176 lines
7.5 KiB
Plaintext
176 lines
7.5 KiB
Plaintext
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This text is (c)1993 by IdMEDIA & Francois Dion
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Gravis Ultrasound JP5 and JP7 mods and specs
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--------------------------------------------
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The JP7 connector is a CDROM audio in. It is line level (10KOhm, 1V is
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a typical signal) and is assigned like this (see diagram A below):
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A= left input (hot)
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B= gnd
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C= gnd
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D= right input (hot)
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The connector to use is a 4 inline Molex millimetric. On the CDROM side,
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you'll have to look in the CDROM manual or technical addendums.
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The JP5 block of jumpers (like JP6 etc...) is on the card for the 16 bit
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daughterboard. But what is it, you'll ask? Let's take a look:
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DIAGRAM A
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---------
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JP7 x x x x /|\ top of the card
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A B C D |
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1 2 3 4
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JP5 [x]x[x]x
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[x]x[x]x as you can see 1 and 5 are connected and 3 and 7 re connected
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5 6 7 8
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Now these jumpers are not for digital selections. An analog signal goes thru
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them. What signal you'll ask? Again, let's check out this with ascii art:
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DIAGRAM B
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---------
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|preamp &|
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Vfilter V
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+------+ right +\
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| | +-----+------||-| >-[]---x x--> to mixer
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| GF1 |--| DAC | +/ 5 1
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| | +-----+---| +\
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+------+ left|--||-| >-[]---x x--> to mixer
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+/ 7 3
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So you can clearly see that if you remove the jumpers, the DAC output will not
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be fed to the mixer. What is the incidence of that? Well there are several!
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1- You can bypass the mixer and connect the output directly to tape or a good
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external amplifier or mixer and gain several dB in SNR! I would not be
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surprised if the SNR was very close to the specs of the Phillips DAC. In any
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case the GUS was already over 85dB so this modification can make it into
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pro gear specs.
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2- You are not feeding back the output to the input anymore in cases like the
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multitrack recorder or gusdelay if you use an external mixer. So this
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definitely has great potential and will correct all problems related to
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simultaneously playing and recording as far as separating signals.
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3- etc...
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Just removing the jumpers however will totally cut any output from the GUS
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so you need to add an output jack. Since my first draft, i have found out a
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way to get better results with less work so i'll first give what i currently
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use, then the first method i had posted. I suggest reading both to fully
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understand the process before actually doing it. Also, using either method
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i'd suggest a ground loop isolator (at Radio shack they have one that looks
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like a small cylinder with female rca jacks on both end).
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Method one
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----------
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You need a 1/4 inch jack (or 2 rca jacks or whatever jacks you prefer). Then
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you will need 2 capacitors of 100nF (like the small yellow caps on your GUS).
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You will also need a piece of shielded twisted pair cable. Last, you may need
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a Molex 2x4 millimetric connector depending on how stock you want to keep your
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GUS.
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__
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1 2 3 4 | |
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JP5 x x x x | | 1/4 (or 1/8) inch stereo jack
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x x x x | | A and B are left and right
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5 6 7 8 | | C is the shield
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_ _ | |
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| || | RCA Jacks |__|
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|_||_| Connect both /|| Use either a 1/4 inch or 1/8 inch jack
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CA CB C
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Now simply solder a 100nF capacitor between A and C and another one between
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B and C. Next, solder the shield of the cable to pin C of the jack and at the
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other end to pin 6 of JP5 (solder side - see method two) or to pin 6 of a
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Molex 2x4 block (or pin 2 of a 3 or 4 inline) if you prefer not soldering
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anything to the GUS. Then solder one wire to A on one side and pin 5 on the
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other or to pin 5 of the molex 2x4 block and the last wire to pin B on one
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side and pin 7 on the other or to pin 7 of the molex 2x4 block.
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With the molex you'll end up with a cable like that:
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____________S__________ __+-+Left
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|__________|_L__________X__| |Shield
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JACK \______________| |Right
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R +-+Not connected
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(not much more complicated than a CDROM cable as you can see)
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So you just plug in the molex on the gus when you want to use the jack,
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else you can easily remove it and it still looks stock. With the daughterboard
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you have to solder on the solder side instead of using the molex cause the
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db uses jumper 5. Also, for the jack (or jacks) use one slot cover, drill a
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hole for the jack. You'll of course loose the use of this slot. Another
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solution is to use an unused serial port hole, or drill a hole in the casing.
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Method two
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----------
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Ok, so you have removed the jumpers on JP5. Now, what we have to do is to
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remove C85 and C86, the two big capacitors near the amplified out jack. This
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will: cut the output of the LM317 from getting to the amplified out jack and
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give us a place to hook up our line level signal from JP5. As a bonus, we will
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benefit from the two caps (from left signal to ground and right signal to
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ground) that are already there. How to remove the caps i hear you say? Well,
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just put the tip of the soldering iron on one leg of the capacitor (solder
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side) and push toward the second leg of the capacitor. Do the same with the
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second leg and repeat for the second capacitor.
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We are almost there... now the most crucial step. You need a shielded twisted
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pair cable (microphone cable should do). Dont get cheap, we are only talking
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about 4 inches long, so get the best oxide free copper you can. Now, before
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cutting anything, note what are your soldering points.
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component side:
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C86
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+ x o (where the capacitors were)
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C85
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+ x o (note that x is actually a square on
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the board. the o is a circle on the
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1 2 3 4 board)
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JP5 x x x x
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x x x x
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5 6 7 8
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What we want to do is to connect pin 5 of JP5 to o of C85. pin 7 will go to o
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of C86 and finally the shield to pin 6. I would suggest using the solder side
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because it is easier to get access to the various pins.
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solder side:
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o x C86 x x x x JP7
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4 3 2 1
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o x C85
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4 3 2 1
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x x x x JP5
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x x x x
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8 7 6 5
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So, cut the cable so it's a tad longer than the distance between JP5 and the
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pair of caps. Remove just enough shield to permit the three wires to reach their
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respective pins and solder them in place. In my case i didn't take any chance
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and did the same at the other extremity soldering the shield to the pin of the
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amplified out jack that's on the edge of the card so i wont have any bad shield
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problem.
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That's it. Just plug your amplified out to whatever line level input and voila!
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Ciao,
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--
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Francois Dion
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' _ _ _
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CISM (_) (_) _) FM Montreal , Canada Email: CISM@ERE.UMontreal.CA
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(_) / . _) 10000 Watts Telephone no: (514) 343-7511
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_______________________________________________________________________________
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