Power Supply

The power supply is essentially very straightforward. The mains enters via a filtered IEC inlet and powers two toroidal transformers. These connect to the power supply PCB, which generates two separate ±15V supplies for the main analogue PCB and the surround PCB. The +5V logic supply and the +12V relay supply is also generated here, and there is a mains-fail detect circuit which controls the output muting relays.

There is no mains power switch as the unit is intended to be left powered up continuously. While in 'standby', all circuits remain powered apart from the VFD display. Fortunately, the preamp doesn't consume much power, so it shouldn't adversely affect my electricity bill.

The analogue supplies are regulated by LM317T/337T ICs. I chose these for their improved regulation and lower noise output compared to 7x15 types, plus, should it become necessary, they provide the option to vary the supply voltages. The 12V +RELAY supply is regulated by an LM2940, which is a low-dropout 12V regulator - meaning that hopefully, a mains dip shouldn't cause the record-out relays to drop out and affect a recording (assuming the rest of the equipment holds up). I felt that it was definitely worth regulating this rail because apart from that reason, it ensures that the relay coils are fed with a clean supply, eliminating the possibility of hum and noise being induced into the audio circuits. Finally, for the logic supply, a standard 7805 is used.

The regulators use the case as a heatsink - they are bolted onto a nicely-machined length of aluminium which is in turn bolted to the bottom panel.

In addition to the basic regulation and mains-failure detection, the PSU PCB serves one final purpose - to separate the serial control connections from the 34-way ribbon cable and pass these through to the control PCB via the black connector on the right.

Top view of the PSU PCB (33KB)
Rear view of the PSU PCB (29KB)

Although it would have been possible to derive the logic and relay supplies from the same transformer used for the analogue supplies, it would have resulted in increased dissipation due to the greater voltage drop across the regulators. The VFD takes between 150 and 200mA, which is significant. Also, purists will assume that this will automatically affect the audio signal, and that isn't totally impossible. The main advantage of separate transformers, or separate windings on a single custom-made transformer in the real world, is ground management. The issue of avoiding earth loops and minimising digital breakthrough is complicated to say the least!

Ideally, the analogue and digital grounds should only be connected at one point. The PGA2310 datasheet suggests that this is close to the IC, so this happens on the main analogue PCB. Having the PIC on a separate board helps, and when the PGA2310s aren't being addressed there is no activity on their serial control bus. A common trick to achieve further isolation is to use opto-couplers to electrically isolate the digital and analogue section - I didn't feel that was required here as I couldn't detect or provoke any digital breakthrough on the PGA2310 test PCB.

The mains-fail detect circuit is based on good old-fashioned transistors. This is because you can predict exactly how they will behave during power-up and power-down, which is not necessarily the case with CMOS or TTL logic! The main function is to control the un-mute relays, as discussed on the control systems page, but there is also an active-low PWR_FAIL output that is passed to the control PCB - this causes a power-fail interrupt in the PIC which writes the current program state to EEPROM during the brief hold-up time before the power supply capacitors discharge. Another option that I have is to use the PIC to pull down the PWR_FAIL line - this will drop out the un-mute relays, disconnecting the power amps from the outputs and reducing power dissipation slightly. This would be used during Standby, after a routine dump-to-EEPROM has occurred (as obviously the PIC won't be able to respond to the PWR_FAIL signal in this state). This neatly saves tying up another port on the PIC and having to run an extra signal between the PSU and Control PCBs, but in practice I haven't felt the need to implement this as yet.