Does anyone care to ask an amplifier designer a technical question? My door is open.


I closed the cable and fuse thread because the trolls were making a mess of things. I hope they dont find me here.

I design Tube and Solid State power amps and preamps for Music Reference. I have a degree in Electrical Engineering, have trained my ears keenly to hear frequency response differences, distortion and pretty good at guessing SPL. Ive spent 40 years doing that as a tech, store owner, and designer.
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Perhaps someone would like to ask a question about how one designs a successfull amplifier? What determines damping factor and what damping factor does besides damping the woofer. There is an entirely different, I feel better way to look at damping and call it Regulation , which is 1/damping.

I like to tell true stories of my experience with others in this industry.

I have started a school which you can visit at http://berkeleyhifischool.com/ There you can see some of my presentations.

On YouTube go to the Music Reference channel to see how to design and build your own tube linestage. The series has over 200,000 views. You have to hit the video tab to see all.

I am not here to advertise for MR. Soon I will be making and posting more videos on YouTube. I don’t make any money off the videos, I just want to share knowledge and I hope others will share knowledge. Asking a good question is actually a display of your knowledge because you know enough to formulate a decent question.

Starting in January I plan to make these videos and post them on the HiFi school site and hosted on a new YouTube channel belonging to the school.


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@bdp24 

Thank you for your reply. Roger commented on your reply, but my question remains somewhat unanswered. Your advice, which is common (though not universal), is why I’ve never tried a 25-50 w/ch “quality” tube amp. I’d love to be able to try some amps at home, but the few dealers within reasonable distance don’t have much to choose from. One guy carries Line Magnetic, but I’m not interested in buying one of those, and don’t want to try something I’m not planning to buy, just to see if it has enough power. I’ve had five different pair of Maggies in the past 32 years, among other types of speakers. I always end up going back to them after trying something different. I don’t listen very loudly, and I don’t feel like I’m using anywhere near 200 watts per speaker, so I asked a tube amp designer what he thought would be an appropriate power output for my speakers. I have a Dynaco st 70 in my weight room system, but it isn’t a “high end” amp, so I’ve never used it in the main system, as I don’t think it would represent what a really good amp could do. I had a VAC 100 w/ch amp in the past. It was plenty powerful, but ended up losing out to a Krell KSA 250 (purely on sound quality) when I was able to do a side by side comparison. I also had CJ Premier 5’s, but they’re very powerful, so not what we’re talking about here, and ultimately proved problematic, so I replaced them with the hotrodded CJ Evolution 2000 tube input/mosfet output amp I have now. I do know that in the late 80’s/early 90’s, iirc, Magnepan had a 1960’s tube integrated playing into their speakers in their lobby... I think it was a Scott, @ 20 w/ch. I’m sure it was played at a very low level. Roger’s RM 10 looks interesting to me, as does the Decware Torii Mk IV. From an electrical stand point, would these drive MG 1.7i speakers playing music at 80-85 dB ? That’s what I’d like to know before I buy something without the benefit of hearing it first. 
@krelldreams

At your listening levels an RM-10 would do the job. I need to talk with my salesperson about arranging an in home demo.

The RM-10 now has a beautiful but expensive wood base which I would not like to have go back and forth.

The RM-10 is very light weight (15# i think) and thus can be shipped economically. What would you say to getting the amp in a simple base and if you decide to keep it buy the newer base or take a discount on the amp with the simple base?


@atmasphere Hey Ralph

Roger, I think if you revisit the above comments you will find them to be incorrect. An OTL has to be able to drive real world loudspeakers and so can produce the same currents at the output as any other amplifier. FWIW, the output tubes in most OTLs can easily blow a 10 amp fuse in certain situations without damage to the tubes.


I still find that OTLs at low impedance are current limited. As to 10 amps without damage. When I put a 6AS7 on the curver tracer and go just a bit above the peak rated cathode current I see flakes of cathode coating coming off like sparks from a sparkler at much less that one amp. WIth the grid being so close they can easily fall into the grid wire and POOF. Horizontal output tubes that Futterman and I use are specified for high peak current about 1 amp. 

I will measure the power of the M-60 at the impedances you suggest. What numbers should I find? At what level of distortion?  The amp is now working well with fresh tubes and all DC voltages confirmed to be in line. I have always liked the WIggins circuit and if I ever produce one it will be with transformer of course. 
@almarg Correspondingly, at my 12 foot listening distance I have measured peak SPLs on those recordings of close to 105 db, with the softest notes being in the vicinity of 50 db. I used a Radio Shack digital SPL meter for these measurements, set for C-weighting and fast response.

My speakers (Daedalus Ulysses) are rated at 97.5 db/1w/1m, and have a very flat impedance curve with a specified nominal impedance of 6 ohms. My 12 foot listening distance corresponds to 3.66 meters. Putting aside room effects for the moment I assume that SPL produced by a box-type dynamic speaker such as those falls off at 6 db per doubling of distance, which means an 11 db reduction going from 1 meter to 12 feet. I conservatively add in 3 db to reflect the presence of two speakers (as I understand it that figure will actually be closer to 6 db at my centered listening position when both speakers are producing similar signals), and I add in perhaps 3 db for “room gain.”

97.5 -11 + 3 + 3 = 92.5 db at the listening position for 1 watt per channel. Let’s call it 93 db.

I add in about 3 db of margin to the 105 db I want my amp/speaker combination to be able to produce at the listening position. So the required minimum amplifier power (into 6 ohms) is:

105 + 3 - 93 = 15 dbW (decibels above 1 watt)

15 dbW = 32 watts.


Thanks for your measurements and math. I think the math will leave most with their head swimming. Perhaps you could add some details to the steps so that others, less math inclined, might work things out. it is nice for people to know how different speakers fall off with distance. Perhaps you could write that up for us. :)

I ask for SPL both at listening position and 1 meter for two different purposes.

I want to get to know the listener and measuring at 1 meter leaves out all those other calculations. One just works off the 1 meter speaker spec and adds or subtracts. If one is 3 db higher than the speaker spec he is at 2 watts, 6 db is 4 watts, 10 db is 10 watts, 20 db is 100 watts.


@krelldreams, you should definitely try your Dynaco ST70 with the MG1.7, just to see if that gives you enough power. The RM-10 is a far better amp than the ST70, but their power output is about the same (right Roger?). But remember, a 35 watt tube amp puts out less power at the Maggies’ 4 ohm (and lower) impedance. The RM-200 doesn’t.

Another option is to have Roger make you an autoformer (maybe even build it into the amp!), to raise the impedance the RM-10 sees.