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Modular CamCorder Audio Mixer

A set of modules to build a CamCorder Audio Mixer, or other general purpose use.

I have a CamCorder. Not the latest flash-memory whizz-bang, but a venerable, though entirely reliable, Panasonic VDR-M55EB with CD-Ram. It’s great for out and about shooting, reliable and easy to use, and when I needed to rehearse the making of a demo of my Guitar Synth project, my thoughts naturally turned to it as a solution.

When you are pointing at seagulls cavorting across Cullercoats harbour, or attempting to catch a sneak profile of a friend, sound doesn’t matter, and I hadn’t realised until now how bad the sound capability of the camera is.

I’m not blaming the on-board microphones – I wouldn’t bother attempting to use these to capture half-decent audio, but when I made up a lead and plugged in two good quality super-cardiod mikes, I expected (a lot) better than I got.

The worst thing, I suppose is the in-built AGC, which allows for huge amounts of background noise on quiet passages, only to distort the audio badly on a loud sequence. The second-worst thing is the absolutely useless control over record volume – there is apparently none, despite this being intimated in the ‘Operating Instructions’.

I set up a test rig, using my existing mixers etc., so that I had control over both Mic signals going into the Camera, and monitoring the volume levels via the camera’s AV cable. Be aware this necessitates two bulky cables plugged into the front of the camera – see photo below. For a static set-up, this is fortunately not a problem, and the tests showed me that it was relatively easy to defeat the crazy AGC problem by feeding in the microphone signals at a reasonable level consistent with monitoring for good-quality audio on the camera’s output. By far the best feature was having a ‘knob’ to turn to control the record volume rather than pushing a fiddly, tiresome push-button.

CamCorder connections

CamCorder connections

I could have used my test lash-up as the final recording rig, but since this will be required over a long(ish) period of time, and the equipment is in use for other purposes almost daily, a self-contained unit seemed to be the solution, and I decided to more-or-less duplicate the units required and design a printed circuit to accommodate them.

Over the last two years, I’ve put together various audio systems, and the thought occurred to me that if I had made each sub-circuit modular, then putting together a ’special’ would be a relative ‘doddle’. Modular audio systems aren’t new, and modular mixers and their design abound on the Web. Most seem to be aimed at ‘professional’ audio recording, with relatively complex FX send/return and mix busses, and extensive tone control, none of which are required for this application. Further, such systems tend to be bulky, and I wanted something unobtrusive, that would sit amongst the plethora of kit I have decorating the Mic stand in front of me as I play. A bonus of keeping things ‘lite’, would be that I could also use batteries with the unit, making it portable.

I jotted down a rough schematic of the circuit blocks I was actually using in my test rig and arrived at the following list, which I’ve hot-linked (where relevent) to the pertinent section in this post:

In addition, you may want to add mains power to the unit to save on batteries. I made a 9 volt version of the unit discussed fully in: Versatile Dual Power Supply Printed Circuit Board on this site.

Note in the above, I could have used the word ’stereo’ in place of ‘2 X’, and in the implementation I do specify the concept of a ‘left’ and ‘right’ channel – but purely for conventions sake. This is demonstrated in the photos of the finished prototype unit below.

Cam Mixer Front View

Cam Mixer Front View

Cam Mixer Back View

Cam Mixer Back View

My minimalist modules would basically be small PCBs supported on the socketry where possible, so that building a bespoke unit would be simply a question of drilling suitable holes in a panel, mounting the unit and adding the wiring to intervening pots and the mixer. The more I thought about this idea, the more I liked it, so set to work drafting the prototype units. A bonus (or so I thought) to the use of small PCBs, would be that I could use up all the little ’scrap’ pieces of FR4 photoresist laminate, lying about in the bottom of my laminate hoard box.

