More than a decade since the original publication by , many readers still actively search for information and share the schematics of this OTL tube project. Tim Mellow's design explores a different output stage using a novel combination of local feedback and current drive in order to achieve good symmetry and cancellation of even harmonics. This article was originally published in , February 2010.
If you have spent $500 or more on 5m or so of exotic loudspeaker cable, have you ever wondered about the 500m of standard copper wire in the output transformers of your tube amplifier? Audio output transformers are large, expensive components that require complicated winding arrangements in order to work properly at high frequencies. They are the prime culprits for the soft bass sound associated with tube amplifiers.
The main causes of this are iron core saturation distortion and the winding inductance which bypasses the loudspeaker at low frequencies. Also, the winding resistance typically wastes 10% of the output power. Hence, a lot of iron and copper are required in order to minimize these problems. An alternative is the output transformer-less (OTL) tube amplifier. However, this concept is not easy to realize in practice, otherwise there would be more of these around.
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My OTL design offers several solutions. First, in order to protect the loudspeakers in the event of a fault, it needed to be naturally current limiting without using auxiliary protection circuits. Another problem was how to realize a symmetrical output stage when tubes do not come as complementary NPN and PNP pairs as with transistors. One option was to adopt the “circlotron” circuit1, which was invented by Cecil Hall in 1951, but that precluded the use of natural current limiting and would have greatly complicated the power supply configuration.
Instead, I designed a non-complementary totem-pole output stage using a novel combination of local feedback and current drive in order to achieve good symmetry and cancellation of even harmonics, as confirmed in subsequent measurements. This configuration has more in common with the Futterman circuit
, except that a long-tailed pair of pentodes is used for the driver stage instead of the concertina phase splitter. The pentodes provide the current drive as well as greater voltage swing than triodes.
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A general aim of the design was to have as simple a circuit as possible with a minimal number of components in the signal path as well as push-pull operation throughout (Fig. 1). Push-pull amplification not only cancels even harmonic distortion, but also provides good rejection of power supply ripple. In a long-tailed pair, the supply current is virtually DC so that the power supply is effectively removed from the signal path.
Above all, I wanted a stable, reliable design that would not need constant readjusting. To this end, I incorporated ample loop DC feedback, which — after initial adjustment — keeps the offset voltage within 20mV between tube replacements. Similarly, the DC bias needs hardly any adjustment over time.
I know that signal feedback is a controversial issue and there are those who maintain that the ultimate goal should be 0dB. However, zero feedback in this design would result in audible noise and an output impedance of 8Ω, which would severely affect the tonal balance of most loudspeakers. I have applied 26dB of feedback, which is a similar amount to most classic tube designs and sets the output impedance to 0.4Ω for well-controlled bass. However, the advantage of a DIY amplifier is that you can adjust the feedback to suit your own taste. The simplest way to reduce the feedback to 11dB is to omit the coupling capacitors between the first and second stages.
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Finally, in order to drive normal loudspeakers, I decided that I needed a power rating of at least 20W. The obvious choice of output tube was the Russian designed 6C33C triode, because a single pair can deliver 2.5A into an 8Ω load from a moderate 150V rail. This enables the amplifier to deliver 25W into an 8Ω load or 40W into a 16Ω load, such as my full-range reflex-port loaded Lowthers. If you can increase the load to between 40 and 100Ω, then you can easily obtain 50W of pure class A power.
I could only measure the distortion without feedback (by injecting the signal directly into the grid of the input tube) because the distortion with feedback was less than that of the signal generator. This gave 0.14% THD at 2W with an 8Ω load without feedback, or 0.007% with 26dB feedback. I am pleased to say that, during the eight years since I built this amplifier, only one fault has occurred, which was an internal short-circuitin one of the output tubes. Luckily, the HT fuse did its job and no further damage occurred.
I have designed and built many tube amps over the years from push-pull ultra linear to single-ended triode, using home-wound transformers. I have even experimented with solid-state, but I tend to be highly critical of my own work and have never been fully satisfied with the results until now. This amp just lets me enjoy the ambience and natural tonal color of a real performance. Unfortunately, it is somewhat unforgiving of recent highly compressed recordings, rather preferring early stereo classical and jazz vinyl LPs made with simple tube equipment.
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The signal from input phono socket SK1 is fed to the grid of V1a via the volume control RV1, C1, and R1. Shunt feedback is provided by resistors R1 and R3, which mix the output and inputsignals to set the overall maximum gain to a value of R3/R1, which is about 29. In other words, an input voltage of 500mV is required to produce 25W into an 8Ω load. When RV1 is set to maximum, the input impedance is about 26k due to RV1 in parallel with R1.
I included capacitor C1 to maximize DC feedback. When there is no offset, the grid of V1a is at the same potential as that of V1b, which is grounded via R4. However, small differences in grid to cathode voltages of each section, due to mismatch, can produce a voltage at the grid of V1a. This also appears across the loudspeaker as a DC offset because the 100% DC feedback path, via R3, keeps the input and output voltages equal. You can adjust trimmer RV2 to null the offset.
An alternative arrangement is to apply the input signal to the grid of V1b with series feedback applied to the grid of V1a. This has the advantage of allowing higher input impedance (1M, for example). However, it would also tend to unbalance the input stage slightly, unless you use an “ideal” solid-state current source in place of R7. Of course, you could achieve excellent noise immunity by using a preamplifier with a balanced output and then applying this to both grids of V1 via C1 and C2.
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The input stage, which is formed by V1 and its associated components, acts as a long-tailed pair phase splitter. The high DC voltage across R7 provides a near constant-current that is split between the two halves of V1. This means that if the current through one half increases by a certain amount, the current through the other half must decrease by the same amount in order for the sum of the two currents to remain constant.
As these currents also flow through the anode load resistors R5 and R6, the output voltage developed across one of them must also increase by the same amount that the voltage across the other decreases. An attractive feature of long-tailed pairs is that the current drawn from the supply is near constant DC. In other words, the supply is largely excluded from the signal path, and this reduces its effect on the sound quality.
The neon bulb N1 serves to limit the heater-cathode voltage on both halves of V1 to about 65V maximum during warmup. It is unlit during normal operation. The balanced outputs from the input stage are coupled to the grids of V2 and V3 via C3 and C4. There is also partial DC coupling via R8 and R9. The driver stage, formed by V2 and V3 and their associated components, also acts as a long-tailed pair. The outputs of this stage are directly coupled to the grids of V4 and V5, which form the output stage. Trimmer RV3 allows the voltage developed on the grids of V4 and V5 to be adjusted in order to set the output stage bias current.
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The choice of bias current involves a trade-off between tube life and distortion In theory, you can bias the output tubes up to 400mA maximum, at which point their anodes dissipate 60W. This gives the lowest distortion. However, you can achieve a much extended tube life with a lower bias current of, say, 200mA. This also reduces the considerable amount of heat produced by the amplifier!
I used pentodes in the driver stage because they can swing greater voltages than triodes and also because they make excellent current sources. The latter ensures symmetry within the output stage. Another benefit of the pentode is the
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