DIY rebuild of the Tek SG505 instrument

This post will describe the building of a complete instrument based on the earlier investigation of the Tektronix SG505 Signal Generator described here: 

https://www.paulvdiyblogs.net/2025/03/diy-build-of-tek-sg505.html

Enclosure and Front Panel

I'm going to use the same enclosure as I used for the SG502, which is a TEKLA KL-22. This is a black plastic enclosure with a metal front panel and back panel. I would have liked to use a full metal enclosure but the ones I found where either too small, too low, or too large and mostly all of them had the wrong dimensions to use a front panel.

To get a feel for the proper handling of the knobs and the overall layout, I typically create a mock-up using a paper front panel with a card board backing functioning as the PCB.

I made a number of changes to the initial layout to make sure I can turn and switch everything without cumbersome finger acrobatics. I also want to group controls together as much as possible, and make the positions logical. Also the position of the BNC output and the main frequency control contraption needs to be positioned correctly.

Here is that mock-up in the final version, after spending a few days relocating and printing and mounting the controls again. (this is not the final version (see the PCB below), some text got moved a bit here and there)

Here is the view from the back:

And here the detail for the mounting of the main potmeter contraption.

This is an earlier version, the mounting holes for the bracket line up in the final one. (;-))

After finishing this, I could finelize the design of the Front panel PCB for The KL-22 , so it has a nice looking feel for the instrument. Here is the 3D viewer result of the final version:

The rectangle in the top is the cut-out for the OLED display that will show the frequency. There are no mounting holes on the OLED display board itself, you have to find a way to somehow glue it to the back of the front panel.

All the holes are isolated from the front and back ground poor to add some more strength and EMI blocking. I will order the black solder mask with white silkscreen lettering as I usually do for all of my front panels.

New PCB Layout

I have also started to create a new layout for the main board, and added the relay's for the range switching, as well as the output amplifier. 

Here are the circuits for the frequency range selection, using a simple rotary switch located on the front panel.

This is the frequency setting for the phase shift amps.

Here are the dampening circuits for the AGC and the Peak Detector.

And here is the circuit for the frequency multiplier rotary switch:

A new Power Supply?

I'm very happy with the Jung SuperReg, but it is more than a bit of an overkill for this instrument. I'm now looking into testing a dual shunt supply that I also very recently used for my Twin-T notch filter and the Victor Mickevics oscillator. I'm using Victor's design to start with. I've used LTSpice to learn a bit more about the principle and I think I now know enough to be dangerous and feel brave enough to give it a try. I also need to add the 12V rail for the relay's and feed the Arduino and LCD display.

This will decide whether I'm going to add the components to the main PCB, or create a separate one, and then add the Arduino counter circuit to it.

Well, after some fooling around with prototypes, I finally got it going on two protoboards.

Here is the schematic of the supply:

The prototype below does not have the transformer snubber and the bridge capacitors implemented.

The power LED is optional. If you're going to use the OLED display, it will show that there is power. During testing, I used an LED to tell me that power is on, otherwise I can't see it visually, and before I added it, to my dismay, I left the unit powered on for a whole night. Not good.

The Shunt Supply principle is quite simple, almost literally taken from my Twin-T notch filter design and I only added trimmers for the key voltages and tweaked the current distribution. I'm now also using a much smaller transformer that I can mount on the PCB.

Here is the first result:

For some reason that I do not understand yet, there are more harmonics of the principle visible and you can also see a bit more of the mains frequency. I'm hoping that a real PCB and complete mains and transformer filtering will reduce that.

The top protoboard on the left is the raw supply circuit fed with the ac from the 2x 28VAC transformer. It has the rectification and filtering plus the LM317HVT that feeds the 44V to the dual shunt supply, located on the lower board. The two boards are connected together using the two 91 Ohm resistors that provide the shunt supply current headroom.

I'll do some more long term testing and look at the temperatures, but it looks like this supply is adequate for this application.

I finished the layout of the new power supply. It will be mounted up-side down from the top of the enclosure. There is a normal transformer located on the bottom for the +/- 17V rails, and the top rectangular in black is the 12V DC power module. That circuit is completely isolated from the +/-17V rails.

And finally, here is a 3D picture of the main board:

The 12V supply for the relays, the Arduino and the OLED display is completely separate from the generator circuits itself.

