|Elliott Sound Products||Project 41|
Opamp Design and Test Board Rod Elliott (ESP)
Opamps are wonderful little building blocks, but quickly building a test circuit is a real pain. Most people will use a 'breadboard', one of those plastic blocks with holes and connections, but I have found them to be a complete pain in the backside. After a while the contacts expand, wires fall out, and you can't actually see what you are doing anyway. One small slip, and the opamp is consigned to the dustbin.
This project is what I nearly always use for a quick prototype, or for just mucking about with an idea. It is easy to make, and only needs a medium size sheet of un-etched printed circuit board (or possibly two). This can usually be obtained from electronics suppliers, and is not overly expensive.
With four opamps to experiment with, even quite complex circuits can be made. Needless to say, you can expand the idea so that six or eight opamps were available (or make two test boards).
For this project, two low-cost dual opamps are used. I suggest that nothing too exotic be used, because the really good, fast opamps will only oscillate because of the lead lengths. This board is for testing ideas, not final circuits, but is tremendously useful.
The layout of the top of the board is shown in Figure 1, and the opamp symbols are simply drawn on the top (non copper side) using a fine felt-tip pen. Mark out where all the holes go for the wire loops these are shown as heavy lines) that provide connection points to each opamp, earth, and the power supply rails. Make the holes the right size for the wire you are going to use so the loops don't fall out every time you solder something to them.
Figure 1 - The Board Top Layout
Now that you have a board with a bunch of holes in it, you need to separate the various pads that are used for everything other than earth connections. There is no need to be too particular about this, and rather than try to etch the printed circuit board, use a small section of hacksaw blade, and simply cut the copper away from the board to leave each of the terminals isolated. The remainder of the board serves an an earth (ground plane), and will help prevent noise pickup.
Although it looks like there are way too many supply connections provided, you will probably kick yourself later if you leave them out. At the very least, leave the ones up the middle of the board.
Figure 2 - The Hacksaw Blade Copper Cutter
To be a success, the hacksaw blade may need to be snapped off so you have teeth right at the end where they are needed. To use the cutter, place a metal straightedge along the line you wish to cut, and draw the cutter towards you using enough pressure to cut the copper. Note the orientation of the teeth in Figure 2. Use a blade with large teeth, as small ones will take far too long to do the job. If you get the right blade, most cuts in the copper should take only one or two passes. You can also use a handheld rotary engraver or similar if one is available.
Figure 3 - Copper Cutting Plan
Figure 3 shows how the copper should be cut - this is looking from the copper side of the board, so you should be able to make a series of straight line cuts as shown. The opamp layout is shown in light grey and the loops are again shown as heavy lines so you can relate to where everything goes. The separate +ve and -ve sections are simply joined together with insulated wire.
When you are done, make sure that you double check that each copper 'land' is isolated from the main board with a multimeter. A close visual check with a powerful magnifying glass will ensure that you don't have an accident waiting to happen. Install all the wire loops first - these should be about 5mm (1/4") high, and installed so they can't fall in or out of the board. Small kinks in the leads can help here.
The opamps should be mounted on their backs, and glued down to the board with insulation below the pins - these should be carefully bent out as shown in Figure 4. Do not bend the leads at the body of the opamp, as they may either break, or crack the case. When gluing the opamps down, use hot-melt glue, or something else that is not too permanent. The power supply can simply be 'sky hooked', with the larger components also glued down and acting as mounting points (filter caps and voltage pot for example).
It may be easier to use a small piece of Veroboard or similar to wire the supply, with the component side glued to the main panel and connections made to the tracks. This is up to the constructor, but it's easier than trying to cut all the required isolated lands into the copper, and is more secure than a fully sky-hooked assembly.
Figure 4 - Mounting The Opamps
When the opamps are mounted, and all the loops are in place, you can connect the opamp's pins to the pads. Run wires from each of the leads to the appropriate connection, and ensure that the lead is not soldered to the wire loops. If you do, when components are soldered to the loops your wires will come off as the solder melts.
Figure 5 - The Circuit Diagram Of The Test Board
As you can see from Figure 5, much of the circuit is as simple as you can get, but only because no external components are in place around the opamps. This happens when you wire up a test circuit. The large dots are connections to the loops on the board, and +ve and -ve supplies go to the opamps and the appropriate pads on the board. It does not matter if U1A is connected to the top left opamp pins on the board (none of them matter), but you must ensure that each opamp symbol is connected to a single opamp unit (in other words, don't mix up the opamp connections).
The power supply uses a 3-terminal adjustable regulator, and is set up to provide equal +ve and -ve supply voltages. The maximum is +/-15V, and this is more than sufficient for most testing. It's also the maximum recommended supply voltage for most opamps anyway. Power can be provided by a transformer, but I suggest that a standard 16V AC plug-pack type supply be used, not only for convenience, but safety. Also note that the DC input voltage to the regulator is right on the limits for the standard device. Most should be OK, but if you want to and can get one, use the LM317HV type, which has a 60V rating.
The rectifier is a full-wave voltage doubler, using 1N4004 diodes. The regulator should be provided with a small heatsink, but if you were to use the earth plane of the board, the remaining copper will be more than enough to keep the regulator cool. The regulator tab must be insulated from the copper with a mica washer or Sil-Pad. Make certain that there is no electrical connection before you wire the circuit.
|Please note that the regulator is not referenced directly to earth, but is 'floating'. The earth connection must be made only to the centre tap of R1 and R2 as shown. No other earth connection can be made anywhere, or the regulator may be damaged. Your power supply must also have a floating output, with no earth/ ground connection.|
The 1.2k resistor (R3) should make the regulator provide an output voltage that can be varied from about ±1.25V, up to just under ±15V. This is not overly critical, but if you want to be able to set exactly ±15V, you will need to experiment with the value of R3 a little. A lower value will cause the output voltage to increase. Any experimentation should be done before the opamps are connected to the supply voltage. The actual voltage will vary a little due to the tolerance of the LM317's internal reference, and also with the actual (as opposed to nominal) value of VR1.
When everything is wired up, make a back for the unit so it won't short out if you lay it down on something conductive. I shall leave this part to the constructor.
You wire the external components for the circuit you want to test directly to the loops on the board. Leads don't need to be cut, so components can be reused many times. Adding a couple of RCA connectors is also a good idea, or use 1/4" phone jacks if you want to make guitar effects. The input and output connectors allow you to use standard leads to connect the circuit into your audio system so you can hear what your brand new circuit sounds like.
Remember that this unit is for testing, and uses very ordinary opamps and long leads, so it will be noisier (and have poorer frequency response and distortion) than the final circuit using good quality opamps. Even so, I have used mine to make guitar reverb units, all sorts of filters and other circuits as well. Many of the circuits shown in my Projects Pages were tested using exactly this method of construction.
An assortment of alligator clip leads is also very useful, since you can use these to extend component leads that are too short, or join different parts of the circuit together rather than using bits of wire.
If you do not have any test equipment other than a multimeter (this is the absolute minimum), make sure that you check for nasty voltages (such as large DC offset or oscillation) before you connect to an amplifier. The idea of this unit is to allow you to test circuits, not blow up the stuff you already have!
When wiring a circuit, make sure that the power is off (although I have forgotten a few times, and mine still works). Unused opamps can safely be ignored - they will not be damaged by not having any connections to their inputs. Make sure that outputs are not shorted to anything - opamps will tolerate this, but will get quite warm.
|Copyright Notice. This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright © 1999. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro- mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Rod Elliott.|