IntroductionThe line level inputs on My Soundblaster 16 crapped out for some reason, so I decided to go out and get an Ensoniq AudioPCI card. I got mine for $30 at CompUSA, but I'm sure you could get one of these babies for half that at a computer show. This isn't the absolute best card on the market and is probably nearing the end of its life cycle, but it seems like a good value and I've always wanted a reason to go out and get one.Note that the card described here is the Creative version of the Ensoniq AudioPCI, and not the original Ensoniq AudioPCI. The original is laid out differently, and doesn't have the speaker amplifer nor the jumpers to select it. I'm not sure, but the Creative version may be identical to the SoundBlaster 16 PCI. Certainly the specs look the same. This page describes some simple modifications to the card that make it more convenient to use for speaker testing.
Undocumented I/OInexplicably, there are several useful features not discussed in the documentation provided with the soundcard. For one, this card can drive speakers! The green output on the bracket of the card does double duty - unbeknownst to the unenlightened (and those that don't look too closely at the silk screening). By jumper selection, this output can be either a line level output or a speaker level output. My card came defaulted to the line level as most probably do. People tend use this output for self-powered speakers these days, so I guess the line level setting makes the most sense. Still, supplying powered speakers via a low-level speaker level output works just fine; you would think the default would be for speaker output. Regardless, they really ought to mention in the documentation that you have a choice here.See the yellow arrow in the figure below. If both jumpers are placed in the lower position, then you have line level output. If the jumpers are placed in the upper position, then you have speaker level output. The corresponding 1/8" output on the mounting bracket is colored light frog green here.
Figure 1. Yellow arrow indicates undocumented speaker/line out select jumpers. Blue arrow indicates Auxiliary input pins. Another thing that can come in handy is more mixable inputs. This card has one line level input on the mounting bracket of the card, and three line level inputs on the inside: TAD, CD, and AUX. See the blue arrow on the figure above for the location of the Auxiliary line level input which is quite handy to modify for the higher voltages encountered speaker testing. The line input on the mounting bracket can be modified in a similar manner. The connector immediately to the left of the internal Auxiliary input is the CD input, which you should go ahead and use for your CD drive. To the left of this is a line input labeled "TAD" (telephone answering device?) which didn't particularly interest me, but there it is.
Double-Duty OutputThe center two pins on the six pin header (pointed to by the yellow arrow in the above figure) go to the output on the mounting bracket of the card. The lower two pins come from the line level output buffer, and the upper two pins come from the speaker amplifier built into the card. Can you use both at the same time? Absolutely! I wanted easier access to the speaker level output, and really wanted the green output jack on the bracket to be line level so that I could plug my self-powered PC speakers in there and just forget about them.
Figure 2. Jumper I/O (left), jumper options (right). To do this, I set the jumpers in the down position (so the mounting bracket output = line level), and then made a cable for the speaker level outputs from an old CD drive audio cable. I removed the individual pin connectors from the plastic four pin connector (each connector has a little tab holding it in place; push it in with a little jewler's screwdriver while pulling on the wire and it should come out easily). Then I put heat shrink tubing over each of the individual red and white connectors and shrunk it with a standard issue disposable butane cigarette lighter. I also shrunk some tubing on the ground lead (smaller diameter around the braid, and larger stuff around the connector). This treatment keeps everything from shorting together when plugged in, and also acts as a strain relief. Finally, I shrunk some larger tubing around the point where all wires emerge from the gray jacket, which just acts as a strain relief at this point.
Figure 3. Connection of new speaker level output wire. Before you plug this cable into the soundcard, you have to ask yourself "where does the ground go?" Well, you can use a short piece of 1/2W resistor lead, or scrounge a similar pin from some other equipment, and then solder it somewhere on the board to provide a ground. Alternatively, you could make the ground wire longer and then plug it into one of the center two pins of the unused "TAD" input. I went ahead and soldered a scrounged pin into the large hole that attaches the upper end of the joystick DB15 connector to the PC board, and that seems to work pretty well. You can see where this pin is located in the figure below - here the black ground wire is plugged into it. Note that the red and white wires plugged into the top two pins of the six pin header are swapped in this photo - the red should be on the left and the white should be on the right.
Figure 4. Here is where to attach the new speaker output wires. The red and white should be reversed (photo is incorrect!). The other end of the speaker level cable should be stripped and tinned, and then soldered to the connector of your choice. I used gold plated phono jacks (Radio Shack 274-0852) on the front panel of my computer. Alternatively, you could put these jacks on the rear of your computer by mounting them on a blank mounting bracket. In the words of the late Bob Ross, "its your little world...".
Auxiliary Input SurgerySince there are a couple of extra line level inputs on this card, I thought it would be nice to adapt one in order to make it more suitable for speaker testing. This involves doing a bit of surgery on the card in the form of replacing four small resistors.The Auxiliary input has voltage divider attenuators at its inputs, but they are not being utilized as such. The series input resistors are zero ohms (a wire) and the resistors to ground are around 40K or 50K ohms (I didn't record their value before tossing them out). This gives no attenuation, but does keep the inputs from flapping in the breeze and injecting unwanted noise on the board. What attenuation would be best if we had our way? I picked -20dB, which makes a 1/10 divider. Low level speaker signals tend to be around 5V peak, and line level signals tend to be around 0.5V peak, so this should work fine. I also decided keep the input impedance around 30K, as on the line input (see below). Looking through my options as to available resistors, I found that 30.1K and 3.32K are common values in this 0603 1/16W package. The ones I obtained are 1% tolerance, which is nice but not essential. The evil, anal-retentive part of me would have preferred the values 27K and 3K since they would give an even 30K input impedance (instead of 33.42K here) but I couldn't get my hands on any easily. Not to worry, it's all pretty much the same to the electrons. Here is a before and after schematic of the Auxiliary input:
Figure 5. Schematic of the Auxiliary input attenuator, before (left) and after (right) surgery. These new resistor values give us an input voltage division of:
Which expressed as dB is:
The 30.1K 1/16 watt resistors should be able to handle:
Of which 1 / 0.9 will be across the entire divider, or:
As a reference point, 48 VRMS produces 288 Watts across an 8 ohm load. This would produce 6.8V peak at the actual soundcard circuitry, which may or may not be good for it. The upshot of these calculations is to show that the new input attenuator is able to withstand the higher voltages associated with speaker testing, and that this soundcard input will now easily tolerate some tens of volts without roasting. I don't think you would really want to connect this modified auxiliary input directly to the output terminals of a 288W amplifier - the A/D converter would clip at a lower level for one thing - but you can safely use this input directly for measuring lower level speaker signals on the order of some tens of watts into an 8 ohm load. My speaker testing jigs provide further attenuation and voltage limiting at this input, making high-wattage testing much safer.
