Roger Modjeski Event
On Sunday, June 23, 2019 SFAS members and guests were treated to an interesting and entertaining event with legendary amplifier designer Roger Modjeski. We were also lucky enough to have in attendance John Curl (another legendary amplifier designer), Allen Perkins of Spiral Groove turntable fame and speaker designer Fritz Heiler of Fritz Speakers. Roger brought his new, soon to be released, Output Transformerless (OTL) monoblock amplifiers, his preamp, a Jolida CD player and two Pairs of Fritz Carrera BE speakers stacked in series.
Fritz and Tony Setting Up the Speakers
- John Fleming developed the Fleming valve, also called the Fleming oscillation valve, was a thermionic valve or vacuum tube invented in 1904 as a detector for early radio receivers used in electromagnetic wireless telegraphy. It was the first practical vacuum tube and the first thermionic diode. The thermionic diode was later widely used as a rectifier — a device which converts alternating current (AC) into direct current (DC) — in the power supplies of a wide range of electronic devices, until beginning to be replaced by the selenium rectifier in the early 1930s and almost completely replaced by the semiconductor diode in the 1960s. The Fleming valve was the forerunner of all vacuum tubes, which dominated electronics for 50 years. The IEEE has described it as “one of the most important developments in the history of electronics”, and it is on the List of IEEE Milestones for electrical engineering.
- Lee de Forest was an American inventor, self-described “Father of Radio”, and a pioneer in the development of sound-on-film recording used for motion pictures. He had over 180 patents, but also a tumultuous career—he boasted that he made, then lost, four fortunes. He was also involved in several major patent lawsuits, spent a substantial part of his income on legal bills, and was even tried (and acquitted) for mail fraud. His most famous invention, in 1906, was the three-element “Audion” (triode) vacuum tube, the first practical amplification device. Although De Forest had only a limited understanding of how it worked, it was the foundation of the field of electronics, making possible radio broadcasting, long distance telephone lines, and talking motion pictures, among countless other applications.
- Edwin Armstrong was an American electrical engineer and inventor, best known for developing FM (frequency modulation) radio and the superheterodyne receiver system. He held 42 patents and received numerous awards, including the first Medal of Honor awarded by the Institute of Radio Engineers (now IEEE), the French Legion of Honor, the 1941 Franklin Medal and the 1942 Edison Medal. He was inducted into the National Inventors Hall of Fame and included in the International Telecommunication Union’s roster of great inventors.
- David Sarnoff was a Russian emigrant, an American businessman and pioneer of American radio and television. Throughout most of his career he led the Radio Corporation of America (RCA) in various capacities from shortly after its founding in 1919 until his retirement in 1970. He ruled over an ever-growing telecommunications and media empire that included both RCA and NBC, and became one of the largest companies in the world. According to Roger “David Sarnoff was a bad guy. He ruined many a man.”
Roger talked about how we moved from radio to HiFi and some of the noteworthy amplifiers developed during the late 1920’s and beyond. Rice & Kellogg worked for GE and developed the Brunswick console with a record player and a moving coil speaker which was a step up in sound quality compared to previous efforts. Later came the Brook amplifiers including a push-pull 2A3 amplifier. Then Western Electric made a 91A that used 300B tubes capable of a whopping 8 watts where earlier amplifiers only yielded between 1/4 to 1 watt. Western Electric also made the first amps for the talking pictures that used 211 tubes (huge) followed by the 91A amps. As many of you know vintage Western Electric gear is highly sought after by collectors.Roger then talked about different tube factories in the US and Europe. During a visit to a Yugoslavian tube factory Roger asked why they weren’t able to consistently produce quiet tubes to which they had no answer. According to Roger, Telefunken seems to be the only company that was able to make quiet tubes with greater consistency.
With respect to tube measurement Roger says that most companies and pieces of equipment erroneously measure and report tubes transconductance as a measure of tube quality. Roger believes the most important indication of tube quality is mu (μ) which represents the simple voltage gain of a tube. μ = rp x gm.
Amplifiers: Roger feels that power amps are much more interesting and difficult to design compared to a preamp. He’s only designed 2 preamps and many, many power amps.
Single ended triode (SET) amps are fairly easy to build and can get away with no global feedback because of the way the tubes are designed. But, if you need more than a few watts, the SET becomes impractical due to the size of the tubes and the amount of heat dissipated by the tubes. Push pull amps are a more efficient design. One can make a triode push pull amp, but pentodes are much more common. Roger’s Music Reference RM-9 amplifier is uniquely designed to enable it to use EL 34 tubes (true pentode) and KT88/6550 tubes (beam tubes). Push-pull amplifiers can be class A, AB, AB1 or AB2 such as the Music Reference RM-200.
Roger’s new OTL amp. Roger came upon a pair of Futterman OTL amps for repair in the 1970’s – something he’d never seen before and was intrigued by its concept and design. In the 80’s he designed an OTL which he sold to Counterpoint. Output transformers, as with most things, have compromises. For output transformers It’s all about bandwidth – how low of a frequency can you produce depends on the size of the transformer (saturation) and the quality (lack of distortion) of the high frequencies is related to how it’s wound (capacitance). For the Music Reference RM-9’s, Roger made and potted his own transformers, 3 transformers per amp. Even with a really good transformer you still can get distortion at high frequencies.
So how do you get rid of output transformers – you direct couple the speakers to the output tubes. To make this happen, among other things, an OTL needs an output tube that can produce high current (tap on the top of the tube). Otherwise, according to Roger, an OTL is a relatively simple design.
Roger at the Whiteboard
Roger’s current version of the Futterman takes the circuit back to the simplicity and reliability of the very first Futterman’s written up in the 1954 AES Journal (audio engineering society) and Electronics World versions. However, those low power (12 watt) versions used 12 small triodes in parallel for output tubes. Roger is using Color TV Horizontal Output tubes capable of 1 amp each in parallel. He has dispensed with the complex screen regulators of the H3 AA amplifier (the only one that Julius Futterman and later NYAL manufactured). He has also made the amplifier switchable from pentode to triode.
This amplifier is a tribute to Julius and takes a right turn toward simplicity where Futterman took a left toward pushing RMS power in the RMS race. Roger believes that Music Power, as measured by tone bursts is the most valid way to rate an amplifier. Music Power was abused in the 1970s and the FTC stepped in and mandated RMS ratings which do not consider “dynamic headroom”. Dynamic headroom or tone burst power are the same way of stating what an amplifier will do with music. The Futterman has about 3 dB of dynamic headroom over its 50-watt RMS rating. Therefore, it will perform much like a 100-watt amplifier with 0 dB dynamic headroom. Roger reminds us we are listening to music, not running a “shaker table”.
The H3 AA amplifier had 7 interacting adjustments that had to me made in the proper order with and external meter and great patience. Without the 2-page typed instructions an H3 AA is almost impossible to adjust. Roger has reduced the adjustments to 2, put them on the front panel and included a built-in meter to make these adjustments a snap. He has also provided a 6-position switch to check the output tube bias on the 6 output tubes individually. The amplifiers are supplied with matched output tubes which Julius did not match nor provide a way for the user to check.
This was another successful SFAS event enjoyed by all that were in attendance.