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For beginners and tinkerers

Loudspeaker repair

October 18th, 2016

At the moment, I have two audio systems at home. About one of them, I have written in my previous post on building a vintage stereo system. The loudspeakers, which are used in that system, are two-way Mordaunt Short MS35Ti speakers. They sound really nice and produce very accurate sound. I was quite lucky when I bought them on eBay because the speakers were still physically in good shape.

So, there was no need to replacing the rubber surrounds etc. However, there are other electronic parts in the speakers which suffer from the ageing and your loudspeaker repair might be needed. Internal cabling might also need an upgrade. Let’s go though the steps.

Upgrading the capacitors in the crossover

Typically, non-polar capacitors are used in the crossover of every speaker. The circuit can be different depending which order filters were designed. Here is my redrawn circuit of the crossover in Mordaunt Short MS35Ti :

upgrading mordaunt short ms35ti crossover

As you can see, there are two capacitors 4uF and 7uF, which are used in the crossover. What happens with the age is that the capacitors either change the value or their ESR (Equivalent Series Resistance) increases.

As a result, the crossover frequencies get shifted and the produced sounds frequencies ranges are cut in wrong (shifted) ranges in first case. Thus, the loudspeakers are not reproducing the audio frequencies which should be reproduced by them.

In second case, an increase in ESR will be equivalent to the increase value of a resistor connected in series to the capacitor and the smaller amplitude signals will not reach the loudspeaker. Both phenomena are pretty bad for the sound.

loudspeakers upgrade mordaunt short ms35ti crossover

In order to upgrade the caps, you need to look for low ESR capacitors replacements e.g. polypropylene film (MKP) capacitors. Typically, such caps are designed/used in the switching mode power supplies. You could use ESR meter to check the ESR values of current capacitors and once you get new – the latest.

After testing, I needed to replace only one cap which is used with high frequency titanium speaker. In my case, the capacitance value was ok while the ESR value was ~0.35. The last one looked a bit too high. As a replacement, I bought the Arcotronics brand capacitors (but if you prefer you can go for higher  grade Solen or similar) from eBay and after a week or so they were laying on my desk 🙂

I measured new capacitors ESR and it was 0.15. That was “only” twice less compared to old capacitors. The new caps Typically, the voltage of the caps should not be less something like 50V.

Keep in mind, that higher voltage capacitors might be bulkier and that can bring some troubles to install in the place of old. The voltage and the price is also some kind trade-off as it might happen that higher voltage caps can be even less expensive e.g. on eBay 😉

Upgrading internal cabling

There are a lot of the discussions on the cables which should be used for interconnects and speakers. I will not go deeper into this topic and let you make this decision 😉 But let’s agree that it makes sense to have the wires inside the speakers thick enough to deliver required current to the speakers. If you think that internal wiring is already good enough, you can skip this step 🙂

loudspeakers upgrade mordaunt short ms35ti crossover

Was it worth of doing all this?

The short answer is yes! I do not know how they sounded as bran new but the speakers have sounded quite nice once they reach me.  However, after doing above steps, the speakers started to sound more detailed, brighter which I did not hear before. At the end, just few words on the rest of the system to which they were connected: the amp Marantz PM-66SEaudio preamp B1 and moddified VIA Envy24 audio card. Have some fun 😉