A reconsideration for the front panel

While working on the final layout for the main board and the front panel, I became more and more agitated with the two screws that mount the potmeter reduction unit that are visible on the font panel.

After some hemming and hawing, I bit the bullet and redesigned the hardware construction such that there are no more screws visible on the front panel. It meant that I had to move some parts on the main board out of the way, and add another bracket and holes to the main board. I also used this moment to slightly change some positions and changed some of the silkscreen text.

This is how the front panel looks now:

And here is the new construct for the potmeter and the reduction unit:

The reduction unit is now no longer mounted on the front panel, but has it's own bracket mounted on the main PCB. The two brackets are made of an L shape aluminum 1mm thick and is 20x30mm. Commonly available in DIY stores.

Here is the main PCB:

On 22-aug-2025 I uploaded the three PCB fabrication files to PCBWay and asked them to produce them under the sponsoring agreement. They already passed their review successfully. I hope to get their approval for the sponsoring soon in which case they can be here in 7-10 days.

Construction Details

The three boards have arrived from my sponsor PCBWay and as usual, look great. Especially the front panel came out very well with the black color of the solder mask and the crisp white silkscreen. Compared to other fab houses, PCBWay uses a special matt black color that is especially interesting for front panels because does not shine or mirror and does not leave finger prints. It is more difficult to clean though, so don't spill any flux on it. I use it on all my front panels and I quickly counted 5 of them on different instruments.

This time I forgot to ask them to put their manufacturing number on the back side of the board. By default, they put it on the front, as you can see here in the bottom left. It's not hindering though.

The Power Supply

I already populated the power supply board (top left above) the same afternoon. The next day I tested the vitals and all the voltages where there. 

One important notice that I now also added to the schematic is that with this supply, you always need to connect a nominal load to the output of the shunt supplies, otherwise the TL431's will get too hot. They will try to reduce the voltage by sinking all the current. I used two 680R 1/4 W resistors as a load, connected to the board as you can see in the picture below.

I could easily adjust the voltages, the trimmers have a good range. The LED that I mounted on the board, to have a visual indication of the applied power, was too bright. I changed the series resistor from 10K to 47K to reduce the brightness.  Now that I know that the supply is working, I can give it a good soak & scrub. 

The Main Board

After knowing that the power supply works, I can start on the main board. I normally keep the prototype in a functioning state, but there are so many special parts on it that I can't do that. I'll simply transfer the parts from the prototype to the new board.

The challenge will be that I used the Tek part identifiers on the prototype, but started anew for the new board. I will have to use the iBOM plugin for KiCad on both PCB's to figure out which is which and where.

Here's a very valuable tip.
If you don't have this iBOM plugin installed yet, you're missing out on a fantastic addition. Highly recommended! My buddy Bud looked at it when I told him, and initially was not impressed. However it took only a little explanation of the features to make him a true believer as well. Now he swears by it.

Here is how that looks like:

The top screen shows the iBOM windows for the prototype with the parts list on the left, and the layout on the right. Everything is linked, so when you select a part in the layout, iBOM parts list or iBOM layout, the cursor will go there and highlight the part(s).

The bottom screen shows the Version 2 board, using a second browser. So I select a certain part value on the prototype, and also select the same part value on the V2 and then know where it is located on the prototype and where it needs to go on the V2 board. I first apply solder paste on the V2 board, and then use my heat-gun to remove the part from the prototype and place it on the V2 board. When everything is done, I can use the heat-gun or reflow plate to solder all the parts on the V2. Easy-peasy in theory, not so easy in practice, but after discovering and correcting a few errors, I could also add the other parts. 

In the parts list, there are checkmarks you can fill in so you can keep track of where you are.

The main board is now ready for a reflow soldering process on my new hot plate. This is the largest board with a lot of parts, so yet another good test of the hot plate design.

Wave soldering went well, although I had to nudge a few tiny (0402 diodes - I ran out of the 0805 types) components in place. I'm now soaking the boards for several hours so I can clean them properly in order to inspect them better with my microscope. With the amount of flux and very tiny solder ball bearings in the way, that's not really possible.

Aftermath of reflow soldering

This is a little deviation, but I wanted to shar it with you nonetheless. I will probably move this information to a new post, but at least you know why I did not present any progress for about a week.

