Mains line filter

An offshoot of my work on the STEPS circuit was that I started researching mains filters a bit. I kept it simple on the STEPS circuit, but decided to do a proper separate line filter PCB as well.

I’ve included something that is missing on the STEPS board, namely a differential-mode filter with an earth connection to serve as the midpoint. I’ve also put a fuse on this board as I often struggle a bit to find suitable space for a mains fuse in my builds – and obviously the fuse isn’t something that should ever be left out of a mains-powered circuit!

However, the STEPS wasn’t actually the only inspiration here: While looking for suitable common-mode chokes I discovered the Murata PLY10-series which is a hybrid containing a common-mode and a differential-mode choke winding in one part. This makes a more compact filter possible which obviously is an advantage (even if it has low-ish current capability and separate chokes are obviously more effective/efficient). The current capability of the PLY10 makes this filter suitable for preamps, headamps and similar circuits only though.

Pictures of the prototype below. To be honest, I am not sure if this is significantly better (or worse) than a normal filtered IEC socket, but it is at least a bit more versatile – and it was fun to make 🙂

An INA217 Mic preamp

Most of what I build is designed to reproduce sound that is already recorded, whereas this is designed to actually record sound for later reproduction. A slight departure from what I ordinarily do then, but bear with me. 🙂

It’s a microphone preamp based on the INA217 instrumentation amplifier chip from TI. The board layout is actually (another) one of my old designs that I’d managed to forget about for years but for reasons I’m not really sure about I rediscovered it and decided to rework it fairly recently. I think I did make a prototype board of the original back then but just never put it together – which is probably a good thing as I found an error in my original schematic when I did the update 😀

This design is also known as “the $5 mic preamp” (google it) since if you really pare it back to the essentials, it could be built for not much more than 5 dollars in components. My version is much more luxurious though, featuring an on-board XLR/TRS combo jack, configurable gain and phantom power as well as a DC servo and all the EMI-filtering and protection circuitry needed to avoid noise and accidents with phantom power. The only feature for this sort of amp that I have left out is the option to pad down the output with a switch – didn’t need that (and besides, no space left over anyway 🙂 )

I’ve also updated my matching PSU board which uses two small EI-core transformers to provide both the +/- voltage and the 48V phantom voltage. The transformer form factor only allows current for a single amp board, but that is OK. Originally regulating the phantom voltage was a bit of a faff, but since there is now an LM317HV with can tolerate up to a 60V-input, that was the obvious choice for the phantom supply regulator. This also means that for supplying more boards, my previous “Triple-PSU” design should be usable.

I’ve tested the preamp board with both a condenser mic and a cheap-ish Beyerdynamic dynamic mic and it seems to work quite well. I don’t have a proper recording setup at the moment though, but the sound quality is definitely good enough to warrant further experiments. I’ve made some updates to both boards and I’ll release the files when they have arrived so you can have a go yourself 🙂

Project files: STEPS clone PSU

What is it?
The board for my “STEPS-clone” single-rail linear PSU as described here. This PSU is suitable for low-power streamers, DACs, headphone amps etc. that run on a single DC-voltage rail and require less than app. 15W maximum. This isn’t really a 100% clone of the original STEPS supply (see here), but I’ve drawn quite a bit of inspiration from the STEPS so I think the credit is well-deserved anyway 🙂

Note that the transformer primary connections are hardwired on the board, so there are separate 115V and a 230V versions of the board files.

How big are the boards?
The board measures 3.95” x 4.7” (app. 100 x 119 mm)

What is the status of the boards?
The published board files are for version 1.0 which is the version I have prototyped. There are a few minor changes I could do, but it’s mostly cosmetic and it might be a while before I get to it anyway so I have decided to publish this version.

EDIT 21st March 2020: As described here I have added files for v1.5 below. Apart from some other minor changes, the board now includes the footprint for the 10VA Talema transformer (so you can use 10-25VA sizes) as well as an extra set of pads between the main capacitors to make space for a larger footprint inductor for the Pi-filter.

Does it use any special/expensive/hard-to-find parts?
If you can order from Mouser, then nothing here is hard-to find. If you can’t, then the only thing that might be difficult to substitute is the Murata common-mode choke and that is optional anyway 🙂

Anything else I need to know?

