KMTech Active Speaker Module Rev 1.03

This is the online user guide for the KMTech Active Speaker Module Rev 1.03

User Guide and Component List

IMPORTANT - ASSEMBLY / INSTALLATION BY QUALIFIED PERSONS ONLY.

RISK OF SHOCK FROM MAINS ELECTRICITY.

 

Thanks for buying this Active Speaker Module from KMTech Design. We hope it will give you many years of listening enjoyment!

Safety

SAFETY FIRST! Soldering can be quite hazardous so please make sure you are taking all reasonable precautions such as wearing PPE (Personal Protective Equipment), and making sure the area is well ventilated so as to avoid inhaling toxic fumes!

If you feel that you do not have the necessary experience or background to assemble this module, seek the help of a qualified engineer!

Keep children and pets away from your workplace!

Overview

This module is a circuit board-based device that is designed to fit inside a passive loudspeaker and thus transform it into a high-performance “active” speaker. The loudspeaker can be a three-way (bass unit, midrange and treble) design, or a two-way, with the bass output taken to an external passive subwoofer.

The module consists of three distinct stages:

1.      The Power Supply. The power supply consists of two AC to DC convertors, or rectifiers. The first rectifier takes a dual 6V supply from an external 6VA-10VA transformer, and converts it to a dual DC voltage suitable for powering the active crossover filter. The second rectifier takes a dual 35V supply from an external 160VA transformer, and converts it to a dual DC voltage suitable for supplying the power amps.

2.      The Active Crossover Filter. This device takes the line-level audio signal from either the balanced input or unbalanced input, splits it into three frequency bands which it then sends to the appropriate power amp sections. The filtering topology is dual cascaded Sallen-Key with a filter slope of 24dB/octave. Individual output levels can be adjusted by means of three trimmer-type potentiometers. Op amps can be NE5532 or OPA2134 (there are several other types that will work).

3.      The Power Amplifiers. There is provision for four 68W RMS LM3886 power amps on this device. One is for powering the treble driver, one for the midrange driver, and the remaining two for the bass driver. The two bass driver amps can be configured in parallel mode or a single amp can be used instead. Note that any passive crossover filters should be removed from the speaker before fitting this module.


The module is designed to be installed inside the speaker cabinet with the LM3886 ICs protruding outside the cabinet to ensure  adequate cooling. Heat sinks of an appropriate size must also be fitted to these ICs or their thermal shutdown protection will continuously activate. We also recommend the use of thermal compound.

Picture of fitted heatsinks:

 

THERE IS A SIGNIFICANT DC VOLTAGE ON THE LEGS OF THESE ICS SO KEEP THEM OUT OF THE REACH OF CHILDREN OR PETS.

 

Component list

WARNING! MAKE SURE YOU BUY LM3886 ICS FROM A REPUTABLE SUPPLIER. THERE ARE MANY SUBSTANDARD/FAKE ICS BEING SOLD MAINLY BY VENDORS FROM THE FAR EAST.  THESE FAKE ICS FAIL CLOSED CIRCUIT AND BURST INTO FLAMES!

Power Amps.

There are four power amp sections (you may only need to populate three of them). These are the components you will need for each power amp section:

·         1 x LM3886 IC.

·         3 x 100µF 50v electrolytic capacitor.

·         1 x 22µF 50v electrolytic capacitor.

·         1 x 1µF polyester film capacitor.

·         3 x 22kΩ 0.25W 1% metal film resistor.

·         2 x 1kΩ 0.25W 1% metal film resistor.

·         1 x 5mm terminal block.

Extra components required for bass section in parallel mode:

·         1 x 5mm terminal block.

·         2 x 0.15Ω 5W resistor.

 Active Crossover Filter.

·         5 x Audio-grade dual op-amps such as NE5532 or preferably OPA2134.

·         5 x DIP-8 IC sockets (optional).

·         1 x Neutrik NC3FAAH2 XLR socket (optional, can use unbalanced input instead).

·         1 x RCA phono socket (optional, can use balanced input instead).

·         5 x 50V 100nF ceramic capacitor (for power supply noise filtering).

·         2 x 47µF 50V electrolytic capacitors (for power supply noise filtering).

·         20 x Polyester film capacitors (for signal frequency filtering, nominal 10 x 33nF/10 x 3.3nF).

·         20 x 0.25W 1% metal film resistor (for signal frequency filtering, nominal value 11kΩ).

·         4 x 10kΩ 0.25W 1% metal film resistor (for balanced input differential-mode receiver, use 0.1% for higher CMRR).

·         1 x 1kΩ 0.25W 1% metal film resistor (for unbalanced input potential divider).

·         1 x 100kΩ 0.25W 1% metal film resistor (for unbalanced input potential divider).

·         3 x 6mm 100kΩ trimmer potentiometer.

Rectifier Sections.

·         8 x 2200µF 50V electrolytic capacitors.

·         8 x 10nF 50V ceramic capacitors.

·         4 x 100nF 100V ceramic capacitors.

·         2 x 5mm 3 way 24A terminal blocks.

