RC-20
Multipurpose Audio Interstage Transformer

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

RC-20 is an R core audio interstage transformer. The excellent magnetic properties of R core and precisely wound coil by the computerized manufacturing system achieve a wide bandwidth, low distortion, and low magnetic loss of the transformer.

Outline

Specifications

Type

Interstage transformer

Winding configuration

Primary 1 + 1 : Secondary 1.8 + 1.8

Primary drive impedance

Primary two windings in series connection: Standard 3.5Kohms, supporting from 1Kohms to 5Kohms
Primary two windings in parallel connection: Standard 1Kohms, supporting from 600ohms to 2Kohms

Secondary impedance

Secondary two windings in series connection: Standard 10Kohms, supporting open (without load) operation
Secondary two windings in parallel connection: Standard 2.5Kohms, supporting open (without load) operation

Frequency bandwidth

Primary & secondary in series connection: 10Hz to 32KHz (-2db) driven by 4V 3.5Kohms
Primary in series & secondary in parallel connection: 15Hz to 100KHz (-2db) driven by 4V 3.5Kohms
Primary in parallel & secondary in series connection: 8Hz to 30KHz (-2db) driven by 4V 600ohms
Primary & secondary in parallel connection: 8Hz to 100KHz (-2db) driven by 4V 600ohms

Primary inductance (H)

Primary two windings in series connection: 30H at 5V, 50Hz

Primary maximum current

50mA(AC)

Primary maximum permissible DC current

30mA

Recommended primary DC current

20mA or below

Maximum passing voltage

Primary two windings in series connection: 200Vrms

Primary DC resistance

Primary in series: 74ohms, primary in parallel: 19ohms

Secondary DC resistance

Secondary in series: 146ohms, secondary in parallel: 37ohms

Dielectric withstanding voltage between primary and secondary

2KVAC

Maximum permissible voltage on primary

1KVAC

R core type

Type R50 50W R core

Shape

Enclosed in square case.
The same mounting dimensions as those of SOFTONE RW-20, RX-40-5 output transformers, and Tango NC-20

Connections

Lead wires

Overall dimensions & weight

W: 83mm, D: 78mm, H: 107mm, 1.5Kg

Price

US$ 120 (excluding tax & shipping)

Winding configuration & phases

RC-20 has two windings each on primary and secondary. The table below shows the turn ratio (voltage ratio) of each winding and the phases of lead wires.

Winding

Turn ratio (voltage ratio)

Positive phase (+) lead wire

Negative phase (-) lead wire

Primary

P1

1

Brown

Red

P2

1

Orange

Yellow

Secondary

S1

1.8

Green

Blue

S2

1.8

Violet

Gray

Winding combination and its characteristics

Series and parallel connections of primary and secondary windings support various circuitries.

Wiring diagram

Connection

Winding configuration

Drive circuit & impedance

Output circuit

Characteristics

Detailed characteristics

Primary
series


Secondary
series

1:1.8

Single-ended

1Kohms to 5Kohms

Single-ended

Voltage ratio 1.8 assures sufficient drive voltage.
Suitable to drive power tubes requiring high drive voltage, such as 300B, 211, 845, etc. Extra strong version of Tango NC-16

1

Primary
series


Secondary
parallel

1:0.9

Single-ended

1Kohms to 5Kohms

Single-ended

No gain in transformer with voltage ratio 0.9, however, this connection can drive successfully the power tubes into + grid field.
Characteristics equivalent to Tango NC-20F

2

Primary
series


Secondary
Push-pull

1:0.9+0.9

Single-ended

1Kohms to 5Kohms

Push-pull

Inverted phase can be obtained and at the same time, the voltage much the same as the primary can be obtained at the grid of power tube to facilitate to drive the power tube.

3

Primary
Push-pull


Secondary
Push-pull

1+1:1.8+1.8

Push-Pull

Between P-P

1Kohms to 5Kohms

Push-Pull

Push-pull circuitry both drive and power stage assures very low distortion output.

4

Inverted interstage transformer coupling

Primary
series

Secondary
parallel

1:0.9

Single-ended

1Kohms to 2Kohms

Inverted interstage transformer coupling

Single-ended

Low resistance windings and large size core can drive successfully + grid transmitter power tubes.
Characteristics equivalent to Tango No.11643

5

Primary
parallel

Secondary
series

1:3.6

Single-ended

600ohms to 2Kohms

Single-ended

Voltage gain 3.6 can be obtained matching to low output impedance components & circuits, 600ohms line, etc.

6

Primary
parallel

Secondary
parallel

1:1.8

Single-ended

600ohms to 2Kohms

Single-ended

Can be used as low impedance and wide bandwidth input transformer.

7

Several combinations other than the above are also available.

