Bridging Digital Audio Output to TTL Input
The genesis of this circuit arose from the need to find an uncomplicated solution for linking a digital audio output to the TTL input of devices like a sound card. Typically, an S/PDIF signal maintains a level of 0.5 Vpp at 75 Ω. However, this voltage level is insufficient for driving an input operating at TTL levels. The most straightforward approach to attaining the correct voltage involves using a small transformer to step up the voltage. To preserve signal integrity, it’s crucial to maintain an input impedance of 75 Ω, minimizing distortion in the signal.
Compact Transformer Selection and Construction
In this circuit, we opted for a transformer featuring an EP7 core accompanied by a former, given its diminutive size, measuring only 10.7 mm by 8.5 mm. To construct the transformer, we chose a T38 core material, readily available from Farnell, providing the transformer with an AL value of 5200 nH. However, due to the significant contribution of windings (copper being diamagnetic), the practical AL value is notably lower, approximately 30 to 40% less than the nominal value.
Precise Winding and Assembly Details
The primary winding comprises 15 turns of 0.2-mm-diameter enameled copper wire, meticulously wound in a single layer from one corner pin to the opposite corner pin. Similarly, the secondary winding encompasses 150 turns of 0.1-mm-diameter enameled copper wire, wound in a comparable fashion between the two corner pins. It is crucial to handle the coil former delicately while affixing the core halves with the metal clip, as it is prone to breakage. The clip not only secures the transformer but also serves as a shield. During winding, the coil former can be stabilized using a 3.5-mm drill bit and a piece of paper.
Ensuring Linear Input Impedance and Proper DC Offset
To maintain a linear and constant input impedance over a broad bandwidth, a 75 Ω terminating resistor is coupled with a 270 nF capacitor in series. At the secondary side, a clamping circuit comprising C2 and Schottky diode D1 is implemented. This circuit imparts the correct DC offset to the AC signal. A captured oscilloscope screenshot of the output signal illustrates the circuit’s performance, recorded at a sampling frequency of 48 kHz. It is evident that the circuit’s operational limit is reached at this frequency; beyond 96 kHz, the logic ‘1’ level diminishes significantly.
Passive Voltage Division Approaches
There is a second viable approach: employing a potential divider between 5 V and ground, precisely positioned midway between the two logic levels. This setup allows an AC-coupled S/PDIF signal (1 Vpp open circuit) to attain a sufficient amplitude for acceptance at the logic input. Alternatively, a third method involves a combination of the previous two techniques. By utilizing a winding ratio of 1:5, the signal quality, particularly the bandwidth, is significantly enhanced, potentially offering superior performance when utilized alongside the potential divider.
Pursuit of Passive Circuitry
The primary objective was to maintain the circuit’s passivity, eliminating the need for an external power supply. Remarkably, this goal has been accomplished, albeit with certain limitations in performance. It is plausible that opting for a different core material and employing a larger core could yield improved outcomes. As it stands, the converter presents ample opportunities for experimentation within a home setting.