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

Wide bandwidth, low distortion, low magnetic loss
Primary 1+1: Secondary 1.8+1.8
Advanced versatility supporting various circuits and impedances by selecting series or parallel connection of primary and secondary lead wires.
Can be used as input transformer for inverted interstage transformer coupling amplifier
High power handling capacity assured by large-size R core can drive powerfully the power tube by using as driver tube a small or medium power tube such as 6CK4, 6BX7, 12BH7, 6V6T (triode connection), 6BM8T, etc.
Drive impedance 3.5 Kohms when the primary windings are connected in series.
DC current can be superimposed in the primary. Maximum permissible current: 30mA when the primary windings are connected in series.
The secondary can be opened (without load). No peculiar peaks and dips are observed in the frequency characteristics even when the secondary is opened.
Enclosed in a handy and beautiful square case. The same mounting dimensions as SOFTONE RW-20, RX-40-5 output transformers, and Tango NC-20F.

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
Primary	P2	1	Orange	Yellow
Secondary	S1	1.8	Green	Blue
Secondary	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

Transformer cover case makes it easy to mount this transformer on the amplifier chassis and brings superior form to the amplifier.
The same mounting dimensions as those of SOFTONE RW-20, RX-40-5 output transformers and Tango NC-20 makes it easy to mount this transformer on various chassis available in the market.
The beautiful finished case is four-side welded construction and black metallic painted.
Mounting and overall dimensions

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.

RC-20 Multipurpose Audio Interstage Transformer