Figure 6. Fuzzy (sorry) view of the Auxiliary input voltage divider resistors. The photo above shows the location of the Auxiliary input voltage divider resistors. These correspond to the resistors in the schematic given above for the Auxiliary port. You want to replace the outer resistors, which are zero ohms, with 30.1K resistors (labeled R46 and R52 on my board). The inner resistors should be replaced with 3.32K resistors (labeled R45 and R52). Replacing these resistors requires a small iron, some tweezers, some solder wicking braid, and a steady hand. Removing them can be tricky since you have to unsolder both ends of the device at the same time. To do this with a single iron, I use the following techique: add a blob of solder to one end of the resistor, and then quickly add some solder to the other end. Use the tip of the iron to push the part off the board. You may have to go back and forth heating one end and then the other before the resistor budges from the board. The solder blob increases the thermal mass at the joint, and therefore extends its cooling-off time constant. Once all four resistors are removed, wick up all of the old solder from the resistor pads with solder braid. To begin the installation process, add a tiny bit of solder to only one of the pads. Pick up the proper resistor with the tweezers (I like to use the spring loaded types that require pressure to release the part) and place it in position, holding it there with the tweezers. Heat the end which lies over the pre-soldered pad until the solder underneath melts. The part should now be secure and you can remove the tweezers. Apply a small amount of solder to the unsoldered end to finish that joint, and then go ahead and apply a bit more solder to the pre-soldered joint. Now install the other three resistors. Finish with some flux removal spray if you have it.
Line Input SurgeryIf you prefer to use the blue 1/8" stereo Line input jack for speaker testing instead of the internal Auxiliary input, it too can be modified to tolerate speaker level voltages in much the same way.Inspecting the card, I found that the line level input on the mounting bracket of the card has 1:2 voltage dividers on its inputs consisting of two 15k ohm resistors for each channel. Here is a schematic:
Figure 7. Schematic of the Line input attenuator before (left) and after (right) surgery. So obviously the plan here is to replace the 15k resistors with the 3.32k and 30.1K (or alternatively 3K and 27K) just as in the Auxiliary mod listed above. These resistors are pointed out by the arrows in the figure below. The ones on the left should be replaced with 30.1K (labeled R2 and R3 on my board) and those on the right with 3.32K (labeled R5 and R6). Use the same techniques listed in that section to remove the old and install the new resistors. Don't confuse the capacitors to the left (labeled C1 and C2) with the resistors that need replacing. If in doubt, check them with a DMM, you should be able to measure the 15K value easily.
Figure 8. View of the Line input voltage divider resistors. If I were making suggestions here, I would recommend that you modify either the Line in or the Auxiliary in, but not both. It's best to preserve a true line level input somewhere on your soundcard.
All Together NowLet's review the options for using this card to do speaker testing (pick one):
I don't like using 1/8" stereo jacks for speaker testing, particularly for speaker output levels driving low impedances, since they sometimes give an unreliable connection. So I picked the final option. The beauty of this option is that the connectors are low impedance and easily accessible, and I can use the line output on the back for my PC powered speakers, or I can run this output to an external amplifier and then to my jig for higher-powered speaker testing. The best of all possible worlds. I used a second CD drive audio cable I found laying to connect the front panel RCA jacks to the Auxiliary input of the soundcard. All four of the RCA jacks fit on a 3 1/2" drive bay panel. It's a bit rickety, but seems to work OK for now. The inputs are located on the left of this panel and the outputs are on the right. Use the color coding LEFT I/O = WHITE and RIGHT I/O = RED for the I/O jacks. Make sure you don't mount these jacks so close together that you can't plug in your average bulky RCA plug. Mine are staggered to avoid this. See figure 10 below for the jack color scheme / staggering format. Alternatively, you might find a hard drive sled or something that fits in a standard 5 1/4" drive bay that you can hack on the blank front panel of. You could mount the I/O jacks there, as well as a volume control for your CD drive, and other gizmos and whatnots. This would be a sturdier solution. I've since added a 1/8" stereo headphone jack to this panel (I play computer games now and then, and I'd rather not expose others to the cacophonous mayhem needlessly). I wired it in parallel with the speaker output RCA jacks, with the tip going to the left output, the middle ring to the right output, and the main barrel to ground. If you do this, remember to pull your headphones out when doing speaker testing, however, else they may confound your impedance measurements. If you like, use a nylon cable tie to fasten the cables to the upper right corner of the soundard (see the figures above and below) and a couple more ties to bind the cables neatly together.
Figure 9. The completed assembly.
Figure 10. The "Tank Commander" is ready for action!
FAQ SECTIONsome junk |
