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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
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.
almarg's avatar
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almarg
9,670 posts
 
Roger Modjeski: The only loss (from light loading) is about 20% of the power rating or 1dB.
Hi Paul,

As we had discussed some time ago, this statement by Roger strikes me as incorrect for most amplifier designs. I have no way of knowing how correct it may be for his own designs.

In general, connecting an 8 ohm load to the 4 ohm taps of an amplifier may reduce maximum power capability by up to 50%, compared to what can be delivered into the same 8 ohm load connected to the 8 ohm taps, and compared to what can be delivered into a 4 ohm load connected to the 4 ohm taps.

A loss of about 50% will occur in cases where maximum power delivery into a 4 ohm load connected to the 4 ohm tap is limited by voltage swing capability, and where the design is such that the voltage swing capability on that tap does not increase significantly as a result of the 8 ohm light loading.

Concerning your basic question, all of Ralph's comments make complete sense to me, and I would expect them to be true. However I would imagine that in some cases, presumably including Roger's designs, the transformer may be over-designed (or designed with the possibility of light-loading specifically in mind) such that the concerns Ralph raised would not have too much quantitative significance, and/or may be outweighed by the benefits Roger enumerated.

I third Viridian's excellent comments about speaker impedance swings, and about the need to try both connections and see what sounds best.
Your experiences with light loading?
FWIW, my speakers are very close to 6 ohms throughout almost all of the spectrum, rising to around 10 ohms at deep bass frequencies. I tried both taps on my amp, and settled on the 4 ohm tap, although the difference was surprisingly small.

Best regards,
-- Al
pubul57 OP
3,712 posts
 
Hi Al. To tell you the truth, I find they sound very, very similar both ways, but was predisposed to prefer the "light load" for the "benefits" in measured performance(but for power)and longer tube life that Roger mentions in his comments. I had no doubt Ralph was right as a general principal, and I do wonder if Roger just took an "roger" approach, as is his way, that made his recommendation valid - for his amp.
clio09
3,343 posts
 
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.

Yes, a genius designed it. Three things the man is known for are: quiet amps (quietest tube amps I have heard), deriving more power from tubes than others have, and electrostatic speaker design.

In all seriousness though Roger specifically states he wrote his own application for the EL-84 tube. The original concept for designing the amp was to power his Quad ESL 57 speakers (not many know this but Roger cut his teeth working for Harold Beveridge designing the ESL amp for those speakers). Clearly the typical EL-84 application wasn't going to cut it.

For those inclined, the 6 Moons review of the RM-10 MkII is a good read with interesting comments from Roger, including the one below, as well as theories on light loading and 2nd harmonic distortion. I'd also recommend reading the RM-10 MkII manual for additional insights from Roger.

http://www.6moons.com/audioreviews/musicreference/rm10.html

"Over the last 10 years, I have written several new applications to get higher power out of some common tubes that I thought could do more. If I were employed by Sylvania in the 1950s-70s, I would have done the same thing. There are an infinite number of applications that can be written for any given tube. They involve variations in B+ voltage, current, grid drive, screen voltage and load impedance. The data books list only a few to give amp designers some ideas. Sadly, that's as far as many designers go. David Manley was a stickler for applications believing that the only ones allowed were the ones in the data books and they must be adhered to the letter. Sadly, he took the ambitious 560-volt ultra-linear application from MOV as reliable and made some amplifiers that damaged a lot of tubes. Not even MOV could make tubes that consistently held up in that application. To this day, there are no tubes that hold up well at those voltages on the screen.

I research my applications very carefully with dissipation studies on all elements. And, although I have a replacement tube business, I do not want to waste valuable tubes (or your money) on poor applications. One application that caused a lot of comment in the Asylum was my getting 35 watts out of a pair of 6BQ5/EL84s [the amplifier under review - Ed]. They said it couldn't be done yet it is done and has proved reliable over the past 10 years in the field. The RM 245 uses a new application that yields 4 watts at a dissipation of less than 8 watts. There is no extra stress on the tube. Typical applications are 10 watts dissipation (the max for that tube). If you look at the following link, you can see the power vs distortion for 3 tubes at 25 mA. and the EML at elevated current where we take advantage of some of its extra dissipation. I have not yet written the maximum application for that specific tube but I expect to get 8-10 watts at low THD.

The above referenced graph of power output vs THD is at the load. You can add about 20% at the plate. The commonly used application from the RCA manual (RC-11 pg. 86, 1933 and repeated thereafter without modification) gives 2 watts at the plate with unspecified THD (shall we assume 5-10%). Two watts was more than enough power for the times and it seems little has been done by modern 45 amplifier designers to consider new applications.

Applications once written are forever left to rest and copied verbatim by every other maker. Remember, an application is often written to entice the designer (of a table radio at that time) into a circuit that fits his technical and economic needs. The low B+ (275V at 36 mA = 9.9 watts dissipation) of this application is attractive. Note they have 2 even lower power ones for those more economical who desired less power back then. This tube was designed to be a step up from the type 71 (max pwr. 0.79 watts) and a practical alternative to the dismally inefficient and hard-to-apply type 50. Note that the 50 was rapidly pushed aside for the much more popular 45. I have a Brunswick Panatrope that uses a 50. It's a monster that sold for the price of 1.5 Model T Fords in 1928. The 50 also had to be transformer-coupled when resistance coupling was coming on fast. My application is 310V at 25 mA =7.75 watts along with circuit and load differences.

I "light load" my RM-10 and by default have no choice but to use the 3 ohm tap on my Music Reference EM-7 monoblocks (my speakers are 8 ohm minimum with 12 ohm peaks). I'm not hearing anything negative like lack of bass, quite the contrary with the RM-10, I thought the bass improved. Having met the man a number of times when I lived in Santa Barbara I can vouch for his engineering knowledge. What he does to make his equipment sound so good is beyond me though. I just know it's relegated some really good equipment to the closet.
atmasphere's avatar
atmasphere
11,575 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 :)
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