Wiring Diagram

This is high quality Preamplifier circuit with built-in USB DAC for my power amplifier Leachamp. Schematics is from datasheet of PCM2902. Circuit includes DAC and ADC, SPDIF output and input and HID part with 3 buttons for MUTE, VOL + and VOL-.

For high quality playback is needed to use an external low-drop voltage stabilizer for the DAC part. DAC is used LP2951CM which was easily available at local stores. Output voltage is set to about 3.7V with two resistors. Circuit board is designed regarding to good ground placement and separating of analog and digital ground. These ground are connected in one point at a USB connector.

In datasheet of PCM2902 is recommended to connect Low Pass Filter to output of DAC for filtering high frequencies above audioband which are produced by oversampling conversion. Integrated circuit includes digital LPF which filters frequency above 100kHz. In application notes for filter on the manufacturer pages are recommended 1st-Order LPF (simple RC) or 2nd-Order with operation amplifiers which works like preamplifier too. I used simple RC LPF with recommended values R 1k and C 4n7. It's better to use roll-type capacitor instead ceramic. I didn't hear difference in a sound between connection with filter or without it, but with respect to other components in a audio chain it is better to use it. For a higher cut-off frequency we can change value of capacitor to 3n3.

USB can be configured for connecting several peripheral devices to a single PC, Since a PC can supply only a limited power to the external devices connected through its USB port, when too many devices are connected simultaneously, there is a possibility of power shortage, Therefore an external power source has to be added to power the external devices. In USB, two different types of connectors are used: type A and type B. The circuit presented here is an addon unit, designed to add more power to a USB supply line (type-A). When power signal from the PC (+5V) is received through socket A, LED1 glows, opto-diac IC1 conducts and TRIAC1 is triggered, resulting in availability of mains supply from the primary of transformer X1.

Now transformer X1 delivers 12V at its secondary, which is rectified by a bridge rectifier comprising diodes D1 through D4 and filtered by capacitor C2. Regulator 7805 is used to stabilise the rectified DC. Capacitor C3 at the output of the regulator bypasses the ripples present in the rectified DC output. LED1 indicates the status of the USB power booster circuit. Assemble the circuit on a generalpurpose PCB and enclose in a suitable cabinet.