One of my (numerous) neglected projects is a version of the “HackerNAP” Naim clone amplifier. Naim is one of those brands that have a very loyal following, and many of their original designs have been analysed extensively by DIY’ers looking for improvements. The HackerNAP is one of those derivatives and the NCC200 from Avondale Audio is another – ebay is awash with other (more or less accurate) versions as well by the way.

Part of the reason why this project hasn’t been top of the list is that did not like the original HackerCAP PSU boards, so with a small delay (of around three years…) I decided to do my own version instead 😀

To be honest this isn’t how I would normally have done a PSU board – if had designed from scratch I would have made a larger board, used two full bridges and a full ground plane – but as the chassis are already drilled for the original boards I kept the physical size as close to the original as I could.

While the board size isn’t 100% the same as the original HackerCAP, I’ve retained the option to configure the design for both “normal” PSU usage and also for CRCRC or CLCLC configurations.

As the PSU boards are now done and tested, I can hopefully manage to do the rest of the assembly in less than three years 😀

Another mains controller…

I’ve designed and built a few control boards for switching on mains (e.g. this and this), because it tends to be a thing that many of my projects need. Good (and good looking!) mains switches are hard to come by, especially for higher currents, so it makes sense to use a lower-voltage switch combined with a relay or an SSR for this duty. An obvious downside to the relay-based approach is that a standby voltage is needed to control the relay, but as described in a previous post there are now several types of switching AC-DC converters able to do that job very cheaply and reliably.

However, more often than not I have found that I prefer to keep the standby PSU separate and so this addition to the control-board portfolio was delberately made smaller and to fit my usual 2”x2” format to make it stackable with my softstart-board. For anything with a large transformer in it, this is a combination that is very useful.

Another addition is an external trigger input (isolated with an optocoupler) which I don’t often use to be honest, but which I could see some potential in anyway. To make this feature a bit more versatile I have opted for the “deluxe-version”, by feeding the optocupler from a constant-current source made from an LM317L. This should mean that it’s not just the usual “12V-trigger” input, but actually it would work with any voltage between app. 3-30V and draw less than 20mA from the triggering device.

“In flight” (or at least on the way) are boards for a matching standby PSU based on the Mean Well IRM power modules – when everything is here and tested I’ll publish some files and more pictures 🙂

Sunday morning chipamps…

It’s been some time since I did an ebay kit, but that doesn’t mean I have given up on them (in fact I bought plenty…) and a cheap kit is still a great thing to play with on a Sunday morning**

It’s a pair of power amps based on paralleled TDA7293 amplifier ICs in the correct “master/slave” configuration as per the data sheet (and this discussion on diyaudio). The TDA7293 and TDA7294 chips are among the few survivors of the “purge” of audiophile components and they should still be available. Unlike the LM38xx-series and its siblings, the TDAs have MOS-FET output stages which means they can run in parallel without resistors to limit current sharing between outputs. The parallel arrangement allows for more current into low-impedance loads, but as the TDA7293 will work on up to +/-50V rails having two ICs also makes for a fairly serious effective power output.

These kits are seriously cheap and although I’ve tried to use most of the components that came with the kit, some parts have been replaced for cosmetic reasons (because that matters to me, sorry!). Even with component replacements though, these kits are so cheap that there is no real excuse for not trying them – even if you don’t need new amplifiers at all 😉

No real sound impressions yet, but I know these chips can sound really good so I am looking forward to seeing how much of their potential can be unlocked for the same price as a takeaway meal 😉

**Yes I know it’s not Sunday today, but as Whit Monday is a holiday in Denmark it felt like Sunday morning 😀

Project files: INA217 Microphone Preamp

What is it?
Board files for my INA217-based microphone preamp and the matching PSU as shown here. The design is meant to be “configurable” with three different gain options and phantom power selectable via jumpers. The amp also has a full complement of protection features. The matching PSU has three rails via two small onboard transformers for a compact “all-in-one” solution.

How big are the boards?
The amp board measures 3.1” x 1.9” (app. 79 x 48 mm.) and the PSU board measures 3.95” x 2.7” (app. 100 x 69 mm).

