M250 - A 250W stereo power amplifier

This amplifier was designed in spring 2001. Click any image for a full view.

Some short points:

Electronic construction details


Note: This schematic shows the complete amplifier, with overload (short-circuit) protection. The DC protection circuit and output relay is shown below.)

Here are some short details about the components used.

Power supply:

DC protection and startup delay circuit (added to this page July 2003) :

This circuit is added between the amp and the speakon connectors shown in the main schematic. The +12V supply was made using a simple small pcb transformer.


Back view. Note the SpeakOn speaker terminals. Only RCA input jacks are mounted at the moment, but 1/4'' jacks or balanced XLR inputs will be added for compatibility with professional filters etc.

Under the hood. The PSU transformer and filter capacitors are mounted in the front of the box. The amplifier PCBs are mounted using strips of copper sheet that also carry the collector current to the output transistors. The output relay is mounted at the center, and the DC protection and power-on delay circuit is visible to the top left of the transformer.

Detail view. Two aluminium L-profiles (actually one 500mm profile, from ELFA, sawn in half) are bolted to the main heatsink (with a lot of silicon grease). These carry a total of 16 motorola transistors. In this figure the bottom profile is just visible (mounted upside-down with respect to the top one). Note that the bias transistor MJE340 (marked ** in the schematic) must be mounted on the L-profile together with the output transistors for thermal stability.

Mechanical construction details

This sketch shows how the output transistors were mounted on the L-shaped aluminium profile. The transistors were placed 12*2.54mm apart, and the copper strips shown in the figure carry the collector current from the PCB. Note the use of insulating washers and plastic spacers, necessary to isolate the transistors and copper strips from the heatsink. The copper strips were made from a 1mm thick sheet.

Sketch of the complete set of output transistors (for one channel) mounted on the heatsink arrangement, viewed from the top. The bent copper strips to the collectors form four supports to which the PCB is fastened. The positions of these supports coincide with tinned copper areas on the bottom side of the PCB. The emitter resistors are all connected to a central copper strip which is bent upwards and forms a fifth pad, which carries the output current back to the PCB. When screwing the copper pads to the pcb, spring washers must be used. Note how the bias transistor is placed. This is soldered directly to the bottom of the PCB. If the hole drilled in the L-profile for this transitor is M3 threaded, assembly becomes easier.

I did not make a PCB layout using a computer and there is thus no transfer-ready UV mask available. I may add this to the page at some point when time allows.


The amp runs smoothly at full volume with 7.5Ohm power resistors connected to the speaker terminals. When doubling the load for an impedance of 3.75 Ohm, the measured output power was 273W rms (driving one channel). Going to lower impedances than that, the short-circuit protection will kick in and limit the output. After all, the idea behind this amp was to make a design that is very rugged under normal conditions, not going after extremely low impedances. Using my homebrew speakers (15'' MGR bass driver + Zomax hi+midrange horn) the performance is great, with a firm 'kick' in the bass.

This amp saw some duty in May 2003, on the goth club 'Black Orchid' (for which I am co-organizer). Just before opening time, it turned out that the venue had not ordered the rental PA for the smaller of the two dancefloors as promised. We had to rush to get the M250 + speakers, (it's nice to have some resources of your own..) and this worked quite well. This was a small dancefloor in a room with around 80 people and a bar, and the M250 could deliver the music for a four-hour DJ set while getting only warm, rather than hot.

Additional comments

This amp is discussed in the DIYAudio forum (http://www.diyaudio.com/forums/showthread.php?s=&postid=51642#post51642).

(c) Marcus Gunnarsson 2001

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