From cadsi.com!kent@elvis.cadsi.com Wed Nov 22 11:35:25 1995 22 Nov 95 14:34:36 +0500 30 Oct 95 12:42:23 +0500 for Movable-parts@smug.student.adelaide.edu.au id KAA06162; Mon, 30 Oct 1995 10:25:25 -0600 From: "Temporary Insanity" Date: Mon, 30 Oct 1995 10:25:25 -0600 X-Phones: (319) 626-6700 X-Snail-Mail: 2651 Crosspark Rd Coralville IA 52241 X-Fax: (319) 626-3488 X-Confusion: Can a filter sweep the noise floor? Subject: (MP) mixer plans CHEAP Submixer So you need more mixer inputs? Don't want to spend hundreds of dollars? I've built a couple of these little boxes and they work fine. You'll be out about 20$ for parts, at Radio Shack, including the case. They can run off batteries, or An AC supply of sorts can be constructed with 2 wall warts for an additional $15. This involves some soldering, but nothing too mentally taxing. Now the more technically inclined among you will think this is the electronic equivalent of baby talk; you should probably not bother with this. But I settled on this circuit after looking at all of the references I could find, and I've actually built and used a couple of these. They do work, and they're cheap. THE CIRCUIT 10K Input 1 o-/\/\/\-----+ 10K | 100K Audio Pot Input 2 o-/\/\/\-----+ +----+ 10K | | V Input 3 o-/\/\/\-----+ +-+-/\/\/\/--------------------+ 10K | | 1N40001 | Input 4 o-/\/\/\-----+ | +---|<----o V+ | | | | 10 microFarad Electrolitic | | |\ +---|(----o GND | | | | \ V+ | + | +----+---|- \---+ | | \ | |741 +---------------------+-------o Output 10K | / 1N40001 +-/\/\/\-------|+ /---+--->|---o V- | | / V- | 10 microFarad Electrolytic | |/ +---)|---o GND o + GND This is repeated twice, once for left and once for right. I've left out pin numbers; you'll have to get those from the chip packaging. You can do without the diodes and bypass capacitors on the power inputs but they do serve useful purposes. The diodes protect the Op Amps from being plugged into power backwards, and the capacitors smooth out the input voltage. WIRING INPUTS To wire up the inputs, get one of those set-of-4 stereo RCA jack females. 1. Run a single wire through all the shield connectors and connect that to signal ground. 2. Solder a 10K resistor to each RCA jack tip connection, and then solder the other ends of all the resistors together and run one wire to the circuit board. WIRING THE OUTPUTS I wired the outputs to one of those one pair of RCA jack female dealies. Again, wire the shield connectors together and run one wire to ground. Depending on how you're arranging your case, you can wire the tips directly to the potentiometer, or run a wire from the tips to the circuit board. WIRING THE CIRCUIT BOARD For the circuit board, I used a little general purpose board that's designed to hold a 20 pin DIP socket. Each pin of the socket has a pad coming out to 2 or 3 holes for you to tack in wires or other components. More soldering points can be had by cutting the foil between the socket pads and an otherwise unused set of holes. The Audio Pot can either be a knob for each channel, or one of Radio Shacks's nifty miniature stereo volume controls. As this circuit is wired, an increase in the resistance of the pot means an increase in the gain. You can theoretically connect a microphone to the inputs and use it as a microphone pre-amp. If you do this, you might consider lowering the input resistors to 1K ohms, to give yourself more gain. PACKAGING I used small cheapo plastic boxes. These are easy to cut holes in with a drill and an Exacto knife. If your leads are kept short (and you use nice thick wire for all your ground connections) you don't need to worry too much about shielding. A lot of circuts use a .1 microfarad capacitor before the resistor on inputs, but you get better frequency response without it. I've even seen circuits where the capacitor is switched in and out, depending on whether you want DC or AC. If you're worried about noise, then run shielded cable between each of the inputs and outputs to the main circuit board. This is less satisfactory with respect to soldering; the shield braid tends to break off if you flex the cable. The only durable solution is to solder the shielded cable into the circuit board so that it lays flat against the circuit board, and then superglue the cable insulation to the board. This keeps from straining the wire where it can break. POWER You can use two 9 volt batteries. In my experience, the circuit doesn't draw very much current when you're not driving sound through it. But as the batteries degrade, you have to turn up the volume, and you gain noise and lose headroom. To wire a bipolar supply from 9V batteries do this | | V+ +------||-----+----||-------+ V- | | | o GND You can build a nice regulated supply for the thing. I've suggested a cheap circuit below under 'SINGLE ENDED POWER.' Best results would be obtained if you set it up for +/- 15 volts. Most 741 class Op Amps can take up to 18 volt supply voltages, and the higher the supply voltage the better your signal to noise ratio. A cheaper (and apparently effective) way to go is to go to your local Target or K mart and buy two generic multivoltage wall warts These can be wired together just as the batteries are above, i.e. wire plus on one to minus on the other to get ground, and then take plus and minus off the free ends. Then use duct tape to tape their power leads together, and run them into your box. These are about $8 at Target, so this solution is about half as expensive (and much quicker) than building a decent regulated supply. If you've done as I have and soldered in 9 volt battery connectors, then you can simply plug the 9 volt connectors on the wall wart cord into your battery connectors. ENHANCEMENTS LEVEL CONTROLS on INPUTS If you want to have individual line controls, you increase the price of the project. I figured I'd use whatever volume control the