After a reflow process, you end-up with a lot of flux that was part of the solder paste. Unfortunately, you need to remove that completely because it will have influences on hf circuits, and is hygroscopic so will attract water that will result in corrosion. Another side-effect of using solder paste is that you end-up with a number of so called ball-bearings. They are very tiny solder balls that are the result of the part sucking down to the PCB and that will push solder from beneath the part to the sides of the component.

Below is a good example of this issue. Mind you, this is after cleaning and scrubbing the board twice already!

The whitish goo is the solder flux and you can clearly see several ball bearings. 

The remedy is to soak the board in a solvent for a while, and then vigorously scrub the PCB with a brush to remove the unwanted flux and the ball bearings. 

Sounds easy, but is not so. The selection of the most optimum solvent is a science by itself, and is highly depending on the solder paste you are using. Some have water solvable paste, most have not. When you soak the board with a solvent, and use a brush, you typically end up with a very sticky board. To remove that, you can use IPA 99% to rub the PCB some more, and then generously poor fresh IPA on the board to let is drop of the board, and hopefully take the sticky goo with it. After that process, you can use clean water to wash the remaining IPA off, and then dry the board. You can use a hairdryer, or use a solder heat gun, as I do, because that will allow you to blow the remaining liquid out of small pockets.

That's the process, and I have been using that for a long time.

This time however, it did not go well at all. First of all, the PCB was the largest I ever used on my reflow hotplate and that meant that the solder in the middle was overly done, and on the edges just about. There was an abundance of flux and I have never seen so many ball bearings. Cleaning as I describe above did not give a good result. Even after soaking for several hours, the goo stayed and the ball bearings with it.

Rather frustrated, I finally bit the bullet on purchasing an ultrasonic bath. I had been eyeing them for a while, but thought I could do without yet another box and also save the money. So I bought one for 2 liters that is just large enough for this PCB (160mmx115mm). It fits, but not in the wire-bucket.

After trying it with my solder paste solvent in the pure state, I tried heating the solvent to 50C and tried it for 5 minutes. That was much, much better, but still not good enough so I tried it for another 10 min. after adding some liquid dish washing soap. That resulted in about a 90% successful cleaning. The edges of the board were not as clean as the middle, but that's because the transducer is in the middle, and I'm pushing the size limit. 

In the process, I noticed that one of the tiny 0402 diodes that I soldered by hand after the reflow soldering got loose (note the circle below). It must have been a cold solder joint, but I'm actually glad that happened because finding that error would have been difficult and I'm warned for the next time.

You can also clearly see the cloud of the solder paste solving into the liquid.

In any case, I'm happy with the results of the cleaning session now.

In the meantime, I discovered the reason for my reflow issues. It turns out that my solder paste is expired. I used SN4258 (Sn42/Bi58/Ag1), 138C melting point with a mesh (size of balls) 20-38um, or size #4. Unfortunately, there is no expiration date.

The effects that I see are exactly what is described when using expired solder paste. I always keep mine in the fridge, but it explains a lot. Unfortunately, I can't find the purchase date anymore but must be less than a year. I ordered it when I was working on my reflow hotplate. Lesson learned...

First Power Up

I connected the 12V rail to my Lab Power Supply to keep an eye on the current and used a jumper to select the frequency ranges, and watched for the relays clicking, which they did.

I then added the +/- 17V rails again using my Lab Supply and set the current to the expected value of 35mA. That didn't work, the supply went into current limiting, so something was wrong. Upon inspection, it turned out that I goofed with the location of two of the NE5534 Opamps, they were upside down. Unfortunately, trying the power again did not result in a lower current. I probably fried them, because after replacing them, I got the expected current result.

By then I could use the current shunt power supply for the SG505 and use that from now on. While checking the voltages, I noticed that the +17 could no longer be adjusted higher than 16.4V, so I lowered the -17V to also 16.4V to have symmetry. I need to investigate that in more detail later on, but I now had the 12V rail and the +/- 16.4V rails.

I then added the three potmeter connections and used my DSO to look at the generator output. Nothing, just some weird low frequency flopping of the trace. That smelled of a bad connection in the overall loop so I hunted for bad soldered parts. It took a while adding some flux (here we go again) and using either the heat gun or my solder iron to go over all the possible parts. Eventually I found the culprit and was presented by a beautiful sine wave. The frequency setting worked, the vernier worked and the frequency ranges worked as well.

