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 Here is another link to the articles from Walt Jung: directory
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.
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 used 499R for R4 and 5K1 for R5 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 2mA. 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 that I need that much current.
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, (to better balance the load on the Opamp inputs). The value for the 4K99 most likely comes from a very early version of the supply, called the Sulzer regulator, shown in figure 7a and 7b, in Walt's first document. In that version they use 51K for R5 and R10 (now R12) in those schematics. The voltage setting resistors (R4, R3 and R9 and R8) were 3K16 each. In a newer version, Walt used lower values for these resistors to improve the regulator. My assumption is that he used a factor of 10 to lower the values, and he used wire-wound metal film THT resistors that were only available in the 4K99 value.
Because I'm going to deviate from the LM329 by using the recommended combination by Walt of two Zener diodes in a back-to-back configuration to keep the noise and the tempco down. It's an old trick to use a normal diode in a back-to-back configuration with Zener, because the tempco's of both cancel each other out. On top of that, a 6V3 Zener is typically at the sweet spot of the tempco for Zener diodes. I also need an output of +17V. So the voltage output divider values of R6 and R7 have to change.
In one of the articles from 2/95, there is a table 1 on page 31 that shows different resistor values for different output voltages. There is a 16V and a 18V row, but not a 17V one. In all cases, R1 is 499 Ohm, just as I want to use. I wanted R7 and R14 to stay close to the 1K value to keep the Opamp balance almost the same with the 499R value of R4 and R11, so I selected a 820R value, which is just about in the middle for the recommended 16V (866R) and 18V (806R) outputs.
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), see actual measured values below.
The parallel combination of R6 and R7 with these values is 495 Ohm which dictates R4 to be 495 Ohm as well. Keeping the 499 Ohm value should be good enough initially, but can be changed later on.
The recommended Opamp is the AD825, Jan Didden mentioned that the NE5534 can be used, or even 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 AD825. You could swap them out if you want as long as you use an SOIC package. I also ordered two AD825's which I'm going to use first.
Most of the parts are SMD, with the exception of the larger capacity electrolytes, and the 10nF film capacitors for the rectifying bridges.
It seems that Walt very carefully selected the series transistors, the D44H11/D45H11 so I'm going to use them too. They are not expensive.
The bias transistors for the series transistors I'm using are the equivalent SMD versions of the BC546/556 TO-92 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 just about 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 small heatsinks, so I will not put a footprint on the PCB, just use the U-shape parts that you can screw on the TO-220 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 of the generator.
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 arrived and I quickly put together a working version, again using my new reflow hotplate.
When I first powered it up and checked the output rails, I was pleasantly surprised with a +/-18.9V output. This is without the trimmer resistors.
However, when I tested the +5V supply I was greeted with a 0V output. Bummer! On careful inspection, I spotted my mistake. I was planning all along to tie all the common grounds to the GND pin of the output BNC. When I was testing the supply, the digital GND for the +5V was still floating and not connected to either common. Well that makes sense. However, when I thought about it some more, I realized that I did not need to connect the digital or DGND to the analog common GND's. I could keep the digital supply completely separate because it will only power the Arduino Nano with the OLED display, and the relays I'm planning to use for the range switching. I could keep all of that "out of the way" from the generator by creating a dedicated and separate DGND. That required a small modification of two diodes and two 10nF capacitors to tap the other two windings and create a negative return path.
While making the changes to the schematic, we now have a V1.1:
Note that the two Opamps are still shown as the NE5534, but I installed the AD825. (KiCad does not have a symbol for it so I'm leaving the NE5534 as a place holder)
The two optional diodes and two filter caps for the separate +5V supply DGND are added underneath the board.
First step now is to tune the voltage rails.
I first measured the Zener diode combination. The plus supply was 6V99 and the minus supply was 7V01. They are very close to each other. When I used a 6K2 trimming resistor for R6b1 and R13b1, the voltages after warming up are now +17V04 and -17V04. Excellent!
The parallel resistance of the 1K3 and the 6K2 are now 1K075, very close to the original 1K. 1K075 in parallel to 820R makes 466 Ohm, I could lower the 499 to 470 Ohm to better balance the Opamp inputs but I'm going to leave the 499 Ohm as is for the time being.
The downside of using hi-efficiency SMD LED's for the current source is that they are very, very bright. I think that I'm going to use a black pen to darken them a bit.
I used my thermal IR camera, and was surprised to see that the current source transistor (Q2) was getting a lot warmer than I anticipated. I am using the SMD version instead of the original TO-92, but even so. There is 1.97V across R1, so with 240 Ohm, that's 8mA of current. The earlier version (Fig 8a and 8b) used a 100 Ohm value that Jan Didden changed to a 249 value. I used a standard 240 Ohm value.
To the right is the positive supply with the (hot) 330 Ohm load, to the left is the negative supply with no load. The green LED's are also contributing to the hot spots on the display.
