GK800RB & Phantom power

[StkIt2DaMan]

Hello Folks,
I know that GK had a phantom power incompatibility issue with the DI o/p on the older 800RB's (c. 1984), however I've received no response to my requests to GK for any info or updates. Does anyone have ANY info that they can share on the subject ? I'm a TV repair tech (plasmas, lcd, etc ) and familiar w/ repair procedures, schematics, etc. If this is an easy mod., I'd prefer to do it myself . . . hate to send anything BTF if possible.
Thanks in advance for your help, Chris.

[Mark Black]

The problem with the older 800RB's is that they have no isolation from the 48V phantom power - it just tries to jam itself into the outputs of the opamp drivers.

Check out their engineering fix on a later model amp (this one happens to be a Backline 115). The main component values and topology of the DI section are very similar to the old 800RB, but with the addition of several components for DC blocking & protection.

Added components:

C45 & C47 - electrolytic - note polarity and the fact they are in series with the opamp outputs - I would use at least 63V if not 100V rather than the 50V specified in the schematic (darn bean-counters). These are the parts that will block the phantom 48VDC, and may actually be all that you need.

C44 & C46 - film type - possibly added for RF suppression or to cure oscillation problems - you may or may not need to add them.

D16, 17, 18, 19 - zeners to clamp any leakage to a value at or below the amplifier power supply rails & protect the output opamps - probably not a bad idea but maybe not essential.

Good luck and thanks for reminding me - I think I need to implement some version of that fix in my old 250ML as well.

[StkIt2DaMan]

WOW, All I did was run a few errands and you've already replied, thanks.

I had a feeling that a couple of isolation caps were needed (I'll use 10uF/100v-105 deg.) to block the +48v, but was unsure how far into the bowels of this thing I'd have to go. The .0047's wouldn't hurt either. Thanks again for the goods & feel free to post an 800RB schematic anytime . Chris.

[Mark Black]

Happy days - someone already posted one that will probably be correct for your amp:

http://music-electronics-forum.com/t13327/

[StkIt2DaMan]

Thank You, again Dr. Black.

Chris.