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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Roger, funny you mention the Fulton J, as that was what I replaced my Tympanis with! After having lived with the T-I’s for a year, in ’74 I heard the Model J at John Garland Audio in San Jose, powered by Fulton’s own mono tube amps (a modified Dynaco Mark III, I believe). I had heard ESL tweeters before (in the ESS Transtatic I---they had used the same RTR tweeters as did Fulton in the J, 3 of them to the 6 in the J), and the Infinity Servo-Statics, and those tweeters made the Tympanis sound SO veiled. I also was hearing and feeling the bottom octave, which was missing in the Tympanis. The J’s midrange (provided by the available-separately Model 80) was great by itself (unusually transparent for a cone speaker), but adding the RTR ESL tweeters and Fulton’s transmissionline-loaded woofer box made it a very full range loudspeaker. I no longer have the Fultons, but I picked up a pair of Transtatics in 1982 for 400 bucks.

Bob Fulton was also a great recording engineer, and his ARK label LP’s are fantastic; very alive (immediacy, like a Decca/London cartridge), transparent, and detailed, with extremely natural vocal and instrumental timbres. He recorded local Minnesota choirs and orchestras, which in their amateurishness are charming.

@tomic601 
first thanks for the input on Ralphs amp, speakers likely the dead ESL-63 I mentioned in another post. I am away from them but I am assuming with ago all the panels are toast, the limited fault tree look I have done so far indicates EHT power supply issues. I am away from them so do not know on panel color, will check that when home next. Thanks all for inputs on rebuilders.
The fault tree should have the panels at the top and EHT at the bottom.  The EHT power supplies are usually NOT the problem in the 63's. The problem is dried out contact cement. All the panels fail over time and temperature cycling is their worst enemy. My friends died in a hot room one summer day. There are many videos on YouTube that show how the glue lets loose and how to rebuild them. Its not for the faint of heart. It is indeed sad that the 57 panels last forever and the 63 panels have a very limited lifetime due to.... GLUE!

I am away from my toolbox also but I will get DMM peak voltages measured today, SPL at 1 M

fun, real data will set you free was our mantra at work for 30 years..

As I listened to the modified and unmodified OTLs the other day on the 57s (that story is told in an earlier post of 11/26), I had the 57 terminals hooked up to the scope. I was constantly monitoring voltage. Music is fun to watch and anyone can get a good scope on eBay for under $100 and learn to use it if nothing else to look at music, look for oscillations, look for offset drift  Its really not hard, really. I suppose I sould make a video. 

I could see certain things the amps were doing. It takes a lot of experience to see and hear music and correlate, but indeed I could see current clipping and hear the mud is produced. I could see bass peaks due to low damping (that took a oscillator or sweep CD). I could hear that the low damping amp produced a boomy one note bass while the one with 4x the damping produced actual bass tones.

Years ago in the Stereophile "As we see it" column the writer asked several people what they listen for. His daughter said she listens to the "beat". Someone else the tonality, someone else the separation of instruments, someone else the soundstage, yada yada yada. 

I think this is important for listeners to think about what they listen for when they judge a system. If his daughter puts on a recording where she likes the beat it might not reveal any of the other things. But she just wants the system that gives her the "beat" 

I particularly wanted to hear high level high frequencies where the low impedance of the 57 would tax the amplifier. If I had Miles Davis I would have used him, but I dont have Miles. I do have Bach organ works with trumpet enchmade. Thats even toughter. 
      https://en.wikipedia.org/wiki/En_chamade

I could see and hear the current clipping. More hear than see. But I also knew I was in the 3% and over distortion area of the amp and 12 dB of simple feedback made a big difference in the clairty of this organ stop. I had a brief experience of being mentored by an old German organ builder so I knew and was facinated how organs worked. This music was chosen precisely to provide an audible test. It was done with levels matched by pink noise and immediage A/B switching.

BTW the 57 has a dc resistance of 0.5 ohms. If an amp has even 50 mV of offset that is 100 mA of output current which is also similar to the bias of most SS amps so at idle either the top or bottom transistors are turned off. This is not good for the speaker or amp. The 57 was meant to be driven by an amp with an output transformer where there is never any offset.

This is the stuff I think people should know that is never talked about. If you doubt me go measure it for yourself.
@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.