"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
Post removed 
There seem to be an awful lot of folks running RM9s for many years for them to be too unreliable. Now Roger did come to think of tubes on boards as being problematic over time so he went to point-to-point wiring on his RM9SE and the RM10s and 200s. Another thread questioned the reliability of Quicksilver amps to which I could only say that all manufactured goods can have a problem form time to time, but I certainly think that at least in my experience, Music Reference, Quicksilver, and Atma-sphere all are very reliable in the aggregate experience of owners.

I never heard the RM9, I owned an RM9 Special Edition and now the RM10 MKII - they are certainly quiet, but I have 89db speakers not 104db, which would proabably be the ultimate test.
I can concur with Pabul57 regarding the noise floor of the RM-10. Virtually non existant driving a pair of 98db sensitive Tannoy Westminsters. Certainly the quietest tube design I have ever heard.
Eccletique, have you tried light loading? I assume, don't ask why, the Tannoys are nominal 8ohms?
I would describe the RM 10 - to steal a phrase... "starless & bible black". Dead quiet background, resolved, tone rich & fleeting fast. It commands your attention & just draws you in. The best audio purchase I've made to date. When I received my new RM 10 from Roger the 4 ohm taps were open where he had personally burned in my unit & listened to my unit for a week & I followed suit. Definitely the way to go.