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This amplifier was inspired by Steve Bench's experiments with the Talema small power toroids as output transformers. I built it in an empty Bombay Coy tea sampler box. This design was published in the May 2006 edition of AudioXpress.
It produces 4W pc in Class A. The output tubes are biased using DN2540 (depletion mode mosfet) constant current sources, carefully trimmed to get the unbalanced DC current in the toroidal output transformers under 0.1mA: this is very important to ensure good LF response from the ungapped toroidal transformers. The cathodes of each output pair are coupled using a (10µF) film capacitor to create power long-tailed pair output stages. This concept results in near perfect signal current balance in the output transformer primaries. The even order harmonic cancellation of such an output stage is superior to that of a conventional cathode or fixed biased push-pull output stage. The toroids used are the largest in the range, 50VA: At $18 each (at the time anyway), the Talema units cannot be beaten for value as output transformers. Furthermore, they are the only transformers I have tested which can reproduce a 20kHz square wave with absolutely sharp corners, no ringing or undershoot! (I think square wave performance is an important aspect, so much of music has a fast attack or transient nature.) The input stage uses a 6GM8 low-voltage triode, one section in each channel. This tube has a screen between the sections that helps to ensure good channel separation. The low plate voltage (20V) permits DC coupling to a long-tailed pair drive stage resulting in topology very similar to the classic Mullard circuit. The circuit departs from the Mullard design by using a DN2540 constant current source (CCS) in the tail, resulting in near perfect balance. (The Mullard design uses a slightly larger resistor in the second triode of the long-tailed pair to compensate for the cathode signal leakage in the cathode resistor. The CCS is a benefit of later technology.) The long-tailed pair uses E88CC's or 6922s operating at a little over 6mA per section. (The plate voltages are around 130V, this means that the 6DJ8/ECC88 type is a bit marginal in terms of plate voltage rating.) The combination of extremely accurate self-balancing drive and output stages means that the amplifier is very undemanding of the power supply; a big bypass capacitor is redundant. I used a 10µF film capacitor that is common to both the drive and the output stages; this economy does not appear to compromise the sound of the amplifier. In fact even 10µF may be larger than necessary, all that may be needed is a very small capacitor to attenuate any HF hash that may be riding on the supply. I may get around to testing this hypothesis. The input stage being unbalanced, does have RC decoupling from the drive/output stage supply rail. The 6AS7s being high perveance triodes need protection from cathode stripping during turn-on. B+ Series-pass fets that are ramped-on using a long time constant filter on the gate reference voltage provide the required slow turn-on. The sound is fine; full, detailed and fast. I have converted three of my PP amps to power long-tailed pair topologies and in every case, the sound became more detailed and lucid. This design produces fine sound yet is very economical to build, given the low cost of the output transformers. Click to see the May 2006 AudioXpress article (WARNING! 4.78MB): |
