RX-40-5
Push-Pull 5,000 ohms 40W type
Audio Output Transformer

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

RX-40-5 is R core audio output transformer for push-pull type amplifier. Excellent magnetic properties of R core as well as precisely wound construction of the coil by the computerized manufacturing system achieve the wide bandwidth, low distortion, and low magnetic loss of the transformer.

What is R core Output Transformer?

R core power transformers are now increasingly introduced for the high-end HiFi components because of its excellent technical advantages. R core is of non-cut construction and has excellent magnetic properties. The magnetic core used for audio output transformer has evolved from EI core, cut core, and to toroidal core achieving successful results to improve the performance of the transformer. R core has several advantages over toroidal core that has been evaluated as the best. In fact, R core enables further technical improvements of the audio output transformer. We have designed this R core audio output transformer and carried out the performance tests for final product. The product packaging is made by Kitamura Kiden Co., Ltd., an originator of the R core transformer.

Outline

Power handling capacity 40W, primary impedance 5,000 ohms, equipped with ultra-linear taps, the best match for push-pull tube amplifier using 6CA7/EL34, 6L6, KT66, 2A3, 300B, etc.
The combination of high performance core and generous turns of coil winding assures high primary inductance presenting good sound quality such as solidness of mid and low range.
Precisely wound coil has no peculiar peaks and dips in attenuation characteristic and excellent response at high frequency, so enabling stable NFB operation and transparent sound quality.
Perfect balance coil design realizes the low distortion factor even with non-NFB amplifier.
Enclosed in a handy and beautiful square case. The same mounting dimensions as those of Tango FX-40.

Specifications

Type	for Push-Pull
Output capacity	40W/40hz
Primary impedance	5,000 ohms, with UL tap (50%)
Secondary impedance	6 ohms
Frequency bandwidth	6hz to 65Khz (-1dB), input=4V, signal source impedance=5,000 ohms
Primary inductance (H)	290H(min.), 640H(max.)
Primary permissible DC current	210mA (for 2 tubes)
Primary permissible unbalanced DC current	5mA (recommended within 2.5mA)
Power loss (6 ohms load)	0.33db
Dielectric withstanding voltage between primary and secondary	2KV AC
Maximum permissible voltage of primary P-P	1KV AC
Core	Type R50 50W core
Shape	Enclosed in square case, the same mounting dimensions as those of Tango FX-40
Connections	Lead wires
Overall dimensions and weight	W: 83mm, D: 78mm, H: 107mm, Wt.: 1.5Kg W: 3.27inch, D: 3.07inch, H: 4.21inch, Wt.: 3.3lbs.
Price	US$120 (excluding tax and shipping)

Shape and Dimensions

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

Detailed Technical Characteristics

1. Frequency and Impedance Characteristics

The first graph below shows the wide bandwidth 6hz to 65Khz (-1db) and plain attenuation characteristic without any peculiar peaks and dips at the high frequency. The second graph shows the impedance characteristic as well plain and quite similar to the frequency characteristic. The flat section of the impedance on the graph reads 4,800 ohms, a little bit smaller value than the nominal specifications.

2. Frequency Characteristic: with different signal levels and signal source impedances

The frequency characteristic of the audio output transformer varies with the magnitude of the input signal and with the signal source impedance that drives the transformer.

When the input signal level decreases, the permeability of the core also decreases, which results in poor frequency response at low end. This means that a minute and detailed low frequency response becomes difficult to present. Only the audio output transformer equipped with good quality core is successful in minimizing such deterioration of the low frequency's sound quality at small signal level (0.4V).
If the high frequency characteristic of the output transformer is poor and such transformer is driven by beam or pentode power tubes having relatively high plate resistance, considerable amount of peaks and dips at the high frequency will present. In such case, the sound transparency would be lost and much worse, stable NFB would be practically impossible. Good quality output transformer minimizes the fluctuation of the high frequency characteristic caused by the different levels of signal source impedance.