A cautionary tale here – don’t do this. If you are going to hoard small pieces of PCB, then write down the date of purchase on each piece so that you have an idea of how old it is. I exposed, developed and etched four of the modules shown below all at the same time, and the resultant PCBs were all of different quality finish – from acceptable, to just barely usable with required remedial re-constructive surgery on some missing foils around the periphery of the board. (The mixer unit was the worst – see photos later) The problem was probably exacerbated by each of the pieces being of different batches – I won’t be doing this again! The Peak Indicator was produced last and separately, from a newish piece of board, and leaves the others standing, in terms of quality of finish. All five units are shown below, with the dual microphone unit being persuaded to sit upright (it’s front-heavy) by the judicious use of a small screwdriver as counterweight.

The Modules: Mixer, Mic Amplifier, Stereo Line Amplifier & Headphone Amp

The Modules: Mic Amplifier, Stereo Line Amplifier, Mixer & Headphone Amp

Peak Indicator Module

Peak Indicator Module


The Modules: Mic Amplifier, Stereo Line Amplifier, Mixer, Headphone Amp and Peak Indicator Unit
Each of the modules is designed so that it can be used autonomously, or together with one or more of the other modules. Provision is made for on-board pots, where appropriate, but connections are also brought out enabling the use of external controls. With the exception of the Microphone module which uses wire-ended 1% 1/4 watt metal-film resistors, all modules use SMD resistors. All of the op-amps are SOIC type TL072. (or equivalent)

No originality is claimed for the design of units, being based as they are on op-amp manufacturers Application briefs (Texas and National Semiconductors).
Remember! A mirror of the top foil is required as a negative for correct production of the PCB. For those unable for one reason or another to access the free student version of Eagle to print out these, I’ve provided 300 dpi exports of the foils, and you should flip these in a drawing package.

Using SMD components
This has been discussed in my posts before, so if you’ve never used SOIC op-amps and the like previously, you could take a look at my Line Mixer and Distribution Amp projects, plus there is plenty advice on the Web. Above all Don’t be afraid! No special tools are required, a fine chisel bit 15 watt soldering iron, fine pointed tweezers, steady hand and maybe a headband magnifier will see you through. Where possible, I’ve slapped a big ‘1′ beside where pin 1 of the SOIC packages is. When you’ve finished soldering, buzz out the connections with a testmeter and clear any shorts with solder braid.

The Mixer Module
Nothing really exciting here, and your final configuration and gain settings (via choice of input and feedback resistor values) will depend on your application. The arrangement of preset and fixed 180 ohm resistors to the CamCorder output allowed me to tap off exactly the correct ratio of the mixer output for correct recording level on the CamCorder.
I gave the Microphone input a boost by making the input resistors only 1K, and after construction and test, also changed the value of the direct line input resistor to 1K, because of a disappointingly low output available from an external voice synthesis unit.

Cam Mixer Module schematic

Cam Mixer Module schematic


Construction of the Mixer Module
I used SMD for the op-amps, fixed resistors and 100nF capacitors. All other components being conventional wire-ended. Everything SMD is soldered on the foil side, everything conventional ‘below’, including the pin headers and preset resistors. The presets I installed were 2K, set to final values of a little under 1K were used, to give correct sound levels to the CamCorder. Although the ‘Mix Out’ header is indicated as going to the Peak Indicator, on my unit I also wired it to the top end of a 10K pot, and made the output of the pot available on the back panel. (see suggestion for wiring modules – my hand-drawn opus, given later) After finishing the prototype I have decided that this unit is too ‘fat’. (see my problems shoe-horning this into position later) A re-design of the board will be made, and the PCB download updated when I’ve done this.
Cam Mixer Module Top Foil

Cam Mixer Module Top Foil


Cam Mixer Module Component Side

Cam Mixer Module Component View


The Dual Microphone Module
Again nothing exciting or outrageous here, just a simple differential amplifier with a gain of around 10. The gain chosen is easily altered by judicious change of the feedback resistors, but I preferred to provide more gain in the mixer unit itself.

