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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
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albertporter
4,927 posts
 
My experience mirrors that of Ralph Karsten. Using the lower transformer tap is typically higher distortion, more compression and frequently less bass.

Viridian makes excellent points about speaker impedance swing, it's a big factor.

I've not heard a RM10, so perhaps there is something going on with it's design that changes the rules. Or perhaps this somehow relates to the amp designers decision as to what's the perfect number of transformer turns for his creation.
pubul57 OP
3,712 posts
 
Yes, Roger does mention: "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."

In my case my speakers Are nomnal 8 ohms and never below 6.5 ohms.

I'm interested in the variance in views between Roger and Ralph on the issue. It might be that Roger's recommendation only applies to his amps because of the way he designs the RM10s, and Ralph's take is true for most other amps.
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viridian
4,184 posts
 
I would side with Ralph's views, but I don't know if that applies to the RM-10. There is clearly something going on with that amp that allows it to output 35 watts from a pair of EL84s when no other amp is even close to the figure. In fact, there are no other EL84 amps that will make more than half that power from a single pair of EL84s. Modjeski is a brilliant designer and perhaps his point applies to his unique topology used in this amp.
pubul57 OP
3,712 posts
 
I suspect you are right, because I have to believe Ralph very well knows what his talking about. It was interesting to read years back on Audio Asylum how many folks simply did not believe Roger's specs for the amp - of course he did do what others thought impossible.
pubul57 OP
3,712 posts
 
Just to add to the conversation, Ralph added this comment in the Vandersteen thread I alluded to:

03-03-11: Atmasphere
Pubul57, the act of increasing the bias on the tubes has the effect of reducing their output impedance. This will change the relationship they have to the output transformer.

In addition, you have to also consider how the amplifier was designed. For example, many power tubes want to see about 3000 ohms plate to plate. But what if you had the transformer designed to be 3500 ohms plate to plate? You might loose a little power, but now you can experiment with different taps to affect the sound in different ways. This is because the transformer really does what it is called- it transforms impedance. The load that is on the output taps will affect the load that is on the tubes. Especially in the last 30 years, its been a good idea to build in a little reserve to deal with the many 4 ohm speakers out there.

So if in the above case you are loading the 4 ohm tap with an 8 ohm speaker, the result is that the tubes might see a load impedance that is much higher, perhaps 6000 ohms. Now in some cases the amplifier will not be able to make as much power, as the voltage that needs to be made across such a load to get the power is no longer available. But it may not matter if the power thus obtained is sufficient.
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