With that result, I can now investigate and test the output amplifier and that's next on the list.

Wired-up all the required parts to make the amplifier functional, and eventually got it working. 

Oops!

I found one serious issue that I didn't see when using the prototype. I never tested the counter and then also looked at the sinewave distortion. In addition, I also used a slightly different pick-up point to tap the output frequency for the counter. That's a mistake, the 1K series resistor loads the feed-back loop of the output amplifier way too much due to the clamping of the diodes on the other side. I even tried 10K and that's still not good enough. Higher values result in no signal at the diodes. 

The fix is rather easy and only involved a single trace cut from RV2 to the 1K of the squarer circuit and connect it now via a jumper wire to the Sine Out test point. I will fix this in the schematic and layout of course.

Tuning the highest range

While it is still lying on my desk and easily accessible, I decided to tune the X10K range because the frequency was a little too high compared to the lower ranges. I replaced the 100pF capacitors for C16 and C26 to 140pF. I didn't have that value, so I used a 150pF X-version. That resulted in a frequency of 105KHz instead of 100KHz. The 140pF COG should get it closer. 

Looking into the rail situation

I also investigated the fact that I could not get the positive rail up to +17V, while the -17V worked. The raw supply is also a little lower than I hoped for with the transformer I'm using, so I adjusted that output to 40.0V. With that, I adjusted the rails now to +/- 16.4V so they are equal. I don't think it will matter much. The original SG505 used +/- 15V rails, I decided to use the SG505 Option 2 rails that are +/- 17V which gives a bit more head-room for the Opamps.

I have not made any distortion measurements yet, I'll wait until everything is inside the enclosure and I have shortened all the leads that go to the front panel to the appropriate length.

Using the front panel

The next step is to mount the controls on the front panel and the PCB in the enclosure. I also need to make a few mechanical adjustments to the front panel because the holes do not match they way I designed it, and I made a mistake with the size for the main potmeter. These turned out to be a KiCad footprint issues and the result is that the holes are too small. Easy to fix but I also need to move the hole for the BNC connector a bit. Obviously, all the changes will be updated in schematics and the the Gerber files that I will publish.

Putting it in the enclosure

I finished the assembly of the front panel and put the main PCB inside the enclosure. It all works as advertised, but I had to make several changes to the front panel design.

Here is a view from the back that shows a few more details.

The power supply is still outside the enclosure as you can see, and I connected it with flying leads that still look untidy.

Now that I have the reduction unit installed, the adjustment of the frequency is very precise and feels great. If you're in doubt of getting one, do it, you won't regret it.

Adding the frequency counter

I also fixed the counter problem that I mentioned earlier so I can now start to add the Arduino Nano, the opto-coupler and the OLED display.

The installation went pretty easy, also because I used tape to secure the display to the front panel.

However, making it all work was more involved than I anticipated, even though I built a prototype that (seemed) to work very well. After a few hours of struggling, I ended-up making several hardware and software changes to make it work reliably throughout the frequency range. 

I eliminated the squarer circuit and replaced the BJT with a MOSFET to reduce the load on the frequency generator. I could now use a much larger series resistor, and placed that very close to the generator output again, instead of using the output amplifier because that signal can be attenuated.

The trace that goes from the generator output to the MOSFET for the counter is very long, so I wanted to reduce the current and hence the possibility for cross-talk.

I also had to power the opto-coupler with the Arduino 3V3 to reduce the 5V p-p banging on the port. That made a significant improvement to the precision of the counter.

Lastly, I tried to get a bit more fancy with the frequency count display, and wanted to add decimals for frequencies below 1KHz. After trying for some time, I realized that the library I'm using is optimal for the higher frequencies, but not so much below 1KHz. For those frequencies, to get a decimal digit, I needed to select a gate time of 10s which makes the counter unresponsive for this application. Selecting a library that fixes that will not work well with the higher frequencies, and I have not figured out how to use both libraries and select them dynamically. It is what it is. The solace is that the counter is a lot better (more precise) than the frequency dial of the original SG505. If you need a higher precision, use a real counter!

The DMM shows the frequency from the SG505 output and can be compared with the counter display. Not a perfect match, but good enough!