·         4 x 1N5401 rectifier diodes.

·         4 x SR5100 rectifier diodes.

 

Power Supply

There are two rectifier modules on this board. The left-hand rectifier powers the crossover filter. The right-hand rectifier supplies the power amps.
For E1-E8 we recommend 2200uF 50V electrolytic capacitors.
C23, C24, C25, C26, C29, C30, C31, C32 are RF bypass capacitors. Use 10nF 50V ceramic capacitors.
C27, C28, C33 & C34 are also RF bypass caps. Use 100nF 100V ceramic capacitors.
Diodes: For D1, D2, D3 & D4 use 1N5401. For D5, D6, D7 & D8 use SB5100.
Transformers: For the Xover rectifier use dual 6V toroidal of about 6VA. For the power amp rectifier use a dual 35V 160VA toroidal.
 

Audio Signal Input

This board has the facility to accept audio signals on either a balanced or unbalanced line. For the unbalanced line there is a facility for a RCA phono plug. For a balanced line there is a facility for a Neutric NC3FAAH2 female XLR connector. Signals coming in through the XLR connector are processed with an on-board differential-mode receiver, which cancels induced noise that might be present on both the Hot and Cold conductors. You may wish to increase the CMRR and therefore the efficiency of the differential-mode receiver by using 0.1% precision resistors for Rcold1, Rcold2, Rhot1 and Rhot2.
Unbalanced or balanced modes are selected by shorting jumpers J3 or J4 respectively. Do not short both jumpers at the same time!


Crossover Frequency

This active speaker module contains a three-way 24dB/Octave active crossover filter in the poular Linkwitz-Riley configuration. The two crossover frequencies are set using a combination of resistors and capacitors.

The formula for the crossover frequency is f = 1 / (8.89 x C x R)
Where f = crossover frequency in Hz, C = capacitance in Farads and R = resistance in Ohms.
 
We recommend that you do not use ceramic capacitors for filtering, as they are susceptible to microphonic effect. Use polyester instead.
Filtering resistors should be metal film 1% 0.25 Watt.
 
If you look at the crossover filter, you will see it is divided into four sections. The upper two sections contain the filtering components for the upper crossover frequency. The lower two sections contain the filtering components for the lower crossover frequency.  If you use the default values printed on the board (3.3nF, 33nF, 11kΩ) you will get crossover frequencies of 310 Hz and 3100 Hz.
 
Op-amp types: Use dual-audio grade op-amps. If cost is an issue use NE5532. We recommend OPA2134 but there are other op-amps you can use with the same pinout configuration.
 
Choosing a Donor Loudspeaker.

There are two options for donor loudspeaker type.

  1. Connection to the three drivers inside a three-way loudspeaker.

  2. Connection to the two drivers in a two-way loudspeaker with the bass output connected to an external passive subwoofer. 

Make sure that any driver connected to this module can handle the power of the LM3886 IC (this applies especially in the case of a driver powered by the two bass amps in parallel).

Make sure the loudspeaker cabinet is physically large enough to accept the module and transformers.

Preparing the Donor Loudspeaker.

 Firstly, decide where you are going to mount the module. The most convenient place is at the back of the cabinet, with the amps and heatsinks protruding out of the back of the cabinet for effective cooling.

Many speakers have the speaker cable connectors attached to a backplate, which doubles as a mounting plate for the passive crossover unit. Taking out this plate leaves a hole which can be opened out enough for the heatsinks to fit through.

When mounting the board, use nylon spacers of around 3mm so that the board is not resting on the cabinet. Don't make them too long or there will be an air gap that might affect the acoustic properties of the cabinet.  

You will also need to consider where to mount the transformers. As they tend to be quite heavy, it is better to mount them near the bottom of the cabinet to keep the centre of gravity as low as possible.

Wiring up the Module.

After you have mounted the board, it's time to wire it up. First attach your transformers (don't plug them into the mains yet). You can use soldered-in terminal blocks, or wire the transformer outlets straight to the board.

Next wire in the speakers. The photo below shows where to wire each driver in the case of the 4-amp setup, with two amps driving the bass in parallel:

Make sure the impedance of your bass driver is appropriate for parallel connection.

The photo below shows connections for the three-amp setup, with one amp driving each speaker. The tweeter and mid-range connections are exactly the same as for the 4-amp setup.

Testing

Before testing the unit, take the following precautions.

  1. Wear eye protection in case one of the electrolytics is the wrong way round and turns into a "party popper".

  2.  Turn all three trimpots fully clockwise to their minimum position.

  3. Be in a position to instantly remove mains power if anything goes wrong.

  4. Leave the unit powered up and monitor it carefully for ten minutes or so before applying an audio signal.

We hope you enjoy using this active speaker module as much as we enjoyed creating it. Don't get wrapped up in the technicalities so much that you forget to enjoy the music!

 

Useful Websites.

Selecting heatsinks: Click here.

Decent soldering tutorial: Click here.

Loudspeaker Design: Click Here.

Our website (KMTech): Click here.