Even without a load, the voltage differential with an input voltage of 36V across the series transistor makes them warmer than I had anticipated. (you can easily see the difference between the left unloaded and right loaded versions)
With a 330 Ohm 1W resistor as a 50mA load, the series transistor gets a little warm but not overly so. Because the generator setup I have now will only need about half of that current, this will be OK. If not, I can use a larger heatsink.
Next step is to make an FFT of the outputs to see how quiet they hopefully are.
This is the positive supply, also with a 330 Ohm/50mA load. Can you believe that I checked and double checked my measurements? This is unbelievable, wow! There is no difference with a shorted input lead, so the supply does not add anything it seems.
With the 50mA load, the input to the regulator has a 1Vp-p mains ripple, that's completely gone, and no harmonics or noise. Too good to be true?
I was planning to use a mains input filter, I'm reconsidering that for the moment. Even in my noisy environment, there is nothing to worry about.
Here is the negative supply:
Walt, I'm mightily impressed, hats off for your design.
I was already formulating plans to modify the +/-17V supplies to a shunt version that Walt published in a later document, to separate the raw DC side even more, but that is not needed in my case.
And now the final test, the generator with the new supply:
A very clean output with only a tiny bit of mains related harmonics on the left.
I do not really trust the measured and reported THD+N number of 0.0056%, but visually, it looks great with only a tiny third harmonic spike visible, and this is still without any shielding whatsoever.
This power supply is great and that part of my problem is solved!
The +5 Volt circuit
I'm going to hi-jack this SuperReg post by showing information about the +5V circuit I added. It has nothing to do with the SuperReg, but I'm adding that adventure here because I put it on the same PCB.
Here is the FFT from the loaded +5V supply with the positive and negative still loaded with the 50mA:
This is with a 51R resistor for 100mA load. The 50Hz mains harmonics comes through but is otherwise very clean as well. Even when I extend the measurement to 100KHz, there is an uptick in the noise floor after 30KHz, but no additional hash.
But...
When I connect the Arduino counter circuit to the +5V supply, I get this...
My solution to create a 5V rail is causing an issue, and with it, the transformer gets very hot so something is very wrong with my design. The diodes must be shortening the two transformer windings or something like that.
The Arduino only draws 14mA, that's not the problem. I need to figure that out.
I now modified the 5V input by tapping only the transformer winding for the positive section, by using two diodes. Of course, the three power supplies still get GND connected on the generator PCB, because I need to tap the sinewave as an input to the counter. Now that I don't connect the two windings of the transformer, there is no current flowing between them anymore, and the transformer stays cool. That part of the problem is solved, but I'm not there yet.
This is the disappointing result:
A bit better, but no success whatsoever. There are two possible solutions I can think of. One is to galvanically separate the pick-up for the counter from the generator, by using a pretty fast opto-coupler. The other possible solution is to use a separate transformer for the 5V supply. That's the easiest solution to try.
Here I'm using a separate 12VDC Wallwart just to give it a quick shot. I also tried my Lab Supply with an 8VDC input, with the same results.
Here is also evidence why I don't trust the THD+N results from the software. You can clearly see more harmonics, but with a better result than the earlier 0.0056% ???
I'm now using an H11L1M opto-isolator, that has an internal amplifier and Schmitt-trigger gate. In order to get enough signal drive for the transmitter, I had to bridge out the output capacitor that you can see in the amplifier circuit above. That works, so the counter is functioning again. However, there are still added harmonics. The solution I think is to use an additional output amplifier buffer, solely for the counter input. As long as it produces a logic level signal, the opto-coupler will be happy.
Here is the result with a hodge-podge of wires and an additional protoboard to test the opto-coupler:
Getting better, but I'm not happy yet. If it turns out that driving the transmitter is causing the added harmonics, I will need to reconsider if I want to continue with the counter. (PS this time the THD+N is more realistic I think. Go figure...)
As a next step, I created a simple sine wave to square wave amplifier and let that drive the opto-coupler to create more isolation with the generator. Now we're finally getting there:
Here is the amplifier section with the circuit to connect to the Arduino Nano:
I'm done with this post for now, and will continue with the SG505 post.
I have purchased another transformer for the SuperReg that I will try later on. It has a copper shield around the windings that can be connected to Earth GND for extra shielding:
Update to a new version
I like this SuperReg very much but because I made a mess with the +5V supply, I decided to create a new version 1.1 that removes that circuit and only has the positive and negative supplies on it. I will create the layout such that these two sections can be easily separated so you can use them individually, and use one extra side to create another supply. To apply the lessons learned, make it more universal and also support higher currents, I will allow space for larger electrolyte smoothing caps for the raw DC supply, create a bit of a PCB heat-sink for the current source, create room for larger heatsinks for the series transistors and provide a ground plane while keeping the "star" GND connections.Visit often and stay tuned, there may be more...
