MT-72 & MT-128
Matching Transformer for SEPP Amplifier

The right on the photo is the matching transformer in bare condition. The product is enclosed in the square case on the left.

Designed and hand-made by Mr. Amenomori, one of the most outstanding DIY amplifier builder in Japan, here comes matching transformer for SEPP amplifier. Accurate and precise design, artistic technique in coil winding, and cut-core of excellent magnetic properties, all of these factors together have produced this transformer featuring wide bandwidth, low distortion, and high power handling capacity. By this matching transformer, you can ideally operate an amplifier having SEPP power stage of vacuum tubes or SIT's.

On the web pages of Mr. Amenomori, a master of amplifiers and transformers, many articles on tubes and projects of amplifier building are available (unfortunately only in Japanese). Please have a look at them.
To Mr. Amenomori's Home Page

What is the matching transformer for SEPP amplifier?

SEPP (single-ended push-pull) power stage of tubes and SIT's has been introduced to OTL (output transformer less) amplifier. It has been, however, difficult to operate such power stage in ideal condition since the load impedance of loudspeaker is too low to be driven directly by this type of power stage. Matching transformer converts the low impedance (4 ohms to 16 ohms) of loudspeaker into 72 ohms or 128 ohms and enables SEPP amplifier to operate in ideal condition.

By employing this matching transformer, you can realize,

• Small volume and light weight
• lncreased output power
• lmproved distortion factor

of SEPP amplifiers.
In addition to the above, if you connect this matching transformer between your tube OTL amplifier and loudspeaker, you can get greatly increased output power compared to directly driving the loudspeaker. Furthermore, you will be able to drive 4 ohms loudspeaker by tube OTL amplifier. The illustration below describes the connections to SEPP or OTL amplifier and to loudspeaker. If the loudspeaker's impedance is not 8 ohms, connect to the tap having the same impedance as that of the loudspeaker you are using.

Outline

• MT-72 72 ohms: 4, 8, 16 ohms and MT-128 128 ohms: 4, 8, 16ohms are available.
• The combination of cut-core with excellent magnetic properties and generous turns of coil winding achieve a high performance.
• High efficient auto-transformer type
• Wide bandwidth 5hz to 50Khz (-1dB)
• High power handling capacity: MT-72/56W and MT-128/50W
• Enclosed in a handy and beautiful square case. The same mounting dimensions as those of Tango transformers.

Specifications

Shape and Dimensions

• Transformer cover case facilitates mounting on the amplifier chassis and assures superior looking of the amplifier.
• The beautifully finished case is four-side welded construction and black metallic painted.
• The same mounting dimensions as those of Tango FX-40, XE-20, and FW-20.
• Mounting and overall dimensions

Detailed Technical Characteristics

1. Frequency Characteristic

The main features are wide bandwidth (MT-72: 5hz to 70Khz -1dB, MT-128: 5hz to 50Khz -1dB) and plain attenuation characteristic without any peculiar peaks and dips at the high frequency. We have attached greater importance to the performance of 8 ohms terminal operation because this impedance is dominant among the various loudspeakers. The frequency response is the best at 8 ohms terminal, then the second 16 ohms, and the third 4 ohms.

2. Impedance Characteristic

3. Square Wave Response

The photos below show the output waveform at 8 ohms terminal (8 ohms load resistance is connected) when 50Khz square wave input signal is given to the terminal 72 ohms and 128 ohms. The upper waveform is input and the lower is output. Good waveform is maintained at the very high frequency.

MT-72

MT-128

4. Inductance Characteristic

The graphs below show the inductance characteristic between the terminals common and 72 ohms (MT-72), common and 128 ohms (MT-128). The inductance greatly influences the low frequency response and varies according to the level of the input signal and of DC current flowing from the amplifier. The more the inductance increases and the less it varies, the more solid and stable sound quality can be obtained.

• When the input signal level decreases, the permeability of the magnetic core also decreases, which inevitably results in the decrease of the inductance. On the contrary while the input signal level increases, the inductance also increases. When it reaches, however, to its maximum magnetic flux density, the core is saturated and then the inductance starts to drop. The output transformer equipped with good quality magnetic core assures higher value and less fluctuation of the inductance.
• If an offset of amplifier output exists and DC current flows into the transformer, the inductance decreases and the frequency response at low end is deteriorated. The graph below shows the inductance fluctuation caused by the existence of superimposed DC current.

MT-72 and MT-128 have good inductance characteristic, maintaining the inductance very high from small signal level and its fluctuation within a small range. They are also strong against superimposed DC current. The inductance decreases a little even with permissible superimposed DC current 20mA.

5. Amplifier Project using matching transformer

On the home page of Mr. Amenomori, a designer and manufacturer of MT-72 and MT-128, there is an article on 25HX5 SEPP amplifier project using matching transformer.

Ordering

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