Gravis Ultrasound JP5 and JP7 mods and specs

This text is (c)1993 by IdMEDIA & Francois Dion

The JP7 connector is a CDROM audio in. It is line level (10KOhm, 1V is a typical signal) and is assigned like this (see diagram A below):

 A= left input (hot)
 B= gnd
 C= gnd
 D= right input (hot)
The connector to use is a 4 inline Molex millimetric. On the CDROM side, you'll have to look in the CDROM manual or technical addendums.

The JP5 block of jumpers (like JP6 etc...) is on the card for the 16 bit daughterboard. But what is it, you'll ask? Let's take a look:


 JP7  x x x x            /|\ top of the card
      A B C D             |

      1 2 3 4
 JP5 [x]x[x]x
     [x]x[x]x    as you can see 1 and 5 are connected and 3 and 7 re connected
      5 6 7 8

Now these jumpers are not for digital selections. An analog signal goes thru them. What signal you'll ask? Again, let's check out this with ascii art:


                        |preamp &|
                        Vfilter  V

 +------+          right   +\
 |      |  +-----+------||-| >-[]---x x--> to mixer
 | GF1  |--| DAC |         +/       5 1
 |      |  +-----+---|     +\
 +------+        left|--||-| >-[]---x x--> to mixer
                           +/       7 3
So you can clearly see that if you remove the jumpers, the DAC output will not be fed to the mixer. What is the incidence of that? Well there are several!

1- You can bypass the mixer and connect the output directly to tape or a good external amplifier or mixer and gain several dB in SNR! I would not be surprised if the SNR was very close to the specs of the Phillips DAC. In any case the GUS was already over 85dB so this modification can make it into pro gear specs.

2- You are not feeding back the output to the input anymore in cases like the multitrack recorder or gusdelay if you use an external mixer. So this definitely has great potential and will correct all problems related to simultaneously playing and recording as far as separating signals.

3- etc...

Just removing the jumpers however will totally cut any output from the GUS so you need to add an output jack. Since my first draft, i have found out a way to get better results with less work so i'll first give what i currently use, then the first method i had posted. I suggest reading both to fully understand the process before actually doing it. Also, using either method i'd suggest a ground loop isolator (at Radio shack they have one that looks like a small cylinder with female rca jacks on both end).

Method one

You need a 1/4 inch jack (or 2 rca jacks or whatever jacks you prefer). Then you will need 2 capacitors of 100nF (like the small yellow caps on your GUS). You will also need a piece of shielded twisted pair cable. Last, you may need a Molex 2x4 millimetric connector depending on how stock you want to keep your GUS.

      1 2 3 4                 |  |
 JP5  x x x x                 |  | 1/4 (or 1/8) inch stereo jack
      x x x x                 |  | A and B are left and right
      5 6 7 8                 |  | C is the shield
    _  _                      |  |
   | || |  RCA Jacks          |__|
   |_||_|  Connect both        /|| Use either a 1/4 inch or 1/8 inch jack
   || ||   C together         A B| or a pair of rca jacks.
   CA CB                         C
Now simply solder a 100nF capacitor between A and C and another one between B and C. Next, solder the shield of the cable to pin C of the jack and at the other end to pin 6 of JP5 (solder side - see method two) or to pin 6 of a Molex 2x4 block (or pin 2 of a 3 or 4 inline) if you prefer not soldering anything to the GUS. Then solder one wire to A on one side and pin 5 on the other or to pin 5 of the molex 2x4 block and the last wire to pin B on one side and pin 7 on the other or to pin 7 of the molex 2x4 block.

With the molex you'll end up with a cable like that:

 ____________S__________ __+-+Left
|__________|_L__________X__| |Shield
  JACK      \______________| |Right
            R               +-+Not connected

(not much more complicated than a CDROM cable as you can see)

So you just plug in the molex on the gus when you want to use the jack, else you can easily remove it and it still looks stock. With the daughterboard you have to solder on the solder side instead of using the molex cause the db uses jumper 5. Also, for the jack (or jacks) use one slot cover, drill a hole for the jack. You'll of course loose the use of this slot. Another solution is to use an unused serial port hole, or drill a hole in the casing.

Method two

Ok, so you have removed the jumpers on JP5. Now, what we have to do is to remove C85 and C86, the two big capacitors near the amplified out jack. This will: cut the output of the LM317 from getting to the amplified out jack and give us a place to hook up our line level signal from JP5. As a bonus, we will benefit from the two caps (from left signal to ground and right signal to ground) that are already there. How to remove the caps i hear you say? Well, just put the tip of the soldering iron on one leg of the capacitor (solder side) and push toward the second leg of the capacitor. Do the same with the second leg and repeat for the second capacitor.

We are almost there... now the most crucial step. You need a shielded twisted pair cable (microphone cable should do). Dont get cheap, we are only talking about 4 inches long, so get the best oxide free copper you can. Now, before cutting anything, note what are your soldering points.

component side:

                                +  x o   (where the capacitors were)
                                +  x o   (note that x is actually a square on
                                          the board. the o is a circle on the
        1 2 3 4                           board)
  JP5   x x x x
        x x x x
        5 6 7 8
What we want to do is to connect pin 5 of JP5 to o of C85. pin 7 will go to o of C86 and finally the shield to pin 6. I would suggest using the solder side because it is easier to get access to the various pins.
solder side:

  o x  C86                     x x x x  JP7
                               4 3 2 1
  o x  C85

                               4 3 2 1
                               x x x x  JP5
                               x x x x
                               8 7 6 5
So, cut the cable so it's a tad longer than the distance between JP5 and the pair of caps. Remove just enough shield to permit the three wires to reach their respective pins and solder them in place. In my case i didn't take any chance and did the same at the other extremity soldering the shield to the pin of the amplified out jack that's on the edge of the card so i wont have any bad shield problem.

That's it. Just plug your amplified out to whatever line level input and voila!

Francois Dion
    '  _   _   _
 CISM (_) (_)  _) FM       Montreal , Canada       Email: CISM@ERE.UMontreal.CA
      (_)  / . _)             10000 Watts          Telephone no: (514) 343-7511