This unit makes an ideal companion for the ‘No-Frills 6-Input Stereo Line Mixer‘ presented in an earlier post.
The unit employs the same SOIC Op-amps (TL074 & TL072) and construction methods, and provided two, 3-input channels (left and right) with independant level controls, together with a stereo (dual) master level control.
The spec is similar to the mixer unit and is as follows:
- Flat frequency response 40-40,000Hz
- Low Noise
- Will run off a single polarity supply as low as 9 volts
- Wide tolerance of input levels
- Nominal Input Impedance ~1KΩ
- Low cost
The Schematic. Note that the dual op-amp should be TL072, and not TL082 as shown.
The methods of construction are very similar to that in the Line Mixer, so I won’t repeat those here. If you require guidance, please read the Line Mixer post.
For the first of these I built, I decided to try out an ultra-cheap sandwich tin as the container. As well as being cheap, the tin provides useful screening, but does necessitate extra work, as the plate is so thin, the walls are too flexible to support controls and (especially) input/output sockets, so both front and back panels were reinforced with thin sheet steel. Another possible drawback is that GND is commoned through the sockets I used to the case itself. This isn’t a problem provided the power-supply (DC) is completely isolated from GND (EARTH). The problem goes away if a split-supply is used. (see notes in Line Mixer article)
Since the cost saved over the Hammond box I used for the Line Mixer is only about £6.00 (GBP), I leave you to judge whether the cost saving or extra time/effort was worth it.
When producing the printed circuit board for the Line Mixer, I cleaned off the photo-resist once the board was etched, and tin-plated the copper foil, which makes it very easy to solder the SMD components. Since the photo-resist is supposedly dual-purpose and also acts like flux, on this board I didn’t bother cleaning it off. The result was disappointing, being harder to solder, and leaving a messy blue residue around each of the solder joints. Also the resist is easily scratched, exposing the bare copper below.(see photo below)
Note that I have added extra pads to the dual-pot front connections. I discovered that the units I had bought were displaced 0.1 inches further forward than the outline supplied in Eagle.
(Note that you should make a ‘mirror’ image of the top foil to use as a negative for the board.)
Finally, a view inside the unit – plenty of space for adding a power-supply or large battery pack!
SMD resistors and Capacitors: A good way of getting a ’starter’ kit of these, is to buy one on eBay. There are several sellers there, and packs of several thousands of resistors and capacitors comprising the full ranges are available for a few pounds including postage.
TL074 and TL072: Both RS Components and Farnell sell these common op-amps.
ESR, my friendly local electronics store, sell everything including PCB stock and chemicals, other than SMD components. They are reasonable in price, and there is no minimum order. They will also ship anywhere in the world. They can be reached here: http://www.esr.co.uk/
All pots are audio taper i.e. log taper.
All electrolytics are 16 volts. If you intend using a higher supply voltage, then these should be substituted for higher voltage items.
A full parts list can be obtained from the Eagle project. The student version of Eagle can be downloaded and installed free-of-charge.
You don’t need anything special here. SOIC op-amps are reasonably easy to solder. I use a headband magnifier, a good pair of fine-point tweezers and a 15 watt iron with a small chisel bit.
Eagle project Files.
The self-extracting EXE contains both schematic and PCB for the Line Distribution Amp, together with version V1.02 of the line mixer. The schematic etc. is here: http://joebrown.org.uk/LineMixer/LineMixerV1.02andLineDistrib.EXE
The FrontDesigner panels are here: http://joebrown.org.uk/LineDistrib/Line Distribution Amp.FPL and here: http://joebrown.org.uk/LineDistrib/Line Distribution Amp Rear Panel.FPL
and full-size JPG images which are scalable of both panels here: http://joebrown.org.uk/LineDistrib/LineDistributionAmpFSFP.JPG and here: http://joebrown.org.uk/LineDistrib/LineDistributionAmpFSRP.JPG