Next step is to mount the power supply in the top half of the enclosure and create the correct length for the power interconnects. By then, it's time for some distortion measurements, some tuning/calibration and see if I need to add shielding to the inside. I think I will...

Stay tuned!

A Github repository is available here it will be updated with information during the project when I have verified the correct operation. I'm still working on it, so there is very little information there at the moment so please be patient.

Building a new 100MHz Differential Probe

This post will describe the DIY build of a 1MHz, a 10MHz and a 100MHz differential probe with several power alternatives.

In 2017, my buddy in crime Bud and I (well, mostly Bud) build a DIY X10 100MHz probe that has been popular because so many other makers build it, or even changed it to their liking, We know of a 1x and a 100x design modification. Our earlier design was described here:

https://www.paulvdiyblogs.net/2017/10/differential-amplifier-probe-make-buy.html

Unfortunately, that 2017 design has some hard to get and rather expensive parts, especially the Opamps, the voltage regulator and also some hard to get capacitors. 

In early 2025, Bud has been looking into making the probe easier to build and make some needed improvements. He investigated the whole concept again and came-up with a main probe PCB that can be used to build a 10x 100MHz, a 10Mhz or a 1MHz version. The other constraint on the earlier design was the external power supply. Now that we have USB-C PD capabilities, Bud added a number of power options that can be added to the probe to your liking. Another main issue that he wanted to address was a 3D printable housing and making the probe slimmer so it's easier to hold while probing around with it.

There are some more improvements and refinements and if you're interested, you can follow along his design and test efforts here:

https://hackaday.io/project/203296-a-slim-10x-100mhz-10mhz-1mhz-differential-probe

Bud is now in the final stages of verifying and testing everything, and when he's done, I will add the information here also and provide the BOM and Gerber information in a new Github project.

If you're looking to build a probe, all I can say at the moment is to stay tuned for a little while longer. It will most likely be finished in September of 2025, so if you can hold-off, I recommend you do so.

What I'm going to build

I already have the earlier 100MHz probe, but wanted to build another one so I can test it with my gear that is a little bit different from what Bud has or uses. More specifically, my DSO is a 300MHz version, and while he has a much more improved and new DSO (12-bit, touch screen, etc, etc. I'm jealous!) it is a 100MHz version. He has a function generator, but that only goes to 60MHz. I have a fast edge (<1ns) generator, and most importantly a VNA.

So I just completed two purchase orders that is mixed between LCSC and DigiKey to get all the parts for the 100MHz version, the different parts for the 10MHz version, and then the USB-C based power supply.

Building the 100MHz probe

The schematic is largely based on the previous design, but with a number of refinements and changes. The input attenuator is changed so we have equal resistors, and that allows us to have equal capacitors as well, and parallel to every resistor.

As with the old probe, during the verification and calibration, you may have to add capacitors to the not placed (NP) C11 and C12 so the trimmers C15 and C16 can properly adjust the AC compensation. There are three values listed in the BOM that need to be ordered so you have the possible values at hand.

The next change is the first gain stage. The positive output now has an optional offset adjustment that can be installed when you need it. When you don't need it, don't install it because it degrades the output a little bit. Both outputs from the gain stage go to a summing amplifier, also a new device.

The rail splitter Opamp is also replaced by a device that is easier to get, and there are some refinements necessary because we no longer use difficult to get Tantalum capacitors. The ones we use now need a tiny resistance to make everything stable.

Next additional circuit deals with a power on LED. Depending on the way you power the probe, or what power supply you use, you can populate these two parts.

Last major change is the power input circuit. Note that there is no "real" connector, but only two pins. The idea behind it is that you can add a selection of tiny power modules to the bottom of the probe, based on your particular taste or need. More about that later in the power section.

Lastly, as you can see from the picture at the beginning of the post, there is a 3D printable enclosure.

Here is the schematic for the 100MHz probe:

The PCB looks like this in the 3D viewer:

You will note that it's a much slimmer design, that will be easier to hold and maneuver when you're probing around in a circuit.

Note also that the middle pin of the front-end will need to be removed to create the creeping space. The 3D model for the header is not modified, so it shows all three pins.

The bottom of the PCB houses the rail splitter circuit, and is also the place where the power supply will be added. That's where those four square holes are for. Bud calls these little boards Daughter Boards.

