Pair 3.5kOhms Single Ended Output Transformers

This is a one-off, exquisite pair of output transformers proposed for Single Ended Tube amplifier (SET). Most likely you have never come across such a product and chances are that you will never see anything like this. It is a truly unique product for the real SET aficionado. It is the ideal pair for those who seek for the absolute best and do not like compromises.

I have been involved in a truly monstrous power amplifier project during the last months and there has been a quad of iron C transformer core pieces that were finally not used, so I decided to wind a pair of one-of-a-kind output transformers. I wanted to manufacture something truly unique and push the limits of what’s possible, as much as I could. These are nominally specified as 8 Watts RMS, 3.5kOhms OPTs but there is more room available to be used as 2.5kOhms or even 6kOhms OPTs without the slightest deterioration of their performance. They were designed around the idea to be used as OPTs for SET amps equipped with the most popular output tube 300B, or any other tube with similar load characteristics, like 2A3 or PX25.

Nominal power output is specified at 8 Watts but due to design considerations, this is a very controversial product. As you might already know, the main compromise related to Single Ended Tube amplification is reproduction of low frequencies. The deeper you go in low frequency reproduction, the size of iron pieces become larger. You often come in front of small-sized transformers specified as e.g. 20W OPTs (a rather big amount of SET power), but it is not possible to use such transformers in order to achieve High Fidelity sound reproduction of low frequencies. In order to reproduce the deeper registers of a grand piano or a double bass through a Single Ended Tube amplifier, you must invest a lot in large transformer cores; in real world, unfortunately it is not possible to deviate from this path!

This unique pair of OPTs is designed around the principle of using very big iron C core pieces. They are of course Double C Core transformers that could otherwise be easily specified as 20W OPTs, but for this application they are specified as 8W. The main difference is that each primary coil consists of an unusually large amount of turns, so through these OPTs you can easily achieve perfect sound reproduction at full power down to 20 Hz or even lower! The iron core pieces and the primary coils are oversized for perfect linearity down to subsonics, where other transformers are not able to provide the nominal power even at 40 Hz! The difference in sound quality, not only in lower registers but in the whole spectrum of sound is astonishing!

The coil formers are mare of carton paper, precisely cut at a CNC laser jet machine, there is no plastic piece anywhere in the former structure. Insulating material between coil layers is only natural handmade unbleached Japanese calligraphy paper, carefully selected among 5 different grades of comparable paper types, with high impact in sound quality. Even the tape connecting the hand cut paper rolls is made of paper! Everything has been intentionally left un-varnished (varnish takes something out of the magic due to dampening effects). Magnet wire has been carefully selected among at least 4 types and of course the best has been used. Banding straps are of course totally non magnetic, and do not affect the unique properties of the core pieces. Non-magnetic mounting material has intentionally been avoided; it is even non-metallic! As you see, mounting plates are made of CNC laser jet cut Baltic birch plywood for best sound and least interaction with coils and cores.

The most unique feature of these output transformers is the adjustable gap inside the magnetic circuit. The transformers are originally supplied with 0.30mm magnetic gap thickness but the user has the flexibility to fine tune the gap thickness to his specifications, needs or preferences up to 50 microns of precision! Nominal power output has been specified at 8 Watts RMS, a typical spec of 300B power tube, but there is more room available. The minimum frequency where the core is able to perform at its full power without the slightest distortion is 17.5 Hz. Although the specified magnetic core material can easily reach 16.000 – 17.000 Gauss before severe saturation starts to appear, full power output at 17.5 Hz does not drive the core beyond 13.600 Gauss at a hundred percent of linearity! That means you can drive a 300B output tube to its absolute power limits, at a frequency well below the minimum threshold of human hearing at 20 Hz, without the slightest saturation effect of the magnetic material, as on the B-H chart of the specified material, saturation starts to develop above 14.000-14.500 Gauss! I feel very doubtful whether such an OPT has ever been designed and manufactured or not… For power aficionados, this design gives the flexibility to surpass the above-mentioned specifications and always stay in full linearity on the B-H curve. For the experienced DIYer, all these can be easily identified due to the relatively large size of the cores in use! And with proper magnetic gap thickness adjustment or fine tuning, you are able to push the performance towards maximum power output at lowest frequencies or towards maximum primary coil inductance/high effective magnetic permeability for more finesse, tonal colors, and low level detail. The banding straps are secured onto the magnetic cores by means of a screw, so the user is able to take it out, completely dismantle the cores, fine tune the gap very easily and then band the cores again.

The secondary coil is extremely complicated, with 32 leads coming out of each coil former. Through proper arrangement the user is able to match every loudspeaker available. There are no output taps and under each scheme all secondary output leads must be connected in order to achieve best power transmission and linear response in high frequencies. Under specified 3.5kOhms of primary impedance, proper arrangements exist for loudspeakers having a nominal impedance of 4, 6, 8, 16, 20 and 24 ohms. Due to the large amount of primary coil turns and unusually large iron cores, the user can select a different load/loudspeaker connection scheme .


·         Core type: Double C Core

·         Material: Cold Rolled Grain Oriented silicon steel cores at 0.1mm ribbon thickness

·         Size: More than 10 square centimeters of cross-sectional area

·         Nominal power output is rated at 8 Watts RMS at 17.5Hz and maximum peak magnetic flux density at 13.600 Gauss. Considering 25Hz of lowest undistorted frequency reproduction capability and at 14.500 Gauss of maximum peak magnetic flux density, these cores can deliver more than 18 Watts of undistorted Single Ended Triode power output at the nominal load of 3.5kOhms!

·         Primary DC resistance is 105 Ohms

·         Maximum suggested anode current is specified at 200mA for continuous and safe operation

·         Cores are unvarnished, with adjustable magnetic gaps for ideal use under a drastically different load/power output/loudspeaker combination than the nominally specified. With the supplied paper pieces the user can fine tune the gap from 0.1 up to 0.9mm with a precision of 50 microns (0.05mm)!

·         Dimensions fully assembled: 126mm(W) x 137mm(H) x 99mm(D) including supplied mounting structure

·         Frequency response with the transformers fully loaded, without DC bias:
Below 10Hz – 60kHz (-1dB)
Below 10Hz – 95kHz (-3dB)