TechFindings

First I want to be clear, this post is about fixing a broken Marshall Stanmore speaker by turning it into an active loudspeaker. It is not about repairing it to its original functionality.

My Marshall Stanmore died after about two years of little use. One day it simply did not turn on. Completely dead. It seems to be a common fate of those loudspeakers and there seems to be no easy fix. I opened up the loudspeaker and quite quickly decided that I would not be able to repair it.

I felt very certain that the loudspeaker elements themselves were not broken. The loudspeaker looks and sounds quite good and it is against my nature to just throw such a thing away. So I started looking for ways to make a working active loudspeaker of it (allowing to use it with an iPhone or as a computer speaker). Since I thought this was a fun project I was willing to put some time and effort into it. But a brand new Marshall Stanmore is 200 Euros so the fix had to be significantly cheaper than that.

2.1
The Stanmore is a 2.1-loudspeaker. It has two tweeters and one woofer. The cutoff frequency is 2500Hz meaning that the tweeters are responsible for higher than 2500Hz frequencies and the woofer for the lower frequencies. There are different ways to properly produce 2.1 audio from a 2.0 signal. If I remember correctly the tweeters are rated at 2x20W and the woofer at 40W. I don’t know the impendance (Ohm).

The thing not to do
It is not a good idea to just simply connect L+R and connect it to the woofer. Regardless whether you do this before or after the amplifier you will drive current into components that are only supposed to produce a signal and this can destroy your equipment (your smartphone or computer pre-amp, or your amplifier).

Cutoff filters
There are special cutoff filters to split a signal into a lower and a higher part. I looked into this first, but it seemed a bit to advanced (expensive and complicated) for my project, and the problem with mixing L+R remains.

2.1 Amplifiers
There are 2.1 amplifiers to buy. The problem is that they are designed for use with a subwoofer (very low frequencies), not our 2500Hz woofer. This may or may not be a problem.

Mono
If I had a mono amplifier (that accepts stereo input and produce mono output) I could connect all the three loudspeakers to the same output. Since the distance between the tweeters is less than 25cm I don’t think the lack of stereo-tweeters will matter. However, it was not very easy to find suitable mono amplifiers (or “bridged amplifiers” that can be used as a mono amplifier).

Two-trick-solution
In the end I decided to go for a simple solution based on two parts.

First, pre-amp, it is very easy to convert stereo to mono. The only thing needed is two resistors (470 Ohm, or something close to that).

Second, a 2.0 amplifier can drive the tweeters on one channel and the woofer on the other (that is 40W on each channel).

Cleaning out the Stanmore
I removed (unscrewed) the back of my Stanmore. When I was done with it, the only thing that remained (and in place) was:

• The box itself (except the back of it).
• The three loudspeaker elements, and as long cables as possible.
  
• The top golden colored control panel (because removing it would not make anything pretty) and the board attached to it (because it was hard to remove).
• The cable (black+white+red) from the 3.5mm connection on top of the loudspeaker.
• The 4 red cables from the on/off-switch.

What I also needed
This is a list of other components I used

• A DC-power source (18V old laptop supply)
• Jtron Stereo 2 x 50W Digital Amplifier
• 2 x 470 Ohm resistors
  
• Cables and connectors

Assembly
I neatly connected everything in a way that it fits nicely inside the Stanmore.

• DC-power to two red cables connected to Stanmore power switch
• The other two red cable to Jtron board (make sure to no reverse!)
• One Jtron channel connected to yellow+black of woofer
• One Jtron channel connected to red/blue+black of tweeters
• Black of 3.5mm connector to Jtron input (middle)
• Red/white of 3.5mm connector connected via two 470 Ohm resistors
• From between the resistors, connect to Jtron input (left and right)

This is what I got:

As you can see the Jtron is pretty small.

For now my laptop DC supply is outside the Stanmore and there is just a little hole in the back for the cable.

Operating
The power switch on top is operational and I connect my audio source to the 3.5mm connection on top. The Jtron knobs work as expected (there is no balance).

About the Jtron
The Jtron was very good price and I thought 2x50W was kind of optimal for me. Also, it is a digital amplifier with high power efficiency (little excess heat). There are obviously many other options.

Serial vs Parallell
I connected my tweeters in parallell. I suppose they could have been used in series instead. Perhaps serial would have been more safe: impendence would be 4x higher, which would be less demanding on the Jtron.

Review
Well, I shall not review my own work. To be honest I have not fixed a new back plane yet and I think not having it in place is far from optimal for audio quality. Despite that, the Stanmore sounds very decent. It plays loud enough for me (perhaps louder than before). You probably want to experiment with bass/treble until satisfied. The way I use it (with an iPhone) I will set preset volume to loud, and mostly use the iPhone to control volume.

What I have lost compared to the original Stanmore is RCA-input, bluetooth and volume/treble/bass on top of the unit. I can live with it.