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John L Stewart John L Stewart is offline
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Location: Toronto
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Smile The Twin Coupled Amp, Crowhursts Unity Coupled Circuit

For the curious & others here is how Crowhurst's solution to the Unity Coupled circuit works.

Crowhurst’s solution is unique. His starting point was the McIntosh configuration where the output load was split equally between the anodes & cathodes. That in itself will provide a DF of 1.5 to 4 when using pentodes or beam tubes before the application of full loop NFB, depending upon circuit constants. In the figure I have used triodes for the sake of clarity to illustrate how his circuit worked. The complete original circuit was published in the May 2003 issue of AudioXpress Magazine. Rather than using an output transformer with a bifilar wound primary of extended bandwidth, he used two ordinary output transformers. These transformers did not require extended bandwidth since they would not be included inside the NFB loop. However, they did require a special turns ratio. That is because one half the load is in the cathode while the other half is in the plate circuit. In Crowhurst’s original circuit each transformer’s secondary got to deliver one half the power to the external load in a parallel connection.

In the Electronics Workbench simulation, each of V1 & V2 is an imaginary triode with a mu of 5.4. Tubes used in the original circuit required a plate to plate load impedance of 8K. That means that each of his output transformers should reflect 4K back to the tubes. The secondaries would each need to be 16 ohms in order to match to an 8 ohm loudspeaker. Crowhurst was unable to find a part of his liking. The universal transformers available to him were all too small. As a result, he had some made up for his project. There are suitable universal output transformers on the market today that can do this impedance transformation & are able to handle considerable power.

If a 100 volt signal is applied to the grids of V1 & V2 as in the schematic the result at the output transformer primaries will be about 70 volts. With the impedance ratio of each transformer set at 4000 to 16 you would get about 4.4 volts into an 8 ohm load. All this is based on ideal transformers.

Crowhurst’s circuit depends on good coupling of the two secondaries to each of their primaries to distribute the audio load equally between the plate & cathode circuits. At the higher frequencies he has overcome leakage reactance effects by using 500 nF capacitors to tie the plate & cathode signals together. The original circuit used 6BQ5/EL84 pentodes so that each of their screens were wired directly to the opposite plate. That way they still operated as pentodes since the screens were held at a constant potential from their respective cathodes. It was also push pull throughout, including the driver & predriver. As a result, Crowhurst was able to use push pull NFB from the output cathodes to the amplifier input.

Also, you should notice that the output stage needs a large driving voltage. That problem needs to be solved in any amplifier where part or the entire load is in the cathode circuit. Bootstrapping of the driver stage is used successfully in both the McIntosh & Circlotron. Crowhurst uses it in his Twin Coupled Amp as well.

Cheers to all, John L Stewart
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