This month’s Roger’s Corner features the second of two articles that Roger wrote explaining Output Transformer Less (OTL) amplifier circuits. This article is a white paper of sorts that describes the circuit of Roger’s OTL-1 amplifier. This is a unique amplifier in many aspects compared to other OTL circuits. It would not be Roger if it were not unique in some way. Also, based on his work in the latter part of his career it would not be unique if Roger did not repurpose some of his existing stock to build the limited number of OTL-1 amplifiers that made it into the hands of their lucky owners (6 in total I believe). In this case the amplifier was assembled using a Music Reference RM-200 top plate, transformer cover, and frame. With regards to the top plate, all the original RM-200 silk screened lettering is still there, with Roger putting a piece of tape over the RM-200 designation and labeling it OTL-1. He also marked up the binding posts to represent the OTL-1 speaker tap options. Kind of DIY one might think, but when you get under the covers and view the circuit you quickly realize there is nothing DIY about this amplifier. Also, Roger liked to tell stories about his amplifier designs, specifically what drove him to design it to begin with. So here is the story about the OTL-1.
About 40 years ago Roger got a pair of Harvard Electronics OTL amplifiers in for repair and up to this time he had never seen an OTL amplifier. So, this was his introduction to OTL circuit design (as noted in Part 1, Roger became a fan of the Futterman OTL circuits). The amplifiers were not identical, which Roger found strange, but he noted the difference was only in the output tubes employed. The owner of the amplifiers had a single page of typewritten instructions on how to adjust the pots on the amplifier (typed by Julius Futterman himself I might add). It also went into detail of what to do if adjusting the pots did not produce the specified results. One of the instructions Roger found a bit curious was about swapping tubes from one side to the other if things would not bias up properly. Now most of us with tube amplifiers have had to swap the tubes around to trouble shoot things at one time or another and know that this takes a lot of time because you have to wait for the tubes to re-stabilize every time (at least that is what we should be doing, some audiophiles being impatient and/or unknowledgeable skip this part) and one has to be careful not to burn oneself on the very hot tubes. From Roger’s perspective adjusting this amplifier was not a simple matter and many owners could not adjust them, so as in this case they sent them to repair techs to be adjusted. Ultimately, Roger found nothing wrong with the amplifiers and nothing had to be replaced. He did note however that although complex and not so user friendly they were well built.
So, as someone who placed a lot of emphasis on designing for reliability, usability, and serviceability, Roger started thinking about how to design an OTL amplifier that would allow the owner to make several adjustments themselves to ensure the amplifier ran optimally. In addition, per Roger the OTL-1 has some unique qualities.
To summarize:
1. First OTL amplifier without paralleled tubes. It is quite common for an OTL amplifier to have several pairs of tubes in parallel to achieve their desired power rating.
2. First OTL amplifier with an impedance converting autoformer. This autoformer extends the response of the amplifier past 200 kHz surpassing the response of the amplifier without it.
3. First OTL amplifier to have impedance taps from 32 to 2 ohms.
4. Second OTL amplifier to have a servo. The first was the Counterpoint SA-4 (originally the Music Reference RM-6) which Roger also designed.
5. Internally DC coupled allowing for self-adjusting bias for the top tube and zero offset at the output.
6. Remote sensing feedback allowing feedback to be taken from whatever tap sounds best in your system.
7. Adjustable gain and feedback to optimize the amplifier in your system.
OTL amplifiers are special, and you will not find their circuits discussed in the tube manuals which are referenced by most circuit designers when they begin designing a product. In some cases, these designers just replicate the circuit, perhaps upgrading or using “boutique” audiophile approved parts in the process. Not the OTL-1. The circuit design of the OTL-1 amplifier represents over 30 years of Roger’s thinking and research and if you ever get a chance to use and listen to one you will clearly understand and hear why it is unique.
A Primer on Output Transformer Less Amplifiers
Roger A. Modjeski
Part 2 – Music Reference OTL-1
HOW IT WORKS:
The Music Reference OTL-1 is a low powered, ultra short path amplifier with minimal parts, no output capacitor, DC coupled internally from input to output, and self-balancing via a servo. It has a single bias adjustment for each pair of output tubes. Via the direct output the OTL-1 can source 1 amp of current. Via the Impedance Converter tapped inductor it can deliver 4 amps from 20 Hz to 200 KHz. It can also supply 100 volts into a high impedance load. If one slows the servo, the amplifier is capable of DC response.
The amplifier uses NOS tubes designed and made at the height of the tube era by the best factories of the 1970's when tube production was at its highest level due to the popularity of Color TV. It is the Japanese invention of the transistorized TV that closed tube factories all over the world. The factories that stayed open the longest were in the countries where tube TVs were kept in service the longest (China, Russia, and Eastern Europe). I tell you this little-known bit of tube history so you can appreciate that we are using the best NOS tubes made that are far superior to the current sweep tubes such as the EL509 and PL509. The NOS 26DQ5 tubes were great while needed for Color TV construction and replacement as they had to be rugged and at Sylvania the goal was to have them last 10 years. So that is the output tube I selected for the OTL-1.
