also true for the 252 or not but. Maybe he'll chime in.
To clarify what MudPuppy was saying (I think), if you connect an 8Ω speaker to the 4Ω tap and leave everything else the same, you'll get half the power, not double, because the voltage at the 4Ω tap should be about 71% the voltage at the 8Ω tap (actually half the square root of 2), 71% the voltage across that unchanged speaker impedance will cause 71% as much current to flow through it. Power is voltage times current and .71 * .71 = about .50.
Click to expand...

We're saying the same thing (to a degree). The meters don't read the actual power however. Reread my post. I'm not explaining theory, rather actual results from an MC-252. It overheated using the 8 ohm taps at way less than rated output (indicated). In other words, indicated power was less than half of actual power delivered. Actual power will create actual heat regardless of what is indicated. I suck at explaining some things.
 
Aside from whether the meters read correctly or not, in general you will get less power if you connect a given speaker to a lower-impedance output transformer tap, not more.

If your "power meters" are actually voltmeters that have power levels printed on the scales based on an assumption about the load, then they will behave exactly as you describe - the indicated power is pessimistic if the actual load is lower than presumed, and higher if the load has higher impedance. If they're ammeters, they will behave opposite; a lower than expected impedance will indicate higher power than is actually being produced. True power meters, whose readings are based on the product of voltage and current - which Ron says the 501's are - will indicate correctly regardless if the load is proper or not.

If your actual load is lower than the design load, the output stage will be asked to produce more current than expected at a given power level. This can happen if your speaker is really, say, 4Ω but you connect it to the 8Ω tap. The output transistors will produce more current than they should, even at lower power levels, and may go into protection (if that safeguard exists), or could fry themselves if not.
 
ByteWrangler said:
Aside from whether the meters read correctly or not, in general you will get less power if you connect a given speaker to a lower-impedance output transformer tap, not more.

If your "power meters" are actually voltmeters that have power levels printed on the scales based on an assumption about the load, then they will behave exactly as you describe - the indicated power is pessimistic if the actual load is lower than presumed, and higher if the load has higher impedance. If they're ammeters, they will behave opposite; a lower than expected impedance will indicate higher power than is actually being produced. True power meters, whose readings are based on the product of voltage and current - which Ron says the 501's are - will indicate correctly regardless if the load is proper or not.

If your actual load is lower than the design load, the output stage will be asked to produce more current than expected at a given power level. This can happen if your speaker is really, say, 4Ω but you connect it to the 8Ω tap. The output transistors will produce more current than they should, even at lower power levels, and may go into protection (if that safeguard exists), or could fry themselves if not.
Click to expand...

Exactly my point. They are obviously not true power indicators unless they're driving the proper load. The amp overheated at WAY less than rated indicated power. McIntosh told me to use lower impedance taps to match the speakers and avoid overheating. A Mac should not overheat, EVER, IMHO. It did. The "autoformers" met their match (more precisely mismatch) and the protection circuitry performed as designed. This may or may not apply to other amps, I'm referring to the MC-252 only.
 
Hmmm...

First, take a look at the last graph in this review of your speaker, below and at:
http://www.hometheaterhifi.com/volume_12_3/b&w-804s-htm3s-speakers-8-2005-part-2.html

You'll notice that the impedance curve of the speaker is anything but flat, varying from 4.2 ohms at about 100 Hz to 75 ohms at about 2500Hz, with some hefty spikes in the low bass. Whatever the nominal impedance, the amp doesn't actually see much of it.

So... my advice:

Never mind all the stuff about power. Try each tap to see which one makes the sound right in your room. If the amp can live with your favorite, you're done.
 

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Thanks for providing a review that apparently is favorable on the 804s, it makes one feel better about there purchase. I was in a store with my wife over the weekend to get some idea's for a wood cabinet with glass doors to put the mcintosh equipment in. When she walked past the B & W 802 speakers, she said " god help the woman who has to look at those in her living room...they look like bowling balls on top of the speakers with a horn coming out of them". I laughed because I really like the 802 speakers and would love to have them.

The last graph shows that between 55 hz and 1 khz the impedance is around 4 ohms. Everywhere else the impedance is higher. It will be interesting to see if I lose some base because of the fact that some else mentioned that power output on the mc252 amp will decrease at the lower 4 ohm tap.

Below was taken from the review article that was mentioned in the above reply. I never knew that distortion of base was a good thing ? Some of my cd's have excellent full bass and others just sound flat .
Editor's Note: I originally reviewed the 804S with a center channel speaker back in 2005, and the bass response was good, so I suspect this is a room placement issue in Jared's case. However, there really is not much option with front left/right speaker placement, so the subwoofer solution is still the one to follow. Also, the amount of bass is colored by the amount of distortion, with more distortion being perceived as "more bass", when you take into account that bass distortion is not necessarily unappealing. The 804S does not have a lot of distortion in the low frequencies, and depending on what one has listened to in the past, the 804S' lack of bass may just be a lack of distortion. I consider the 804S to be superb, and I wanted to see how it performed in a more conventional consumer-type environment, so I asked Jared to review them as a follow-up to my original one.
 
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