What is the status of the boards?
The amp board is version 2.1. Version 2.0 was my update of the original design as showcased in the previous blog post (link) and 2.1 adds a few minor tweaks including an LED to indicate directly on the amp board if phantom power is on or off.
The PSU board is version 2.1 as well for much the same reasons (although the v2.1 “tweaks” consisted mostly of fixing a couple of fairly serious mistakes in component labelling 😀 )

Does it use any special/expensive/hard-to-find parts?
Not really hard-to-find as such, but still worthy of some attention 🙂

  • The regulator for the phantom supply regulator must be a LM317HV type which allows for a greater in/out differential. You can use the standard version as well, but a short will then kill the regulator.
  • As for the INA217: I am not sure if there are fakes about, but buy from reputable sources just in case. Anything in an 8-pin DIP is an easy target for fakes really.

Anything else I need to know?

  • This board adds nearly all the bells and whistles described in this paper from THAT corp on instrumentation amp IC-based microphone preamps. These extra components for short-circuit and EMI-protection are optional, but definitely recommended.
  • The board has a Neutrik A-series Combo-jack onboard which is very practical and versatile. Unfortunately it means that if you use the TRS it shorts the phantom voltage to ground if it is plugged/unplugged while the amp is on. Protection features have been added, but this scenario is best avoided so only (dis)connect the TRS while the amp is off.
  • See the INA217 datasheet for gain calculations. While you can add a switch to select between the different gain settings, doing so may add quite a lot of noise so it’s not recommended.
  • Voltages for transformers: The two transformers will have to be 2×12-15V and 2x18V respectively. They are usually single-primary, so choose the ones that you need. Note that with transformers in this form factor you will not be able to deliver more power than is required for a single mic amp. If you need a triple PSU that can supply more than one amp board, this design should work just fine (with external transformers.
  • Replacements for the INA217 are mainly the THAT1510/1512, but there are some differences so I am honestly not sure if they are a drop-in replacement. Refer to the files under “related information” if you want to check for yourself.

Download design files here

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

Before you start I strongly suggest you read through the INA217 datasheet. Please also refer to the aforementioned paper from THAT on this type of microphone preamps, this THAT design note and the datasheet for the THAT1510/THAT1512 ICs.

(Yet another) anniversary!

Yes, it’s that time of the year again – and this year it’s the fourth anniversary of the blog 😀

Not a lot to say that I haven’t already said the last couple of years, but I still expect to continue writing as much as time allows. I am also still very excited and greatly appreciative of your questions and comments, so keep it up 🙂

Picture below is of what is (currently) sitting near the top of my project pipeline, namely four 4U diyaudio special-edition pre-drilled heatsinks. These are specifically intended to accelerate (as much as possible) the completion of my Pass VFET project as well as one other Pass project using boards from the diyaudio store that I have wanted to do for some time now 🙂

Project files: Little helpers – Alps PCBs III

What is it?
These are “little helper” boards for the Alps RK168xx series of motorised potentiometers. These pots are not quite as good quality as the standard Alps “Blue Velvet” RK27-series, but they are cheaper and smaller. They are also used in many commercial products, so they should work fine for many diy projects. The motor also means that the pots have a nice mechanical feel to them 🙂
There are two board versions, a 2-channel (for stereo with the RK16812) and a 4-channel (for balanced amps with the RK16814). Alps also makes a six-channel version of the pot and adjusting the footprint to fit these should be relatively easy, but I have no need for these now so I couldn’t be bothered 🙂

How big are the boards?
Both boards measure 1.85″ x 2.0″ (app. 47 x 51 mm) and the rear mounting holes are in the same place on both boards.

What is the status of the boards?
Both are version 1.0 since they are exactly as my prototypes.

Does it use any special/expensive/hard-to-find parts?
Mostly there’s only one real part on the board and that is the pot itself, so not really 😀

Anything else I need to know?

  • These are “preamp” style boards have a ground plane and a ground pad that can be used if you grounding scheme requires the shaft of the pot to be grounded. Use a piece of wire connected from the ground pad to either one of the screws on the back of the pot or soldered to a ring terminal wedged between the pot and the chassis. You can also use the grounding pad on the bottom instead.
  • The boards can also be used to make separate, passive preamps. In this case, a 10k potentiometer should be used.
  • The screw clamps are standard 5mm pin spacing types, but of course it is possible to solder bare wires to the boards as well.
  • The basic Eagle footprint for the RK168 was one I found in a diyaudio-thread, so I can’t take credit for that. All I have done is modify it to match the Quad-version as well.

Download design files here

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

As usual, please remember to consult the manufacturer’s datasheet as well.

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.

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.

Download design 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.

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.