Dual Microphone Preamplifier Module Schematic

Dual Microphone Preamplifier Module Schematic


Construction of the Dual Microphone Module
Note that in my implementation, I used external 10K log pots at the outputs of each module, and no exception was made here, so that the on-board presets can be omitted.
The only SMD components on this module are the 100nF capacitors and the TL072 op-amps – everything else is conventional, the main reason is I do not carry stocks of 1% tolerance SMD resistors. In reality, the size of the wire-ended resistors is not a problem, dwarfed as everything else is in the module by the two relatively huge Neutrik sockets. These are a superb combination socket which allow for plugging in an XLR plug, or a stereo jack plug, or a mono jack plug. The only drawback is their size, but despite this, I managed to fit both on board in line, between 1 inch centres. (with no space to spare between them.) The foil patterns I have drawn are based on measurements taken from the socket used, as I was unable to find an outline in the version 4.16 Eagle software I use. I suggest you mount these sockets 1st, as the rather springy tags need gentle persuasion to locate correctly.
Dual Microphone Preamplifier Top Foil

Dual Microphone Preamplifier Top Foil


Dual Microphone Preamplifier Components

Dual Microphone Preamplifier Components


The Headphone Output Module
This doesn’t need much comment either, the TL072 giving more than sufficient drive to a reasonably good set of ‘phones of around 26-36 ohms impedance. I’ve put in a 15 ohm resistor to protect your ears, and also brought out the headphone output separately via a 100 ohm resistor, as an auxilliary output. This circuit also carries extra decoupling capacitors of 100uF, but with shortish supply runs from the batteries/PSU, these are probably not really necessary.

Headphone Amp Module Schematic

Headphone Amp Module Schematic


Construction of the Headphone Output Module
Same comments as for the other modules here regarding placement of components. This module is another candidate for ’slimming’, and another draft of the PCB will be done when time permits.
Headphone Amp Module Top Foil

Headphone Amp Module Top Foil


Headphone Amp Module Components and Links

Headphone Amp Module Components and Links


The Stereo Line-In Module
I set the gain of this unit to around X6, to match the signal level coming from the CamCorder AV cable. Choose a higher value for R5 if you require more gain. The design allows for the use of an on-board preset to fix the maximum output. If you are going to use a front-panel ‘gain’ control, just leave out the preset(s).

Stereo Line-In Module Schematic

Stereo Line-In Module Schematic


Construction of The Stereo Line-In unit.
I used SMD for the op-amps, fixed resistors and 100nF capacitors. All other components being conventional wire-ended. Everything SMD is soldered on the foil side, everything conventional ‘below’, including the pin headers and preset resistors. The board can be configured to use a stereo jack socket, or two separate mono jack sockets, one on-board, the other off, connected by pin header J1. The link marked ‘L’ should be installed for stereo jack use, and removed for mono. For mono jack use, the connector which usually connects to the ‘ring’ on the jack plug should ideally be connected to the GND connector with a small wire link.

Stereo Line-In Module Top Foil

Stereo Line-In Module Top Foil


Stereo Line-In Module Components and Links

Stereo Line-In Module Components and Links


The Peak Indicator Module
This design of this module is largely based on information from National Semiconductor Application briefs. After I had completed the 1st draft of the schematic, I sat looking at it and started to laugh. All of this circuitry to drive a 20 pence LED? (£0.20 GBP) However, the finished result is small, neat and very effective, flashing as it does from green on an acceptable-level signal, to yellow approaching the danger zone, to red at a potential overload level.
The 1st part of the circuit is a precision rectifier – suggested originally for driving an LM3915. This elegant little circuit converts a small AC signal into a voltage level across C4. Gain is conveniently set as the ratio of resistors R5/R4, and in my version was X10.
The second part of the circuit combines two comparators in wired-OR to drive the green LED section, together with another comparator controlling the red LED section.
The green LED section is lit in the ‘window’ set between lower and upper value reference voltages chosen by the preset resistors R2 and R3, whilst R1 determines when the red LED section is lit.
I incorporated a simple voltage reference chain using 2 X 1N4148 silicon diodes, which gave a voltage of around 1.44 volts. If you require a higher reference, substitute a suitable zener diode for these. With a supply of 9-0-9, correct on-off operation of the LEDs was obtained using LED series resistors valued at 620 ohms. Be prepared to fiddle!
It is up to you to decide where and what point you wish monitored. I chose to monitor the peak voltage across the output of the mixer op-amp, which is a direct ratio of the (fixed) lowered output to the CamCorder. To clarify further – in my setup, the mixer main volume control does not affect the output to the camcorder or Peak Indicator.

