This month’s Roger’s Corner features the first of two articles that Roger wrote explaining Output Transformer Less (OTL) amplifier circuits. Roger’s favorite amplifiers were OTLs or direct drive. If he could avoid the use of an output transformer in a circuit, he would. His experience with OTL designs dates to the 1970s with Harold Beveridge Inc. (HBI) whose Beveridge speakers used direct drive OTL amplifiers. While Roger certainly studied the circuit he is often erroneously credited with the design. The truth is Roger only tested those amplifiers and had no input on their design. In the early 1980s Roger designed the OTL circuit that was intended to become the Music Reference RM-6. In addition to direct coupling the tubes to the load, the circuit employed a servo to automatically balance the DC output to zero. Other OTLs required the user to periodically manually adjust the DC balance. This OTL circuit may very well have been the first to offer an auto DC balancing feature.
Roger was never quite satisfied with the RM-6 design. Instead, he licensed the design to Mike Elliot at Counterpoint who brought the amplifier to market as the 4-box Counterpoint SA-4. Those that have heard a properly sorted SA-4 consider it to be one of the best OTL amplifiers ever. Sadly, many of the SA-4s produced were quite finicky and suffered from reliability issues. While Roger went on to design many well received output transformer coupled amplifiers, he never stopped thinking about OTL amplifiers. I am not sure exactly when, but sometime around 2012 Roger began playing around with OTL designs again. He made several breadboards experimenting with 6AS7 and 6C33 triodes in both totem-pole and bridge configurations, but none of the designs were satisfactory. However, Roger finally discovered a tube he could build an OTL circuit around, the 26DQ5. This led to the creation of the OTL-1, a low powered pentode OTL amplifier, that could also be wired in triode if desired. About 6 people were lucky enough to acquire an OTL-1. I have two, one pentode and one triode and both produce glorious music on my Quad ESL speakers.
In 2018, when Roger was first diagnosed with cancer, he took it upon himself to start work on a new OTL design. One that had its roots in the Harvard Electronics/Tech Instruments H-3 (Futterman) circuit. As you will read Roger was a fan of the Futterman approach to OTL circuit design and he was hell bent on simplifying the process of adjusting the amplifier (we have the complicated hand typed adjustment instructions by Julius Futterman in our files) while making other improvements. We got one pair up and running but before we could sort everything out Roger passed away. He did provide a succession plan to finish the amplifier but due to reasons I will not publish here, the amplifier never came to fruition. So, feel free to enjoy the articles. OTL amplifiers are much maligned in the uninformed world audiophiles exist in, but well-designed examples are known for their unrivaled speed and transparency versus other transformer coupled and transistor amplifier circuits. Hopefully these articles will shed some light upon the reader as to why.
A Primer on Output Transformer Less Amplifiers
Roger A. Modjeski
Part 1 – OTL Circuit Basics
First, let us consider what the output tube in an OTL circuit must do that is different from amplifiers with output transformers. In output transformer coupled circuits most tubes want to work at 400-600 volts on the plate, with an idle current of 50 mA, and see a transformer that is 20-30 to 1 in turns ratio. Whether the amplifier is single ended (SE) or push pull (PP) the same conditions apply. All SE amplifiers are Class A. PP amplifiers can be A, AB1 or AB2 which is just a matter of how much idle current they run. Note I did not say bias because bias is the negative voltage on the grid.
A typical 25 to 1 output transformer steps down the voltage by 25 and steps up the current by 25. People tend to think mostly about the voltage, but the current is just as, if not more, important. People talk a lot about matching the impedance of the speaker to the impedance of the tubes. This is foolishness. The tubes do not have a desired impedance, that is determined by the voltage and currents in the circuit. The typical load for an EL-84 is 8,000 ohms however the transformer in the Music Reference RM-10 is 13,000 ohms for the same tubes, but the voltages and currents are quite different. The difference allows me to safely get 35 watts out of a pair of EL-84s.
On the matter of impedance, the output tube I am using for my OTL circuit has an impedance of thousands of ohms, but it is more capable of driving an 8-ohm load than the 6AS7 which is just a few hundred ohms. So, I hope you can see that impedance matching does not matter. If you start reading a paper about matching impedance note that it only affects damping and is usually not low enough to provide adequate damping without feedback. Feedback is necessary in OTL amplifiers for them to have decent performance. Feedback is not evil if it is properly done. The advantage of OTL amplifiers is that there is no transformer in the feedback loop and, if the loop is short (as mine is), feedback is easily applied and not problematic. People who foolishly add 2 dB of feedback to prove feedback harms the sound as I have read twice now in Stereophile reviews, have apparently added it poorly. Why even bother with 2 dB, it does nothing when you consider it takes 6 dB to cut distortion in half and double damping.
