The need for a single mono balance monitor for use during mixing cannot by understated.
Whilst you can switch your signal to mono and use your nearfield pair, it is nowhere near the same as having a true mono single speaker right in front of you at the center postition.
This project accepts a stereo balanced input and contains a small power amplifier to drive the mono signal to the 5-inch 4 ohm driver.

This project uses a fairly common 5 inch 4 ohm driver and almost any small driver can be used. The enclosure is sealed, since we are mostly interested in the midrange frequencies for balancing. In the back section of the cabinet is an amplifier, power supply and a balanced stereo to mono summing module.


The enclosure consists of a front sealed enclosure for the driver of volume 3.9 litres.
A rear section is present to house the electronics and power supply.
It is built using 12mm MDF which was dowelled and glued together. 20x8 radiate pine edge supports were added for sealing and mounting purposes.
The back plate is made from 3mm aluminium sheet with a bracket to mount the modules on. It could be made with 2mm aluminium or 1mm steel if desired.


backplate topsection











Internal view of backplate assembly showing balanced input summing module and power amplifier

Enclosure Drawing (pdf)


Balanced Summing Module (BIM-2)
This module takes the balanced stereo pair as XLR inputs and uses the SSM2141 line receivers to crete a single unbalanced signal for each channel. These two signals are then electrically summed and buffered using an NE5532 device.
A trimmer allows adjustment of up to around 8dB of gain to compensate for the summing network losses. To calibrate this, apply the same signal to both inputs at 0dBu and calibrate the output of the summing module to 0dBu.
















 BIM-2 Module

BIM-2 Circuit (pdf)

BIM-2 PCB (pdf)

BIM-2 BOM (pdf)


Power Amplifier (PA-1)
This handy little power amplifier module uses the LM3886 device, whcih is capable of up to 68W into 4 ohms with an appropriate power supply.
This device can be powered from +/-10V up to +/-42V and in this case a +/-20V rail is used. This equates to approximately 30WRMS max into a 4 ohm speaker.
It is unlikely that it will be used beyond 1-2W RMS. Since the PSU rail is only +/-20V, it is recommended that a 4 ohm driver be used.

DSC01264 pa1















PA-1 Power Amplifier Module

PA-1 Circuit (pdf)

PA-1 PCB (pdf)

PA-1 BOM (pdf)


Power Supply (PSU-14)
This power supply module provides an unregulated +/-20VDC for use with the PA-1 power amplifier as well as a regulated +/-16VDC for use with the BIM-2.
The transformer input to this module is 15-0-15 (ie 30V CT). A toroid is recommended due to its relatively close proximity to signal modules and tight space.
I used a 50VA toroid that I had on hand, but anything that fits from 50VA upwards should do.
The module regulates the 20VAC rail using LM317/LM337 pair, fixed to a +/-16V output with appropriate resistors. The regulators will need insulation from the heatsink using the appropriate mounting and insulation kits.


DSC01267 psu14


















PSU-14 Power Supply Module

PSU-14 Circuit (pdf)

PSU-14 PCB (pdf)

PSU-14 BOM (pdf)


All of the modules and power components fit on the back plate, allowing it to be asembled and tested prior to fitting to the cabinet.
The back plate is also used as the heatsink for the power amplifier and the power supply voltage regulators. The voltage regulators are insulated from the back plate when mounting using insulation washers and thermal grease.
If you managed to get an insulated tab LM3886 then you don't need additional insulation for it.

The toroid is bolted to the back plate adjacent to the IEC connector and switch. The IEC connection contains the fuse as well. I use a 2A fuse to allow for the toroid inrush current.
When the mains wiring, transformer and power supply are connected, check the power supply outputs before wiring to the modules. Make sure that they are reading the correct values. After checking, hold a 1K 5W resistor across the +/-20V terminals for a minute to discharge the large capacitors.

Wire up the power supply to the BIM-2 module and check that it is operational by feeding a 0dBu sine wave into each input separately. Trim the output to 0dBu.

Wire the power supply to the PA-1 and the signal lead from teh BIM-2 to the PA-1. With no input signal, power up and check that there is 0V at the PA-1 output. Check that there is no DC at the output either.
Now the driver can be wired to the PA-1 and tested with signal.

Hopefully everything is operational now. There is very little that can go wrong, provided the correct values of components are used and inserted the correct way around.

backplate inside























speaker box























Speaker: Krix ML2115 5"
This is a relatively common general-purpose 5" 4 ohm driver available on eBay and other places. It has a cast aluminium chassis and quite a large magnet.
No specifications were available for this driver, so I performed measurements to derive the Theile-Small parameters.

Krix ML-2115
Diameter: 10.50 cm
Fsa: 45.20 Hz
Re: 3.60 ohms
Vas: 17.41 litres
Qms: 2.01
Qes: 0.30
Qts: 0.26