During the hot days of late July and early August I finally found time to create something new. For a long while I’d wanted to work on interstage and line transformers, partly because a few years ago I had a bad experience with a famous, overpriced commercial transformer. On paper it promised miracles, but on the bench—paired with its recommended tube—it was a disaster, rolling off by an embarrassing –1 dB already at 9 kHz.

An unacceptable result—especially after spending €300 on a pair of iron bricks unfit even as doorstops—and I had to argue with the seller for a refund, since the parts didn’t even meet their published datasheet.

So, last weekend I got to work: calculations and simulations until four in the morning, and two days later I was already at the winder building a prototype to see whether practice matched theory.

The prototype passed every measurement without issue. No double-C cores, exotic materials or marketing gimmicks like “capacity traps”: just a carefully designed transformer with a classic small EI laminated core—affordable even when sold by me, at roughly one-tenth the cost of some hi-end products.

Here are the main specs. It’s designed to work with an ECC88 in push-pull configuration at 5 mA per side, but it will also run well with tubes having an internal resistance around 2.6 k?. The primary impedance is a substantial 60 k?, and the secondary offers multiple outputs at 32/64/128/256 ohm.

These values aren’t random: with such a high primary impedance, you can—for example—connect 50 ohm headphones to the 64 ohm tap, or 120/130 ohm loads to the 128 ohm tap, or 250/300 ohm loads to the 256 ohm tap, all without mismatch. Frequency response is 20 Hz–22 kHz (–1 dB) into 250 ohm, with no negative feedback, at 100 mW output.

The primary inductance measured at 100 Hz confirms the design target: about 48 henry—remarkable for a transformer of this size.

This is the simple test circuit used:

It’s a very basic “paraphase” push-pull, probably not the best for deep bass, yet listening in mono—joining both channels of my DT880 Pro on the 128 ohm tap—was surprisingly good. Note: with two transformers, two ECC88/6DJ8 tubes and this simple circuit, you already have a complete headphone amplifier; just add a power supply capable of 150 V and 20 mA.

This transformer isn’t limited to headphone duty: it can serve as an interstage (including phase-splitter) or as a balanced or unbalanced line transformer providing galvanic isolation. Below are two application examples:

My plan is to supply it fully encapsulated for PCB mounting. I hope this project will attract those looking for an interstage or line transformer that truly delivers great sound at an honest price.

If you are looking for an audio transformer that is truly suitable for an R2R DAC output, the Split-3k transformer is not a generic solution, but a component designed and optimized specifically for this application. In particular, the Split-3k was developed in collaboration with the designer of the chiurutu R2R DAC boards, available on eBay through his official store https://www.ebay.it/usr/chiurutu.

This transformer represents the official solution approved by the board designer himself. It has been tested, measured, and validated directly in real-world use as the best line transformer for interfacing his R2R boards. All comparative tests carried out with generic line transformers showed uncertain or non-repeatable results, with issues such as weak bass, attenuated highs, noise, interference, or clear impedance mismatches.

The cause of these problems is simple and technical. Generic transformers are not designed taking into account the real electrical characteristics of the R2R output of these boards, which operate with signals up to 5Vpp and require a well-defined load. The Split-3k transformer was created precisely to address these aspects, ensuring correct conversion from balanced to unbalanced, or vice versa, without introducing tonal alterations or instability.

Technical specifications:

• Turns ratio: 1:1
• Primary impedance: 3k with center tap
• Secondary impedance: 3k with center tap
• DC resistance: approximately 1k, consistent with the nominal impedance and with the correct loading of the DAC output
• Primary inductance: approximately 48 Henry at 100Hz
• Frequency response (secondary loaded with a 3.3k resistor):
  • 20Hz -0.2dB
  • 28kHz -1dB

Every parameter of the Split-3k transformer has been defined to operate correctly with the output of chiurutu R2R boards. The high primary inductance allows real low-frequency extension even with correct loading, while the nominal 3k impedance avoids unnecessary stress on the DAC and prevents the typical anomalies encountered with non-specific transformers. The result is a linear, stable, and repeatable frequency response, without coloration or loss of bandwidth.

Why choose the Split-3k transformer?

• Dedicated design: it is not a universal transformer, but one developed specifically for chiurutu R2R DAC boards.
• Real-world validation: it is the official transformer approved by the board designer, tested directly in application.
• No compromises: it eliminates the typical problems of generic transformers, such as missing bass or highs, noise, interference, and impedance mismatches.
• Long-term reliability: built with materials and design criteria focused on electrical and sonic stability.

If you are using a chiurutu R2R DAC board and want a truly correct output interface, the Split-3k transformer represents the most technically sound choice. For information, availability, or application details, you can contact me through the site form. I will be happy to answer any technical questions.

The 1626 tube, originally designed as an RF power oscillator, is an intriguing component in the world of thermionic valves. First introduced in 1941, the 1626 was intended for high frequency stability, operating in class C with strong negative grid bias. This setup allowed the tube to generate short bursts of power in the tuned anode circuit, which then oscillated to produce sinusoidal waves.

The 1626 is also known by the U.S. government code VT-137. Although originally developed for RF applications, it has recently been successfully used in audio amplifier projects.

When employed in audio, the 1626 typically runs at about 250 V with a maximum bias current of 25 mA. One of its key features for audio use is its low internal resistance. This not only makes it an excellent driver for output tubes but also facilitates the design of high-performance interstage transformers—essential for top-quality sound.

In short, although the 1626 started life as a class C RF transmitter tube, its ability to perform well in audio applications makes it a versatile and valuable choice for tube-amp enthusiasts.

Continuing our look at 1626 applications, we present the i5k-1626 interstage transformer—a key component to fully exploit this tube’s audio potential. The i5k-1626 is specifically designed to work with the 1626, offering a nominal primary impedance of 5 k?.

A major strength of this transformer is its flexibility with reflected load. The secondary load can vary from 4.7 k? to 6.8 k? without significantly affecting system performance, making the i5k-1626 highly adaptable to different amplifier configurations.

The transformer features a 1:1 unbalanced ratio, meaning input and output voltages remain equal while preserving signal integrity. Thanks to this ratio, the i5k-1626 can deliver an output swing exceeding 120 V peak-to-peak. This translates to about 0.4 W RMS of clean power—ideal for driving subsequent amplifier stages.

In conclusion, the i5k-1626 interstage transformer is essential for anyone wishing to use the 1626 tube in audio. Its 5 k? nominal impedance, tolerance for varying loads, and ability to provide high-voltage output without distortion make it a versatile, efficient component. These qualities ensure optimal drive for output tubes, helping create high-quality tube amplifiers. Below is the frequency-response graph:

Note: Following the i5k-1626 design, customers can request versions with different secondary impedances. This flexibility means the transformer can be used not only for audio power amplifiers but also for transformer-coupled preamps and headphone amplifiers. Custom builds ensure optimal performance and high sound quality tailored to specific needs.

When purchasing output transformers together with other components such as chokes or power transformers, significant quantity discounts are available compared to single-unit pricing. See the discount scale below.

Model i5k-1626
	Type: Single Ended Compatible tubes: 1626, 6V6 wired as triode, 6BX7, 6BL7 and others with internal resistance ? 2500 ohm Primary impedance: 5000 ohm Secondary impedance: 5000 ohm Primary resistance: 570 ohm Max power: 0.5 W RMS Leakage inductance: 11 mH Bandwidth: 20 Hz – 35 kHz (–1 dB)
	Dimensions (mm) L 60 H 70 S 72 F 49 F2 37

Dimension diagram