Peak Indicator Module Schematic

Peak Indicator Module Schematic

Construction of The Peak Indicator unit.
This module has little or no ‘options’ in terms of leaving out components, although it is configurable for gain and signal magnitude. Once again I used SMD for the op-amps, comparators, fixed resistors and 100nF capacitors. All other components being conventional wire-ended. Everything SMD is soldered on the foil side, everything conventional ‘below’, including the pin headers and preset resistors. Although I have shown the LEDs installed on the PCB, in my build I installed 2 X 3-pin headers instead, with plugs and short wiring to the LEDs themselves, as there was insufficient space to mount the board sideways behind the front panel.

Peak Indicator Module Top Foil

Peak Indicator Module Top Foil


Peak Indicator Module Components and Links

Peak Indicator Module Components and Links


Connecting it all up
The heart of the project is of course the Mixer Module, whose main function is to gather together all inputs and pass these to the CamCorder at a suitable level. Because of the importance of the last (italicized) statement, the Peak Indicator unit and Headphone Amp, are actually an important part of the mixer itself.

My original test rig set-up allowed me to send (at a controlled level) to the CamCorder, and at the same time monitor what was being received by the CamCorder. This modality I have labelled ‘Cam’, and in my case I saw no reason to include a mixer final gain pot in this functionality.

My next requisite was a facility to re-mix the CamCorder audio together with anything I wanted to add, and send this to a separate audio recorder. I have called this functionality ‘Mix’.

It is important that at no setting of any switches or controls, that the CamCorder output is fed back via the mixer to the CamCorder itself, and so the third setup, which I have called ‘Dub’ assumes that the CamCorder output is disconnected, and the Stereo line input is in use by another signal input.

To give all this facility was beyond the capability of a simple, cheap 4-pole, 3-way switch, so I compromised, leaving ‘Dub’ to implement another day.
Although I have provided the facility on most modules for an on-board level-setting preset, I have not installed these in this unit, fitting external 10K log pots to the output of each unit instead, the slider of which goes to the relevent resistor on the mixer module.

You will probably want to connect up any unit different to me, but I provide below two suggestions, one using a simple switch (with limited facility) and also a more general, easily expanded solution using good-quality analog switches.

Apologies for the mixed technologies in the following drawings – If I ever find a drawing package that allows mixed-level abstractions of electronics circuits I’ll be a truly happy man. Until then I tend to prefer to rely on my freehand 50’s ‘Tech Drawing skills, because I find using ‘general’ PC drawing packages are so tedious. (Grateful thanks go to Mr. Gray for teaching me to do a half-decent block print.)

CamCorder Mixer Simple switched wiring

CamCorder Mixer Simple switched wiring

Suggestion for more refined set-up

Suggestion for more refined set-up


Power supply wiring suggestion

Power supply wiring suggestion

Full size 300dpi images of these scans are here: http://joebrown.org.uk/images/CamMix/CamMixDrawings.zip

Main Construction
Below: The modules from top-left in a clockwise direction as follows: Dual Microphone unit, Stereo line input from CamCorder, 2 mono-sockets for Aux line-in, Mixer module, c/w mix out stereo socket above, and (hidden) output to CamCorder, Heaphone Amplifier unit and Aux output socket. Just visible is the slide switch to select between the mains-power and battery supplies, and completely hidden behind the batteries is an Aux power-out socket. The 2 X PP3 9v batteries sit in a small box constructed from thin glued card, and a small piece of foam jammed next to the small mains transformer, holds the batteries in place. Next is the 9-0-9 power supply. This is a 9 volt version of the unit discussed fully in: Versatile Dual Power Supply Printed Circuit Board on this site. The mains cable to the terminals on the PSU is just visible between the RH inside of the box and the PSU. A small piece of card is glued in place (with hot-melt) over the PSU mains terminals as a precaution against someone inadvertently sticking a screwdriver where they shouldn’t.
From right to left on the front panel are the two power indicator LEDs, monitor selection switch, stereo headphone volume control and peak indicator LEDs, fixed in place with hot-melt glue.
In the centre of the box, a small Paxolin/SRBP/Tufnol panel acts as distribution board for the dual power-supply, and to the left of this is the Peak Indicator LED drive unit.
I had originally intended for the latter to be mounted in place behind the front panel, but decided there was enough clutter, and plenty space on the box floor.

