Transformer coupling
An input transformer provides the gain and an interstage transformer links the stages. No coupling capacitors, no cathode bypass capacitors, no feedback.
Power amplifiers · Flagship
300B Two-Stage Monoblocks
Our flagship: four chassis per pair, a directly heated driver feeding the 300B through an interstage transformer.
$9,000 – $16,000 per pairTypical lead time 4–6 months
This is our most ambitious amplifier and the one that shows how deeply we work with a circuit. Each channel uses a directly heated driver feeding the 300B through an interstage transformer, so there is no coupling capacitor anywhere in the signal path. The driver and output stages each get their own power transformer, rectifier and choke filter, built on a separate chassis so hum and vibration stay away from the signal. One of the circuits we build is Bruce Berman's 45 → 300B, which he kindly helped us with by answering our questions. We now build it with current-production ISO transformers from Tokyo, which makes it easier to service than a design that depends on old stock.
Typical specificationSoundaries
- Topology
- Two-stage, transformer-coupled, single-ended
- Example circuit
- 45 → interstage → 300B, after Bruce Berman
- Chassis
- 4 units per pair: 2 amplifier + 2 power supply
- Transformers
- ISO (current production) as standard; Tamura or vintage on request
- Rectification
- 83 mercury-vapour + damper per stage, LCLCLC choke filter
- Filaments
- DC constant-current regulators
- Power
- ≈ 6 W per channel (300B)
- Wiring
- Point-to-point
Popular options
Other driver tubes: E280F, D3a, 310A, 6SN745, 2A3, 50 or 300B-XLS outputTamura or vintage transformersMeters and custom finish
Includes 45 drivers and rectifier tubes. 300B pair quoted separately or supplied by you.
How it works
Two stages, nothing in the way
Our flagship monoblocks keep the signal path as short as possible: a directly heated driver feeds the 300B through an interstage transformer, and each stage has its own power supply in a separate chassis. Here is one of the circuits we build, step by step.
The right load
A 5 kΩ output transformer on the 300B lowers third-harmonic distortion by about 6 dB compared with the usual 3–3.5 kΩ, trading a little power for a cleaner sound.
Quiet, fast supplies
Separate B+ supplies for each stage, mercury-vapour rectifiers with a soft start, three-section choke filters and DC-heated filaments. Silent even on 100 dB horns.
≈ 5.5–6 Wper channel from 1 V input
25 / 75 mAdriver / 300B plate current
4 chassisper pair: two amplifiers, two supplies
The designer
Bruce Berman's circuit, with his kind suggestions
Bruce Berman has built directly heated triode amplifiers for more than 35 years. His article on a Type 76 preamplifier in Sound Practices became one of the most popular the magazine ever ran, and his amplifiers follow the Japanese single-ended school of Susumu Sakuma: few stages, transformer coupling and very quiet power supplies.
His two-stage 45 → WE 300B amplifier is described in detail in Jeff Day's “da Vinci Files” on Positive Feedback. When we built our first two-stage 300B monoblocks, Bruce kindly answered our questions and suggested how the circuit could be adapted for a client whose speakers needed a little more power.
Bruce is not part of Soundaries and does not work for us. The circuit is his design, and we credit it to him whenever we build it.
Current-production transformers
Built with ISO, not only old stock
Many classic designs call for transformers that stopped production decades ago, such as Tango, Peerless or UTC. New-old-stock parts are rare, expensive and of unknown condition, and if one fails, an exact replacement may never be found. We now build with ISO transformers from Tokyo, which carry on the Tango line and are made today. Bruce Berman's own updated notes name ISO's NC-20FⅡ and FC-40-5S as the equivalents for his design.
AvailableEvery part is in current production, with published specifications.
ConsistentLeft and right channels are matched, build after build.
ServiceableIf anything ever fails, we can supply the same part. We are ISO's authorized distributor in Thailand.
Your mainsPower transformers are wound for 220–240 V or your local voltage.
| Position | ISO model | Replaces | ISO price |
|---|---|---|---|
| Input / line-to-grid | ISO NP-126 | Replaces Peerless 4665 / Altec 15095 | ฿8,400 |
| Interstage 1:1 | ISO NC-20FⅡ | Successor to the Tango NC-20 | ฿15,300 |
| Output, 5 kΩ | ISO FC-40-5S | Replaces the discontinued Tango X-5S | ฿22,700 |
| Filter chokes | ISO LC-10-200DⅡ | 10 H 200 mA, low DC resistance | ฿5,100 |
Prices per piece in Thai Baht, for reference. Vintage or NOS transformers (Tango, Tamura, Western Electric) are still available on request.
Your options
One flagship, many circuits
The two-stage monoblock is a platform, not a single recipe. We choose the circuit with you, based on your speakers, room and taste.
Driver and coupling
45 → 300B with interstage coupling after Bruce Berman, or other drivers such as E280F, D3a, 310A or 6SN7, transformer or capacitor coupled.
300B and beyond
300B for most speakers. A 45, 2A3, 50 or PX25 for very sensitive horns, or 300B-XLS when you need more power.
Transformer choice
ISO throughout as standard. Tamura, Muse Coils or vintage Tango and Western Electric on request.
Rectification
Mercury-vapour 83 or 866Jr, or 5U4G / 274B for a different flavour, always with soft-start sequencing.
Matching your preamp
600 Ω transformer input pairs well with our passive TVC/AVC preamp. High-impedance input versions are also possible.
Chassis and wood
Powder coat in your colour, Thai hardwood frames, meters, and an engraved Pali blessing if you wish.
How it is built
From your room to your rack
01Your room
We start with your speakers, your room and the music you play, then draw the circuit around them.
02Chassis and wood
Panels are cut, drilled and engraved here, and the frame is made from Thai hardwood.
03Hand-wiring
Every joint is wired and soldered by hand, point to point. You get photos as it comes together.
- 04
Burn-in and listening
Each build runs for days on the bench and is measured and listened to before it ships.
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