The GXL3000 has been on my short, short list for while. I've actually got one disassembled on my side R&D bench as a current project and am just about finished my work on it. So that mic will be my next modification offering (to be followed by the V69).
To recap the 3000: it uses a 34 mm K67-style capsule, has three patterns, LF attenuation and 10dB pad switches, 2SK170 FET, a bipolar stage and an output transformer.
The Problematic Issues -
The brightness designed into the K67 style capsule we know about.
Next, I'm not wild about the rats nest of wiring (stray capacitance losses) used to accomplish pattern, pad and filter switching.
To their credit, the capsule-to-FET capacitors are better than the ceramic types the vendors have used in the past.
The common 2SK170 FET, while quiet, does not have the best transient response available due to its high input capacitance (Ciss) (which is an inherent FET characteristic and essentially circuit independent).
The biggest issue (besides the bright K67-style capsule not being compensated with a HF de-emphasis circuit) in this mic is the second stage bipolar transistor - incorrect bias causes this stage to go into nasty-sounding asymmetrical hard clipping 7dB before the FET even begins to symmetrically soft clip. Instead of having a nice "tubey", soft clipping FET sound this mic gets harsh fast. And of course this is going to occur in the high frequencies first because of the rising HF output of the capsule. So it doesn't sound like what a nice FET mic should sound like when pushed hard.
The good news is I just investigated the frequency response, distortion and phase response of the output transformer. Looks like some money can be saved by keeping the stock transformer. Its phase response is excellent - less than 2 degrees input-to-output phase shift at 20Hz and 20kHz. It is down only 1 dB at 20Hz and goes flat out to 20kHz. Once I re-biased the bipolar transistor I got the FET to (soft) clip first, followed by the transformer (again soft, saturation clipping) then the bipolar transistor - a nice sounding gain staging arrangement. Remember, this is a FET mic with much lower voltage swings than a tube mic. The transformer is not the first amplitude limit in this mic's stock circuit - the bipolar transistor biasing is the cause of early onset harshness in the top end.
The Solutions -
A K47-style capsule installed in my signature single-layer headbasket is going to give better results. Every time I've replaced the brighter, stock K67-style capsule I've preferred the results. The GXL3000, like most inexpensive Chinese mics, uses a flat-response signal path from FET through output. In other words,
these mics do not have a HF de-emphasis circuit that Neumann intended to be used with the rising-response K67 capsule.
I'll definitely change the input caps to my Premium Electronics standard part and clean up the wire routing as much as possible while in that area.
I just love the transient response, detail and lack of "congestion" that comes with upgrading from the 2SK170 to a better device and re-biasing for best operating point. BTW - Jim Williams and I are almost on the same page here - he likes the J305 FET because of its fast performance and low capacitive loading. I'm not using the J305 but I agree with his hearing and tactics - a preference for faster transient response and a lower capacitive-load FET.
The bipolar transistor needs to be re-biased to increase headroom 7 dB and get rid of asymmetrical hard clipping.
That leaves getting rid of the polarized electrolytic capacitors in the signal path.
Pricing? I guess you'd have to look at this mod from the price perspective of the discontinued, transformer-coupled AKG 414 B-ULS, which is what I think you have in mind as a goal. Being that I'm more of a Neumann-sound guy than an AKG-sound guy I would go with a Peluso PK-47 capsule (list price $245) especially since this mic lacks a HF de-emphasis circuit. Then there's the headbasket, wiring and electronics mods. It remains to be seen whether or not there is a market for such a comprehensive suite of modifications to a $100 mic. One of the reasons the MK-219 and MK-319 mics are such great values is the capsule and transformer are wonderful-sounding and don't need to be upgraded.
So yeah, I'm deep into the GXL3000. I expect to come up with a mod suite that will be an excellent price / performance value and will add it to my site soon. But in the meantime I'd like to hear some feedback from folks as to where the price / performance sweet spot should be. I mean, for lot's of people the ideal price / performace spot is a $100 stock GXL3000! For others its a Brauner, FLEA, Lawson, Manley, Pearlman, Peluso, Telefunken, Wagner, Wunder (insert your favorite) mic. But maybe start another thread for that topic