To obtain low distortion, low noise, and good damping a circuit was needed that would have the shortest and fastest path possible. It is desirable in an amplifier to have DC coupling to produce the best bass and wide open-loop bandwidth so that feedback can be applied and stability into a variety of difficult loads is guaranteed. The OTL-1 amplifier has a bandwidth from below 1 Hz to above half a megahertz. As such, this amplifier is unique in the field of past and present OTL amplifiers. Distortion is lower than most conventional amplifiers at 0.5% or less. The damping is higher at 20. The frequency and power response are far wider as stated previously. The tubes deliver their power directly to the speaker with no intervening coupling capacitor or relay.
Also unique to the OTL-1 is the inclusion of an Impedance Converter (IC) that allows the amplifier to output 12 watts into a variety of speaker loads from 1 ohm to 32 ohms. While the IC can be bypassed this reduces the amplifiers power to 3.5 watts for 8 ohm loads and doubles as the impedance doubles up to about 32 ohms. The IC along with a fuse for each output tube provides excellent protection from harmful DC present on the speaker. The amplifier will not oscillate into any load. The circuit description has more details of how this was accomplished.
Thanks to the advent of more sensitive speakers, it is possible to achieve enough power with only one pair of output tubes per channel. This reduces the power consumption of the amplifier to 140 watts, a fraction of what most OTL amplifiers draw. Tube replacement is easy and matched tubes are not required as the servo adjusts for unmatched grid bias. However, for best performance matched tubes are recommended and are available at a fraction of the cost of modern output tubes such as the KT-88, KT-120, EL-34 etc.
CIRCUIT DESCRIPTION
The driver consists of a single 6GH8 tube for each channel. The pentode section of this tube provides the entire voltage gain of the amplifier which is about 500x or 56 dB. The triode section is connected as a bootstrapped split load phase inverter which (contrary to some misinformed individuals) provides equal drive to the triode connected output tubes. These tubes provide the current gain and can source one ampere from the positive or negative supply.
Feedback is a controversial subject among audiophiles. It is considered more bad than good, and many amplifiers tout the fact that they have no feedback. In recent issues of Stereophile Magazine there have been several reviews of amplifiers which have feedback switches so that the listener can determine for themselves the effects of feedback. In the examples I have read the feedback is 2 dB which is virtually nothing. In each case the reviewer reported that this small amount of feedback did not please them and they preferred the sound without feedback. I cannot imagine properly applied feedback of 2 dB would worsen the sound of the amplifier as reported unless it was very poorly applied. It would take careful listening to hear it at all being vanishingly small.
The application of feedback is a tricky thing. Amplifier circuits do not have a place marked, “put the feedback here.” One must create a place to inject the feedback. I imagine the amplifiers previously mentioned might have created this injection point with the switch thus modifying the circuit when the feedback is switched in. The designer of the $75,000 amplifier admits putting the feedback switch on the amplifier to show people how bad feedback is. Well, one can certainly install a switch to make an amplifier sound bad.
In the OTL-1 the input tube literally rides on the output terminal. So as the input rises the output follows it perfectly in phase. Rather than being an injection point for feedback this point exists in the fundamental circuit. This allows the amplifier to react immediately. In conventional amplifiers the feedback comes through the output transformer (with considerable phase shift that is load dependent). The feedback then needs an injection point which is often the cathode of the input tube, or grid of a differential amplifier. Internal delays in the loop (phase shifts at low or high frequencies) can cause the negative feedback to become positive and make the amplifier oscillate at low or high frequencies, or both. Many amplifiers are unstable without a load, and many have low frequency instability that causes the woofer cone to wander about its rest position.
The power supply consists of a 300 VA toroid transformer, bridge rectified and connected to two 2700 uF filter capacitors for exceptional energy storage. There are separate supplies for the driver, bias, and servo. The servo is a simple integrator that always keeps DC at the output below 20 mV. It does this very slowly and much slower than the lowest bass note from any instrument. Bias is set on the negative going tube and, due to DC coupling the positive tube adjusts itself to equal idle current.
A high inductance, low loss tapped choke provides protection from DC should a tube fail. Fuses prevent damage of other components if that should occur. Music Reference amplifiers are known for their excellent application of fuses. With only a pair of output tubes per channel tube failures are reduced and locating a bad tube is simplified. The choke is in parallel with the output of the amplifier to ground. It has three taps which allow for more power and lower distortion that would otherwise be available without it. It does not harm the sound of the amplifier in any way. For those who want to confirm that claim there is also a direct output that bypasses the choke.
SUMMARY
Previous OTL amplifier designs have attempted to achieve large power levels of 100 watts or more. This has made them complex and, in some cases, unreliable. The latter giving OTL amplifiers in general a bad reputation. To achieve these power levels often takes eight output tubes per channel. This number of tubes produces a lot of heat and problems for the user. The OTL-1 is not one of these amplifiers and a great deal of consideration to the user has been employed in its design. My hope is that this amplifier will begin a new chapter in the history of OTL amplifiers. The OTL-1 is the simplest circuit with the fewest tubes that has ever been made. It takes advantage of an important fact that dispels common audiophile misconceptions of amplifier power ratings: Small low power amplifiers used within their power range sound better than large high-power amplifiers. This is simply because in the making of a large high-power amplifier, certain trade-offs must be made to achieve the high power which often makes their low power range less than ideal. The first watt must be good for the amplifier to have high resolution. It is sad to see that many amplifier circuit designers often overlook this to satisfy the audiophiles misconception that high power amplifiers are better because in their minds more is better. The OTL-1 was designed to show the truth of the matter, that less is more.