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3-Channel Radio Microphone Mixer

A while ago I put together a portable 3-channel mixer suitable for using 3 wireless receiver-connected microphones. I had posted photos of the mocked-up unit on my photo gallery for the intended user, then subsequently posted photos taken during development and construction. I also posted photos showing mods to a cheap (illegal in UK) Microphone Transmitter and Receiver bought cheaply on eBay.

In a totally different approach to my usual method of documenting small projects, I am presenting these photos alone, and referring any putative constructor to my articles on my No-Frills 6-Input Stereo Line Mixer article, which uses a hybrid SMD op-amp/conventional component approach, for guidance on construction. The Eagle schematic and PCB files are included at the foot of this post.

Feel free to email me with any enquiries and/or comments, either as a comment via the comment form, or direct email. I endeavour to reply to all relevant enquiries.

A link to the V2.00 schematic is here:

A window on the photo gallery is below. Either navigate to the site (here:, or use this window below, but in both cases expand view to full screen, as all photos are in high definition.

The RMM-Notes document carries the following text:(as well as the Dire Warning notice!)

RMM-1 Radio Mic Mixer & WR-601 Mic Transmitter/Receiver Notes
10th Jan 2011.

Always follow the instructions on the unit regarding switching power off, and removing lid:

Additionally, bear the following in mind when fixing the RMM-1 using screws:

The die-cast box is easily drilled, and a sharp drill will produce burr-free holes. However, die-cast aluminium is brittle (un-tempered) and reckless force with a centre-punch and hammer will easily crack the case. Similarly, do not over-tighten any screws, since as this will be used in a vibration-free environment, there is no need.

Setting up your WR-601(s) with the unit.
• Ensure power is switched off on receivers and the mixer.
Plug in the receiver(s).
Advance the channel volume controls CH1 – CH3 (as required) to ‘4′.
Switch on receivers.
Switch on transmitter(s).
Ensure each receiver ‘receiving’ LED is lit, indicating a signal from it’s relevant transmitter.
Advance the ‘Master’ volume control, turning the unit on. Ideally, this should be turned almost fully clockwise (position ‘8′), and individual channel volume controls retarded if feedback results.
DO NOT fully-advance individual channel controls, whilst keeping the master control at a ‘low’ setting – this will result in overloading the mixer when a strong signal is present.
• The unit has a relatively high gain – over 40db (X100), so with judicious use of the controls, signals from line-level (600mv) down to about 40mv are catered for.

A circuit diagram has been provided for reference and future maintenance. The unit uses very-low-cost SOIC op-amps which are widely available should replacement be necessary.

If for any reason the gain of the unit needs to be changed, R6 on the schematic (marked ‘G’) on the PCB sets the overall gain of the mixer stage, while R7, R8 and R9 from the wipers of the channel controls set the individual channel gain. Currently these resistors are all 1K in value and R6 is currently 22K, giving a gain of around 22. Since each input stage has a gain of about 6, the overall gain is a little over 120.

The gain of the mixer stage is the ratio of feedback resistor R6 (’G') over an input resistor (R7, R8, or R9), ie. 22/1. So, to change the overall gain of the mixer for example to 10, R6 can be made 10K. There is no need to use SMD resistors, an 1/8 watt component soldered carefully in place will suffice.

Since the total resistance of 2 resistors (a) and (b) in parallel is given by Ra X Rb divided by Ra + Rb, we could solder a 22K resistor across the existing SMD resistor resulting in a new resistance value of 11K.

e.g: (22 X 22)/(22+22) = 11.

This would halve the current gain of the mixer stage.


The Eagle schematics (V1.00 and V2.00) and board layouts (V1 only) are here:

An OpenOffice document containing (the above) instructions for use is here: Notes.odt

The panel images are here:
and the actual FrontDesigner projects are here: Mic Dire Warnings.FPL Mic Mixer Back.FPL Mic Mixer Front.FPL Mic Mixer Side.FPL

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