input devices provide themselves and keep it cheap. The basic circuit involved is this: 10K +----/\/\/\-----o To Op amp input | V o-----/\/\/\---------o 10K Aud. | Pot _____ ___ _ SINGLE ENDED POWER There are both single-supply Op Amps, and single-supply amplifier circuits. The circuit above would change only in that there would be no negative power connection to the Op Amps, and you'd eliminate the diode and capacitor. You can also use a 'Virtual Ground' (suggested by Bruce Bowling): Vin+ o---------+---------------------o Vout+ | 47K +---/\/\/\--+ | +---------o Ground 47K | +---/\/\/\--+ | Vin- o---------+---------------------o Vout- If you start with 9 volts on the input side, then Vout will be Vin/2. This doesn't give you just tons of signal headroom; If you're assuming line level inputs at around +/- 1 volt, a 9 volt supply would start to distort at a gain of 4.5 (-4.5 actually since this circuit inverts). If you do this and use a cheap wall wart, get the highest voltage supply you can find. A good choice would be the 14 VDC 700 mA supply that Parts Express is selling. The catalog number is #120-380, and it costs $3.95. Their number is 1-800-338-0531. You will still probably want electrolytics in the 10 microFarad ranges between V+ and V- and ground for bypass. Barry Klein also suggested buffering the power with an Op Amp (i.e. a unity gain follower). If you're really ambitious, try building a regulated supply. See Andertons "Electronic Projects for Musicians." His circuit is actually more complicated than it needs to be, since you can now get voltage regulators at the voltage of your choice. +-------+ | 7809 | +--+---+--+IN OUT+--+---+-o 9V+ | | | | GND | | | | | | +---+---+ | | | - | | | - | ^ | | | ^ +----+ | | | + | | + | |XFMR+---|--+ CAP1 | CAP2 | | | | | | | | | | | | _ | | | | | | | ^ | | | | --| | | | | | | | 110 VAC AC | 24V+---|--|---+------+------+---+-o GND --| AC | | | | | | | | CT | | | | | | | | | - | | | | - | | ^ | | | | ^ | | | | | | | | | +---+ | | + | | + | +----+ | | CAP1 | CAP2 | - | | +---+---+ | | ^ | | | GND | | | | | | | 7909 | | | +--+---+--+IN OUT+--+---+-o 9V- | | +-------+ This schematic may be kind of hard to read, but here's the deal 7809/7909 Positive/Negative 9 volt voltage regulators. CAP1 -- 1000 microFarad Electrolytic CAP2 -- 10-40 microFarad Electrolytic or Tantalum - All the diodes (the ^ glyphs) are 1N40001 1 amp power diodes. You'll probably want a switch and a fuse in there as well! This sort of thing will cost you more than the mixer. DE-ENHANCEMENTS If you're plugging this into another mixer, then you've got a level control at your fingertips already. Replace your stereo 100K potentiometer with two fixed 90K Ohm resistors. You've just saved yourself 2 bucks. Questions and Suggestions to: Kent Williams kent@cadsi.com In No particular order, here are some of the feedback I've gotten on this circuit: --------------------------------------------------------------------------- Some comments on your mixer circuit: 1. You may want to have an inverter after your circuit to restore input to output phase relationship. 2. You may want a small (like30pf) cap in the feedback of the opamp to prevent oscillation. The capacitor would go on the pcb from the "-" input to output, close to the chip. The lines to the gain pot should be twisted to help eliminate oscillation too. 3. You may want a series resistor on the output of 1K or so. The 1K or whatever resistor protects the output from unexpected potentials when you plug it in. You can then ground the output jack when not in use. You won't get a "pop" when you plug it in to something if it starts out at ground potential. Not really necessary if all stays in a fixed location and hookup. 4. You may want AC coupling in the input resistors/pots. The AC coupling is necessary if you are summing audio signals. Control voltages no. If you don't do this you'll get an output offset that may cause hum or clipping of the opamps output with high level signals. 5. The 741 is out of date. Look for something with a higher slew rate. I don't know what RS sells. But Digikey/Mouser sell faster quiter op amps. All depends what you want to spend as far as what to pick. That's the basics for now.... >From KLEIN_B@A1.WDC.COM Fri Oct 14 11:06:28 1994 Barry --------------------------------------------------------------------------- 1) For my mixer, I used an LF356 JFET dual opamp. (Far better specs than the 741) 2) I used 100K summing resistors, with a feedback resistor roughly = to the amount of inputs * feedback resistor. (820K for 8 inputs left and the same for the right side). My positve OA input goes straight to ground. Question: I was a bit warry of hooking up 2 wall warts in a " +V GND -V " configuration. Has this been tried/tested over a long period? I was just worried about the posibility of doing damage to the wall wart that's acting as the negative supply. (A friend of mine that I built a smaller similar mixer for, wishes to do this). Good to see such an article posted! It may get the few who haven't wanted to start such a project going, after seeing how easy it is. (If there are any of us like that. Just guessing. :) ). Ric --------------------------------------------------------------------------- For dual op-amps I would recommend the NE5532 (also comes with other prefixes). Quad op-amps the RC or XR 4136. The main reason I recommend these is because they are designed with low noise in mind. Romeo -- ms20@u.washington.edu --------------------------------------------------------------------------- -- kent.williams@cadsi.com [Kent Williams/CADSI/2651 Crosspark Rd/Coralville IA 52241/(319)626-6700] 3 Reasons Why Not to use OS/2: Half of the prisons in the United States are run by OS/2. The federal judicial system in many states is run on OS/2. Nearly every computer in police cars across America run OS/2.