Minipre in a box…

A while ago I presented my “MiniPre”-project of a simple op-amp based preamplifier. Now I’ve had occasion to put it into use as a small standalone preamp/active monitor controller.

The design is very simple, so not much to be added there (whatever you need is probably already in the original post), but it’s basically a standard dual op-amp in non-inverting configuration.

The power supply will be in the form of a small DC-DC converter (a continuation of my previous experiments) so that I can feed the box from a single 5V supply and keep the case size down. Because of this I’ve managed to cram everything into the smallest available hifi2000 case, so it will fit nicely on a desk 🙂

The advantage of this simple design is the the selection of opamp tends to have a noticeable influence on the sound signature, so this is one place where there is room to experiment whether different options have better synergy than others.

Project files: DC-DC converter boards

What is it?
PCBs for DC-DC converters as described here. There are three sizes, for 1”x1”, 1”x2” and 2”x2” converters respectively. These footprints are industry-standard so you can use converters from a variety of manufacturers such as Traco, Recom, Murata and many others.

How big are the boards?

  • 1”x1” PCB: 1.875″ x 1.475″ (app. 48 x 38 mm)
  • 1”x2” PCB: 2.85″ x  1.475″ (app. 72 x 38 mm)
  • 2”x2” PCB: 2.85″ x 2.5″ (app. 72 x 64 mm)

What is the status of the boards?
All the boards are in version 1.1, meaning they have been prototyped and minor tweaks made to silkscreen etc.

Does it use any special/expensive/hard-to-find parts?
The ceramic caps between the primary and secondary sides should typically be rated for 2-3kV which can be a bit difficult to find. Mouser/Digi-key obviously have them but your local parts suppliers might not. Otherwise, apart from the converter itself, not really.

Anything else I need to know?

  • The external components are for EMI filtering and (usually) not required in order for the converter to work. All the caps on the primary side have 1812 SMT footprints.
  • The two component positions on the secondary side can be used for decoupling (required for stability with some converters) or for voltage trim if your converter supports that. These have 1206 SMT footprints.
  • Not all converters have enable-pins and some has the functionality, but wired as “always-off” instead of “always-on”. In this case you need to wire the enable-pin to the negative input voltage in order for the converter to turn on (you can of course also use the optocoupler here, but with the logic inverted).
  • If you use a 4:1 input range converter and you expect to actually use that input range, you need to be a bit careful with the value and power rating of the LED resistor, at least on the two small boards. Both the LED and the resistor are 1206 SMT here. On the 2”x2” board you can fit a 1/2W or 1W leaded resistor and then there should be no problems.
  • Many converters are sensitive to the capacitive loading on the output, so remember to check the datasheet for maximum allowed capacitance. If you exceed this limit it is possble that the converter will refuse to start up.
  • You can sometimes find DC-DC converters as cheap surplus items. Normally that is absolutely not a problem, but remember that even if the footprints are industry standard there can be quite a few differences between manufacturers. I recommend that you do not buy anything that is so obscure that you can’t google your way to a datasheet/application note for it 😀

Download design files here

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

Always remember to refer to the manufacturer’s datasheet and application notes for specifics on pin connections, external component values etc.

EDIT 20-08-2014: Added comment on capacitive loading.


DC-DC converter experiments…

A while ago I started looking at DC-DC converters for powering some of my audio circuits. I am not quite sure what triggered this to be honest, but mainly it was because I wanted to be able to make some more compact headphone amps and preamps than what the usual linear supplies can provide. At least around here there are very few external dual supplies available and so building a preamp or a headphone amp with an external PSU can be a bit tricky. Another option is a single-rail amplifier design, but they are often compromised because they need big input and output caps, so what do you do then?

Enter the DC-DC converter, a switching device that chops up a DC voltage and then transforms it up or down as needed. Most types include regulation of the output and galvanic isolation as well. The DC-DC converter gives tremendous flexibility because instead of needing a 2×12-15V PSU with a transformer, rectifier/caps and a wasteful linear regulator that must be heat sinked, you get a very small PCB that only needs a single DC input from a wall adapter or other DC source to work.  Some, like the Traco THN-series I have used offer a 4:1 input range and up to 20W output from a 1″x1″ brick, meaning you can get a fairly powerful regulated +/- 15V PSU in a tiny form factor and with just a single 9-36VDC input  – that’s pretty impressive I think!.

Another useful feature is that many of the converters have a remote on/off feature to put the converter into standby-mode. This means that if you are using a circuit that already has a micro-controller built in you can turn the converter (and the circuit it powers) on and off using the uC or even by a simple low-voltage mechanical switch.

The downsides of this power and versatility are mainly that DC-DC converters can be quite expensive and then that they (like most switching supplies) have rather a poor reputation for audio use. In some cases that is probably well-deserved, but I don’t like ruling out a whole technology just because of a few poor examples – in my world there are very few absolutes like that in audio. However, using switching technologies of any kind doesn’t really eliminate problems, it just replaces one set of well-known problems (heat, space, weight etc.) with another, perhaps less common set (EMI, high-frequency noise, inrush currents etc.). Depending on the application these “new” problems may be significantly harder or easier to deal with than the “traditional” ones.

So, after a bit more digging, reading datasheets and application notes etc. I have built some prototype boards for various DC-DC converters. Below are some pictures of my prototype boards, using industry standard footprints for 1”x1” and 1”x2” converters (more versions in the works). I’ve followed the application information from a few different Traco and Recom application notes but the boards will generally fit converters from a variety of manufacturers – one of those pictured are some old Tyco ones I bought a few years ago without really being sure what I wanted to use them for.

As for audio quality I can’t really say anything just yet, but as one of the pictures below show I have tried to power my universal preamp/headamp from a +/- 12V converter and I think it sounds pretty good. There are no audible artifacts such as hum or noise from the converter and the remote on/off feature works great as well. So while more testing is definitely needed, I am already starting to look at look at some of my current projects in a new way 😀 If anyone has used standard DC-DC converters to power audio circuits I’d love to see some examples 🙂