Technics SB-MX100D upgrade

Technics SB-MX100D vintage speaker crossover upgrade

Technics SB-MX100D – large-format bookshelf speakers. 2-way design with a bass reflex port. In the basic factory version, they have a good sound with acceptable tonal balance, which is quite a rare phenomenon among vintage speakers from the 90s. The mid-bass driver cone is made of mica with an external lacquer coating, the tweeter dome is also made of mica with a titanium coating. The combination of rather exotic materials in the speaker design is typical of many speaker manufacturers of those times. What can’t be said about crossover elements, modern audiophiles are better off not looking at the crossovers of vintage speakers. Crossover elements in beautiful wrappers are a trend among manufacturers of modern speaker systems, which, in my opinion, is also not always justified.

What’s wrong with the sound, what’s missing? Subjectively, there is a lack of expressiveness in the midrange, particularly in the upper midrange. What is clearly visible on the frequency response measurements:

FR before upgrade

The frequency response smoothly drops from low to mid frequencies and then rises a couple of dB from mid to high. The overall frequency response unevenness is +/- 4 dB.

FR of drivers

Above are the measurements of the frequency response of the drivers separately for the midbass and separately for the tweeter without filters. Their frequency response is also difficult to call uniform. The mid-bass driver has a low-to-mid frequency slope with a small dip around 400…800 Hz and a huge peak around 4…6 kHz. The tweeter’s frequency response has smooth declines around the center frequency of 4 kHz and is more like the frequency response of the midrange driver.

FR after upgrade

By more carefully tuning the filters, I tried to overcome the aforementioned frequency response problems and, accordingly, the sound quality of these speakers. Formally, the unevenness of the speaker’s frequency response after the upgrade fits into the +/- 2 dB corridor, but a small drop of -2 dB in the lower midrange of 400…800 Hz still remains. On the one hand, subjectively it is not noticeable to the ear: a small positive slope of the frequency response from the lower to the upper midrange (+1…+2 dB) adds expressiveness to the sound and is perceived much better than the reverse slope of the frequency response. On the other hand, the additional complication of filters that minimize minor frequency response artifacts is not always justified. Optimization of the frequency response and its fine tuning in general usually give a positive result that is much more significant than eliminating small local irregularities in the frequency response. So the good initial sound of these speaker systems with standard filters became more transparent, neutral, and expressive after the upgrade.
Below are graphs of filter transfer functions, impedance, schematic and wiring diagrams of the new crossovers: