Mar 11th, 2015
The Mod for the ART Pro MPA II is very simple. More shielding, rescaling the gain control for more correct range, noise suppression in power supply, and a few cap changes. It's not about serious problems of quality (ART unit design is better than $1000-$2000 units!!!) It's about some minor issues with attaining super spec. If curious a phone call can explain more details. (617)926-8020.
The Art MPA 2 is in fact a very serious pro design, better than $1000 and $2000 products which usually have very basic circuits in them but are built well and look beautiful. Changing the MPA tubes can actually lessen the intended quality of the performance of a preamp. Original tubes are in all reality more consistent, reliable and achieve a very specific calibrated tube harmonic enhancement. My modification allows user adjustment of tube harmonic with a calibrated "Cathode Bias" control, thus allowing a harmonic (slightly warm/soft or slightly present) forward sound. I mostly modify the MPA and pro channels because it's easier to do.
The problem is on MPA II there is limited space on face plate to add a control and to actually find one that fits in position and still and get into the circuitry with mostly add on wiring. But so many people have asked that, so I guess I will do this now The cost is $135 each channel (plus the return shipping costs to your area) previously it cost $200ea channel
Otherwise put the original correct tubes back in so the machine will run with the correct performance. I like vintage things, but modern tubes are superior in most parameters. While it is true certain types of tubes may in fact create some obvious differences in old or new designs, the problem is too much blind luck will never get the job done. I have 20 telefunken tubes here and can get only a few to match each other. But 18 of 20 Sovtec 12Ax7 will match so close. Remember now, we are speaking of getting a tube with specific circuit voltages, parts to set gain of the tube to carefully create a really nice 2nd harmonic and soft limiting effect. To be blunt, virtually none of the fancy names actually do this trick as well as my ART modifications. Also on this note, I personally dont consider paying the high cost D.A.C. modification are that valuable because the effect has very minimal performance improvements. Whereas a correct selection of microphone and preamp is truly noticeable. In my measurements a modified D.A.C may attain another 1 db of noise floor or an extra point 0.01 extra distortion, neither is audible whereas voicing a tube circuit to create .2 to 1% harmonic distortion of second harmonic is clearly audible and usable to slightly color a recording. Also a similar concern with expensive Chinese mic updates. I feel that only very basic tweaks are needed with such products. See my Chinese mic Blog.