The Studio 220 is rated at 220 watts at 4 ohms, however, on the amp, it says minimum load 2 ohms. So... At 8 ohms its 140 watts, 220 at 4 ohms... so what is it at 2 ohms? I've read that you divide by .667 to get the power rating when you half the impedence. That would give 330 watts. So.. does it actually just stop at 220 watts (swr says that is the minimum power at 4 ohms) or does it push more power with the lower impedence? Anyone know? I've run it quite a bit with a 4 and an 8 ohm in parallel which gives 2.66 ohms impedence and it is definately louder but obviously, it's pushing two cabs so it would be louder. The clip light on the master volume comes on at a lower volume level with just the 4 ohm cab than when I use 2 cabs. I havn't run it at 2 ohms yet because I like the amp a lot and don't want to break it. Has anyone run it at 2 ohms for extended periods? Any failures?

hm...i read the same thing after downloading the manual some time ago and had the same thoughts. i don´t think you should be running it at 2 ohms or even 2.7 as it will be getting hot. i have tried the same thing as you did for a short time(2 ohm) and it worked well but started flashing(pwr amp-clip) quite early. havent done it again as i really like it.

The SWR 220 has a world class preamp, but not much power. Consider getting an inexpensive power amp for the 220 to drive.

Yep, ditto that. I wouldn't run that amp at 2 ohms. It has a really nice preamp though, and when I was using a Studio 220 (for about five years or so) I ran the preamp out through an external power amp. Once or twice I bi-amped it to, so I'd use the internal crossover, and feed the bass output into the external power amp, and the treble output back into the internal power amp on the 220. That worked pretty well, you don't need much power for the highs but the lows tend to suck up the watts.

Thanks all for the replies. I know more power is always nice, it's more of an electrical question though really. If the amp is rated to drive 2 ohms, can it or can't it? And what is it's wattage at 2 ohms? I understand being cautious of course with the amp, it's old and out of production. Let's just say in general, speaking of any amp that is rated at X watts at 4 ohms, it's rating would be at X*? watts at 2 ohms?

I don't care any label says. The SWR Studio 220 is rated a min. 4 ohms. You'll pop the amp if you push it any lower.

To answer the question about magic formulas for power ratings...there's more involved than just a set factor...I don't know the electronics language...but things like power supply capabilities make a difference. As an example, my SWR SM-400 has 400 watts bridged into 8 ohms but only 420 watts into 4 ohms.On a single channel, it has 300 watts @ 2 ohms, 240 Watts @ 4 ohms and 140 Watts into 8 ohms.That's probably similar to the amp on the 220. OTOH, my GK 700RB has 225 watts into 8 ohms, and 350 watts into 4 ohms. That's more usual.

I have no idea how they designed their power sections since many of the SWR amps can drive 2 ohm loads but don't put out significantly more wattage. This is unusual compared to other amps on the market that can drive 2 ohm loads. My Baby Blue II is rated 120 watts into 8 ohms, 160 watts into 4 ohms and 180 watts into 2 ohms; if you do the math the power increase is less than 1 dB (for all intents and purposes inaudible). At 2 ohms the amps runs extremely hot (no fan or heatsink fins) and audibly clips before the clipping LED lights (SWR service told me the LED triggering is unreliable at 2 ohms). After trying it on a few gigs with an extension cabinet I stopped doing it. I would treat 2 ohm use of your 220 as more of an emergency mode then something you want to depend on gig after gig. That's my $0.02. Another thing about SWR that is weird is their power specs can vary depending on what document you look at. My BBII manual says max power is 180 watts at 2 ohms. The online manual version (newer, post-Fender since it lists AZ as the company address) says 180 at 2.6 ohms and 200 at 2 ohms!!!! I own two BBIIs; silkscreened on the front panel of one is "160 watt power amp" but the OTHER one is marked "150 watt power amp" Of course even if there was a difference of 10 watts between the two, it would be inaudible.

As a long time S220 owner I've followed most posts about the amp. A while back someone posted that they had run theirs at 2 ohm for several years without frying it. I believe they used a clip on fan to force air through it. However, the general consensus would be to not do it. Get a power amp and use the S220 as a pre if you need the mega watts. That's what I'm doing most of the time these days. How much power? As Brian has noted, the ratings on SWR amps at different ohms can be kinda screwy. I've noted a couple of different published power ratings for the 8 ohm output of the S220 (140 and 150). At 2 ohms I'd agree with tornadobass that it's close to the specs he posted for the single channel of the 400s

With a "perfect" solid state amp, power would double as impedance is halved. This is because of this rule derived from Ohm's law: power = (voltage x voltage) / impedance In the real world few amps actually double in wattage as impedance is halved. A 50% increase is more common. As the impedance drops, the amp must be able to drive more current to keep the voltage constant: current = voltage / impedance In order to maintain the same output voltage when the impedance drops by two the current supplied has to double as well. An amp cannot generate infinite amounts of current so as the load impedance drops it gets harder and harder for the amp to deliver enough current. A rating of 800 watts at 2 ohms means the amp must be able to supply 20 amps into the speaker load!!!! Consider the SWR Baby Blue amp I mentioned above: with a rating of 120 watts into 8 ohms would suggest 240 watts into 4 ohms. The amp really only can put out 160. You would also expect 480 watts at 2 ohms but the amp only puts out 180. Like I said, a 50% increase is more typical, check the specs for these amps: Peavey Bass MkIV: 130@8, 210@4, 300@2 Walter Woods MI-225-8: 225@8, 350@4 Walter Woods E.A. High Power: 300@8, 450@4 A.M.P. BH-260: 160@8, 260@4 A.M.P. BH-420: 220@8, 400@4 GK 400RB(I): 130@8, 200@4 QSC PLX1602 (stereo): 300@8, 500@4, 800@2 Tube amps use transformer matched outputs so will deliver the same power into any load impedance that they allow.

if this isnt mentioned already ( im lazy and dont read full posts) swr puts these crazy things in their poweramps so it wont overheat or something like lets say you could go 400 watts at 8 ohms then add another 8 ohm cab but it would still be 400 watts. thats only on a few amps though.

This forum rocks! What a bunch of great feedback, I really appreciate it. Tornadobass, you got me thinking there with the sm400 comparison. I've got swr's schematics and the power amp sections are identical part for part, except for the r/c circuit biasing of the first stage of amplification. Apparently, that change increases output from 140@8; 220@4 to 150@8; 250@4 and 300@2. So the components of the power amp section can handle 2 ohm loads, as it says on the case, and probably put out roughly 275 watts. (I didn't say "safely" puts out 275 watts, just that's what I think it would actually do based on the schematics.) The output transistors (2SC3264) can each handle 17 amps of current max and each only has to draw half the waveform (push-pull I think it's called) so that's not gonna break. The two other variables, since the power amps are the same, would be the power supply, which the schematics don't give detailed info on, and cooling, which, other than a fan on the second generation of sm400, looks the same. Anyway, most people probably (and should) find this all very boring and to those who responded, thanks again. Enough of this, back to playing. (I'm sticking with my 2 cab, 4 ohm/8 ohm setup (2.7 ohms), I'll call it 250 watts for fun and I bet I'm not far off).

Gee, I thought this was fun...my guitar player and harp player use tube amps, so things about impedance and power output don't interest them...and they rarely even think about impedance loads on an amp. But yes, playing is generally more interesting! T.