Push-Pull 5,000 ohms 80W 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-80-5 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.



Type Push-Pull Output Transformer
Output capacity 80W/35Hz
Primary impedance 5,000 ohms, with UL tap (50%)
Secondary impedance 4 and 8 ohms
3rd. cathode feedback winding
16 ohms X 2
Frequency bandwidth 2.7Hz to 50KHz (-1dB), input=4V, signal source impedance=5,000 ohms
Primary inductance (H) 120H(min.), 860H(max.)
Primary permissible DC current 280mA (for 2 tubes)
Primary permissible unbalanced DC current 6mA (recommended within 4mA)
Power loss 0.23dB 8ohms
DC resistance Primary P1 to P2 : 168ohms
Secondary 0 to 8 ohms : 0.17ohms
3rd. 0 to 16 ohms : 4.18 ohms
Dielectric withstanding voltage between primary, secondary and 3rd. . 2KV AC
Maximum permissible voltage of primary P-P 1KV AC
Core Type R160 160W core
Shape Enclosed in square case, the same mounting dimensions as those of Tango XE60,FC-30
Connections Lead wires
Overall dimensions and weight W: 110mm, D: 100mm, H: 150mm, Wt.: 3Kg
W: 4.33inch, D: 3.94inch, H: 5.91inch, Wt.: 6.6lbs.
Case color black, silver
Price US$220 (excluding tax and shipping)

Cathode feedback winding

When using the cathode feed back winding, the connection of primary winding and cathode feedback winding is as follows. In the case of self-bias, a self-bias resistance and a bypass capacitor are connected between K1-Brown and a ground and between K2+ white and a ground.

Calculating the closed-loop gain and feedback ratio
The load of the output tube is both the primary winding and cathode feedback winding in the cathode feedback circuit.
The load impedance of the output tube is not 1.25K ohms of a usual pp circuit (1/4 of 5K ohms).
The load impedance is as follows.
  Rl = ( SQRT(1250) + SQRT(16) ) **2 = 1548 ohms
    SQRT : Square route operation
    ** : Factorial operation

The open-loop gain on the primary side before feedback
  Aopen = gm * ( 1548 // Rp )
    gm : Transconductance of the output tube
    Rp : Plate resistance of the output tube
    // : Parallel value operation

The feedback factor β is a voltage ratio (a winding ratio) of load Rl and the cathode winding .
  β = SQRT ( 16 / 1548 ) = 0.102

The closed loop gain on the primary side after feedback
  Aclosed = Aopen / ( 1 + Aopen * β )

  Feedback ratio = 20 * log( Aopen / Aclosed ) dB

Value that uses EL34(6CA7) for output tube
    gm of EL34 = 11000uS
    Rp of EL34 = 15000 ohms

The open-loop gain on the primary side before feedback
  Aopen = gm * ( 1548 // Rp )
       = 11000 * 10**-6 ( 1548 // 15000 )
       = 11000 * 10**-6 * 1403
       = 11 * 1.403 = 15.43

The closed loop gain on the primary side after feedback
  Aclosed = Aopen / ( 1 + Aopen * β )
         = 15.43 / ( 1 + 15.43 * 0.102 )
         = 15.43 / 2.57 = 6.07
The feedback ratio
  Feedback ratio = 20 * log( Aopen / Aclosed ) dB
             = 20 * log( 15.43 / 6.07 ) = 8.2 dB

Shape and Dimensions


1. Frequency Characteristics

The first graph below shows the wide bandwidth 2.7Hz to 50KHz (-1db) and plain attenuation characteristic without any peculiar peaks and dips at the high frequency.

2. Frequency Characteristic: with different signal source impedances

The frequency characteristic of the audio output transformer varies with the signal source impedance that drives the transformer. In general, the bandwidth broadens in a lower signal source impedance. The bandwidth narrows in a higher signal source impedance. The figure below is the frequency response when it drives by 1.2K ohms signal source impedance like triode power tubes and when it drives by 20K ohms signal source impedance like beam power tubes.

Frequency bandwidth by signal source impedance

Signal source impedance Bandwidth
5K ohms (Standard) 2.7Hz to 50KHz -1dB
1.2K ohms (Triode tubes) 0.7Hz to 70KHz -1dB
20K ohms (beam tubes) 3.8Hz to 35KHz -1dB

3. Phase Characteristic

4. Primary impedance Characteristic

5. Cathode feedback winding Frequency Characteristics

6. Cathode feedback winding Phase Characteristic

7. 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.

The primary inductance of RX-80-5 audio output transformer is very large, being 120H at minimum and 860H at maximum. Furthermore, the fluctuation of the inductance is quite small assuring good total performance.
However, the existence of the unbalanced DC current reduces drastically the primary inductance. Although the service limit of the unbalanced DC current is 6mA, we recommend reducing this unbalance down to 3mA or less in order to make maximum use of this R core output transformer.