Suitable drive tubes

Suitable drive tubes for RC-20 are small/medium power triodes having plate resistance 1Kohms to 5Kohms or small/medium power pentodes in triode connection.
Example) 6AH4, 6BX7, 6CK4, 6S4, 6BM8 triode connection, 6F6 t-connection, 6L6 t- connection, 6V6 t-connection, 12BH7A

Shape & dimensional drawing

Application Notes

Detailed characteristics

1.Primary in series connection, single-ended input & secondary in series, single-ended output

The two graphs below show the characteristics of the transformer when both the primary and secondary are in series connection for single-ended input and output, using RC-20 as input transformer 1:1.8
The features are wide bandwidth 10Hz to 32KHz (-2dB) and plain attenuation characteristic up to 100KHz without peculiar peaks and dips. When the secondary is opened (without load), the bandwidth is narrowed but no peculiar peaks or dips are observed. The frequency above 100KHz rises due to the capacitance between the primary and secondary that passes the signal.
The impedance characteristic also represents a plain curve without peaks or dips.

2.Primary in series connection, single-ended input & secondary in parallel, single-ended output

The graphs below represent the characteristics when the primary is in series connection and the secondary in parallel for single ended input and output using RC-20 as input transformer 1:0.9.
The features are wide bandwidth 15Hz to 100KHz (-2dB) and plain attenuation characteristic at high frequency. When the secondary is opened (without load), the bandwidth is narrowed but no peculiar peaks or dips are observed. The impedance characteristic also represents a plain curve without peaks or dips.

3.Primary in series connection, single-ended input & secondary push-pull output

The graphs below represent the characteristics when the primary is in series connection for single ended input and the secondary in push-pull connection for push-pull output using RC-20 as input transformer 1:0.9+0.9.
The features are wide bandwidth 15Hz to 32KHz (-2dB) and plain attenuation characteristic up to 100KHz without peculiar peaks and dips. P-P balance between positive and negative phases is also remarkably good up to 40KHz. A small difference is observed at 40KHz and above. The frequency above 100KHz rises due to the capacitance between the primary and secondary that passes the signal.

The graph below represents the characteristics when the secondary is opened (without load). The bandwidth is narrowed but no peculiar peaks or dips are observed. P-P balance is also kept remarkably good.

4.Primary push-pull input & secondary push-pull output

The graphs below represent the characteristics when the primary and secondary are in both push-pull connection for push-pull input and output using RC-20 as input transformer 1+1:1.8+1.8
The features are wide bandwidth 12Hz to 50KHz (-2dB) and plain attenuation characteristic up to 100KHz without peculiar peaks and dips. P-P balance between positive and negative phases is also remarkably good up to 100KHz. The frequency above 100KHz rises due to the capacitance between the primary and secondary that passes the signal.

The graph below represents the characteristics when the secondary is opened (without load). The bandwidth is narrowed but no peaks or dips are observed. P-P balance is also kept remarkably good.

5.Primary in series, single-ended input & secondary in parallel, single-ended output, namely at inverted interstage transformer coupling operation

The graphs below represent the characteristics when the primary is in series connection for single-ended input and the secondary is in parallel and inverted connection for single-ended output using RC-20 as input transformer 1:0.9 for inverted interstage transformer coupling operation.
The features are wide bandwidth 8Hz to 60KHz (-2dB). The inverted interstage transformer coupling operation tends to have peaks and dips because the inverted electro magnetic transmission and non-inverted transmission generated by the capacitance between the primary and secondary cancel each other. For this, a slight (+-1dB) peak and dip is observed between 60KHz and 70KHz. When the secondary is opened (without load), the bandwidth is narrowed but the peak and dip do not change for the worse.
The impedance characteristic also represents a plain curve with only slight peak and dip between 60KHz and 70KHz.

6.Primary in parallel connection, single-ended input & secondary in series, single-ended output

The graph below represents the characteristics when the primary is in parallel connection and the secondary in series for single-ended input and output using RC-20 as low impedance input transformer 1:3.6.
The bandwidth is 10Hz to 32KHz (-2dB). The features are plain attenuation characteristic up to 150KHz without peculiar peaks and dips. When the secondary is opened (without load), the bandwidth is narrowed but no peculiar peaks or dips are observed. The frequency above 150KHz rises due to the capacitance between the primary and secondary that passes the signal.

7.Primary in parallel connection, single-ended input & secondary in parallel, single-ended output

The graphs below represent the characteristics when the primary and secondary are in parallel connection for single-ended input and output using RC-20 as low impedance input transformer 1:1.8.
The features are very wide bandwidth 8Hz to 100KHz (-2dB) and plain attenuation characteristic at high frequency. When the secondary is opened (without load), the bandwidth is narrowed but no peculiar peaks or dips are observed. The impedance characteristic also represents a plain curve without peaks or dips.

8. Primary inductance characteristics

The primary inductance of the transformer that can be superimposed with DC current on the primary winding has the constant inductance characteristics. This constant inductance makes it possible to drive the power tube steadily without being affected by the fluctuating AC input voltage. The less the inductance fluctuates under various operating condition, the more solid and stable sound quality can be obtained. The inductance of RC-20 is nearly constant, quite independent from the various levels of input AC voltage.

The primary inductance of RC-20 is 30H without superimposed DC current. The more superimposed DC current increases, the more the inductance drops and the low-end bandwidth narrows. Although the service limit of DC current is 30mA, our recommended operating condition is within 20mA.
The graph below shows the inductance characteristic of RC-20. You will notice its excellent performance. The inductance remains unchanged quite independent from the various levels of signal and superimposed DC current unless the core is saturated at 200V input.


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