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

@kellydreams   . I did try a passive preamp in the main system recently and did Not like how it made the system sound

@fsonicsmith   And while a solid state input stage may very well offer better measurements, where is the proof that it sounds better? At the end of the day, isn’t it indisputable that it is cheaper to produce and less complicated?

@bdp24  The J’s midrange (provided by the available-separately Model 80) was great by itself (unusually transparent for a cone speaker)

@almarg  . I recently changed to a Pass XA25, which is specified as a class A amplifier rated at 25 wpc into 8 ohms and 50 wpc into 4 ohms. Per JA’s measurements, though, it is capable of 80 and 130 wpc into those impedances. I presume that most of that increase represents the amp’s capability after leaving class A, although per JA’s comments some of the increase apparently reflects differences in the distortion percentages the ratings and measurements are based on.

@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.

@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.

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.

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.

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..

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.

Roger I happened to make my first visit to a new hi-fi shop just opening in Livermore, CA in 1973, Audio Arts. It was a 1-man shop, that man being Walter Davies, now the maker of the great Last Record Care products. And as luck would have it, that was the same day Bill Johnson was delivering and installing a complete ARC system in his new dealer Walt’s listening room; a Thorens TD-125 MK.2, a Decca Blue mounted on a prototype ARC arm (it never went into production), an ARC SP-3, and Tympani T-I’s (as you said, at the time distributed by ARC) bi-amped with a D75 and D51.

I was just a kid, and spent a couple of hours getting an education in high end hi-fi. A couple of months later I had that exact system (with a Decca arm) in my own room. All set up and connected, I pushed in the power switch on my SP-3, and immediately heard a "poof" and smelled the aroma of something burning, which turned out to be a resistor in the power supply. Welcome to the wonderful world of using under-rated parts!

Thank your for the walk back in time. Anyone who is not old enough to have your experience I encourage to read every word above. Thats exactly how is was back then. My store in Richmond VA was called Audio Art and is still in business. Manufacturers visited us. Nakamichi flew in on their private Mitsubishi plane. We ate, we drank, Bob Fulton told the story how the monkeys behaved when they heard sounds carried by different speaker cables.He said bad cables confused them. No kidding. In his Fulton J system, one of the best things going back then, he used Ford solenoid coils for woofer chokes. He was delightful.

The J was cool because you made it in pieces as your funds allowed. First you bought the bookshelf speaker which was a fine speaker in itself. Did what a good "monitor" should. Then you could either buy the 8 Cu Ft woofer box or the RTR ESL tweeter. When you were done you could please the missus with a dark brown cloth and wood "hat" that covered the whole thing. Stood about 4-5 ft tall. Marvelous. No bullshit, even the monkey story is better than most cable claims today. He made some good wire when this current crowd of cable makers were babies. I think some of them are still babies the way they rant and bend science.

As to your "POOF of smoke" thats is simply an unprotected power supply. Bill didnt seem to get the point that the down side filter capacitor looks like a short every time you turn on the preamp. Rather than take the time to discover how to make a Short Protected power supply he just added band-aids. It was always claimed to be some "sonic improvement" to make it easier to swallow. Most times it was a fixing a failure mode in an unfinished design. Power supply failures were never mentioned. As I recall the SP-6A upgrade was $150 for a little circuit board with two diodes and a resistor. He always kept the price you paid plus the upgrade cost equal to the price of the newer unit. Of course you had to send the preamp back, wait 6 months and some claim they got someone elses preamp back. ARC database has lots of information, though no stories. http://www.arcdb.ws/ It is the work of a group of dedicated ARC fans. Good place for ARC info on prices, modification dates, some schematics. 

Back then the joke was. If you buy an ARC preamp you have to get two because one will always be in the shop and by the time you get that one back its aready one or more behind the latest version. People like Harry Pearson turned that inconvenience into a virtue.

If you dont believe me ask bdp24. He has obviously been around the block. Im sad for the horrible choices newbies have to make. The pressure to buy expensive cables and power cords and fuses and products that do nothing. Those did not exist when I got into this. We had to make real stuff. However we had coloful fellows like Bob Fulton, Jim Whiney, Matt Polk and Co, Arthur Jansen was still alive and I interviewed him as well as Saul Marantz and SId Smith. And last but not lease Harold Beveridge, my mentor.

Funny that the latest ARC amps I've heard sound greyish, muted dynamics, etc. My friend has an ARC 75 which is no match for his Classic 60s or my voltage regulated amps. Worse, his amps, Ref 5 preamp and $10K CD player all have had significant break-downs after only a year or two. Yuk!

My question remains, I'd like to know what you opinion is of the VAC IQ (continuous autobias) amplifiers. They sounded great wherever I heard them and they appear to be conventional designs well executed. They must have good impedance outputs to make them adaptable to drive many types of speakers like your own amps.

