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.
BAT Preamp Output Design Changes - Why?
I’ve been looking at the recent preamp offerings from Balanced Audio Technology (BAT). I notice that they are now touting transformer coupled (2nd generation) outputs – moving away from capacitor coupled.
When the company first debuted (to much acclaim I might add) their highly regarded preamps: the VK-3 and VK-5 (and variants), they were transformer coupled designs, but then BAT moved away from using output transformers for many years. They have now moved back to transformers. 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?
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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. |
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.
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