As I write this, I'll be updating the language for our mod on the WA-CX12, and I'd like to take a moment to let folks know that we are now taking these in for upgrade and have a mod worked out for them that truly gives the mic the great sound it deserves.
I'll be honest about something, due to how busy we are here, sometimes a daunting project has to be put off until there is ample time to do it justice and that was surely the case with the WA-CX12. Although we've offered a mod for the mic available on our site since the beginning of the year; the truth is that about 4-5 of them have sat here in my shop waiting for the rainy day when I had time to truly dig deep into this design and try to overcome the debacles first observed on a cursory evaluation of it last year. the folks who sent theirs in early were aware of this, and were promised that it would be worth the wait. I believe it was, and the reports I'm getting back now verify that they are as satisfied as I was with the end results.
I knew that this would be a challenging one because there were some choices (or accidents?) made with this design, particularly with the power supply, which just really befuddled me. I couldn't understand why they chose to run the mic at such a low B+ voltage, and why they would set up the PSU to produce the required Bias voltage and the mic to recieve it, only to short that bias voltage to ground due to the generic cable configuration and essentially give it a bias voltage of 0v. The heater voltage issue is the same as with the others, and which we already correct via our custom made discrete voltage regulator courtesy of Sparkos Labs... But this mic was 0 for 3 in terms of delivering the right power rails. The end result, I felt, was a boxy sound that had a really high self-noise floor and low headroom/low output, and none of the clean warm smoothness I associate with the C12 sound. I worried that it was something that would just be unfixable due to the limitations of this design; but after considerable work, I'm very happy to say that it was all fixable and that we now have a VERY comprehensive, thorough mod for this mic that should absolutely knock your socks off, giving it that BIG smooth, clean, silky sound that a C12 should rightfully have.
Before we get into the technical details, I want to start off by saying that its not the purpose of this article to disparage any product or company, or to discourage anyone from picking up the mic, but only to inform and better serve our customers here. We can't do well unless these manufacturers we offer mods for also do well. I think the WA-CX12 has a lot going for it. It represents a good value, its presentation overall is probably the nicest and most impressive of any mic they've done thus far, it actually has a really good tube in it (selected EH 12AY7, which honestly would have been my choice for production as well, and thus we don't change it), and it has a really good large transformer in it by AMI (cleverly repurposing the same double-wound T14 used in the WA-47) which, while not historically exact to the larger variant T14's used in the 251 and C12, is plenty close enough and in some ways is perhaps cooler in the sense that its even bigger than the biggest of those. The board for the mic is laid out pretty cleanly without clutter, and it has a good quality cable, briefcase, shockmount, body, etc.
What I've realized about the contract manufacturer who makes most of these for the company (and for many other brands as well) is that they detest manual calibration, and have gone through a lot of hoops to provide a maintenance-free/interchangeable type of PSU system. I think this makes sense for mass production and for the semi-pro level. On most of the other PSU's (WA-47, 251, et al), they use a zener diode network on the high voltage side to provide essentially a fully regulated B+ supply, with the H+ supply already regulated by an actual IC fixed positive regulator (which we upgrade to a discrete regulator set to the precise tube heater voltage). Due to the unique historical properties of this kind of supply, it really needs a calibrated high voltage supply and should be set to or just under about 120v. What I've come to determine is that they are sort of 'fixing' the calibration on the output to about 75-77 volts (based on the 4 I measured) using a fixed resistor, which more or less ensures that even under the worst voltage conditions (say, 250vAC input set to 230v) it will not overload the mic or damage the capsule. But unfortunately, that desire to make the PSU foolproof and ubiquitous causes it to under-serve most of the mics under normal operating conditions. I was measuring about 75v for the B+ that should be 120 and about 32-34 volts at the capsule that should be close to 60. This results in what I considered a noisier and less dynamic sound. We remove the fixed resistor which clamps down the B+ output voltage and install a trim pot and attenuator circuit which allows us to dial in something just shy of 120v, to give it a bit of margin for safety. For instance, we will give it about 116v when presented with a 120v input, which gives it a safety margin of about 4volts, considering 124v is at the top range of what our domestic power here in the states will swing to. When we modify an overseas unit, we will likewise take consideration with what the voltage expectations are for that territory and calibrate accordingly.
The heater voltage is corrected, as we usually do, by installing a 6.3v fixed discrete regulator made specifically for us by Sparkos Labs in the USA.
The bias voltage needed for the WA-CX12 is restored by reconfiguring the Gotham 7 pin cable to prevent pin 7 (bias) from shorting to ground on the mic, the chassis, or in the cable itself. We place the 2 heavy gauge leads of the cable where they are most needed (heater voltage and heater return), and instead short pin 3 to ground lug, so that the cable becomes a star ground point to bridge the chassis and circuit grounds. I believe that is what was 'intended' but they may have not considered that it requires a different cable configuration than their modus operandi.
Aside from that, we do dampening mods for the tube, transformer, and polystyrenes to reduce microphony, we clean and re-seat the capsule, we install a ground wire to the mic chassis, we add a safety ground wire to the PSU board inside the PSU, and we replace all capacitors in the PSU as well as the extensive mod we do the PSU circuit. And much much more.
MORE UPDATES
Always striving to improve, we now add several RF suppressor ferrites to capsule terminations on the WA-47 after observing some very low level RF interference on some units. The RF suppressor ferrrites seem to do a good job of preventing any RFI/EMI from entering the high z circuitry.