I have a Question About Light Loading Speakers

[Quadaptor!]

I just caught a discussion about "light loading" speakers on PS Audios YouTube channel and its news to me. I wasn't sure if this thread should have been posted here or over on the speaker Forum so I put it here because the critical item in my discussion is potential harm to my beloved Fisher 800B receiver. Do any of you "guys' have experience with light loading speakers on tube amps? Or in other words, running 8 ohm speakers off the 4 ohm taps on tube amps. Per the YouTube clip, this posses no harm to the amp and it could actually improve the sound to some "measurable' degree. This appeals to my basic cheap nature as I'm always all in for getting something for nothing. However, if if causes harm to my amp then I end up being little more than a schmuck /chump/moron for having done it. In my situation, I use a Fisher 800B to drive a set if Klipsch Heresys that, in turn, provide power to individual powered SVS subwoofers. That is, my amp powers a set of Heresys and a stereo pair of powered subs off the 8 ohm taps. Would moving the taps to 4 ohms really make any difference in sound quality or create a dangerous environment for the amp and speakers? Many thanks and I appreciate your thoughts.

 

[Ken Boyd]

Not sure if I am understanding you correctly. But to get carlity. your not really using the bamp section to drive both the speakers and sub, not if the subs are powered and your just sending a line level signal and the subs internal amp is driving the subs. And Heresys are very efficent so you don't need to use a 4 ohm output. I don't think you get anything extra by connecting to the 4 ohm taps, I stick to the 8 ohm tap for 8 ohm speakers. If your speakers were rated for 6 ohms I would probably use the 4 ohm taps.

 

[Selmerdave]

By connecting to the 4 ohm taps you make the speakers look like 16 ohm speakers to the amp effectively. It won't harm the amp, there will be a reduction in power and it's possible that in some cases distortion will improve slightly, but generally speaking best measured performance will come from matched impedance, so 8 ohm speakers on the 8 ohm taps.

 

[Quadaptor!]

By connecting to the 4 ohm taps you make the speakers look like 16 ohm speakers to the amp effectively. It won't harm the amp, there will be a reduction in power and it's possible that in some cases distortion will improve slightly, but generally speaking best measured performance will come from matched impedance, so 8 ohm speakers on the 8 ohm taps.

Thanks. A very straight forward answer!

 

[wparks]

I use a Fisher 800B to drive a set if Klipsch Heresys that, in turn, provide power to individual powered SVS subwoofers

If this is correct, the powered subs are only getting a line level signal, but are not drawing any current from the output so don't count as loading.

In general, this is snake oil along the lines of many "get something for free" videos you find on Y-tube. The engineers at Fisher knew what they were doing- by pairing the speaker to the correct impedance tap, you are ensuring the best performance that the design engineers intended. It's not just about distortion, it's also about available power output, and also amplifier stability, which have all been optimized to provide the very best "compromise" across all design vectors. You may improve one, but you harm the others.

In reading about the Klipsch Heresy speakers, there is one low frequency where the impedance can drop as low as 4 ohms, but for most of the frequency spectrum the impedance varies widely at much higher impedances. Other users on Audiokarma indicate you may actually get a better balance moving to the 16 ohm taps. If it were mine, I would trust the Fisher engineers more than a Y-tube content provider and stick with the 8 ohm taps.

 

[Quadaptor!]

If this is correct, the powered subs are only getting a line level signal, but are not drawing any current from the output so don't count as loading.

In general, this is snake oil along the lines of many "get something for free" videos you find on Y-tube. The engineers at Fisher knew what they were doing- by pairing the speaker to the correct impedance tap, you are ensuring the best performance that the design engineers intended. It's not just about distortion, it's also about available power output, and also amplifier stability, which have all been optimized to provide the very best "compromise" across all design vectors. You may improve one, but you harm the others.

In reading about the Klipsch Heresy speakers, there is one low frequency where the impedance can drop as low as 4 ohms, but for most of the frequency spectrum the impedance varies widely at much higher impedances. Other users on Audiokarma indicate you may actually get a better balance moving to the 16 ohm taps. If it were mine, I would trust the Fisher engineers more than a Y-tube content provider and stick with the 8 ohm taps.

Thanks

 

[gadget73]

More specifically the primary impedance that the tubes "see" is doubled. An 800B is ballpark 6.5K plate to plate, if you run an 8 ohm on the 4 ohm, it will be about 13K plate to plate. The problem with the high impedance is the voltages can swing more wildly at the output stage, and thats hard on the tubes.

For comparison here, the early 400 had the same 6.5k-ish that the 800B has. The late 400 has a 10K transformer. To get that to actually work ideally, the screen voltage needs to be lowered in order to make the tubes properly match the transformers and not be so abused.

It would be the same thing going from 6.5K to 13K, but the screen voltage should be even lower to make that run right.

 

[Quadaptor!]

More specifically the primary impedance that the tubes "see" is doubled. An 800B is ballpark 6.5K plate to plate, if you run an 8 ohm on the 4 ohm, it will be about 13K plate to plate. The problem with the high impedance is the voltages can swing more wildly at the output stage, and thats hard on the tubes.

For comparison here, the early 400 had the same 6.5k-ish that the 800B has. The late 400 has a 10K transformer. To get that to actually work ideally, the screen voltage needs to be lowered in order to make the tubes properly match the transformers and not be so abused.

It would be the same thing going from 6.5K to 13K, but the screen voltage should be even lower to make that run right.

So I guess the old saying "don't fix it if it aint broken" applies here. Leave it alone. Thanks

 

[zackthedog]

Subwoofers with high-level inputs are designed to not present a significant load when run off the amp's speaker taps in addition to normal speakers. Typically they might have a 100-ohm input, which has negligible effect in parallel with a 4 or 8 ohm speakers. I would strongly advise using the appropriate taps for your Heresys. While changing taps might not pose a hazard, the amp will perform as designed when you match the speakers to the appropriate taps on the amp and you'll get the best sound.