First, I don't have a Mackie 1604VLZ (I do have some Mackie monitors that I rather like, though). Regarding the Mackie design, I would start looking at a few things:
0. The Mackie will almost certainly be built with SMT parts so you'll want a nice soldering iron to do this work and some practice first... Also, I'm just going to indicate where I'd start to look at. In other words, things that look a bit funny to me.
1. the RF filtering - L48, C315,316,110,79 seem to me like they are actually going to cut into the audio band. In theory a common-mode choke should not affect the audio but really it will. If you're using it on stage, this is probably ok. In the studio I would expect some wierd phasings happening up high which I think would result in a smeared sort of sound. Likely all the caps are ceramics which would be ok for a 10pF or 150 pF stabilization cap but nothing close to the audio band. Even a 150 pF is pushing things for me in a ceramic. Those are close to the audio band.
2. Capacitors C349, C350 (phantom blocking caps) - need some looking at. So does C94 (the output cap).
3. Current sources Q64, Q80, and Q63 - they are a bit wierd. I would look at ditching R483 and replacing it with another DL4148. This should lower the impedance of the current source base node... at the expense of thermal stability of the design. Better yet, look at replacing D80 and R483 with a standard red LED. It has a 1.8 volt drop but the same temperature coefficient as a transistor. You can get an SMT LED to fit where the DL4148 went, and a 0 ohm jumper where R483 went.
4. C383 and C384 seem wierd. They are very large for this circuit. This to me indicates a strange frequency/phase/CMRR response thing may happen. I would expect to not need them. Or maybe a stabilization cap of 150 pF or something like that. But not 0.01uF!!!
5. Q8 and Q24 - I haven't had any experience with the 2SA1084, so I don't know hot it compares to other low-noise transistors. I've had good luck with the BC860's (similar to the BC560's but a SMT version of it). Fortunately, SMT transistors generally have the same pinout so it would be easy to swap. I'm not sure if that would make much of a difference. Matching the gains of the two transistors would probably be helpful. That's a bit hard to do with SMT transistors, though.
6. Last, I would look at the op-amps. U17A and U25A look ok to me, U35A looks a bit strange in terms of capacitor values. I would expect C214 and C237 to need to be different to get a good response. To test this I would probably apply a common-mode square wave (to both pins 2 and 3) and check the response here with a 50 MHz or higher scope.
The datasheet indicates the NJM op-amp should be ok here, but I'd get a handful of different op-amps to try. You can get a lot of audio op-amps in SMT, it's just a pain to swap them. I think I'd wire up a socket temporarily and do my testing with DIP's. When I chose one then I'd solder in the SMT version and re-check it.
Comments?
