PRODUCTION BUILD

     The Prodigal uses 3 Torroidal Power Transformers instead of the cheaper option of a single one with lots of output windings that magnetically couple. This means that the VU and relay circuits are completely isolated from the analog audio circuitry - for super silent running. A third transformer feeds only the P48 supply.

Rail voltages produced are:
T1: +48 phantom
T2: +15, +9, -9, -15 audio
T3: +5 digital metering, +12 relay switching.

     Note the Star Ground Post on the rear panel, which is the only point where Safety, Analog, Digital, and Chassis Grounds meet. From any point in the Prodigal's circuitry, there is only one path back to here, which eliminates internal ground-loops and current noise from neighbouring circuit nodes. This simple idea can be challenging to implement, but is a big factor in creating the Prodigal's scarily quiet noise floor. The Monitor Section alone boasts a signal to noise ratio of 104dB!

     Keen observers will also notice there is no electric fan to annoy you during mixing. Fans in audio gear, well, blow. The Prodigal is designed to run cool, and only needs the four small black heatsinks for the audio rails, to which an additional cooling fin will be added later in the build process.

     Prior to assembly, all boards are tested against a massive checklist to ensure there are no mistakes in the build, and that they operate within their designated electrical parameters.

     Voltages, current consumption, signal levels, gain and switching are checked and adjusted where applicable.

     One of the finer settings performed is alignment of the electronics behind the 0 to 30dB Input Gain knob, which is tweaked to a tolerance of one hundredth of a dB. Also, to overcome the tremendous imprecision of logarithmic pot tapers, this gain section boasts an innovative design wherein a linear potentiometer enslaves a cascading pair of op-amps in a highly mathematical way. Across 30dB of adjustment, the gain curve tracks to within 1dB of a perfect log law.

     Special matched components are also installed at this time, such as the gain-reduction FETs (one for the De-Esser and one for the Compressor) which need to operate in perfect concert between Left and Right channels. Pre-calibrated parts are soldered in sequence, then each board pair is numbered and kept together during the rest of construction.

4. Pre-Assembly Test and Alignment.  Microphone  Preamplifier   (egghead warning!)

     Special treatment is given to the commissioning of the Microphone Preamp board.

      The Prodigal has no input transformer to deliver a free level-boost prior to amplification. The Mic signal is taken directly into the preamp, so every part of the front end has to be sweet and clean.

      Transistors are hand-matched and thermally paired for super low-noise operation. Input resistors are also matched, and impedances are kept on the low side of the IEC permitted range. That's because we think audio signals sound better when driving a lower impedance. Raise an eyebrow if you like, but we've done double-blind tests and folks can hear the difference; now it's part of the Prodigal sound. Unlike transformers, resistors also present a uniform input impedance across all frequencies. We like that too.

      With Mic inputs, switching is important. You need to be able to change level and impedance between mic and line, apply a 20dB pad, invert phase, and apply 48v phantom. With the main inputs at rear, mic signals would have to travel a long way exposed to other circuitry before arriving at the front-panel switches, and in cheapo devices that's how it works. In the Prodigal we take the switches to the back panel instead – via high quality relays. The switches switch the relays, and the relays switch the audio. That keeps the signal path short, and delivers the cleanest possible soundwave prior to amplification.

      Mic levels are very small, and fluctuations on the power lines can cause large harmonic distortions if they bleed through. On each mic amp power supply, the Prodigal uses a zener-referenced regulator circuit which subdues about 10,000 times more voltage flutter than an on-chip regulator. Zener diodes are classified as being of a fixed voltage, but the volts actually vary  with the current passing through them – as the graph on the left shows. We use 1N4735A vintage Motorola diodes (the blue line) which are not made any more. You can see as current increases there is a flat spot where the voltage doesn't change. We run the Motorolas right in that spot, at 58 microamps bias. That means Thor (the mighty god of ending your career) can zap your power supply with a 50% voltage spike, causing a 50% change in diode current, and the result will be (at 38 or 87uA) NO change of zener voltage and the microphone amp won't even notice.

      We continue to blow our lunch money testing modern diodes (even with the SAME PART NUMBER) and so far they're all useless – no flat spot, so no gig. Five are on the graph, but we've tried heaps.

      Pre-assembly testing looks at all the above, plus a bunch of less interesting housekeeping. I like the photo of the Motorola diode. He's a sexy boy.

