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"Why do we need to send in the 7 pin cables?"


I get often asked why someone needs to send in the 7 conductor tube mic cable with their Sterling or Warm Audio tube microphone upgrade... I figured I will answer this here, so I don't have to answer it as much individually.


The short answer is, that we re-work these cables. On every model, there is something I do called 'retensioning' which strengthens the connection to ground between the ground lug (the virutual '8'th pin') and the outer barrel of the XLR on both ends, as well as ensuring the jumper is in place between the shield pin (ground lug) and the actual pin which represents the ground/drain wire on the 7 pin header of the XLR. I also will re-terminate or adjust the cable if the jacketing is stripped back too far so that the teeth of the XLR connector's bushing are 'biting into' the shield and conductors of the cable (very common manufacturing issue on the Warm mics).


I also do a modification to each of these. On the Sterling mics, there is one un-used pin which should be tied to ground, and I add a jumper to each end of the cable to do this (tying pin 7 to ground lug on both ends). I do the same on both the microphone's 7 pin XLR connector and the power supply's 7 pin connector, thus just ensuring better ground contact, and less intermittent behavior.


On the Warm mics, the cable tweak is more extensive. The jumper between pins 7 and the ground lug is already present (as well as each having their own wire lead), and this is a good thing. What I disagree with is using the two heavier gauge leads to represent B+ voltage and capsule polarization voltage, respective. The 2 heavy gauge leads of a tube mic cable will do the most good, in terms of helping extend the life of the mic, tube, and power supply) being applied to the filament (heater) voltage (H+) and the heater return (H-), in whatever way that is handled. In the WA-47, the heater return is kept separate from signal ground and carried by chassis ground, which thankfully already has plenty of mass, being the spiral shield, allowing me to put the other heavy gauge lead on the circuit ground pin. Chassis ground and circuit ground wires should always be of heavier gauge. In the old Neumann tube designs, they often used every un-used pin and wire to bolster ground. In the WA-251, the heater return is carried by the circuit ground pin, which needs to be a heavy gauge lead just like the H+. The pin configurations for each mic are not the same, and thus if a customer sends in both the WA-47 and WA-251, I will label the cables accordingly because one will be configured for the 251 and one for the 47! While they will 'work' if swapped, they will not be optimal.


Why does this matter? Well, if you've ever done any electrical installation for business or residential, think back to why you would pick 10 gauge, 12 gauge, and 14 gauge installation wire. The gauge of the wire doesn't change based on voltage... it doesn't care whether you are doing 120 or 230v appliances... it changes due to current! The higher the amperage of the breaker and the appliance at the other end, the heavier gauge the wire needs to be. The principle is the same here. In a tube microphone, almost all of the current draw is to the tube heaters. that represents about 90 to 95% of the current draw of most tube microphones. The 5751 or similar variants used in the WA-47, for instance, have a heater current draw as high as 360mA on some tube models... yet the overall current draw of the mic is probably well under 400mA. Most of it is the heaters. Even the power supply designers in Asia know this because you can see they use thicker wires on that side of the PSU front end than on the high voltage side. So, while the B+ and capsule voltage rails coming out of the PSU may be 'high voltage', they are extremely low current, and simply don't benefit from a thicker wire. The heater voltage side, however, does. This wasn't something I was always as concerned about; but when we began implementing our own discrete regulator for the H+ section, and knowing that this regulator could deliver 1amp of current (way more than will ever be needed), it became more apparent to me that we needed to 'tighten up' other aspects of the system. Thus we now basically have to move 4 wires around on the cables, where as before we were merely inspecting the cable... The result is just an even more rock solid design.


This is just to show, that we are always striving to make these mods even better and getting them closer and closer to perfection.

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