Top view of completed wiring

Top view of completed wiring

Below: Despite careful measurement and planning, completely forgetting the position of the rear centre support post and screw boss necessitated feeding the mixer module a half-bottle of cheap vodka so as to persuade it into place, albeit at a less-than-flattering angle.

View of module wiring

View to rear of module wiring

The Rewards(?) for frugality.
I mentioned the variable quality (and I use this word advisedly) of the four PCBs I made from oldish scrap pieces of FR4 laminate. The CamMix board was by far the worst, and I show it below as a salutary reminder that ‘many a mickle, makes a muckle‘, and/or ‘look after the pennies, and the pounds will look after themselves‘, must be countered by remembering ‘you can’t make a silk purse out of a sow’s ear’!

The inevitable results of a canny, penny-pinching philosophy

The inevitable results of a canny, penny-pinching philosophy


Front & Rear Panels
These were done using FrontDesigner. The FPL files are included in the download section, together with high definition exports for those without access to FrontDesigner.
Suggested Front Panel

Suggested Front Panel


Suggested Rear Panel

Suggested Rear Panel

Component Sources

Transformer
Power Supply 9-0-9 volts. The transformer I used is in the range from ESR

Order Code Specification Price each (GB Pounds)
300-655 Encapsulated Transformer 0 – 9, 0 – 9V (2 x 278mA) 4.31

ESR can also supply all ’standard’ components and materials for making PCBs, but do not stock SMD components.

Sockets
The Neutrik XLR/Jack Sockets used are type NCJ9FI-H (XLR/switching (normalling) stereo jack) and are available from RapidOnline and RS Components to name two suppliers. A PDF datasheet of this item is here: http://joebrown.org.uk/images/CamMix/NeutrikXLRSock.pdf
The jack sockets are widely available.

Controls
I used the small 16mm ‘commercial’ grade pots with splined 6mm shafts cut to size. These are reasonable in quality, and most importantly, small in size and readily available. The knobs to fit them were obtained from Technobots, who I am pleased to recommend for price, availability of a wide range of electronic and mechanical components, and who offer a timely and prompt service.

SMD Components
SMD resistors and capacitors are widely available as kits on ebay for a few pounds. (GBP) Both op-amps and comparators are available for a few pence each from RS components and Farnell, amongst others.

The Box
I used an ABS box obtained from ESR, type MB6 (Black ABS Case Order code 400-575) with the following dimensions: 220mm x 150mm x 64mm external. (8.67 inches X 5.9 inches X 2.5 inches)

Here are some free plugs for my suppliers:

ESR
Technobots
Rapid

Downloads
Front & Rear Panels.
The FrontDesigner Front panel project is here: http://joebrown.org.uk/images/CamMix/CamMixFrontPanel.FPL
The FrontDesigner Rear panel project is here: http://joebrown.org.uk/images/CamMix/CamMixRearPanel.FPL
A full-size export of front panel is here: http://joebrown.org.uk/images/CamMix/CamMixFrontPanel_FullSize.JPG
A full-size export of rear panel is here: http://joebrown.org.uk/images/CamMix/CamMixRearPanel_FullSize.JPG

Printed Circuit Boards
Eagle files (Schematics & Boards) for the 5 modules are zipped up here: http://joebrown.org.uk/images/CamMix/CamMixEagleFiles.zip
Full size PNG exports of the schematics and top foils for each of the modules are zipped here: http://joebrown.org.uk/images/CamMix/CamMixSchematicandFoilsExports.zip

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