View Single Post
Old 20th February 2012
  #64
Here for the gear
 

having re-read the first post of this thread and reviewed the content of the mod, i wanted to make a few more comments and suggestions. this is not in any way meant to discourage folks from modding! just corrections and a little direction.

the repair work the author did was necessary because of the failure of the power transformer. there is no doubt he was correct about that. a new power trans was in order! but there are several assumptions after that which need to be clarified. lack of documentation and the "instruction manual" would have made this difficult, hence the "educated guesses".

the first is that the B+ and the anode voltage are not the same thing... they are not. the ad copy for the pre that states "200 volt supply", refers to the B+ mains, and not the anode voltage, which should be 85 - 95 volts. the current source for the differential stage is adjusted (it has a trim pot) to accomplish this. adjusting this for 200 volts at the plates of the 12AY7 is absolutely incorrect. that means the tubes are completely cut off (little or no current going through them at all). they will definitely distort with any signal. as i mentioned before, setting the anode voltage for 150 volts is also not good for the 12AU7s. bad idea.

the next is an assumption that preamps normally supply unlimited amounts of phantom supply... that is grossly untrue. until recently, most pres and even mixers guesstimated well under 10mA per mic. there weren't any mics that needed that much. that "standard" doesn't fit today because of the proliferation of inexpensive direct coupled condenser mics from china. these typically draw more than 10mA. plug 5 of them into a mackie mixer made before 2006 and see what happens. most people wouldn't own more than 2... so that is unlikely to happen. i do take responsibility for this pre not having enough current for those mics, though. it doesn't, and no amount of changing the the zener string will help... a larger trans/wall wart could help, however. as would a dedicated phantom supply. but where to squeeze it in?

if the blue LED extinguishes upon turning on the phantom, you can be sure the mic needs more than the B+ in the preamp can supply.

the next assumption is that the 1/4" jack is unbalanced. it is not. it is a TRS jack and is balanced out. the stage associated with it has a small amount of gain and was never intended to be used as the primary output. the second pot controls that. if the gain on the pre is high (say with the clipping indicator blinking), and the monitor is also turned up, it will of course distort like crazy. it states this clearly in the instructions, but these could be lacking in a used ebay purchase, or if you throw them out when you take the thing out of the box.

i thought this second output would be useful as a rough monitor send, for headphone checks... old school. this was my biggest mistake, as way more users of the pre assumed it was a main out. i would never do that myself, and i assumed no one else would either. i am old and old fashioned... if you plug an unbalanced cable into that jack, of course hum can be coupled through. one has disabled the common mode rejection of the preamp.

so that brings me back to the mod... one thing that can be done to maximize the current available to the phantom supply AND the buffer, without altering the power supply much, is to do as the author suggests and move the power LED to the heater rail. grounding the 12AY7 grids removes them from association with the phantom supply, and without great penalty for the performance (there is a reduction of headroom and small increase of distortion, but the H pad in front of the 12AY7 provides a solution for that). the main reason for doing this would be because you own, or intend to, inexpensive modern condenser mics.

i cannot attempt to discourage you enough away from installing large value resistors in place of the 33K cathode resistors for the 12AU7 buffer. if there was more current available in the B+ mains, i would recommend reducing them! 22K or even 18K would make more sense. MORE current is what you want. here is what i would suggest as a compliment to this mod:

move the LED as suggested, ground the 12AY7 grids as suggested, readjust the current source so that the 12AY7 plates sat at 85-95 VDC... change the cathode resistors on the 12AU7 to 18K (from 33K). remove the 2 pin DIN connector and replace with a 4 or 5 pin DIN connector. remove the zener string that supplies the phantom supply completely, and carry a wire from the 50 volt pad on the string out to the new power connector. include a ground connection from the connector to power ground on the PC board. make a new "wall wart" with a 1.2 amp 12VAC toroid and a 48V @ 20mA minimum AC to DC converter module (these are made to supply vacuum fluorescent displays), and a new cable with a 4 or 5 pin DIN to match. a small "bud box" with room for an IEC connector, switch and fuse as well as the new trans and converter... done. perhaps a midi cable and 2 DIN connectors could be used as a ready made power umbilical? one could double up (use 4 wires) for the 12VAC, and 1 + shield ground for the 48V phantom... if you want to go 5 pin DIN.

last and very importantly, only use the 1/4" TRS jack with a balanced cable, or not at all.

my 2 cents...