minor tweak : re-capping the Alesis Monitor One (MK I) studio monitor speakers.

These speakers have been a favourite of mine for ages. I got them over a decade ago second hand for $600. Given they are now at least 12 years old and probably more I decided to replace the capacitors in them. As stock they are KSC 50V non polar electrolytics.

First I removed the crossover and checked the capacitor values:

Now the values are 5uF, 16uF and 24uF.. these aren’t commonly available values so I made up close matches of these values by combining various capacitor values from the polyester and polycarbonate capacitors available from Rockby Electronics.

  • C1: 16uF – made up using 2 x 6.8 uF and 1 x 2.2 uF
  • C2: 24uF – made up using 2 x 12uF
  • C3; 5uF  – made up using 4.4uF  and 0.22 uF.. just to be obsessive as one measured a bit low I added an extra 0.1 uF so they matched more closely.

Here is the crossover circuit as mapped out by tracing the PCB.

  Left Speaker   Right Speaker  
  original value replaced with original value replaced with
C1 (16uF) 18.7 16.1 18.3 16.4
C2 (24uF) 25.7 24.1 26.8 24.7
C3 (5uF) 5.4 4.8 5.3 4.8

C2 especially has drifted up in value to be out of spec on each side, 17% and 14% off respectively.

The crossover looked a lot bulkier after the replacement:

Measurement: using a quick and dirty measurement setup I took some before and after ARTA impulse measirements, here they are plotted using Speaker Workshop. There does appear to be a measureable difference after replacement (right speaker measured).


(click to get higher res image)

Listening: I haven’t really had much time to listen to them in detail, on quick eval they feel less shrill but that could just be that I want them to sound less shrill. But at least with these capacitors the drift in value over time has been arrested.

I used a dab of hot glue to stick the capacitors in place so they dont vibrate, as they are now assemblages of multiple capacitors protuding from the PCB somewhat.

Also hot glued the corners of the terminal plate for a bit of strength, the plastic corners had become crumbly with age and had just fell off when I unscrewed them, though not looking as neat the hot glue will be an adequate replacement.

4 thoughts on “minor tweak : re-capping the Alesis Monitor One (MK I) studio monitor speakers.

  1. Update: interesting to compare the response measured further away in the “Cheapowave MK II” article on this site of these same speakers – obviously such a close up measurement of the speakers in a cluttered environment (speaker was surrounded by junk) had an effect with various reflections.

  2. Hello, quick question, I just became the owner of a pair m1 , I normally mix on Yamaha Ns-10s, which normally are really bright speakers, but this guys are just over the top I guess around 1k to 1.5 (your graph confirms), so… my questions is I wonder if the caps can be change to minimize that freq. I try TP over speaker (my regular solution for bright monitors), the crossover according to Alesis is 2.5 but I wonder if bringing that to 2k would help.. food for thought let me know if you have any ideas/solutions

  3. Hello- just checking the circuitboard on my Monitor Ones….on your circuit diagram you appear to have the values of R2 & R3 reversed. Am I correct?

    • You could be! I was just trying to trace from the crossover PCB so perhaps I made an error, I was only looking at replacing the capacitors and didnt go near the resistors.

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