Bias and monoblocks

[AnotherJohnson]

I’ve got a pair of LP275M monoblocks that sound wonderful. No complaints.

I had some ground loop issues when I first installed them. JF told me to run them from the same duplex outlet. I reluctantly followed this advice, and as usual JF was right. The ground loop disappeared and has never returned.

I check bias twice a year, and I watch bias lights regularly … maybe every few weeks. There are never really any issues.

But I have noticed something that I deem odd.

When I shut the system down at the end of a listening session, I turn the amps off first. I turn off the right amp first, and then go to the left amp 9’ away. After the right amp is off, the bias lights on the left amp are often mostly on. I turn off the left amp, then the preamp, then the sources.

When I turn the system on, I go in reverse order, with amps on last. There are never any bias lights on at turn on. Nor during play. But at the end of the session (typically 2 to 3 hours later), I see the same pattern. Lights turn on on the left amp after the right amp is turned off.

I suppose this could be due to the line voltage change or other transient with the shut down of the paired monoblock.

This behavior was not impacted by the addition of the Niagra 1200.

Anyone else who is running monoblocks from the same duplex see similar behavior?

[admin]

My ART monoblocks will have some unpredictable patterns with turning on and off. They are right next to each other so I usually turn them on/off almost simultaneously. The truth is that I don't pay too much attention as I don't see any bias lights on after warmup and playing music. I'll have to look at it more closely but I may see some of them on after a listening session but with no active signal.

I also do the every 6 month re-bias procedure and of course the entire set if there is a tube change.

As a side note, you mentioned in another post that you were having large voltage swings in the power to your home. Any chance this behavior is related?

[roberto]

"I suppose this could be due to the line voltage change or other transient with the shut down of the paired monoblock"...

This is the reason, and no harm is produced. Sometimes, when you have a too tight the bias adjustment to the almost led on adjustment position, the leds might come on with an AC transient like a motor, or the compressor of a fridge, a cloth dryer machine, etc, even a neighbour near by could cause it. Also along with the music they could lit too.

The turning on and off sequence of yours is the right thing to do. This is the safest way and all users should follow it.

The voltage varying is another reason why the bias leds could be lit. The leds are very dependant with the house voltage variation. For readjusting the bias on all tubes, do it late at night.

Happy listening!

[AnotherJohnson]

As a side note, you mentioned in another post that you were having large voltage swings in the power to your home. Any chance this behavior is related?

I don’t think they’re related for several reasons, the main reason of which is that this is a virtually “every time” occurrence and every time is always on turn off of the first amp.

[Big Dog RJ]

Yes! This does happen with monoblocks and especially monoblocks plugged into the same AC mains line. I had the same pattern with the Premier 8s, Prem 12's, LP140's, LP260's and LP140 used in bi-amp mode with a huge Threshold amp for bass on the Apogee Diva's.

So, I was advised by LJ and Ed D at the time to wire the monoblocks into separate dedicated AC lines if possible, that would be ideal. And so I did!
Fast fwd 23yrs... and now in Melb I did dedicated wiring for the audio system and problem solved! No bias LEDs flickering, no unwanted noise, dead silence and super quiet.

I was also told to first power up the left monoblock, wait 40secs and then power up the right monoblock, wait another 40secs for tube heaters to adjust and settle in. Wait another minute or two... and then start playing after about 5mins power on. This allows grid voltages to settle in the amps properly and if bias LEDs light up then do minor adjustments if necessary. If they don't light up within the first 5mins just leave it!

During power down, again start with powering off the left monoblock first, followed by the right monoblock last. Before you power off the amplifiers, make sure preamp is set to 20 and is in MUTE then power off the amplifiers one at a time starting from left to right.

This is due to the monoblocks acting like a circuit, as one power amplifier. So all we're doing is just shutting off one channel at a time... therefore voltage swings will always travel to the channel that's still powered up. The left channel acts as the ground channel, so it's always advised to power on & off the left channel first. They went onto explain it in technical terms but lost me half way...I had too many beers that night.

So, I'm still not sure what's the main difference between the two channels other than being dedicated power amps. In the case of a stereo amplifier, when powering on / off, both channels are activated and shut down simultaneously. Voltages go in at the same time unlike with monoblocks.

Hope that makes sense!
Cheers, RJ

[AnotherJohnson]

So, I was advised by LJ and Ed D at the time to wire the monoblocks into separate dedicated AC lines if possible, that would be ideal. And so I did!
Fast fwd 23yrs... and now in Melb I did dedicated wiring for the audio system and problem solved! No bias LEDs flickering, no unwanted noise, dead silence and super quiet.

I started with separate circuits and Jeff said to power the whole system from one duplex (with a power strip). That solved my problem.

I think it’s funny … maybe the best advice is if there’s a problem, switch.

All those instructions about powering up and down are sort of like a liturgical ritual to pacify the power (literally) behind the music.

[admin]

I think it’s funny … maybe the best advice is if there’s a problem, switch.

In my personal experience this actually has been the answer. I've had ground loop issues in the past and when you go online to read about what to do, it's all over the place, use multiple outlets, use one outlet, flip the plug, ground a unit without a ground prong, lift the ground (don't recommend), etc... Frankly, it's been trial and error. Just try to isolate the unit, and then try altering the connections until the problem is resolved.

[AnotherJohnson]

FWIW, different amps respond differently too. I installed dedicated 20 amp circuits for the Krells.

The Krells were dead quiet … but every VPI TT I tried to run in that system hummed.

The MF2200s and MF2550SE were quiet. The LP275Ms were not.

I was surprised when it was suggested that everything should go into the same circuit. But it solved the problem.

[admin]

Yup, completely unpredictable. That has been my experience.