diy (ddddo iiiit yyyyourself) project by e. · pdf filediy (ddddo iiiit yyyyourself) project...

DIYDIYDIYDIY (DDDDo IIIIt YYYYourself) project

by E. Wijkamp


by E. Wijkamp


by E. Wijkamp


by E. Wijkamp

Introduction For amplifying several musical instruments like electronic drums, acoustic bass, clarinet saxophone, acoustic guitar, bass guitar, mandolin, violin and saz, I wanted to make use of a combo-speaker and a clip-on electret or dynamic low-impedance microphone. This BX1200 combo contains speaker and amplifier both. For my purpose It should be necessary to connect low-impedance (200 Ohm) dynamic microphone in a normal way (with the use of a jack 6,3 mm connector) to this combo box and it should have power enough for several great performances. Connecting a 200 Ohm dynamic microphone to this amplifier with use of the normal front inputs isn’t possible, because of the fact that those 2 front (jack) connectors want to see high impedance signal on both active and passive inputs. Bass guitars can have high impedance pickup elements (ceramic, piëzo) and the input stage (T5) is specially designed for this purpose. This combo is tested with use of a impedance transforming unit and a Di-box, connected between microphone and combo. The sound is good but mismatching makes that the sound is not as loud and powerful as one can expect of a 120W unit. So this is my DIY project. Re-designing the front stage without any documentation isn’t easy (and I’m not a Behringer designer too…) so I designed my own front-stage amplifier. With this modification it’s possible to inject the signal at a certain point in the normal front-stage of this amplifier (front board). It is a simple project but the quality of sound is nice and the set will be useable for many different jobs. Combo? At < www.Thomann.de > I’ve found a BX1200 bass-combo (price: 159,- euro in Jan. 2008) and that should be the victim! Building some kind of amplifier is not a problem for me, designing and constructing a cabinet is also a piece of cake, but for these costs it isn’t possible to design and build one from scratch. So I’ve bought 2 of these and decided to rework them for special use. The BX1200 amplifier is designed with 2 bridged output amplifier sections, the well known LM3886 modules. The same components are used in the famous ‘GainClone’ amplifier designs. Dismantling these combo’s means ‘no guarantee’, but for this price it’s no problem for me. The new designed pre-amp section This new pre-amp stage uses 5 volts, available at the front board. I’ve located the LED power (5 volt) and this is enough for this project (RED dot). It is buffered by a 1000uF capacitor, this makes that power comes up slowly and no other sounds are being heard when powering on. Muting When being used, it’s necessary to insert a dummy connector into one of these two front inputs. The muting circuit is then passed by and the signal is directly available. The connector for a low-impedance element becomes available at the backside (6,3 mm jack PREAMP OUT).


by E. Wijkamp


by E. Wijkamp

Modifying the BX1200 combo, step by step

01) 4 Screws on top and 2 screws on both sides should be removed.

02) The amplifier unit can be pulled out at the front side. If not, push the backside left/right.

03) Locate the ‘new’ input connector at the backside (upper rear). It’s named PREAMP OUT

04) Remove the 2k2 chip. Use a soldering iron and some WK chemwick-lite. A soldering iron and some soldering wire will do the job too.

05) On the upper left, above the flat-wire connector, the 2k2 SMD resistor is visible. This resistor should be removed (has a greyish colour as one can see on the picture)


by E. Wijkamp


by E. Wijkamp

06) And here… the SMD resistor is removed! (It now has a brownish colour because it’s removed and some brownish resin that is left over, is visible)

07) This is the simple amplifier that I have used. See last page.

08) Keep it simple, build it on matrix board

09) The wire on the right is for connecting the input of the circuit to the input connector (jack).

10) As seen on the upper side of the PCB

11) Now the wiring should be done. I’ve used a simple wire type, symmetrical-shielded, but other types of shielded wire can do the job too.

12) As seen sideways… How to connect the matrix board to the input board.


by E. Wijkamp


by E. Wijkamp

13) As seen from top, from the upper side with chip resistor removed. The long wire (now on the left) is connected to the input (jack).

14) Locate the front printed connection board

15) RED dot: The LED has on the left side a continuous +5 Volts supply and on the right it is switched on/off, so the left side should be used. WHITE dot: The input stage is injected with the ‘new’ signal on this point.

16) Connect the circuit: Power to the red, signal to the white. Glue the wire to the chassis with a single dot and… ready you are!

17) These two input jacks (on the front) switch the input signal to ‘mute’, when not connected. Use a dummy connector to switch off muting (as seen on the left side of this picture)


by E. Wijkamp


by E. Wijkamp

18) Now these 2 units are ready and it’s time to show that these are modified by me. How to do this? I’ve printed a nice sheet of paper and laminated it. On the left and right I’ve glued a (black) foam block and then I’ve shifted it on it’s place…

19) Then the amp should be mounted in the cabinet with the 8 screws and connected to the speaker with the 6,3 mm jack. That’s al!

20) For several purposes it’s necessary to add a tweeter to this combo. It should be added parallel to the speaker output and can be any tweeter (8 ohm). Connect a capacitor (4,7 uF 100V) in serial with the + connector of this speaker. Polarity: + to the amplifier + (tip of the jack connector).

The preamp that is used as a buffering stage for low impedance signal:

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