Upgrade MA Bronze 100

New crossovers for Monitor Audio Bronze 100

I was approached by a customer with a problem: the sound of his speakers Audio Monitor Bronze 100 seems “compressed”, vocals and live instruments sound indistinct.
So let’s understand what the problem is and how to fix it.
To begin with, a brief description of these speaker systems.

Monitor Audio Bronze 100 – classic shelf speakers, 2-band configuration with 8-inch mid-bass speaker and high-frequency speaker with aluminum dome diaphragm. The design of the cabinet is traditional: a rectangular MDF box, with one vertical stiffening rib and the rear placement of the bass reflex port. An interesting solution for mounting a low-frequency speaker: one long bolt on the rear wall of the cabinet passes through the entire cabinet and twists into the core of the magnetic system woofer, thus attracting it to the front of the cabinet.

Test listening.
On first impressions, the sound seems quite normal, large-scale, dynamic, tonal balance is generally correct, no complaints. But after some time of listening carefully to recordings of various music, the impression of a somewhat simplified, detached sound. Especially vocals and live instruments lack expressiveness. But these are subjective impressions, let’s look objectively at the measurements of the amplitude-frequency characteristics of these systems.

Measurements show a typical “saddle shape” frequency response. What does it mean? The graph shows a gradual decrease in midrange relative to the level of low and high frequencies, ie the shape of the frequency response resembles the shape of the saddle. In fact, this is the reason for the lack of expressive sound, because good midrange frequencies are the basis for expressive sound, and it, as measurements show, is understated by -4 dB. The non-uniformity of the frequency response is ± 3 dB
Other measured characteristics, transfer functions of filters and impedance:

The graph of the frequency characteristics of the filters clearly shows that thanks to the filters the speakers have the so-called “saddle frequency response”: the low-pass filter (green curve) is tuned in such a way that the level of medium frequencies starting from 200 Hz is reduced, and the filter circuit also provides a notch circuit tuned to 4.3 kHz. The high-pass filter also generates a rise at high frequencies relative to the midrange. The impedance graph shows a minimum resistance of 4.5 ohms, the sound coils of individual speakers also have a resistance of 4 ohms, although the resistance of the speaker system is specified by the manufacturer 8 ohms, I think this is a mistake.

After analyzing the configuration of Bronze 100 filters, I note that the engineers of Monitor Audio have made a very competent scheme that is optimal for these speakers. But engineers have also deliberately created a “saddle-shaped frequency response” for these speakers, carefully sinking the mid-frequency range to -4 dB. This may not seem like much compared to other speakers, such as B&W 683 S2 (read about tuning crossovers of these speakers at the link).

New crossovers for Monitor Audio Bronze 100
The schematic diagram of the new crossovers repeats the basic one with minor changes. But by carefully tuning the new filters, I reduced the unevenness of the frequency response, applied other denominations of parts and installed better filter components: Vajd Audio inductors, Monacor capacitors.

Measured characteristics of Monitor Audio Bronze 100 speakers with new crossovers:

With the new crossovers, the unevenness of the frequency response has been improved to ± 2 dB and, most importantly, the midrange frequencies now have no decline in relatively low and high frequencies.

Sound after tuning crossovers.
Vocals and live instruments have returned to the forefront, now they sound full and expressive.

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