[Clio09]

I have an RM-10 MKII and really enjoy it. One of the things I enjoy about it is the user manual. It's one of the greatest reads IMO. Lots of great information on history, design philosophy, and how to get the best from the amp. One of the topics of interest for me is the idea of light loading. In this set-up if you have 8 ohm speakers, using the 4 ohm speaker taps on the amp will reduce it's power somewhat, but increase linearity and head room, while lowering distortion.

My speakers are rated between 8 and 12 ohms (Audiokinesis Jazz Modules) and in reality the they operate closer to 12 than 8. I'm wondering if anyone has any advice as to which tap might be best for light loading. Last night I was using the 4 ohm tap and it sounded quite good. In the past I've used the 8 ohm tap and that seems to work fine too. My speakers don't need a lot of power as they are 92db efficient. I've run them with as little as 10 watts, although I've noticed from amp comparisons 20 watts plus is what they prefer.

Any thoughts? Anyone else doing light loading?

[Ericus Rex]

I believe Roger says you should use the lowest ohm tap that gives you adequate volume without clipping.  If you can run the 4 ohm taps with your 12 ohm speakers to satisfying volume then do it!  The added benefit is it is easier on your power tubes to do so and so they'll last even longer.

I run the 4 ohm taps with my 8 ohm speakers on my RM-9 and it works great.

[Clio09]

Spoke with my speaker manufacturer and the impedance curve is very benign and averages out to 9 - 10 ohms. He indicated the 4 ohm trap should present no problems. So I'll keep listening with the 4 ohm connection until I can get a better feel for whether I like it or not.

[pubul57]

I raised the issue of light loading as a general proposition on Audiogon. Ralph Karsten of Atma-sphere said:

03-02-11: Atmasphere
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.

He then followed up with regard to the RM10, suggesting that his general comments might not apply to the RM10 with regard to the light loading principle:

03-03-11: Atmasphere
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

So, if light loading makes sense, it it generally true, or true for Roger's amps?

[Clio09]

It is not a general rule although amp manufacturers who put multiple outputs on their amps will tell you to feel free to experiment. I know the manual for my VAC Auricles Musicblocs recommends trying both taps. I have heard the results of amps not designed for light loading. Lack of bass being a key result.

However, the RM-10 appears to be designed with light loading in mind.