BAT Preamp Output Design Changes - Why?

Fascinating. Thank you for this insight!

Any idea on why this flip-flop in preamp design? Afterall, neither transformers nor capacitors are new technologies – why such a major change in design approach?

@captc2 By using coupling capacitors, the preamp, although balanced, was unable to support the balanced line standard also known as AES48 (which is a connection standard, also defines how the grounds are connected). There is no such standard for single-ended operation FWIW.

When coupling caps are used, the ground becomes part of the circuit return path, which means the ground is involved with the signal. This opens the preamp to ground loops, something that isn't supposed to happen with balanced operation.

A transformer OTOH easily supports the standard. This is beneficial as it allows the preamp to drive longer cables and to reduce interconnect cable artifact.

Historically speaking one of the founders of BAT was a customer of ours; evidently the BAT designer took inspiration from our designs- at the time we were the only manufacturer in high end audio using a circuit called the Circlotron. We obtained a patent for its use at the preamp level and I suspect they didn't want to get involved with infringement; our patent allowed for a reliable direct-coupled output that supports AES48.  So they used coupling caps but lost a lot of the benefit of balanced operation since the standard wasn't supported. I suspect this move is to reclaim that benefit- transformers are very good at this sort of thing.

Gentlemen,

Thank you for your thoughtful replies. Your comments confirmed my suspicious as to why the move back to transformers, which were: 1) market access - compatibility with more amps; 2) The sound (bass roll-off), and; 3) reliability as caps sooner or later will need to be replaced. While I agree there has been some evolutionary movement in materials/caps/transformers/wire/cores, etc., it does make me wonder why BAT moved away from transformers in the first place – seems like the first approach was the right one. Regardless, I’m looking forward to hearing my first BAT with transformer coupled outputs very soon.

Cheers.

The tech for both high-end capacitors and transformers has evolved in 20 years. Perhaps BAT feels the ground has now shifted in favor of transformers. The break-in time and longevity concerns are also in favor of transformers. Line level transformers will simply never wear or fail, and all the horror stories like suddenly passing DC or "600 hours minimum for these to sound good" resides squarely with output caps.

Wiring quality has an solid effect on transformers, and there’s is exceptional purity wiring available today that wasn’t 20 years ago. Same with cores - amorphous materials, nano-crystalline, and even better lamination tech is more widely available today.

Using line transformers can also allow a reduction in tube count and active circuitry complexity (caps don’t provide the same benefit). The VAC Master and Statement preamps use Lundahl transformers for both inputs and outputs, and this allows them to only use a single pair of 6922 tubes. With concerns about tube availability, and dwindling populations of vintages greats (especially 6922 types), a reduced tube count is an asset.

Edit: @imhififan has an excellent point about ESR too. It takes very large (and expensive) output caps to drive a low impedance load without bass roll-off. The classic pattern of tube preamp into SS amp sometimes runs hard into this issue. The transformers facilitate a more "one size fits all" approach for the output stage without sacrificing additional real estate, budget elsewhere, or parts quality. 

BAT preamp output coupled with capacitor design always suffered low frequency roll-off if driving low impedance load. I think this is the main reason they moved back to using output transformer, so their preamp can be compatible with more power amplifier in the market.