  • The original idea was that the board should be able to slide into a eurocard-sized enclosure (that’s also the reason for the two extra mounting holes). However, in practice this isn’t possible as the primary pins of the transformer are way too close to the enclosure walls to make this safe. My recommended enclosure is the GX1xx-types from modushop, but there are many other options. If you have more devices, you can of course use larger enclosures to hold multiple PSUs.
  • The transformer secondaries are in parallel, so with the standard Talema range from 7VAC to 22VAC, it should be possible to make the STEPS with outputs from around 3-25VDC.
  • The 2-pin header near the output can be used to connect a volt meter to display the output voltage (or it can be used for something else – your choice! :D).
  • The solder pads on the board can be used either as test points or to tap the AC or unregulated DC-voltage from the board to another PSU board for an AUX-voltage of some sort (additional circuit, trigger voltage etc.). Remember to watch the total load on the transformer and the maximum heat dissipation in all regulators.
  • You can use my spreadsheet here to calculate the adjustment resistors for various output voltages. This will show you the upper/lower limit voltages if you use a trimpot for variable output, and also the power dissipation in the adjustment resistors which you need to be careful with at higher outputs.
  • The only really tricky bit of this circuit is (potentially) managing heat dissipation if your load draws a lot of power on a continuous basis. You’ll have to balance the heat dissipation in the regulator and the pi-filter resistors, while still keeping the voltage to the regulator high enough so that it doesn’t drop out – even if the mains voltage varies a bit. A little tip can be that if your load device isn’t sensitive to output voltage, then turning up the output by app. 0.5-1V will shift some heat away from the regulator. Be sure that you stay within the specs of whatever you are connecting to the PSU at all times of course!
  • As usual for these circuits, you can use both standard and LDO (low-drop regulators). The low-drop types are normally not “better”, but can be a bit less tolerant of circuitry and load conditions so it’s actually better to stick with standard LM317 unless you have a good reason to use an LDO.
  • The only time it really makes sense to use a 3A rated regulator (LM350 or Lx1085 types) would be if your PSU is 5-7V output with a 25VA transformer. If your output voltage is higher or the transformer is smaller, the 1.5A+ current limit of a standard LM317 (or Lx1086) should be just fine.

Downloads:
Download design files here

EDIT 21st March 2020: Download v1.5 files here

Related information:
1) Read the original STEPS page linked above. Even if the circuit isn’t completely the same, there is still lots of great info about the LM317 type regulators and how to get the most of them.
2) Read the manufacturers datasheet for the regulator that you are working with. Pay specific attention to recommendations around output capacitance and bypassing of the adjust pin as there are some differences between regulator models and manufacturers here.

Note: Always read the “intro post” for additional important information about my designs.

 

Project files: LED tester

What is it?
The PCB files for my version of Håvard Skrodahls LED-tester as described here.

How big are the boards?
The board measures 2.0” x 1.6” (app. 51 x 41 mm.).

What is the status of the boards?
This is version 1.0 as everything (for once) worked the first time 🙂

Does it use any special/expensive/hard-to-find parts?
None, really. The 16mm pots can be bought from ebay and everything else you should be able to get from multiple different sources. If possible, I would suggest using a stereo 5k-10k pot and the fully-isolated version of the LM317. The former gives the best adjustment range and the latter helps protect against mishaps with flying test leads 😀

Anything else I need to know?

  • For information about how the circuit works, read the hackaday-post linked above.
  • Output current can be calculated as 1.25V/Rtot. For max. current Rtot = R1 and for min. current the value is Rtot = R1 +  the pot value (with the decks in parallel if you are using a stereo pot obviously)
  • There is a difference between Lin/Log pot as described in the build article, so you’ll have to decide up front which adjustment profile fits you best (or keep the pot offboard so you can change – or just build two boards 🙂 ).
  • If you want to use the “high-current” mode, populate R2 as well and short the jumper. Remember that power dissipation in both the resistor and the LM317 regulator increases with higher current. The calculations for min and max current above have to be adjusted to reflect the fact that R1 and R2 are in parallel.
  • The connection for the ammeter is required as it is in series with the LED being tested. If you don’t want the ammeter, bridge If+ and If- connections as shown in the picture. The connection for the voltmeter is optional.  Note that I have tried using a cheap LED meter from ebay for the ammeter and I had some problems with it, whereas if i connect my normal multimeters everything works fine – YMMV.

Downloads:
Download design files here

Related information:
Be sure to read the original post for the exact circuit description, information and tips.

Note: Always read the “intro post” for additional important information about my designs.