In this thread, Roger says the RM-10 is a class AB1:
http://www.audioasylum.com/cgi/t.mpl?f=tubes&m=61190
http://www.audioasylum.com/cgi/t.mpl?f=tubes&m=61190
"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?
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?
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- 34 posts total
Michael (Swampwalker), the "1" and "2" suffixes are applicable to tube amps only. "1" indicates that the grids of the output tubes are always negative. "2" indicates that they can go slightly positive, causing grid current to flow. That in turn results in some increase in output power capability, at the expense of an increase in distortion. Best regards, -- Al |
"Yes, a genius designed it." Sometimes design and execution are different things. The same genius designed the RM-9 amp, and the Counterpoint OTL amps, if memory serves. I'll let Ralph address the particulars of the OTLs. I owned two of the original RM-9 amps. Both prone to problems and not particularly quiet amps, either. It's comical how he takes pot shots at David Manley for unreliable amps; must not be any mirrors in Roger's lair. I have come around to the realization that one bias pot for the four tubes in each channel was nonsenical, at best. Except if you are a tube merchant, I suppose. Tubes can start off matched, but that is clearly not how they end up. One always had to have extra fuses and cathode resistors handy. And yes, tubes, as well. |
Maybe we can save Ralph the trouble since you actually addressed the SA-4 here: http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1141537350&read&keyw&zzcounterpoint To quote: 03-05-06: Viridian |
What does sound quality have to do with reliability? That's what Ralph may want to address. The RM-9s sounded great, when they worked. That's why I took a chance on the second one. Likewise The RM-10 sounds great, as well as the SA-9. But it is unfair to blame Roger for the issues with the Counterpoint amp as he did not spec the amps, only designed them, like the Beveridge preamp before that. The RM-9 is all his baby though. It was neither quiet, nor reliable. |
- 34 posts total