My Thoughts and Know-how

23 Feb 2022

I bought one speaker out of curiosity. I wanted to see the value the speaker delivers for just $199 (in 2022!).

B2031A

What a great first impression unboxing and taking it apart!

These things are designed (thanks, Genelec!) and built (thanks, Behringer!) really well: the cabinet is dense and solid, the driver frame is cast, the circuits have good layout and soldering. Speaking of circuits: 3 TDA7293 chip amps power it, fed by a toroid transformer. At the ±42V supply voltage, the power these chips can deliver is over 300W. This is not the 20-year old B2031A design you see reviewed online: on the outside it looks the same, but the PCB is a new revision of the now discontinued B3031A (which I have had for 10 years and like a lot).

The B2031A electronics.

I particularly like the front facing bass ports and the down facing rear connectors that make placement easy.

But then you turn it on... Right out of the box, my B2031A sounded bright. Sure enough, a sweep confirmed the tweeter is 4-5 dB too loud above 4 kHz. They say it is factory calibrated... Did the technician stick the microphone in the woofer?

17 April 2017

I put my barely-taken-out-of-the-box JBL Charge 2+ bluetooth speaker to the test and I was quite impresed!

JBL Charge 2+ frequency response.

The DSP changes the response with volume. This review has detailed measurements.

8 August 2016

The JBL LSR310s sub is so good I had to take it apart. I hesitated buying it since I wasn't a fan of the downfiring and not grill-protected woofer, but having the LSR305s I decided to be brand loyal and wow: -3dB at 32Hz and quite flat above. It is audible with uncolored bass lower, though at much diminished SPL, which my equipment cannot properly measure, so let's say -10 dB at 27 Hz.

Board shots follow, let's go thorugh!

2 June 2015

The JBL LSR305 has a space age looking waveguide that indeed does provide subjectively omnidirectional tweeter pressure, while the wide flared port has practically no wind noise. The resulting sound lacks a bit of detail, but is otherwise outstanding at its $130 price point. Surprisingly, the speaker does not offer over-power protection or any power saving capabilities, both of which could have been implemented using the existing chips. There is also a peculiar bug, which might be destructive. I am particularly interested in the design because of the switched mode power supply, digital filtering and class D power amplification.

Here are the guts, let's dive in!

24 May 2014

How do we add clipping protection, overpower protection and automatic standby to a studio monitor? ...Using a microcontroller! ;-)

Completed board.

Functionality

Block diagram.

The input and output of the LF amp are fed to the differential ADC input of the microcontroller (ATTiny85). When clipping occurs, the output fails to follow the input causing the MCU to mute the power IC (LM4776) until the input signal decreases to a lower level. Similarly when the HF amp overpowers the tweeter (I set the threshold to 15W), the system is also muted.

30 Jan 2014

There are numerous advantages to self-powered bi-amped speakers, including more efficient and quality cross-overs, amplifier-to-driver matching, driver response equalization, and lower distortion. These benefits, however, come at the cost of making these systems complex - a professional studio monitor has 10+ amps, split among the filter sections, protection circuitry, gain stages, etc. These in turn require at least low voltage and high voltage power supplies, leading to a complex design that is hard to build as DIY, more prone to failure, has higher distortion and noise, etc.

Here I present a system of my own design with minimal number of components, yet delivering all benefits listed above. It is based on the popular in studio monitors 4th order (24 dB/octave) Linkwitz-Riley cross-over since it allows for phase aligning the tweeter and woofer at the cross over-frequency. The novelty here is in adding significant filter gain in the pass-band region, which allows us to use the power amplifier as the active element of the active cross-over. This eliminates the need for any opamps and extra voltage supplies. The resulting filter is imperfect (in fact each channel has many poles and zeros), but due to the strength of the power amplifier (compared to an opamp) a wider range of resistor values can be chosen to qualitatively eliminate the imperfection. The frequency response is further shaped by frequency dependent impedence in the feedback loop to boost the bass and cut the supersonic frequency content.

Here is the complete circuit diagram:

A Bi-Amp system with just one active element - a stereo power amplifier IC.

The schematic is concise because components play multiple functions. The block diagram should clarify what is happening.