@ramtubes, Roger I happened to make my first visit to a new hi-fi shop just opening in Livermore, CA in 1973, Audio Arts. It was a 1-man shop, that man being Walter Davies, now the maker of the great Last Record Care products. And as luck would have it, that was the same day Bill Johnson was delivering and installing a complete ARC system in his new dealer Walt’s listening room; a Thorens TD-125 MK.2, a Decca Blue mounted on a prototype ARC arm (it never went into production), an ARC SP-3, and Tympani T-I’s (as you said, at the time distributed by ARC) bi-amped with a D75 and D51.

I was just a kid, and spent a couple of hours getting an education in high end hi-fi. A couple of months later I had that exact system (with a Decca arm) in my own room. All set up and connected, I pushed in the power switch on my SP-3, and immediately heard a "poof" and smelled the aroma of something burning, which turned out to be a resistor in the power supply. Welcome to the wonderful world of using under-rated parts! High End? Not by my definition!

I think the debate between cone and electrostatic is reminding me of the great cable interconnect debate. Could it be that this is just personal preference? Or even that my ears are just trained and accustomed to whatever cones do to the ears? I’ve listened to my share of electrostatics. Some were very impressive. Some were better than my Altecs in many respects. But none moved me.

I really like the sound of SET over PP.You can see my audio history in the below link.https://forum.audiogon.com/discussions/the-most-musical-sound-in-my-40-year-s-audio-historyThanks again for your kind advice

@krelldreams, though you asked Roger for a recommendation of a low-powered tube amp for use with Maggies, I hope he doesn’t mind if I chime in on the question. As a long-time Maggie owner (my first bought in 1973, the original Tympani T-I; I now own Tympani T-IVa’s), I can assure you a typical low-powered tube amp is about the worst kind you can partner with a Maggie. But what do you consider a "lower" powered tube amp? Under a hundred watts? Under 50? Maggies are very insensitive speakers, and benefit from a "higher"-powered amp. Another factor working against tube amps with Maggies is that the speaker present a 4 ohm and lower load to an amp, and almost all tube amps prefer a higher-impedance load, producing less power at higher distortion into lower impedances than into higher.

While this may strike some as inappropriate in an answer to a question posed to the designer/manufacturer of Music Reference amps, I’m doing it anyway ;-) . There is one tube amp I can suggest for use with Maggies, the one I use: the Music Reference RM-200 Mk.2. Unique amongst tube amps, it produces slightly higher, not lower, power into a 4 ohm load than into an 8 ohm one. About 100 watts/ch into 8 ohms, a little more into 4. Most 100w tube amps produce only 60 or so watts into 4 ohms. The RM-200 is also a real good amp in general terms. Low distortion ("clean" sound, including at bass frequencies, The Achilles heel of many tube amps), low output impedance (no tube amp colorations, such as added "warmth" and/or "soft" highs), stable into reactive loads (usable with ESL’s and ribbons), long tube life (some popular tube amps burn through a set of output tubes in as little as a couple thousand hours, and a replacement set are not cheap), and tasteful, classic styling, all at a reasonable price. Review, both subjective and objective, available on the Stereophile website.

you can’t possibly mean that preamps are easier in general and I am unaware of any accolades for your preamps (whereas you are lauded for your amps). Obviously you mean that for your tastes and purposes, preamps are easier than amps.

A great preamp does more than attenuate a signal. It breathes life into music. A great preamp is quietly powerful, while a great amp simply provides grunt.

 By “more power” I’m just going by where I set the volume control before and after. It was a surprise, as I thought there would be less, and I was pleased because in my layman’s view it meant less stress on the tubes and circuitry.

Also, I’m not sure what you mean by better technology than cone speakers. First off, the Altec is a horn/cone hybrid. That aside, while electrostatics have their virtues, they don’t move air the way a cone does, as someone here also observed. That’s what I mean by the dynamic punch of the Altecs. I’ve heard some great horn speakers, with prices in the stratosphere.

I figured it out by listening and trial and error over time and based essentially on those parameters as a guide. However specifications alone seldom tell the whole story. The devil is always in the details much of which is never specified. Detailed measurements like those in Stereophile help a lot but I have found there may still be surprises playing real music even with very comprehensive test measurements at ones finger tips, though if done correctly, measurements certainly help with the decision making process of what to try next or not.

I still love to "visit" electrostatics (the Quad ESL 57s being my favorite), for their unique qualities. But every time I listen to an electrostatic, of any make, I come away happy to have moved on to cone speakers.That includes every hybrid I’ve ever heard: The cones seem to add some body, but only within their frequency range. As the frequencies climb up to where they are handled by the panel, the sound character changes to my ears to that ghostly quality, so I am always aware of this discontinuity.

Your own decision to go hybrid with your higher powered amp and to go true balanced with your higher powered amp but not your lowered power amp is a head-scratcher. If-as you state-monos are more susceptible to hum than stereo amps, why do you implement RCA-only vs. offering true balanced in the opposite direction? And while a solid state input stage may very well offer better measurements, where is the proof that it sounds better? At the end of the day, isn’t it indisputable that it is cheaper to produce and less complicated? You on the one hand have little good to say about Rogue and yet when it comes to hybrid tube amps, I think of Rogue (and Musical Fidelity though I don’t count their "tubes" as tubes).

