AXT Systems

Munro Egg150 Amplifier Mods
I've had a pair of these fantastic nearfield monitors in the studio since 2013. Constantly amazed at how smooth and detailed they sound.

First Problem:
Recently the AC mains fuse started blowing on switch-on.

If you don't know where to find the AC input fuse, it is in the IEC socket itself.

You need to remove the IEC lead and, using a flat bladed screwdriver, gently prise out the fuse chamber. You may see that the fuse has vaporised.

Since the line voltage here is quite high (closer to 250V than 230V) the likely culprit is inrush current to the 250VA toroid transformer.

None of the secondary fuses were blown on the internal fuse board, and the main board looks fine. I replaced the 2A fuse with a 3.15A fuse to allow for a little extra inrush headroom.

Powered up OK and checked the secondary voltages - all OK (close to 19-0-19 and 28-0-28 here with the higher line voltage).

With the amp reconnected to the inputs and speakers everything is working as it should again.
Second Problem:
The annoying switch-off thump.

Since the amplifier is well out of warranty, having it open on the bench enabled me to examine ways to sort this problem out.

I have a small module that provides a turn-on delay and instant turn-off when power is removed, which I've used in many other amplifier projects.

Fitting one of these appeared to be simple. AC detection comes directly from the 25.5V AC secondaries on the secondary fuse board and the 35V power from the main amplifier board.

Then the 4 relay contacts are used to interrupt the 4 speaker outputs (LF and HF for Left and Right).

These photos show the little module between the heatsink and rear panel, right next to the speaker outputs.

Works well, a 2 second turn-on delay allows power supplies and circuitry to stabilise befor attaching the loads and an instant turn-off means no more thumps.
Egg150 Amplifier
Amplifier internals showing module location at the rear
Egg150 Amplifier Mod
Close-up of the de-thump relay module