Block diagram


Bode plot

6 Nov 2013

The 2-way Boston Acoustics Classic Series 26 speakers (6" woofer, 1" tweeter) offered mostly faithful reproduction of sound at a low price ($125 each). Measuring their frequency response yields relatively flat pass-band except for an audible notch at 3kHz, resulting from a poor cross-over and destructive interference between the woofer and sound reflected from the back wall of the speaker.

CS26 Frequency response*. Ignore the sub-100Hz part due to a limitation of my equipment.

Speaker/room EQ can easily correct for the dip, so the real weakness of the speaker is lack of deep bass, which here we will correct (I listen to them without a subwoofer).

The speakers' bass reflex port can be improved in 2 ways.:

• The port was originally tuned at 55 Hz. I wanted to bring the resonance lower for deeper bass - basically consciously trade off SPL at 50-60 Hz for reachability of 30-40 Hz. 
• The port is right behind the woofer, passing the mid-range straight trough. These waves then reflect back from objects behind the speaker, creating a noticeable interference pattern, as seen in the below figure. That's quite common with all straight port designs - play a 1 kHz tone through your speakers and move your head around - the notches and peaks will be very obvious.

The bass reflex passes mid-band signals straight-through, which interfere with frontal sound.

The port length is roughly inversely proportional to the square of the resonance frequency. I wanted to bring the resonance to around 45 Hz, or a 50% increase in port length. That would make the port longer than the depth of the speaker. Fortunately, the way I solved this, also mostly eliminated the mid-band pass through problem - I made a turn inside the port, midway through.

Serpentine bass port has lower resonance and blocks unwanted signals. I used sound blocking rubber on the outer side of the turn.

I constructed the new port from 2" ABS (aka gutter) pipes and ABS cement (which actually disolves the plastic and makes the final pieces appear molded).

15 Sept 2013

RX-V2500 is a powerful receiver. The respectable 130W/ch at 0.04% THD is well complemented with the user tweakable 7-band/ch programmable filters that allow for changing their frequency, gain and Q-factor (or YPAO, in Yamaha lingo).

Although the receiver delivers outstanding audio performance, it operates rather inefficiently and has a mild temperature problem. The Yamaha engineers went for a linear power supply and a traditional class A/B power amplifiers making the unit dissipate much more energy as heat than what is delivered to the speakers (at "normal" listening levels). The operating temperature is further exacerbated due to obstructed air convection. The heat channel cools the unit effectively only when the fan spins which never happens in practice, due to an overly aggressive acoustic design point.

The unit is equipped with a fan blowing air from the transformer into a channel made from the aluminum heat sinks of the power amplifiers and LDOs.

I was determined to make the receiver run cooler. Altering the efficiency, particularly at idle, would increase harmonic distortion, so I was left only with the option of modifying the fan rpm vs. temperature curve. I opened the receiver up and studied the fan control circuitry for a tweak. For a surprise, the design, though simple conceptually, was implemented in literally 10x the number of components I would have expected. To make matters worse, the simple fan control was distributed to several boards.

With details abstracted, here is the fan control diagram:

21 April 2012

Patiently hunting on eBay and craigslist, I have acquired 4 Bose 501 series IV speakers from 1985. They were made when Bose had focused on features and quality rather than marketing and so they sound very well. Some of the notable features include a 5 sided speaker cabinet to minimize standing waves and a back-facing tweeter to make the sound spacious (unison effect). The frequency response is relatively flat, excluding a minor resonance at about 8 kHz.

My bedroom A/V entertainment area. The 501s are the 4 speakers on the sides.

Being about my age, these vintage speakers needed some repairs. The first point of speaker failure due to age is usually the foamy speaker surround. The 501s were not designed to be easily user serviceable and so after fixing 4 speakers, I developed a procedure to replace the surrounds that works well. Prerequisite materials include new 10" surrounds and a bottle of glue that bonds with metal, rubber and paper.

Step 1: Removing the speaker cloth and its frame

Using a knife gently raise the cloth frame

Reach the staples with the tip of a chisel or a flat screw driver. Then using a shim for support, press down on the chisel handle to pull the staples out. If a staple cuts through the frame and stays on the cabinet, take it out.

Step 2: Replacing the surrounds

Take the woofer out. Cut through the remaining surround if parts of it are still in tact and then reach between the cone and the paperboard ring. Wedge your fingers under the ring and take it out.

Clear any glue and surround remains and apply a ring of new glue on the metal, slightly towards the center.

Centered the new surround on top of the layer of glue and add another ring of glue on top of it.