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EQUIPMENT FORUMS >> SE84UFO >> Is Power output the same driving 8 ohm vs 4 ohm?
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Message started by flargosa on 07/07/17 at 20:19:14

Title: Is Power output the same driving 8 ohm vs 4 ohm?
Post by flargosa on 07/07/17 at 20:19:14

Is power output the same driving a 8 ohm vs 4 ohm speaker? Does rms wattage increase driving a 4 ohm speaker? I know some amps have two wattage specs depending on the speaker impedance.

Title: Re: Is Power output the same driving 8 ohm vs 4 ohm?
Post by Steve Deckert on 07/28/17 at 04:08:36

Hi flargosa,

The current models with the UFO transformers have an impedance switch so the power is the same for 8 ohm as it is for 4 ohm. Prior to the UFO transformers there was no switch so the stock transformer put out more power into 4 ohms than it did into 8... and more power into 2 ohms than it did into 4.

-Steve

Title: Re: Is Power output the same driving 8 ohm vs 4 ohm?
Post by flargosa on 07/30/17 at 23:50:25

So driving a 4 ohm speaker with the switch at 8 ohm will increase wattage output from 2.3 to about 4 watts? Not that 3 db increase in volume makes any audible difference, just curious.

Title: Re: Is Power output the same driving 8 ohm vs 4 ohm?
Post by Steve Deckert on 08/01/17 at 03:19:12

On the new UFO output transformers that feature the switch, it is possible to run a 4 ohm speaker on the 8 ohm tap or visa versa. Impedance is reflected, so when you show an 8 ohm tap a 4 ohm load, that load is reflected through to the primary winding causing it to drop. When it drops, the output tube sees a lower primary transformer which causes it to put out more power with less headroom. Doing the opposite, running an 8 ohm speaker on the 4 ohm tap would raise the primary and thus cause the tubes RMS power to drop, but the headroom would increase. In addition to power, tone is also effected. The lower the primary impedance, the more midrange tone and bass weight. The higher the primary impedance the better the top end transparency and the tighter the bass. This is why the best way to adjust the switch is to not know which way it works, and simply choose the best sound for the particular listening volume your at. I say it that way, because when your back is up against the wall, and you are pushing the amp into the beginning stages of clipping, one of the two positions is always going to hold together longer and it may not be your preferred setting during more relaxed volumes.

Steve

Title: Re: Is Power output the same driving 8 ohm vs 4 ohm?
Post by alper_yilmaz on 08/01/17 at 17:06:22

Steve,

On another note, how does the SE84UFO work with 16-ohm loads?

Best,

Alper

Title: Re: Is Power output the same driving 8 ohm vs 4 ohm?
Post by Steve Deckert on 08/03/17 at 04:47:41

The power drops by about 20% into 16 ohms. This is an insignificant amount unless the 16 ohm speaker is not of high efficiency. A 20% power loss can be easily compensated for by a 20% increase in efficiency of the loudspeaker, which is not uncommon since 16 ohm speakers tend to be more efficient.

Steve

Title: Re: Is Power output the same driving 8 ohm vs 4 ohm?
Post by alper_yilmaz on 08/03/17 at 16:38:28

Thanks Steve... This is very helpful.

Best regards,

Alper

Title: Re: Is Power output the same driving 8 ohm vs 4 ohm?
Post by flargosa on 08/07/17 at 16:47:15

Thanks for the detailed response.