When driven by the signal source impedance=5,000 ohms

The graph below shows the frequency response of the output transformer driven by the signal source impedance=5,000 ohms, which represents the case that the transformer is driven by beam or pentode power tubes having relatively high plate resistance. You can first notice a wide bandwidth 6hz to 65Khz (-1dB) and plain attenuation characteristic at the high frequency without any peculiar peaks and dips. The low frequency response does not attenuate even with small signal level (0.4V). This proves that the enough inductance is kept even at such low signal level thanks to the high performance magnetic core. The high frequency response attenuates in a plain manner except only a small dip between 90Khz to 100Khz. Such characteristics are ideal for stable NFB.

When driven by the signal source impedance=600 ohms

The graph below shows the frequency response of the output transformer driven by the signal source impedance=600 ohms, which represents the case that the transformer is driven by triode power tubes having relatively low plate resistance. The bandwidth further extends compared to the above case with the signal source impedance=5,000 ohms and the low frequency down to about 10hz is almost flat at every signal level. The high frequency extends up to 85Khz (-1dB).

3. Primary Inductance Characteristic

The primary inductance of the push-pull output transformer very much influences the low frequency response. This primary inductance varies according to the magnitude of the signal level and unbalanced DC current level. The more the primary 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 primary inductance. On the contrary while the input signal level increases, the primary inductance also increases. When it reaches, however, to its maximum magnetic flux density, the core is saturated and then the primary inductance starts to drop. The output transformer equipped with good quality magnetic core assures higher value and less fluctuation of the primary inductance.
In order for push-pull type output transformer to perform the best, the DC current difference between P1 and P2 windings (unbalanced DC current) should be minimized. For this reason, a matched pair of power tubes is used for push-pull amplifier. The graph below shows the primary inductance level caused to decrease by the existence of unbalanced DC current.

The primary inductance of RX-40-5 audio output transformer is very large, being 290H at minimum and 640H at maximum. Furthermore, the fluctuation of the inductance is quite small assuring good total performance. This inductance value is in fact equal to that of 100W class conventional type output transformers. However, the existence of the unbalanced DC current reduces drastically the primary inductance. Although the service limit of the unbalanced DC current is 5mA, we recommend reducing this unbalance down to 2.5mA or less in order to make maximum use of this R core output transformer.

4. Characteristic when installed on our test amplifier

We have installed RX-40-5 on our test Power Amplifier and carried out the measurements. This amplifier is EL34/6CA7 push-pull type. EL34's are in pentode operation and the design maximum output is 25W. The reason to employ EL34/6CA7 push-pull amplifier for packaging test is that EL34 is a pure pentode tube having high plate resistance that consequently poses a severe testing conditions to RX40-5. If good performance is obtained with the pentode operation EL34/6CA7 push-pull amplifier, much better performance can be surely obtained with the beam tubes having lower plate resistance or triode tubes.

Frequency characteristic with non-NFB

The low frequency extends down to 10hz (-1dB) within 1W output. At 10W, the frequency response below 20hz decreases because of the core saturation. From this test result, the maximum permissible output at 20hz of this transformer is estimated approximately 10W. Since the maximum permissible output is proportional to the square of the frequency ratio, the permissible output at 40hz is 40W, which is exactly the design value. The high frequency extends up to 40Khz (-3dB), which is quite a good figure considering that the transformer is driven by the pentode operation tubes. The high frequency attenuation curve is very plain without any peculiar peaks and dips.

Distortion factor with non-NFB

The distortion factor is graphed as below. They represent a total distortion performance of the pentode push-pull amplifier and not only that of the output transformer itself. However, you can notice that the distortion level is very low as of a non-NFB pentode push-pull amplifier. From this fact, you can evaluate that the distortion of the transformer itself is kept at low level. The excellent magnetic properties of R core is proven by the fact that the distortion curve of 100hz is not so much degraded compared to that of 1Khz.

Frequency characteristic with NFB=8dB

The graph below shows the frequency response with NFB=8dB. The low end is very flat up to 1W output. The same as the above case, the frequency response below 20hz decreases because of the core saturation at 10W output. The high frequency extends up to 60Khz (-3dB). Since the high frequency curve is very plain without peculiarities, no phase compensation to stabilize NFB operation is necessary.

Distortion factor with NFB=8dB

The graph below shows the distortion factor with NFB=8dB. The distortion curves of each frequency are quite same within the output range below 10W that falls in usual service power rate. This again proves the high performance of this audio output transformer.