Warm Audio WA-CX12 tube microphone upgrade

The WA-CX12 is the latest tube offering from Warm Audio, and is an affordable replica of the classic AKG C12 microphone which originally had a green body and gold headbasket.  Their rendition is a somewhat accurate reproduction of the mic circuit and slightly less exact but still similar reproduction of the power supply.  

There are things I actually did like and some things I take issue with in regards to some of the design choices made on this mic.  I want to keep things simple and positive, and just go over what we do in the upgrade.

In the mic:

One of the things that called out the most loudly to be addressed was the use of a standard (automotive/electronics grade) WIMA film capacitor for the output coupling cap in the mic design.  I use WIMA caps for plenty of things; but this just isn't one of them.  I'm of the firm belief that this is the most sensitve location in terms of capacitor choice, and it needs to be a really good one, preferably tubular, and suitable for microphones.  General purpose capacitors will sound one-dimensional, dull and lifeless; and often the so-called 'high fidelity' caps will sound shrill, harsh, fatiguing, and lacking depth.  I would put this situation in the former category.  We will be performing the upgrade with a NOS vintage Sprague PIO capacitor that comes in a hermetically sealed metal can.  They are pre-tested and burned in to ensure quality.  When we run out of these from our private stash, we may move to a JB Audiophile series film capacitor, which is the closest modern thing to the capacitors of yesteryear; but while supplies last we will be pairing this with a vintage cap that is era-appropriate for the design.  The results are instantly audible... much smoother, richer, more 3D.

The tube will be checked and replaced as needed.  The unit we evaluated had an EH 12AY7 valve in it, with a pretty decent self noise measurement.  I don't know if they are selected tubes or not; but the noise profile compared favorably to my quietest replacement tube.  I have used new production EH tubes before, and find them quite decent (thanks EveAnna Manley for the hot tip!), so we will make a judgment call on this, on a per-mic bases.  In all cases, the tube is inspected, pins lubricated, and a tube damper anti-vibration ring added.

The transformer is wrapped in kapton tape to ensure consistently perfect operation under different temperatures.  we also add astrip of foam neoprene tape to either side rail to acoustically dampen the frame against the transformer package.  a dedicated ground wire is added to the mic chassis.  The 7 pin XLR is inspected and re-terminated if needed, pins sprayed, and a special adhesive added to the polystyrene caps to eliminate microphonics.  The capsule is inspected, cleaned if needed, terminals tightened if needed.  

The mic cable is reworked, as we now do standard to all WA mods.  We put the 2 heavy gauge leads where they will do the most good: on the H+ (filament) voltage and on the circuit ground/heater return.  These are where you have the most current, even if the lowest voltage, and this is where heavier gauge wire will grant some relief to the power supply.  We lift the jumper tying pin 7 to ground lug so as to not short out the provided BIAS voltage to the tube, and move the jumper to go between lug and pin 3, bridging the chassis and circuit grounds in the cable.

The power supply is EXTENSIVELY rebuilt and tidied up.  All electrolytic capacitors are replaced with the best available options.  Nichicon or Panasonic high temp/long life parts for most values, and the four 47uF caps are replaced with 105c/10,000hr rated parts that were custom made for Signal Art by SuperTech in Taiwan.  These are the same caps used on the handmade Signal Art U47 tube mic.  We add a dedicated ground wire to the PSU board for safety (instead of relying on a chassis ground connection which may be questionable), and add a calibration circuit to the B+ voltage section to allow us to safely raise the B+ voltage to near its ideal operating voltage of +120v (stock unit delivers a much lower B+ voltage, contributing to a possibly noisier, lower headroom performance).  

Everything is re-wired, tidied up, tightened, cleaned, lubricated where needed, and inspected.  Special adhesive is put on the electrolytic capacitors and even the regulator heatsink to keep anything from moving or vibrating.

The 6v DC regulator for heater voltage is replaced with our own custom made 6.3v Discrete Regulator made for us by Sparkos Laboratories right here in the USA.  These parts cost as much as 50x more than some off-the-shelf IC voltage regulators; but deliver the cleanest power possible and up to 1 amp of current, so every tube of any make will absolutely shine and the mic will overall perform at its best.  They are also fixed to deliver exactly 6.3v, the textbook requirement for heater filaments on these small signal tubes; which corrects the longstanding design limitatation of having to choose between 6 or 6.6v from a generic fixed regulator.  This particular update, which we have wanted to implement for many years, solves a very longstanding issue and greatly extends tube life and tube stability in these products.  ...and you get the side benefit of having a fully discrete power supply, once the single IC component is removed.

The result is a somewhat smoother, cleaner, and certainly longer lasting and more reliable mic that should hold up for decades with the proper care, making an already good mic something to truly be proud of in your studio.