Sterling Audio ST66/69 tube microphone mod-upgrade

The legacy ST66/69 are both variations of the same LDC; the former being a carryover from the retired Groove Tubes product catalog, and the latter being an updated, more original design commissioned for the Sterling brand.

The first time I heard Sterling mics, I felt there was potentially something really good there.  Even though the mic was overly bright and shrill and harsh, and lacked real bottom end; something about it came through that told me it could be sort of saved from itself.  Because aside from that, it sounded and felt solid; and didn't suffer many of the hallmarks of cheap asian mics that I consider to be dealbreakers.  The main problems can be summed up simply: too much top end, and not enough low end.    There is some engineering going on, but not proper voicing.  

 

The lack of bottom end mostly has to do with the poorly done Chinese output transformer used in these mics.  It just doesn't pass all of the available low end, and seems to have a higher output impedance than a a properly wound T8-style transformer should.  We worked with our partners to commission our own T8 part for the mic, that has the right amount of bottom end and also just a bit better articulation and less 'smear' that is sometimes associated with cheaper magnetics.  I'll definitely give it to Sterling; they find a part and they just go with it.  Every mic I've evaluated is based on the same kind of transformer, same kind of tube, same kind of capsule... All the better for us; it allowed us to make a small run of custom transformers that can service any of their microphones.  So we remove their transformer in place of our own, carefully re-wrapped and put back into the internal sheild cannister, with the wire leads kept as short as possible and twisted in pairs.  

 

The reason for the harsh, extended top end is purely laid at the feet of the Sterling's 'disc resonator' contraption, added to the termination lead of each diaphragm of the capsule.  We remove this nonsense with extreme prejudice.  A disc resonator is basically a tuning baffle, something used in ribbon microphones over the years, which reflects back a specific frequency range to create an artificial shelving boost.  In the case of ribbons, this is sometimes done to make up for a sharp decline in higher frequencies.  Royer did this to great effect on the R121.  However, even with ribbons, we know today how to address this more naturally, at least to some extent, by changing the ribbon's geometry, dimensions, thickness, etc.  but I digress...

 

In the case of LDC condenser capsules, literally no one in their right mind would tell you that they need an additional top end boost.  If anything, they already have too much; particularly so many asian capsules and their ubiquitous 10k bump.  Most audio circuits, particularly tube, endeavor in some fashion to tame the existing top end of LDC capsules.  Every Sterling microphone is based on a K67 type capsule, and K67's/87's traditionally already have a top end lift built into their design, called the pre-emphasis.  The classic U87 mates to this capsule so well because it's driver circuit contains the 'de-emphasis' components, which serves as the antidote to this and balances out the sound.  I don't know to what degree Sterling's circuitry employs the de-emphasis, if at all; but suffice it to say, these mics do not need the added brightness experiment.  If you really need more top end on a track, its better to add some selective EQ to just the track needed, rather than record a number of tracks with an overly bright microphone.  It's harder to remove excess top end from a recorded track without sounding dull and losing detail, and the cumulative effect of layering such tracks can hurt a mix (I know this from experience).  It can also cause a degree of acclimation to the project where you no longer are objectively aware that the tracks are too bright, as they're all competing for space in the busy top-end of a mix.  Put it this way: you will never see a high end professional mic made by a leading manufacturer that has a tuning baffle stuck on their LDC capsule.  They'd re-design the capsule if they had to, or find out why the circuit was losing top end.  Even beyond the acoustic issues, the addition of the disc resonator raises the capacitance of the K67-type capsule far beyond what is normal for that capsule type (by nearly an additional 50pF).  This probably altars the sound as much as the physical presence of the resonator; as this type of change to a capsule will typically rob low end and increase presence and sibilance.  

 

Luckily, however, we can remove the disc resonator and re-terminate the capsule with fresh (and far better) silver teflon wiring, very carefully, and the capsule will be no worse for wear.  In fact, it's real quality will be revealed.  This surgery also brings the capsule's capacitance reading back down to normal measurements for a K67.  One of the reasons we knew that Sterling mics were worth our attention to mod is because they actually 'DO' have good capsules.  All of their capsules are made by the elusive Frank He, a veteran of Chinese capsule manufacturing who was originally trained by the German engineers brought to China to train the first generation of mic manufacturers.  Though he's always done work for affordable brands of microphone such as Groove Tubes, M-Audio, Alesis, and others, he's always consistently done very good work; and in the right circuits (and with disc resonators removed), his capsules really do shine.  It's for this reason that we don't upgrade the capsule.  We perform the necessary surgeries to it; but we only replace it if it is damaged.  We do carry a small supply of Frank He K67's in that situation.

 

Because each of these mic PCBs are populated a bit differently, it's impossible to make statements as to semiconductor replacement... We've opened up ST51's with completely different (even in color) PCBs, and all 3 have a somewhat different PCB, populated differently.  I've opened up one that was all WIMA film caps and decent quality mica caps and electrolytics, and one that was all generic where I replaced a number of things.  Suffice it to say, we will do what is necessary to make it sound better.  If we see a WIMA film cap in the floating high-z section where there should be a polystyrene (and we did, on one!), then we will correct that.  If everything looks decent, we will not.  I will say that on at least every one I've opened, I did at least replace the cap in the high z section to a polystyrene.  this allows the mic to be more open and less 'dense' or rigid sounding, and is the appropriate part to use.  

 

With regards to the power supply for the ST66/69, we do a few tweaks to it that are mor