BAT VK30 Bride of son of zen rippoff?

[mullardel34]

The BAT Web site only offers ambiguous hints at the specifics of the underlying topology, so I greatly appreciate your willingness to directly state the fundamental topology of the BAT Unistage (differential pair, high-level bias via current-source, capacitor-coupled output, no global-loop feedback). Combining this core design information with hints provided on the BAT Web site and information gleaned from BAT product reviews, a bit of insight into the BAT designs becomes are bit more apparent.

The Stereophile review of the BAT VK55SE (Stereophile: Balanced Audio Technology VK-55SE power amplifier) indicates that the baseline (non-SE) model uses resistive current sources while the upgraded SE model is configured with vacuum-tube–based (6922) current sources. The BAT Web site does note that the REX pre-amplifier allows the owner to select from one of three options for the differential-pair current-source (based on either a 6C19, a 6H30 or a 5881 vacuum-tube) in order to "tune/tailor" the sound of the REX pre-amplifier. According to a comment from BAT to Stereophile, "BAT claims, [the vacuum-tube current-source] provides "a more soulful portrayal of musical texture and instrumental timbre."

So a BAT Unistage is a single-stage design, comprised of a differential pair (based on a range of vacuum-tube models (including the BAT in-house "SuperTube")), powered by current-sources (again, using various vacuum-tube models and/or resistive-sources), driving resistive plate loads, capacitively-coupled to the output (via the BAT custom in-house paper-in-oil capacitors) with no intervening buffers (or global-loop feedback). The specific components/vacuum-tubes are selected to voice the BAT Unistage for a specific sonic character in a given application.

As a student of the art of audio-engineering, this is great information. Thanks!

[reinhard]

Again, there are models (VK30) with solid state current sources. The current source is a so called ring of two composed by a MosFet and a BJT working on a supply of minus 15 Volts.

Reinhard

[daly41k]

Erno Borbely wrote an article in glass Audio back in the 90's about a balanced tube line stage pre amp which I built many years ago and sounds great. You can review it on his web site Home Page. It is basically close to the mosfet version but uses mu followers following the balanced tube input.
It seems like the Bat design.

[reinhard]

No, Borbely´s design is a low powerd differential and buffers.

BAT´s design is a high power/current differential and NO buffer. The plate load resistors are between 2K and 500 ohms depending on model, tube and tube quantity.

Reinhard

[mullardel34]

Originally Posted by reinhard

No, Borbely´s design is a low powered differential and buffers.

BAT´s design is a high power/current differential and NO buffer. The plate load resistors are between 2K and 500 ohms depending on model, tube and tube quantity.

Reinhard

Reinhard,

Let's see if we can clarify the situation; is the attached simplified schematic close to representing a BAT UniStage configuration?

* The input/output connections are always balanced XLR connectors.

* The differential-pair is typically based on a 6H30 dual-triode in the BAT UniStage, configured for a high level

of Class-A bias current to enable a low output impedance.

* The current-source could be based on either solid-state or vacuum-tube elements.

* The differential-pair plate resistor values range between 500-2000 ohms

* The output coupling-capacitors are usually BAT custom oil capacitors.

Sometimes, a picture makes is easier for everyone to visualize the circuit topology...

[reinhard]

Yes correct, the quantity of 6H30 tubes range from one to four per channel.

Reinhard

[mullardel34]

Originally Posted by reinhard

Yes correct, the quantity of 6H30 tubes range from one to four per channel.

Reinhard

Excellent! And very interesting; so the 6H30's are paralleled for increased transconductance and lowered noise (ideally, about 3dB lower noise for each doubling of the tube count)? Hmmm, so about 25mA of standing bias current per triode section? Ah, then a single tube would have 2K plate resistors, a pair of tubes would have 1K plate resistors and a quad of tubes would use 500R plate resistors, right?

[Jen-B]

Don't forget the volume control.

[reinhard]

There is some excellent information on I-series shunt volume control and VK 60 autobias on their homepage, look for recources/white papers,

Reinhard

[mullardel34]

Originally Posted by Jen-B

Don't forget the volume control.

The volume control hasn't been forgotten; however, the BAT UniStage is employed throughout the product line, not just in pre-amplifiers, so the BAT attenuator is logically a separate point for discussion. The core UniStage circuit topology is the focal point of the thread.