Ultimate Placid for Buffalo II
I have retired my old faithful OPUS DAC and I am going to reuse the Placid supply I had used to power the analog section of OPUS. I’ve used the following components to build the “ultimate bang-for-buck” power supply:
- Case from removable power supply for a server-class PC. I like it because it includes a mains EMI filter
- “12V DC” wall-wart transformer with added shielding. The rectified output is 14V DC when loaded, which is perfect for “dual regulation”: we want 5.5V final output, 3V headroom for Placid and 3V required by the LM317 adds to 11.5V, so 13-14 volt is optimum.
- LM317 pre-regulation module. Standard configuration LM317 set to 9V output.
- Original Placid v 2.0 set at 5.5V output and updated as described below
Updating Placid 2.0 to the latest circuit (2.01) diagram
- Removed compensation network R7/C4
- Increased compensation capacitor C3 to 220 pF
- Also modded the current source to the LEDs by shorting R6 and using a 2SK170BL JFET where I measured the current to be ~ 10 mA when Vgs=0
- Bridge rectifier is not installed because I’m using pre-regulation
I set it for 5.5V, 350 mA output. The placid power transistor are not warm at all. The LM317 runs warm, but not hot to the touch.
Advantages of double regulation
- Additional PSRR (Overkill, I know, but it is very cheap to implement)
- Better control of input voltage into Placid and thus minimize power dissipation through the power transistors
- Can use my salvaged 12V transformers 🙂
More on EI transformer shielding
Originally I just put some metal on 3 sides of the transformer, but after some reading on shielding this type of transformer, what is needed is a “flux band” around the windings as specified in this guide (see figure 13.4) and this post. According the the guide,
In order to reduce the radiated flux from an E-I transformer core, you will sometimes see a copper or brass band* wrapped around the winding and the outside of the core, as shown in Figure 13.4. This acts as a shorted turn to the leakage flux only, and greatly reduces magnetic interference to adjacent equipment. Such measures are not needed with toroidal transformers, as leakage flux is very much lower, and the core is completely enclosed by the windings.
(I added a second transformer for a 5V supply for future projects such as adding an Arduino). Here is an interesting article comparing EI transformers vs toroidal transformers for audio: link
Here are a couple of pictures or commercial EI power transformers with a flux band. Left is a Luxman product, right is a Marantz product. I’ve only seen flux bands installed in power amp transformers such as these.
Maybe stray magnetic field in low power transformers like the ones I am using are very weak and thus the flux bands I installed would completely eliminate any potential noise problem (which because of the frequency would manifest itself as a hum).
Below is a picture of the Audiosector DAC which uses a dual transformer similar in type to EI, and does not use a flux band.
Comparison Placid 2.0 vs 2.0.1 board layout
The 2.0.1 board layout is a nicer, more compact layout. I believe it leverages both sides of the board with nice GND planes on both sides. Maybe Russ had a summer intern do the first layout 🙂