Manufacture digital to analogue converters the Dutch way

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Manufacture digital to analogue converters the Dutch way

Metrum Acoustics makes a small but remarkably well formed range of digital to analogue converters, but that is not all that sustains the company. The All Engineering production facility behind the brand makes ‘all’ kinds of electronics for different customers at its base in Zeewolde, Holland. Cees Ruijtenberg, founder of Metrum Acoustics (audio products) and Transient (development and OEM), started his career in audio with electrostatic loudspeaker panels and amplifiers. He has been expanding his company since 2010 when his first DAC made its debut in a market that’s grown year on year because of increasing interest in music streaming.

 

Cees Ruijtenberg

The relatively simple NOS Mini DAC was the first Metrum product I encountered, a competitively priced DAC that shocked the hi-fi community because it offered such high quality. This converter was the first to use a high speed R2R ladder converter made for industrial rather than audio purposes. The Mini DAC was soon followed by the more sophisticated Quad and Octave DACs, the latter using external power supplies. One step leads to another and having started with two DAC chips (Duo), Cees moved onto four (Quad), eight (Octave) and finally the sixteen converters in the Hex, a DAC that was extremely well received in the audiophile community. The Aurix headphone amplifier for the expanding personal audio market became available in 2013. The latest introduction is the Pavane converter, the product that prompted me to visit Cees Ruijtenberg and see for myself what he’s up to.

 

Inside the Pavane DAC

The first Metrum DACs were literally made on the kitchen table, but they became so successful that Cees approached an engineering company to take over production. Not much later this cooperation led to a new partnership. This company works for industrial suppliers and OEM companies and is capable of meeting the high quality standard set by Metrum Acoustics. Both companies gained from the partnership; Metrum Acoustics no longer needs the kitchen table and is able to rely on modern production and logistics capacity, meanwhile All Engineering uses Cees for some of their design and consultancy work. Metrum Acoustics delivers its products to any place in the world through a dealer network, or direct to customers if no dealer is available. Worldwide delivery is just one of the reasons that Metrum products need to be extremely reliable, no one wants to send repair jobs around the world.

 

Octave Mk2 DACs

Ribbons of SMD components feed the machine

From meeting Cees in his office, where he’s surrounded by components, circuit boards, future developments, pre-production models, prototypes and test equipment, we move on to the production facilities. Cees explains that circuits are designed from scratch, followed by board layouts and pre-production samples. As soon as the definitive circuit board is ready for production and components are ordered, a third company starts producing the empty boards. When these come into the warehouse they are made ready for SMD components by using a template to apply a paste of glue and solder to the spots where the small parts will stick. A large machine using rolls of SMD parts, places each chip, capacitor, resistor etc on the board. The next step is to put the stuffed board in an oven, where the glue particles vanish and the components sink into the solder. This is the basis of what is called Surface Mount Technology (SMT) which is very reliable, parts can be small and processors measuring only 17x17mm with 256 connection points on the bottom can be soldered in place. Bigger parts like power capacitors need to be placed by hand and fixed using a solder wave bath to make the connections.

 

Transient modules

The finished boards are visually inspected by comparing them against a photographic reference. A job done by a machine that shows if parts are missing, incorrect or not exactly in place. This comparison is far more reliable than can be done by eye, primarily because machines never get tired. Connections are checked from samples under a stereo microscope, which is also used to make minor adjustments on the boards when they deviate from the master.
The next step in the process differs for Metrum Acoustics finished products and Transient (OEM) boards. OEM boards are electrically tested without attaching wires, using pins that connect to the printed circuit boards from a template and connecting them to measuring equipment. The Metrum Acoustics products are tested as soon as they are built into their housing and all wires are in place, soldered or pushed into the connections. This is followed by a 24 hours burn-in for each device. After the burn-in the same measurements are repeated before the casework is closed. It’s a lot of labour intensive work, but it makes sure that failures are kept to an absolute minimum.

 

Optical board checking

Back in the office Cees shows me some measurements done on the Pavane DAC and competitive DACs from other manufacturers. The display shows the harmonic distortion of the Pavane being only 0.00567% and 0.00538% for each stereo channel. Other results showed dynamic range of 140dB and an indication of extremely high linearity over the audio band. Frequency response is flat as can be, given sample rates of 44.1 to 192Khz. Roll off at the upper limit is of course necessary. These test results relate to the technical performance of the Pavane DAC, musicality can only be determined by ear, and my ears indicate that Metrum Acoustics has done an excellent job in that area too.

 

Pavane casework

The Pavane DAC is the first to use Cees’ Transient modules. These contain industrial R2R ladder converters and additional circuits. A small piggyback board holds the FPGA based Forward Correction Module, splitting the 24-bit digital signal into two 12-bit streams before actual conversion to analogue takes place. The FPGA works at a 400MHz clock speed and applies the algorithm Metrum Acoustics uses for this technology, an approach that I have never seen before in conversion products for (high end) audio.
Leaving that day meant I had to leave the Pavane DAC that I had had for a couple of weeks. But not before I could take some pictures of the inside and drool over the boards. The one man company producing the NOS Mini DAC has grown into today’s high end digital to analogue and headphone amplifier manufacturer. The never ending stream of ideas inside Cees Ruijtenberg’s head will bring more exciting products in the future, all it takes is time.

René van Es