Bias LED. When is it too much? CJ MV125

[Robbie]

Hello, I'm currently using a Conrad Johnson MV125 to drive my Sonus Faber Extrema.
The ensemble really sings and I enjoy it a lot. Im just a little bit worried that I'm asking too much to the amplifier because the Extremas are definitely power hungry and the bias led in louder passages are constantly lit up. I don't perceive anything wrong from the acoustic point of view,
But I'm worried that I am pushing the tubes too hard.

Check this video (it could be a lot worse, with other music, this was not that bad, actually)
https://youtu.be/_aEPhOMXeQE?si=3eGcQIejYaEbIHyf

I set up the bias as per user manual, of course. Where they say that led flashing is normal within certain amount. But this is way over a "led flashing".

Is this a consequence of the fact that the 2x125 watt of Pv125 are not enough? Am I overthinking?

[admin]

Short version: You're over thinking it. It's fine.

Longer version: The SF speakers are rated 4 ohm, 88 db sensitivity. The MV125 is a high quality amp and will be able to handle the load without any issue. You are not getting any distortion or clipping. In other words, no acoustic distortion. The LEDs can flicker and this is not a problem. That is why biasing is done without active signal (just silence) as otherwise it would be impossible to bias the tubes with constantly shifting load. If you tubes are not failing then you are fine. Lastly, enjoy the LED light show, this is CJ's equivalent of a VU meter!

[Big Dog RJ]

Yes, that pretty much sums it up!

The MV125 has plenty of power to drive the SF Extrema's, no issues. And you've correctly biased the Output tubes as recommended, so really nothing more you can do there. With the bias circuits, what's taking place is you're adding just the right amount of idle current for the Output stage and in this case, a tube Output stage. Even though you're probably going on louder levels and the LEDs light up, it's reaching its maximum idle, that's all. Those tubes can certainly push way more than what's factory set, which is slightly lower than Max output. So in actual terms, the Output stage is somewhat cruising along... it's not under stress as being maxed out towards clipping. If it were the case, you'll definitely hear it and your SF's will definitely feel it!

OTOH, if you really did want to pump out those tunes and listen continuously at louder levels, where you're fully energising your room in every square inch... then you may want to upgrade to a more powerful design. The Premier series monoblocks will offer much more power, as well as the high powered LP series amplifiers and of course the ART150 and ART300's, if you really wanted to go this route.

Having said that, even though these amplifiers are real power houses with finesse, even during the loudest passages, it's no where near 100s of watts... only around 45 to 50w the most. The human ear cannot tolerate 100s of watts continuously, you'll go deaf in a matter of few days! And eventually lose your precious hearing by the end of the week.

Therefore, this is only domestic Hifi and not rock concert levels we're referring to, so no issues driving these speakers in a home setting. Like I said, if the fact that those LEDs are lighting up constantly and it's bothering you... then only thing to do is upgrade to higher power with a much larger output stage. Larger output stages also have larger power supplies, and this is where more of everything comes into play: more power, more current, higher voltage, higher tolerances on the power supplies and output stages. So not just watts but more of everything.

That kind of upgrade can only be determined by you! If you feel nothing's breaking up... no distortion in playback and you're enjoying the music, then that's all that matters. So simply put, the tunes are fine!

I certainly hope so. Cheers
RJ

[Big Dog RJ]

Oh! One more thing I forgot to mention, is that these output stages are based on Class AB not Class A. Such that, if the MV125 was a Class A design, then your Output stages are always ON, which is full throttle... and this can deteriorate tubes quicker than a Class AB design.

In Class AB output stages, those Output devices are ON & OFF, which is also referred to Push- Pull in either ultra linear Pentode design or Triodes. Most SE type designs are pure Class A with a handful of watts, typically rated from just 8w to about 20w Max.

Class AB is also far more efficient than Class A. As I mentioned before, Class A is always in ON mode, continuously driving full tilt. There's quite a difference in the two when you experience playback, and in certain cases on difficult loads, such as full range Stats with notorious impedence swings like those older Apogee's, driving in Class A is more suited. Also offers far less stress on tube Output stages, plus using monoblocks is even more effective. Just one power amp dedicated to drive one channel.

Having said that, JF has managed to find a way in Push-pull config to offer Class A output stages, this is quite remarkable! These designs are found on CJ's top of the line ART27A the ART108A's, they're really top flight gear, and a beautiful sound. It's the first time that CJ has ever offered Class A output designs. In all their gear from the 60's onwards, has been in Class AB.

These latest Class A designs will have absolutely no issues driving such reactive loads but they come at a price. It's a pretty penny!

Cheers mate, and do enjoy those finest tunes!
RJ

[AnotherJohnson]

Hello, I'm currently using a Conrad Johnson MV125 to drive my Sonus Faber Extrema.
The ensemble really sings and I enjoy it a lot. Im just a little bit worried that I'm asking too much to the amplifier because the Extremas are definitely power hungry and the bias led in louder passages are constantly lit up. I don't perceive anything wrong from the acoustic point of view,
But I'm worried that I am pushing the tubes too hard.

Check this video (it could be a lot worse, with other music, this was not that bad, actually)
https://youtu.be/_aEPhOMXeQE?si=3eGcQIejYaEbIHyf

I set up the bias as per user manual, of course. Where they say that led flashing is normal within certain amount. But this is way over a "led flashing".

Is this a consequence of the fact that the 2x125 watt of Pv125 are not enough? Am I overthinking?

It does look like a bit much. Back them off an eighth of a turn and try it again. If it is just on this music, I wouldn’t worry too much. But if it is pretty much a regular occurrence, you might try to tame it a bit.

If they’re on all the time regardless of setting, it’s usually time to replace the tubes.

[Big Dog RJ]

Actually yes, that's the best advice!

As AJ suggested, a very slight turn- further down- will keep those LEDs off. And this is only IF they're continuing to light up. Changing the Output tubes may also be in order, in which case you'll probably notice fresh tunes! Like a whole new sense of life and far more energy. Maybe you've already replaced the Output tubes... ? don't know.

See how you go and please do report back so that others with similar experiences can learn from.
Cheers, RJ