You might recall a while ago when I needed a +/-15V supply for a vacuum tube module. The idea was to power the entire thing using a common 12V DC “wall wart”, regulating the 12V down to 6.3V for the tube heaters, and boosting it to +/-15V for the rest of the circuit. It turns out that that I was able to use one of those little presoldered modules that you can find in AE, after filtering the audible noise (likely coming from the inductor) with a couple or RC lowpass filters.
So I thought. Why not put some of those other cheap DC-DC modules with a dual output to a simple test to see if they can be any useful for audio/synth circuits requiring a dual symmetrical output?
The setup was as follows. A suitable DC adaptor was used as the input of the DC-DC module, and the module output was used to power a Rene Schmitz’s “VCA-2” built on a PCB. I fed the inputs of the VCA with audio and a CV from a synth, and monitored the VCA audio through a desktop computer type speaker, while I monitored the output voltage of the AE module with a multimeter. No oscilloscope measurements, nothing fancy. Just a simple test to see if these modules work and sound “right” in a simple test that emulates a likely use case. Noted, I do not expect that any of these modules will replace whatever you use to power your modular. Rather, the test is to see whether they can power a couple of individual modules which need a dedicated supply for whatever reason (because of tubes, or because of a need to be boxed outside the modular, in a rack, a desktop case, a guitar pedal, etc).
I started with the familiar module based on the XL6007 IC (approx. 2.5x1.5cm, ~ EUR 2.00 in AE). which uses a “double SEPIC” topology to derive the negative rail. With a 12V input, I got the promised +/-15V output (there are versions with +/-12 etc) and, surprisingly, I could not detect any noise as when I used it in the tube VCA. It seems that the gain of that circuit was much less forgiving than this VCA.
Next was a module based on the SB6284 IC (approx. 2.2x1.2cm, ~ EUR 1.00 in AE). I couldn’t find the dual output topology in the datasheet, but it likely involves a diode as there is roughly a diode drop difference between the positive and negative outputs. With a 9V input, I got something like +12.15V and -11.65V. The module is advertised to handle up to 100mA. I would not trust this with anything more than a single module, but it sounded fine with the VCA despite the difference in the voltage.
The next one was even slightly cheaper, and based (allegedly) on the TPS61040 IC (approx. 2.5x1.2cm ~1.00 EUR in AE). I say allegedly because this is supposed to be a TI part but it’s anyone’s guess if the part is legit. I think that the TPS61040 is meant to be used with lithium batteries so the input voltage range is much lower. With a 4.5V input, however, I got something similar to the +12.15V and -11.65V above, which worked satisfactorily with the VCA. The TPS61040 supposedly can handle up to 400mA but, again, I would not trust this for anything besides a couple of modules.
Switching from boost to buck converters I tried a board based on two XL5430 ICs. The XL5430 is the Chinese version of TPS5430. The board was much larger and more expensive (approx. 3.5x4.5cm ~ EUR 4.00), but it also came in a very nice plastic box. I got the +/-15V version but, aparently, the boards can be configured for +/- 5, 12, or 15 by making the corresponding tiny solder bridge on the board. With a 19V input from an old laptop “brick” I got a stable output without any difference between the positive and negative rails. The product page lists 8 and 7mV output ripple for the +/-15V respectively, measured without a load. It also claims to be able to handle a 3A output, but this is very doubtful. In any case, I could see this powering a few modules as it worked fine with the VCA.
Next was another board based on two XL5430 ICs (approx. 5x5cm ~ EUR 4.50), which has a different number of parts, although I doubt that the topology is different. The differences between the two boards are primarily practical as this one has an on/off push button and it comes with presoldered screw terminals. The board I got is a +/-12V version, and just like in the other, the 19V input gave a well balanced and stable output. In this case, I even tried a slightly heavier load with a couple of 80mA incadescent bulbs on either side and the output voltage did not budge while the VCA worked just fine.
Finally, I tested a buck/boost module based on the XL6019 IC (approx 6x3.2cm ~ EUR 3.50). This one features an adjustable output, but given the open (and rather flimsy looking) inductors and some bad reviews, I did not have high hopes. According to the product page, with 1.4A at 18V input I could get 790mA at each of the +/-12V rails. In this scenario, however, if I understand the description correctly, you also have an Inrush current around 3.5-4.2A. Ouch! I would not imagine that anyone would seriously try to get this to deliver nearly 1.6A of output though. The test results were rather disapointing as the negative rail was all over the place. When I tried adjusting the output, the negative rail took several minutes to settle and there was a noticeable (1V+) difference in the output between the two rails. Surprisingly, the VCA worked fine, which made me think that I might have selected a very forgiving circuit for this test. In any case, I will probably revisit testing this module by putting a load of at least 15mA (as stated in the product description) while monitoring the inrush current.
Tentative conclusions:
Given how well the XL6007 module handled inrush current and the output RC filters in previous tests, I will continue to use XL6007 boards as needed. I also plan to test the XL5430 board with the vacuum tube VCA (which is probably less forgiving when it comes to inductor noise) and, if everything goes well, I will pair one with a 19V laptop “brick” in the modular to power the tube VCA and a couple assorted modules. The +/-12V XL5430 board with the switch will become an alternative breadboard supply to test new modules. This will allow me to test several use cases for these boards. The SB6284 and TPS61040 boards might find their way into some individually boxed module, but I don’t think I will be using the XL6019 board anytime soon. At least not before I run a few more tests with it.
I might buy a couple more modules that I have seen around and post a second installment, and I have a plan to test a couple of single output boost converters for higher voltages that I bought sometime ago. Until any updates, your thoughts and your tests on these or similar modules are very welcome!
