Who needs another DI box? A quick online search turns up dozens of models — from units under $20, to high-end tube DIs like the Tonecraft 363 [Tape Op #103] for $1,495. And now, TELEFUNKEN Elektroakustik (South Windsor, CT) has thrown its hat into this crowded ring with four U.S.–made models. What sets these T-Funk DIs apart? Transformers. With decades of manufacturing high-end microphones, TELEFUNKEN has built up a fair degree of expertise in transformers.
Let's start with the two passive models — TDP-1 mono and TDP-2 stereo — for those of you who prefer the simplicity and isolation of passive transformer DIs. These use a custom-wound, large-format, nickel-iron OEP/Carnhill transformer. Tranny geeks will recognize the Carnhill name as the brand used in vintage and newer Neve consoles and gear. Next, the T-Funksters decided to build a high-quality and durable enclosure that could handle tons of daily abuse. I'm pretty sure you could drive a car over these super-rugged aluminum boxes, and they'd work just fine. The components inside are also no compromise — ENIG circuit boards with through-hole components from Amphenol, WIMA, and Nichicon. All the switches and jacks are metal, instead of plastic; and each unit is soldered and assembled by hand — and individually tested to meet spec before leaving the Connecticut factory. Input impedance of the TDP DIs is 20 kΩ, so they won't load down passive pickups. All this attention to detail results in a frequency response of 10 Hz – 70 kHz ±1 dB.
The active TDA-1 mono and TDA-2 stereo DI boxes share all the high-quality components of their passive siblings, but rely on a phantom powered, discrete, Class A, FET input stage that is based on the design of the company's M60 FET mic. But what sets the TDA boxes apart from most active DIs is the use of an output transformer — also an OEP/Carnhill. The TDA boxes have a higher input impedance of 30 kΩ, and a less extended frequency response of 20 Hz – 50 kHz ±1 dB. Each TDA unit is hand-tuned for lowest THD before it leaves the factory. All four models have beefy Amphenol XLR jacks for their mic-level outputs, and recessed switches for ground-lift and −15 dB pad; and they all benefit from the transformers adding some harmonic saturation if you push them hard.
To evaluate the sound of these DIs, I used a MIDI-sequenced Minimoog bass line, with a little bit of filter attack dialed in, so I would have some highs in the signal. Because the bass part was sequenced, each performance was identical, eliminating any variation in playing technique. I recorded the same bass part four times through the TDP-1, the TDA-1, an ACME Motown DI [Tape Op #116], and a DIY box I built from a vintage Triad A-10 J transformer. Once the four tracks were recorded and level-matched in Pro Tools, I used X-Or solo mode to carefully listen to each track.
The Motown DI has been getting good feedback from engineers here at Panoramic House. Both the Motown and Triad DIs were very similar in sound, with a slightly more robust lower midrange, and a tiny bit less extension in the lows and highs. The passive TDP-1 was similar in the midrange to the Motown DI, but it felt slightly fuller in the sub-bass area, and it definitely had a bit more top end. The active TDA-1 was in a completely different class, as expected. It had way more gain, and even with the −15 dB pad engaged, I had to significantly turn down the synth's output to keep it from overloading the signal chain. The TDA-1 was also the highest fidelity of the group, with a greater amount of highs, as well as deeper and fuller lows.
Bottom line? All these DIs sounded really good, and I could use any one of them to record a great-sounding bass part. Plus, they are competitively priced and super durable. I recommend the TDP models if you are looking for a vintage transformer vibe. On the other hand, if you need a transformer-output DI with high gain and low SNR, the active TDA models are an obvious choice.