Razor-Flat Blue Line of the Vintage Motorola Diode

5. Assembly  All Audio Boards

     Installation of the Prodigal circuit boards is fairly straightforward, but they still want to be handled like new-born puppies. There are three control boards for the front panel, six in/outs for the rear, and the power supply makes an even ten. Once they're all fitted, six IDC ribbon cables similar to those used in computers provide the complete network of inter-board power and signal busses. Prior to insertion, connection pins are cleaned, and connectors are sprayed with electro-lube.

      Much has been done to suppress or eliminate electromagnetic interference within the unit. Each board has a guard-ring etched around its perimeter to shunt radiated energy away from audio and back to ground. Along the ribbon cables, no two signals travel side by side; instead, they are always separated by a grounded strand that guards against one signal-pair injecting crosstalk into another. There are actually more strands used as guards than for any other purpose. Furthermore, power and signal are never allowed on the same ribbon.

      As hinted above, audio travels along the buss in balanced pairs, which is an unusually fussy arrangement for board-to-board signals. This is so any received radiation not whisked away by the guard-strands can be cancelled by a balanced receiver at the end of each line.

      All the careful design work which focussed on the extraction of unwanted signals has paid off in silence, with input noise way down at -118dBu, and remarkably, mains hum levels (unmeasurably) even lower than that.

All Buss Lines In 

All Boards In  

6. Testing and Aligning  All Systems

      Nobody wants to get testy during testing, so a dummy front fascia is attached to ease the headache of wondering which knob is which. Afterwards, the dummy will be replaced by a pristine real one. Knobs and other scratchable externals also stay under wraps until the last possible moment.

       First up, the power buss is broken out into its seven separate rails, which are checked against expectation under various load conditions, and any Prodigal inconsistencies are corrected.

      When we are satisfied that the heart is beating correctly and the blood is pumping where it should, we move on to inspect the athletic and artistic properties of the Prodigal's audio circuits. There are tests for each side of the channel strip, more tests for the monitor section, and it takes about four and a half hours to run through the full checklist. Every input and output is examined, and a number of internal settings are tweaked for critical performance and optimum channel match.

      The main audio signal injected is a pure sine-wave, which facilitates reliable analyses of amplitude, bandwidth, headroom, equalisation and distortion. Besides inputs and outputs, readings are also taken at the L-R mix buss, and other internal nodes. Sexy and highly accurate test equipment is employed, to measure levels, baffle our friends and relatives, and make us look very clever. (Trivia sidenote: even though the idea has been workshopped by every technician in the last fifty years, not a single one of these machines dispenses coffee.)

      Eventually the form below is transcribed into a forest of ticks and scribbles, all the audio paths are tuned to perfection, and we are ready for final assembly.

Graphing Bandwidth

7. Final assembly

      First among the last things, we add an additional cooling fin to the power supply, to draw away as much heat as possible and direct it towards the rear vents. Then we put the lid on, and try to make the Prodigal hotter than you are ever likely to experience it! The unit is warmed up for thirty minutes with all LED switches on, audio clipped beyond saturation, and the headphone amp turned up loud but playing into a short-circuit failure condition. We take its temperature at specific external and internal nodes, and are yet to witness a complaint. As we said earlier, we didn't want a fan, because fans make noise, and the Prodigal is dead silent.

      Following that torsion ordeal, the unit receives the final standard appliance safety test, to double-check for insulation and earth continuity compliance.

      Hooray! Now we get to fit all the good-looking bits, and enjoy the finished product.

      Off comes the dummy fascia, and on go the potentiometer nuts. They've been kept aside till now so that any board requiring inspection can be easily removed. A shiny new fascia and two gold rack ears are unwrapped and fastened with hex-head machine screws. Now the unit is finally looking like a Prodigal.

      Knobs and switch-caps all get the cotton-glove treatment... and suddenly we are done!

      But wait, there's more. Before sending our baby out into the world, there's still an important box to tick: a live listening test with microphones, playback, headphones and nearfield monitors. This is our favourite test, partly because a love of listening to (loud) audio is why we started doing this for a living in the first place, and partly because after all that painstaking work it feels like we're just kicking back and sipping a coffee that took us a week to get to. mmmm. Stairway to arabica!

      When the neighbours start banging on our windows, the Prodigal gets a quick polish, goes into a plastic bag and foam protectors, and is put snugly to sleep in a double-corrugated cardboard carton for shipping off around the world.

      We are extremely proud of our Prodigal Channel Strip, which while still in its relative infancy has been used to create some outstanding recordings, and is waiting to help you with yours.

      I hope you have enjoyed this blog, and I thank you for reading it. If you would like to try a Prodigal, or have any comments or questions we would love to hear from you.

- Andy Szikla

Shiny New Fascia...

Bagged And Foamed, Ready For Boxing