Updated 14th December 2010
(A companion article detailing a 6-output Line Distribution Amplifier can be found here: http://www.joebrown.org.uk/wp/?p=5283 )
I wanted a small (physically) mixer to accept line signals that was both simple and low-cost, for use as both an input expander for PC Soundcard input and also for general use in recording. Looking around the web reveals a whole host of designs, most of which are either over-blown, badly-designed or are just rubbish. My spec was simple 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
Normally this sort of thing is best realised with a few well-chosen low-noise op-amps, and this is the method chosen here. However, such an approach is somewhat boring, so I decided to give myself a little more practise at assembling SMD circuitry and implemented the design using SOIC op-amps. I realise that this puts off some who would otherwise be interested in building the project, so I’ve included the Eagle project, (see bottom of post for download) and it would be a simple matter to substitute the SMD op-amps with their ‘normal’ PDIP cousins if required, as there is plenty of room on the PCB. The circuit also lends itself to expansion (or contraction!) if required.
Schematic. In the schematic above I used 2 X TL074 for the input stages and power-supply splitter, and an LM833 for the output stages. The latter is what I had to hand, and a suitable substitution would be the TL072 (rather than the TL062 given on the schematic). Gain of the input stages is set to around 6, so that the application of a 600mv signal will result in about 3.5v output with a a given pot set to maximum – about as much as is possible with the low supply voltage.
The supply itself is split by a spare op-amp IC3D to give a common GND that is exactly half of the supply voltage, which should applied +ve to pin 1 on SV2, -ve to pin 3 on SV2. If you already have a split supply, then break the connection from the output (pin 14) of IC3D to the junction of the two 100nF capacitors, and connect GND directly to pin 2 of SV2. The two 10K resistors R36, R37 could in that case be removed.
Construction. I’ve used a mix of ‘normal’ through-hole design and SMD for the components. The ‘top’ of the PCB has the SMD op-amps, resistors and 100nF capacitors, whilst the ‘bottom’ of the board has the electrolytics and other components including the level controls. Be careful when you print the photograph from Eagle that you ‘mirror’ the output for the top foil. Also be aware that as shown in the photo above, the controls are mounted below the PCB and run from left to right. (Input to output) I fitted a 5mm LED in series with a 560 ohm resistor across the power supply as an indicator. I also fitted a 1N4001 diode (not shown on schematic) in series with the +ve supply input, to protect against inadvertent polarity reversal.
(Update 14th December 2010. Polarity protection diode and LED now incorporated on board PCB (version V1.02 – see downloads section at end of post))
As you will see in the following, I am no expert in neat SMD assembly, although I did manage to get one or two of the 0603-sized resistors on the board at right-angles. However, it passes the first requirement – it works. The photos below were taken with my Agfa 2030 12Mp, no flash in closeup mode.
I left the choice of input and output socketry open. I was going to implement it with phono jacks on the PCB itself, but decided I wanted a 1/4 jack version as well, so didn’t bother.
Front and rear panel suggestions.
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.
LM833: Both RS Components and Farnell sell these. RS Components sell one priced at around £0.13 GBP each (in packets of 10)
TL074 and TL072: Again both suppliers above stock 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. Probably my biggest fault is ending up with too much solder on each joint – but I need more practice!
I do not sell components, but if I did, they would be Stella Artois – not Carlsberg!
Updated 21st December 2010
A different version of the front panel to accommodate full-size pots has been made as per photo below. The downloadable jpg file is here: http://joebrown.org.uk/LineMixer/Line Mixer_Bigger_Panel.JPG and you should scale this to suit. (I scaled mine to 130%) and the FrontDesigner project is here: http://joebrown.org.uk/LineMixer/Line Mixer_Bigger_Panel.FPL
Updated 14th December 2010
Version V1.02 of the schematic and PCB includes both polarity protection diode and LED etc on board, plus a few cosmetic changes. No changes are required to the front panel. The self-extracting EXE also contains both schematic and PCB for my Line Distribution Amp, which is discussed in a separate post: http://www.joebrown.org.uk/wp/?p=5283. The new schematic etc. is here: http://joebrown.org.uk/LineMixer/LineMixerV1.02andLineDistrib.EXE”
I’ve packed the Eagle project into a self-extracting EXE file (created with IExpress) here: http://joebrown.org.uk/LineMixer/LineMixerPCB.EXE
The front panels I used were designed using FrontDesigner, the project files are here: http://joebrown.org.uk/LineMixer/Line Mixer.FPL
and here: http://joebrown.org.uk/LineMixer/Line Mixer Back Panel.FPL