Reference1.3 Transformer Passive Attenuator Line Stage

Reference 1.3 is a passive preamplifiers, more precisely passive control units to change the output level of the passive control in multiple steps to control the volume of a high fidelity system. It offers a material improvement over all previous such schemes based around resistive attenuation.


Control Functions: Gain, Input Select, Volume, Grounding
Harmonic Distorms (for 2V RMS at output): 0.005%(10Hz~100kHz)
Frequency Response: (at -1dB) 10Hz-73kHz (0dB), 10Hz-65kHz(6dB)
Input Interfaces: 3 groups (RCA), 1 groups (XLR)
Output Interfaces: 1 groups (RCA), 1 groups (XLR)
Gain at 1kHz: 0dB and +6dB swicthable
Attenuation: -52dB
Dimensions: 430(L) x 160(W) x 100(H) mm
Weight: 10kg


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The R1.3 represents the ultimate transparency as expected with a passive attenuator, but with dynamics. Many listeners have been amazed at the dynamic shadings even at the lowest listening levels. The difference compared to resistor based passive attenuators is that resistors throw away signal as heat were as the R1.3 uses transformer volume control. This trades current against voltage by stepping up or down with up to +6dB of gain. Ideal match for the Cyber mono block power amps.

Modern Sources, such as CD-Players, DVD Players, SACD Players and others generally offer output levels sufficient to drive power amplifiers to full power (usually 2V RMS or more for digital full scale) and also offer sufficient drive for external devices and cables. Many CD-Players and similar devices have output impedances lower than 1kOhm, some are materially lower. Whilst “passive preamplifiers” initially created notable interest as a sonically extremely pure method of controlling volume and selecting inputs, they have soon faded back into obscurity, not surprisingly as most of these devices suffered from substantial impedance mismatches with either sources or loads. If for example a 5k Ohm resistive volume control were to be employed in a passive control unit the source would be required to drive all the time a quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance the worst case output impedance of the combination would be 1500 Ohm at -6db attenuation, while at -20db attenuation the output impedance would still be around 540 Ohm. If combined with around 1nF load capacitance (easily found in longer, high capacitance interconnects), this leads to a 0.3db attenuation at 20kHz for a 20db attenuation setting, practically showing the absolute permissible limit for load capacitance. The worst-case attenuation at 20kHz almost reaches 1db!!! If, to provide our source with an easier load we choose a 50 kOhm resistive volume control we must either accept drastically higher levels of roll off at 20kHz or we must limit the load capacitance to less than 100pF. Such a level of capacitance (100pF) can easily be found with only 1m of high quality interconnect cable and is often exceeded by the input capacitance of many amplifiers!

Thus the resistive volume control employed in passive control units must navigate a course between the loading the source, leading to increased distortion and high output impedance. The introduction of the magnetic volume control R1.3 neatly cuts through this tangled Gordian knot of contradicting requirements, making possible passive control units that actually work more effectively and in a much wider range of environments.

Reference 1.3 is a high performance core component for a magnetic volume control. It is a multitapped transformer designed to be used with a 23 position switch, the transformer is housed in a Mu-metal shielding can that measures 57.5mm in diameter x 65mm in height, excluding the connecting leads.

The full primary has around 200 Ohm DC Winding resistance. The same applies for the secondary winding. The primary inductive reactance at 20 Hz is in excess of 50 kOhm (400 H Primary Inductance) and thus provides an input impedance of more than 50 kOhm across the audio band if the secondary loading is infinite.

The primary winding has a tap that allows the transformer to give 6dB of gain when the output is set on the 0dB tap. This is especially useful with power amplifiers having a minimal number of stages and therefore lower gain than common.

The secondary (output) winding offers a number of taps allowing the following attenuation values:
0 db, 3 db, 6 db, 8 db, 10 db, 12 db, 14 db, 16 db, 18 db, 20 db, 22 db, 24 db, 26 db, 28 db, 30 db, 32 db, 34 db, 40 db, 46 db

With these values the steps by which the volume is changed over the majority of the range is smaller than the commonly acknowledged limit of audibility (3db), giving consistent fine control over the volume, when compared to continuous controls.

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