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"Light Loading" Amps - Music Refence and others...

On another thread, a discussion started regarding light loading amplifiers. Since it was a discussion on Vandersteen speakers, I thought it merited an new thread, especially since there is some difference of opinion.

The principle of light loading was prposed by Roger Modjeski of Music Reference.

He said (with reference to his RM10, but applicable to all his amps):

"The amplifier is flat within 0.1dB and has low distortion of 0.3% when played below clipping on average level material. At the recommended bias current of 30mA/pair, the idling dissipation is nine watts or 75% of the tubes' rating. I estimate tube life to be 5,000 to 10,000 hours. Although higher idling currents will reduce distortion, it can also be reduced by light loading. Basically, light loading reduces the output current demand on the output tubes, allowing them to be more linear. It also reduces noise, raises damping factor, reduces distortion by 78% and allows for 80% more peak current when needed. The only loss is about 20% of the power rating or 1dB." Light loading means connecting the speaker on the tap that's one half its nominal impedance rating (i.e. the 4-ohm tap for 8-ohm speakers). For 4-ohm speakers, the he recommends running two RM-10s bridged to 70-watt monoblocks.

In the aforementioned thread, Ralph Karsten of Atma-sphere said:

"If you use the 4 ohm tap on an amplifier with a speaker of higher impedance, the output transformer will be inadequately loaded, and so it will express less of its winding ratio and more of its inter-winding capacitance. This can result in the amplifier no longer having flat frequency response. In addition, the transformer can 'ring' if inadequately loaded, which is another way of saying that it will distort.

The Merlin is an 8 ohm load, with a dip to 6 ohms or so. Its a benign load and an amplifier with an output transformer, if the transformer is designed properly, will likely work best on the 8 ohm tap. This will minimize the artifact of the transformer."

Two views. And different views from listeners, somew thinking light loading works magic, other saying differently.

What do you think of the priciple. The two technical arguments? Your experiences with light loading?
pubul57

Showing 3 responses by atmasphere

atmasphere's avatar
atmasphere
11,640 posts
 
I suspect that Roger has a really robust driver circuit in the RM-10. IOW, its probably a class AB2 circuit.

Depending on the way the transformer is spec'ed out, I could see a loss of 50% or a loss of 20% of power if you 'light load' the amp. We build a guitar amp that uses a pair of 2A3s in push pull for about 16 watts (and is otherwise a clean version of the Marshall Plexi for you guitar nuts); loading on the amp has to be correct for the amp to make full power.

I don't have any problem with anything I have read here or on the other thread so far (I just geek out on it is all...). There are a lot of variables in an amplifier design, the transformer being a rather obvious one. But I have found that just because one designer says you can't do it is no panacea for it not being possible :)
atmasphere's avatar
atmasphere
11,640 posts
 
Seems to me Scott made an EL-84-based amp that made 25 watts. It too was class AB1. To get 35 watts though would require one to jump through a lot of hoops design-wise (most EL-84 designs don't make more than 17-20 watts, but a good portion of those are class A). Something will have to give somewhere. I suppose if you ran enough B+ you could get that kind of power without running grid current, but OTOH you might have problems with tubes holding up, and it would not surprise me that the base of the tube was putting a lot of heat into the tube socket.

This latter issue may explain why the move away from circuit boards. But I would expect that the tubes may well have to be pre-selected, or certain versions of the tube may have to be avoided.
atmasphere's avatar
atmasphere
11,640 posts
 
Al, that's what I get for relying on my memory for something like that.
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