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Thursday, September 21, 2017

Tamura STU-001 Input Transformer

Tamura STU-001 Input Transformer

Tamura Input Transformer - STU-001, 150,300: 100KCT (Push PULL). Tamura STU-001 is widely used in designs by Sakuma-san, in Sunaudio amplifiers, and designs in MJ Audio Technology magazine. 

Unfortunately, STU-001 input transformers are out-of-production now(at time of publication), and is quite rare to be found now. We've received quite a lot of inquiries on similar high step-up ratio input transformer such as the Tamura STU-001 and have decided to open it up for mass market instead of making them quietly in the background. 

Below are some applications designed and built by HIFI enthusiasts from the East. It can be used as single ended step-up, or phase splitting for push-pull use. You will also notice that directly after the Tamura STU-001 input transformer, is power tubes, such as 211, or 300B, meaning that it is some sort of a Supercharger amplifier, or power amplifier. 

We have blogged about supercharger amplifier in the past - you can refer to this post:


- transformer is a ratio machines, with such high step-up ratio, the secondary load needs to be very high impedance, and the primary source needs to be very low impedance, to keep the bandwidth wide enough for audio applications
- 300:100,000 equals a step-up ratio of 1:18, or impedance ratio of 1:333. Load will be reflected accordingly to the primary and the source will need significant drive strength to ensure satisfactory performance
- volume pot is not recommended at the primary of such input transformers for higher fidelity

Let's take 250K and 500K (maximum grid resistance 300B and 211 can tolerate in self-bias circuit, fixed bias will be much lower) secondary load for STU-001 as example.

- primary reflected impedance = 500K / 333 = 1.5K Ohm 
- primary reflected impedance = 250K / 333 = 750 Ohm

You can see that it is not a really friendly load for weak (high output impedance) tube preamplifiers. Most common-cathode topology simple tube preamps will not apply since the output impedance will be high. We would need tube preamps with low output impedance, such as those with Cathode Follower output stage to drive such STU-001 input transformers satisfactorily. Or, tube preamps with output transformers, that has low secondary impedance, such as 50-100 Ohm, or maybe lower, will work nicely. That reminds us of tube headphone amplifiers!

On the other hand, high swing low output impedance solid state preamplifier will come in nicely here. Therefore, choices are plenty. 

In some cases, secondary load may not be required, if there are no oscillations / overshoot or undershoot at the output of STU-001, and therefore, the secondary of STU-001 acts as a grid-choke for the driven tubes! 

One may asks, so what's the benefit of doing so? You can set the power tube free from the coloration of the driving stage! You get to hear the real sound, real character, true flavor of the power tube with such high gain input transformers. Imagine if the power tubes are WE WE 300B, 242, WE 211, WE 212, GEC & Telefunken power tubes and various other exotic power tubes. Such input input transformers such as Tamura STU-001 and the likes will come in very handy.

Note: Standard STU-001 may not have that much swing for high power tubes. STU-001 with larger cores (diff price) is needed for big power tubes. 

Hear the unheard! Sail the uncharted! 
Long live custom made audio! 

J&K Audio Design

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