Behringer B2030A / B2031A Defects and Modifications

[Artyaya]

Behringer B2030A / B2031A Defects and Modifications​

These Behringer speakers are essentially copies of the Genelec 1031A, but there are several mistakes in the circuit design. I’m opening this thread to discuss them with everyone. I’ll start with the obvious faults and talk about sound-quality improvements later.

Power-on thump​

The B2030A produces a loud “thump” at power-on. The LM3886TF amplifier IC already has a soft-start circuit, so why does the thump still occur?
The reason is a factory error: the charging resistor and the discharging resistor were swapped. Simply swap R4 (2 kΩ charging resistor) with R3 (27 kΩ discharging resistor). This reduces the charging current to about 1.8 mA, and the power-on thump disappears.

This mistake has remained uncorrected from the 2004 version through the 2021 version, even after five revisions of the schematic.

Power-off thump​

Even after changing R3 to 27 kΩ to ground, there is still a power-off thump.
The fix is: remove R4 entirely, lift the ground end of R3, and insert a 1N4148 diode in series to the AC supply. Connect the diode’s cathode to the 21 V AC secondary, and the anode to R3.
This applies the negative supply directly to pin 8 of the LM3886 mute circuit, bypassing the original problem where the large filter capacitor discharges slowly after power-off and cannot cut off immediately.
After this modification, you should no longer notice any power-on or power-off thump.

B2031A notes​

As far as I remember, the B2031A also has a power-on thump. It has been so long since I modified mine that I had forgotten the details. If you do hear a power-on thump, increasing R22 to 33 kΩ should fix it.
I later made additional modifications so that the B2031A can enter full automatic standby and completely switch off the main transformer. After that modification, there is no switching thump at all, as shown in the picture below.

I also want to apologize to @Davey2156. The B2031A preamp regulator IC does have an overheating issue. In my case it did not overheat because I had already changed it to an independent power supply. It has been almost 20 years, and I had forgotten; I only remembered after opening it up today.

I used a small transformer with a 220 V primary in series with a 9 V winding, giving a single 13 V secondary. I then used a dual half-wave voltage-doubler rectifier to generate ±18 V, feeding the inputs of the LM7815 / LM7915 regulators. I removed the original large dropping resistor at the regulator inputs. With only about 3 V of dropout, the regulator ICs no longer run hot.
Because the small transformer supply uses only 220 µF of filter capacitance, it loses power faster than the amplifier section when switched off. This allows the original LM3886 mute circuit to activate in time, preventing any power-off thump.

The small transformer remains powered continuously to supply the preamp stage. I used the B2031A’s automatic power indicator to drive a MOC3081, which controls the main transformer so that it turns on automatically when music is detected.

Each speaker originally drew about 20 W in standby; after the modification, standby power consumption dropped to about 5 W.

 

[Artyaya]

I'll start by posting a mod diagram for the B2030A preamp stage. I'll explain the reasoning behind each modification one by one when I have more time.
In this diagram, the black text indicates the original factory values for the B2031A, the brownish-red text shows the original factory values for the B2030A, and the red text marks the values that B2030A need to be changed.

 

[Artyaya]

Blocks A and B: IC12A handles the balanced-to-single-ended conversion, and IC12B provides signal gain. This stage benefits from an op-amp with a higher slew rate, such as ST’s TL082. Since the gain is not very high, there is no need to use an AD827.

The remaining op-amps are part of the active crossover circuit and do not provide gain, so they do not need to be replaced.

For C51, the factory used an electrolytic coupling capacitor. Replace it with a non-polarized capacitor, 1 µF rather than 2.2 µF. The reasoning: the Behringer B2030A bass-reflex port has a resonant frequency of 40 Hz, so setting the high-pass cutoff at 30 Hz is appropriate.

In the calculation table below, the 6 kΩ load resistance is (VR4 + R88) in parallel with (R27 + R83).

Using a polarized electrolytic capacitor as the coupling capacitor in an unbiased input stage is the main culprit behind degraded sound!
Anyone who has worked with music knows this: frequencies that cannot be reproduced must be filtered out for the sound to improve.
The coupling capacitor must be accurately calculated to roll off the low frequencies that the speaker cannot handle, preventing them from disturbing the circuit’s optimal operating point.

 

[Davey2156]

@Artyaya
A lot of versioning with these speakers. I think you need to be much more clear in your referencing.

Also, as I'm sure you know, most of the B203X speakers in the field now use TDA7293 chips, not LM3886's.
The TDA7293's have an entirely different scheme for muting/standby.
In fact, the LM3886's don't have a standby feature at all.

Regardless, I'm not sure who you think your audience is here. These speakers have already been discussed at length on diyaudio.com (and other forums) through many years.

 

[NEUTRINO]

@Artyaya

Regardless, I'm not sure who you think your audience is here. These speakers have already been discussed at length on diyaudio.com (and other forums) through many years.

I made a request. When I was modifying the B3031A, I lacked sufficient knowledge of electronic circuits, so I simply swapped the components for high-quality ones.

 

[NEUTRINO]

@Artyaya

I was able to understand the parts that were unclear with the help of AI.
I look forward to the upcoming updates.
Thank you once again!

 

[Bamboszek]

This stage benefits from an op-amp with a higher slew rate, such as ST’s TL082. Since the gain is not very high, there is no need to use an AD827.

Can you refer to any source to back up this statement?
For line level audio slew rate of 1 V/us is more than enough.

Even if we assume purely theoretical 192 kHz hires signal with 0 dBFS ultrasonic peak near end of the pass band, slew rate of 4 V/us would completely suffice.
Of course, it is doubtful that any music material contains such high-frequency, high level components.

However, the proposed TL082 is a very poor audio opamp by today's standards.
It has orders of magnitude more distortion and noise compared even to the cheap 4580, which can actually be audible.