This will be a post to describe my efforts to more or less regurgitate and resurrect the Walt Jung/Jan Didden power supply design that gained a reputation as a SuperReg.
I want to use this supply for my SG505 project because it deserves and will hopefully get a very quiet and responsive power supply.
A lot has been written about this design over several decades, and many attempts have been made to improve on it, mostly for audio applications. The majority, at least the ones I found, were tailored to hi-end audio power amplifiers, with an obvious focus on supplying lots of power. I need less than 100mA for both rails, which is more in line with pre-amps, DAC's etc. For these type of applications the original design from Walt Jung is more than sufficient. It does not need a lot of special parts, and is deceivingly simple.
The secret to success is to stay close to the original design principles as much as possible, which is what I'm planning to do.
If you want to have some more background on the original Superreg design, the performance measurements and the evolution over time, here is a link with all the information that will keep you busy for a while. SuperRegs
I will follow the latest version (2.3) and try to replicated that. The information can be found in the above archive, but this is the document I'm using as the reference for my copy. Version2.3
Here is another refinement/update for the reference: article
If you're interested in buying the original PCB's, you can do that here.
And finally, there is a discussion at diyaudio.com that can be followed here.
Below is a picture of the V2.3 PCB that Jan Didden designed.
I'm going to use SMD components where possible, hopefully without messing-up, and create a version with +17V and -17V outputs and also add the +5V rail I need.
Here is the schematic design I came up with:
Even though I'm hesitant to deviate from the original, there are a few required differences with the original.
First of all, I added the rectification and reservoir capacitors. This means that I only need to feed two AC transformer winding connections, and leave the transformer, on-off switch, and the EMI filters in another separate enclosure, away from the more sensitive parts.
Because I need to make some changes, I didn't see the need for 499 (R4) and 4K99 (R5) resistor values (that I don't have in stock), so I changed them to 510R and 5K1 for now, because I'm also not going to use the LM329. It's only available in TO92, and it's pretty expensive. The 4K99 value will cause a current in the Zener of about 2.01mA, according to LTspice. According to the specifications, the device also works with 1mA. However, the recommended combined Zener current by Walt should be around 4mA, so the value for R5/R12 should be 2K7 which sets the current to 3.8mA. The schematic still has the 5K1 value though because I'm not sure yet.
The original information by Walt Jung in the first article about the Superreg also talks about the 499 Ohm for R4 as having the same value as the parallel value of R7 and R6, both 1K, ( my addition: to better balance the load on the Opamp inputs). I'm guessing that the 4K99 value of R5 is 10x the value of R4, but I have not been able to get that verified yet. Because I'm planning to deviate from the LM329, using the recommended combination by Walt of two Zener diodes in an anti-parallel configuration to keep the noise and the Tempco down and I need an output of +17V, the values of R6 and R7 have to change. I was also changing the value of R5 to give the reference more current (from 2mA to 4mA as recommended by Walt). Before I make a mistake, I will connect two BZX84C6V2 Zeners in the anti-parallel configuration and have a look at the voltage and current with my Curve Tracer.
To get the +/-17V (+/-1%) output voltage with E24 values, I needed to use a trimming resistor, so R6a has a companion of R6b, the same as with R13a/b.
The formula to calculate the resistor values is : Vout = Vref x (1+ (R4/R3))
This means that we need a 1K25 value for R6 and R13 with the 6V9 reference (although that value needs to be confirmed).
The parallel combination of R6 and R7 with these values is 495 Ohm which dictates R4 to be 495 Ohm as well.
The recommended Opamp is the AD825, Jan Didden mentioned that the NE5534 has to be used for higher supply output voltages anyway, and since I already use them in the SG505, I might as well use them here too. They are a lot less expensive at 1/10th the price of the AD824. You could swap them out if you want as long as you use an SOIC package. I also ordered two AD825's so I can test the difference with the NE5534.
Most of the parts are SMD, with the exception of the larger capacity electrolytes, and the 10nF film capacitors for the rectifying bridges.
I'm still debating the use of the D44H11/D45H11 series transistors. They are not expensive but I think I can get by using a BD138/139 combination, which I have, because I need a lot less then 50mA for each rail. I have ordered the D44 and D45 so I can experiment with them.
The bias transistors for the series transistors are the equivalent SMD versions of the BC546/556 TO92 version.
Because the required output is +/-17V, I raised the voltage for the series Zener diodes (D2 and D7) to 7V5, to keep the Opamp output centered within the rail voltage, as is recommended. The Zener values are not overly critical, I just happen to have this value in stock.
Note that the recommended Panasonic HFQ series for the 120uF/35V electrolytes have been discontinued for a very long time, so I selected decent quality replacements.
I will need very small heatsinks, so I will not put a footprint on the PCB, just use those one of the U-shape parts that you can screw on the package.
I've also added the +5V section, and tried to keep that out of the way of the other two supplies as much as possible. That's why I tap the AC inputs from both transformer windings, the keep them balanced and separate the two supplies at the AC level, and also create a "digital" GND for the Arduino Nano and the relays I will use with the next version.
The layout is now done, and the request for sponsoring from PCBway for the production and shipment has been done. They generously support my activities so I can spend the money on parts.
Don't worry about the apparently reversed picture of the terminal blocks, they will be installed correctly. (;-))
The parts and the PCB are on order and it will take about a week before everything arrives, so stay tuned for more...