It seems to me that JA has a tendency to disagree with his own reviewer’s perceptions as to which output tap is best rather than criticizing the design of this amp and it’s output tap measurements per se.

This may dismay you quite a bit from an engineering standpoint, but I take measurements of tube amps to be very analogous to measurements of DAC’s; the best measuring DAC’s don’t often sound the best.

 Preamps are much easier and to me not so interesting as power amps.

 Is it harder to design a amp or preamp?

Never bought by power. Always purchased by sound.

HI, Roger here with a question.

I would like to hear how each of you figured out how much power you needed to buy in watts?

I would appreciate the following information in your response.

Your listening level LOUD SPL (preferably measured at 1 meter from the speaker with a REAL SPL meter. Your low listening level. If you are using a cell phone app then you have confirmed it?

Your speaker sensitivity?

Listening Distance from speakers?

How many watts at your load is the amp is rated to supply?

I would like to hear how each of you figured out how much power you needed to buy in watts?

Hears what JA had to say in Stereophile.. The Audio Research Reference 75 measures well for a classic tube amplifier design with a single pair of output tubes for each channel and a modest degree of loop negative feedback. Its output transformers are also of good quality, the only proviso being that the amplifier should not be used with loudspeakers whose impedance drops significantly below the nominal value of the output transformer tap
Read more at https://www.stereophile.com/content/audio-research-reference-75-power-amplifier-measurements#jbpCd1H...

All OTL amps like high impedance because they have lots of voltage but limited current. Since current is the limit use the forumla

Power = current squared x impedance. The amplifier max current is the same for both speakers but 16 ohms gives you twice the power of 8.

Two versions of the Futterman circuit. East coast/West coast. Harvey was a hoot. I visited him and his gang around this time. Sadly or not, he folded too. Is there a Futterman curse?

Since I dont believe wire has a sound I prefer colors. I use all 9 colors in my amplifiers. With colors you can actually start to see the circuit without a schematic.

If you look carefully you will find some long wires repeated so you could simplify things there.

If my amps are being run at levels that do not bring on distortion, why do they still have that classic "tube-amp" character even at those low listening levels? If it’s not the clipping characteristics that are coming in to play...what is it that produces that classic tube sound as I described it?

I have a 3 disc LP set of Theodorakis’s “Canto General” that Atmosphere produced, and I found the engineering to be disappointing. It has been muddy, and there has been no bass to speak of.

I would like someone to tell me why spend big bucks on cone speakers when there are better technologies.

Ramtubes 11-26-2018:

HI, Roger here with a question.

I would like to hear how each of you figured out how much power you needed to buy in watts?

I would appreciate the following information in your response.

Your listening level LOUD SPL (preferably measured at 1 meter from the speaker with a REAL SPL meter. Your low listening level. If you are using a cell phone app then you have confirmed it?

Your speaker sensitivity?

Listening Distance from speakers?

How many watts at your load is the amp is rated to supply?

As I mentioned earlier in the thread I listen to a lot of classical symphonic music that has been engineered with minimal or no dynamic compression. Two such recordings, which I believe have just about the widest dynamic range in my collection, are Stravinsky’s “Firebird Suite” on Telarc (Robert Shaw conducting the Atlanta Symphony), and Prokofiev’s “Romeo and Juliet” (excerpts) on Sheffield Lab (Erich Leinsdorf conducting the Los Angeles Philharmonic). I have examined the waveforms of those recordings using a professional audio editing program (Sound Forge Pro), and by doing so I have found the difference in volume between their loudest notes and their softest notes to be approximately 55 db, which is (to me) amazing.

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.

To answer your question about my amplifier, for several years prior to just recently I was using a VAC Renaissance 70/70 MkIII, rated at 70 wpc. I recently changed to a Pass XA25, which is specified as a class A amplifier rated at 25 wpc into 8 ohms and 50 wpc into 4 ohms. Per JA’s measurements, though, it is capable of 80 and 130 wpc into those impedances. I presume that most of that increase represents the amp’s capability after leaving class A, although per JA’s comments some of the increase apparently reflects differences in the distortion percentages the ratings and measurements are based on.

Best regards,

-- Al

Many people believe that SET tube amplifier give more mid range magic than Push pull tube amplifier.Do you agree on the notion as designer of tube amplifier?If so, could tell me why it is so?

• They have very simple circuits
• THe output transformer runs on only one side of the BH curve never crossing through zero.
• They are class A by nature.

• Distortion vs level and frequency. His helps avoiding a really bad one.
• Damping doesnt matter so much if you are using it in a bi-amp situation where it only drives the midrange/tweeter. Use an adequate SS or tubes amp for the bottom.
• Some are noisy. Many don't spec noise. Somehow you have to find out.

Many people believe that SET tube amplifier give more mid range magic than Push pull tube amplifier.