To make a good OTL you need a tube that can conduct appreciable current close to 1 amp and not be damaged in the process. The 6AS7 will pass 1 amp but little sparks come off the cathode like a 4th of July sparkler. Those cathode flakes rapidly reduce the emission and thus the current. I recently tested a set of 16 tubes for an Atma-Sphere OTL. They looked physically great. Good getters, no sign of overheating, yet they had less than 40% emission. That means 40% current. I assume that heavy play had sparked off 60% of the cathode material. Horizontal output tubes (mistakenly called "video tubes") were designed to conduct high peak currents and do not shed cathode material even at 1.5 amps. So, I use those tubes.
Now one must arrange the tubes in some kind of circuit that connects them directly to the load. Since output transformers are out standard Push Pull is also out as it requires a center tapped transformer. What is left is to either stack the tubes or put them in a bridge. While one can make a single ended OTL, that is foolish as the idle current must equal the output current. The dissipation in the tube would be too large. The single ended push-pull (SEPP) is another obvious choice, but the self-bias resistor is a problem. My direct drive ESL amplifier is SEPP, but it runs at 2500-5000 volts. SEPP amplifiers work great up there but are worthless at voltages needed for 8-ohm speakers. A SEPP is a stacked tube output circuit where the top tube is self-driven so that one only needs to drive the bottom tube. No phase inverter is needed. It is a great circuit when the parameters are right, but they are not for an 8-ohm speaker.
Essentially, when it comes to OTL amplifiers we come down to two circuits. The first one we will discuss is called the totem pole and was the preferred circuit of Julius Futterman. The totem pole circuit has a top tube that pulls the speaker up positive and a bottom tube that pulls it negative. This is exactly what happens in a standard transistor amplifier. However most solid-state amplifiers have P and N complementary transistors, we have only N type tubes. Therefore, we must figure out a way to drive the two tubes equally even though the top and bottom tubes have very different drive and DC requirements. The main accusation leveled at totem pole outputs is that the top tube never gets the proper drive, which is equal to the bottom drive plus the output voltage (which is large and varies with the load). If the Futterman connection is looked at casually that appears to be the case, and many assume that the Futterman circuit uses large amounts of feedback to correct this situation. However, a properly constructed Futterman circuit has equal drive with no feedback at all. What I notice with Rozenblit, Karsten, and others who have written on the subject is they first must damn the Futterman circuit to present their "correct drive" which sometimes is not correct at all. I see these mistakes all over the internet by somewhat respected sources.
Perhaps they do understand the circuit and hope you will not be able to argue their points (many Japanese authors have joined them in anti-Futterman articles). Who knows enough to contest their technical argument especially when it is in a respected magazine like Glass Audio? Last I looked Rozenblit had some complex way of driving the top tube differently than Futterman. So, his amplifier circuit goes into the totem pole camp although now I see single ended amplifiers are prevalent on Transcendent Sounds website. One uses a 300B which makes no sense at all as it does not like delivering current. We would not want to see our Emission Lab 300Bs in that amplifier as they would not be there for long. Some of these less than good ideas get past those who listen at very low levels. Those who push them usually find tube life to be that of a Mayfly. Here is a typical Rozenblit analysis of the Futterman, it is entirely untrue: http://www.stereophile.com/content/transcendent-t8-otl-monoblock-power-amplifier-futterman-redux. Unfortunately, Wikipedia does not have a good article on the Futterman circuit.
The other OTL configuration is taken from Alpha Wiggins Electro-Voice Circlotron circuit:
http://en.wikipedia.org/wiki/Circlotron. This brief article is worth reading and has a schematic and some interesting numbers. The circuit utilizes a symmetrical cathode-coupled bridge layout of the output stage, as well as output transformers to couple the relatively high output impedance of tubes to low-impedance loudspeakers. Ralph Karsten of Atma-Sphere eliminated the center tapped output transformer to design an OTL amplifier using the Circlotron circuit. In any event, in the Wiggins circuit you can see it takes 2 or 3 dual triodes to drive the output tubes. My circuit does it with one 6GH8 pentode/triode in a very short path and DC coupled internally. Futterman had internal capacitor coupling and a big capacitor on the output which limited the low end. The last thing we want is a big electrolytic between our amplifier and speaker.
Hopefully you now have a basic understanding of OTL circuits. In Part 2 we will discuss the Music Reference OTL-1 amplifier circuit and it’s features, a number of which are unique to OTL circuit design.