 

Building the Power Supply

There are two components to the power supply. One is the input, and one is the regulator.

The USB-C input board

The input voltage for the probe regulator can come from a variety of supplies. The most optimum is a USB-C PD board that fits into the probe. These boards are very inexpensive, and are widely available, but you have to select a certain type and size. 

These boards are available in different voltage configurations, like 9V, 12V, 15V and 20V. Although the voltage you order is fixed for the board, you can still change it by closing or opening a bridge ( a 0402 0 Ohm resistor or a solder blob). So 9V can be configured to 12V by adding a bridge, and 20V can configured to 15V by removing a bridge. 

The board in the picture is a 20V version, and if you remove the resistor in the top right, it will be configured for 15V.

The 100MHz probe requires the 9V version, and the 10Mhz and 1MHz probes require the 20V version.

These boards need to be glued on to the probe regulator board and the output of the board needs to be connected to the probe regulator board by two wires that are soldered on the pads.

This construction will fit snugly into the 3D printed enclosure.

Obviously, these USB-C input boards need to be fed by a USB-C Power Delivery (PD) supply.

The diff probe does not draw a lot of power such that a low wattage supply will do. I'm using a 100W supply myself, because I want to use it for many other applications.

The probe voltage regulator

Bud create a number of voltage regulator boards to satisfy your particular need or preference.  There is a switched power supply for two different kind of chips, an LDO linear regulator and a discrete voltage regulator.

There are two output voltages required required for the regulator, one for the 100MHz probe, and a different one for the 10MHz and the 1MHz probes. The reason is that the 100MHz probe requires an input voltage of 5.3V (+/- 5%), to create accurate 2.62V positive and negative rails for the Opamps.

The 10MHz and 1MHz probes require an input voltage of 15V to create the 7.5V (+/- 3%) positive and negative rails.

The VREG SOIC-8 daughter board (100MHz probe)

The VREG SOIC-8 Daughter Board (DB) is intended to be used with the 100MHz probe, because it outputs 5.3V and requires an input voltage of 9V. 

It can be configured with a number of LDO voltage regulators. I have decided to use the LP2951 LDO voltage regulator. It comes in an SOIC8 package, and Bud designed a VREG Daughter Board specifically for these devices.

Here is the schematic:

As you can see, there are a number of voltage regulators that can be used on this PCB.

Here is the PCB for it:

The parts are located on the bottom of the PCB, here is the top:

And here a 3D picture to make it a little bit more clear:

The VREG DB

This voltage regulator is a discrete design that can be configured for different voltages.

It can be use for all three probes.

This board has an optional maximum voltage protection in case you directly use a USB_C PD board that outputs a voltage that is too high for the probe. We've found that the output voltage of some of these PD boards are not reliably outputting the correct voltage.

By changing a few components and not populating a few, this regulator can be configured to output 7.5V for the 100MHz probe, and 15V for the 10MHz and 1MHz probes.

The Buck Convertor DB

There is also a Buck convertor design available.

Although this board has many possibilities, Bud is still in the process of investigating and verifying the devices. As an example, the TPP parts looked functional, but Bud discovered that the ripple output in the Pulse Skipping Mode is unacceptable for our application, so they are no longer recommended.

From the list, the NEX40400B is still our first choice, although I will built one with the RY8310 to give that a try.

Building the 10MHz probe

The basic schematic is the same as the 100MHz probe, so it can be built on the same PCB, but there several part value differences.

One of the major reasons to build a 10MHz and a 1MHz probe is to reduce the output noise of the probe, which is quite substantial for the 100MHz version. initially, Bud was able to find two alternative Opamps, the OPA2810 and the OPA810, that are 1/3 the cost of the LTC6268/9 pair of the previous model. 

Building the 1MHz probe

As with the 10MHz probe, this version can also be built on the same PCB, and again there are part value differences to make it a 1MHz probe with the least amount of output noise. It uses the same Opamps because Bud could not find less expensive ones that had the right specifications.

Stay tuned for more...

The SuperReg Power Supply

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.

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 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.

Here is the new schematic V1.1a:

This is the still 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.

A lot better, but not good enough yet. It proves to me that I need to galvanically separate the output amplifier of the generator going to the Arduino counter.

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...