In audio, the acronym OTL has been around since the 1950s. It refers to a tube amplifier usually, one without an output transformer.
In the world of audio, tubes are often docked for poor bandwidth and high distortion by the advocates of transistors. But it turns out that a lot of that distortion can be traced to the presence of an output transformer, which also limits bandwidth. IOW if you can get rid of the transformer, you have the possibility of reducing distortion and increasing bandwidth.
This is indeed a fact, not just a possibility, but like anything else in life comes at a price. In designing such an amplifier, it is rapidly seen that only a few types of power tubes are suitable for OTL service. Such tubes usually have high plate current capacity, reduced plate resistance and are fairly capable of operating with good linearity at lower plate voltages (some examples are the 6AS7G and variants, the 6C33, EL509, EL519/PL519, along with some out of production types like the 6LF6, 6336 and 7241). Of course, the output transformer serves to match the high impedance of the typical power tube to the low impedance of a loudspeaker, and removing it, even with the high current tube types mentioned, usually means that the resulting amplifier is going to have a fairly high output impedance when compared to a transformer-coupled amplifier. For this reason many OTLs often employ high amounts of global negative feedback, in order to allow the amplifier to operate more as a voltage source which is typically required in the operation of many loudspeakers (although this is not by any means a universal truth- more on that later).
There are two main types of OTLs in service today. The Futterman (named for Julius Futterman) circuit and its variants represents one approach; the primary identifier being that the output section uses a 'totem pole' approach, wherein the output circuit is push-pull, but one tube is driving the speaker with its plate connection, while the other uses its cathode. This approach means that the output impedance varies depending on whether the amp is pushing or pulling so to speak, and this usually requires a fair amount of feedback to linearize the circuit. The second type of circuit employs an output circuit known as a Circlotron, wherein there are two banks of power tubes balanced against two power supply banks of equal polarity in a bridge circuit. This approach has a lower output impedance than a totem pole and lower distortion as it is a true symmetrical push-pull circuit (and is more expensive to build due to the need for two separate power supplies).
Both circuits have special driver circuit requirements as usually the grids of such power tubes (and bear in mind, usually there are a number of power tubes in parallel to increase current capacity of the output circuit) have a considerable amount of capacitance and in the case of the Futterman, also have unequal drive requirements (to deal with the fact that the output section is asymmetrical).
If a similar amount of feedback is used in these circuits, the distortion will be similar or lower than is seen in most traditional transistor circuits. Bandwidth will also be considerably wider; its no problem at all to have full power bandwidth from as little as 1 Hz to well over 500KHz. Usually bandwidth has to be limited to prevent problems with RFI and oscillation, not unlike a solid state amplifier. So in this regard, OTLs are quite successful!
More in succeeding posts...
In the world of audio, tubes are often docked for poor bandwidth and high distortion by the advocates of transistors. But it turns out that a lot of that distortion can be traced to the presence of an output transformer, which also limits bandwidth. IOW if you can get rid of the transformer, you have the possibility of reducing distortion and increasing bandwidth.
This is indeed a fact, not just a possibility, but like anything else in life comes at a price. In designing such an amplifier, it is rapidly seen that only a few types of power tubes are suitable for OTL service. Such tubes usually have high plate current capacity, reduced plate resistance and are fairly capable of operating with good linearity at lower plate voltages (some examples are the 6AS7G and variants, the 6C33, EL509, EL519/PL519, along with some out of production types like the 6LF6, 6336 and 7241). Of course, the output transformer serves to match the high impedance of the typical power tube to the low impedance of a loudspeaker, and removing it, even with the high current tube types mentioned, usually means that the resulting amplifier is going to have a fairly high output impedance when compared to a transformer-coupled amplifier. For this reason many OTLs often employ high amounts of global negative feedback, in order to allow the amplifier to operate more as a voltage source which is typically required in the operation of many loudspeakers (although this is not by any means a universal truth- more on that later).
There are two main types of OTLs in service today. The Futterman (named for Julius Futterman) circuit and its variants represents one approach; the primary identifier being that the output section uses a 'totem pole' approach, wherein the output circuit is push-pull, but one tube is driving the speaker with its plate connection, while the other uses its cathode. This approach means that the output impedance varies depending on whether the amp is pushing or pulling so to speak, and this usually requires a fair amount of feedback to linearize the circuit. The second type of circuit employs an output circuit known as a Circlotron, wherein there are two banks of power tubes balanced against two power supply banks of equal polarity in a bridge circuit. This approach has a lower output impedance than a totem pole and lower distortion as it is a true symmetrical push-pull circuit (and is more expensive to build due to the need for two separate power supplies).
Both circuits have special driver circuit requirements as usually the grids of such power tubes (and bear in mind, usually there are a number of power tubes in parallel to increase current capacity of the output circuit) have a considerable amount of capacitance and in the case of the Futterman, also have unequal drive requirements (to deal with the fact that the output section is asymmetrical).
If a similar amount of feedback is used in these circuits, the distortion will be similar or lower than is seen in most traditional transistor circuits. Bandwidth will also be considerably wider; its no problem at all to have full power bandwidth from as little as 1 Hz to well over 500KHz. Usually bandwidth has to be limited to prevent problems with RFI and oscillation, not unlike a solid state amplifier. So in this regard, OTLs are quite successful!
More in succeeding posts...
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