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SIGNAL CHAIN BASICS #64: How a poorly designed 20W amplifier can destroy a speaker system

(Editor's note : Signal Chain Basics is an ongoing and popular series; click here for a complete, linked list of all installments.)

Many engineers who played with speakers and amplifiers will tell you a similar story from their youth. When they pushed an amplifier too hard, somehow they blew a driver in their loudspeakers. This usually included some tale of turning the bass knob higher and higher, or increasing the volume knob significantly. So what happened?

They probably blew the tweeter driver in their loudspeaker. But, why did it happen? Most tweeters are designed to drive between 10W and 15W. Only a small amount of energy is needed at high frequencies to drive them. Mid-range and woofers typically are rated for the average power of the overall loud speaker (50W, 100W, and so on.).

Consider what happens when adding gain to a sine wave in an amplitude-restricted system, or music with fixed supply rails. At some point, the signal starts to clip. If you drive a signal beyond clipping, the wave begins to look more like a square wave. With a frequency domain view, we start getting input signal harmonics. With large amounts of clipping come much higher amplitudes on the harmonics. Now many higher order harmonics can easily make their way from bass and midrange drivers to the tweeters, if you have a passive crossover

Because tweeters are rated for far-lower power, the chances of causing damage are much higher. This is a real problem in many systems, especially those running with simple analog processing like operational amplifiers (op amps), or digitally-controlled analog EQ systems. Here are two good solutions:


1. Bi-amping the system

If in an enclosed system such as an active speaker, consider bi-amping your system. Bi-amping lets you drive the tweeter from a separate amplifier. Providing the split between tweeter and woofer is done before the gain on low frequencies, you can isolate the tweeter from damaging high-frequency content of the clipping bass channel.

A bi-amped system allows you to continue running a mostly analog system with the added flexibility of digital tuning. The downside is added cost for the extra amplifiers. However, tradeoffs must be made between a good passive crossover and the cost of the extra amplifier. Using a digital crossover in the digital-to-analog converter (DAC) or codec can ease some of this pain.

Tuning your crossover digitally is much simpler than swapping out different passive components. This also allows the same PCB design to be reused for different size cabinets and speaker drivers. Note that this kind of system only works where you have direct access to both drivers separately.


2. Smart post-process clipped bass signal

Some developers rely on “soft limiting.” It’s a very simple concept, but is rarely seen in home audio systems. Typically, we give the most post-processing to boost low-frequency bass frequencies. Some developers throw 24 dB of bass boost in an attempt to compensate for the poor frequency response of a small two-inch driver.

If the frequencies in the boost are mainly low ones, try adding a low-pass filter after the gain stage to roll off clipping-induced higher frequencies. In an analog system, creating this kind of low-pass filter with a high enough cutoff rate usually requires a multi-order filter, making the system large and expensive. However, in a digital processing system this can be done with ease, providing there are enough MIPS available in the audio processor.

Figure 1 shows an example of a process flow where soft limiting takes place, with digital range control (DRC).

 

Figure 1: Process flow with high-gain DRC and low-pass filter.

Programmable miniDSP products, from portable audio class such as TLV320AIC325x series of devices to newer PCM514x home audio-grade audio miniDSP DACs, have the ability to implement soft limiting. Smarter implementations are based around the ingenuity of the system developer. Each device integrates a fully programmable miniDSP core, freeing developers from the confines of a fixed processing flow that ties them to someone else's concept of a good audio system. A power-limiting circuit (PLimit) sets a limit on the output peak-to-peak voltage, Figure 2 .

 

Figure 2: Using PLimit to limit the output

and thus ensure no high-frequency harmonics.

(Click here to see enlarged image)

For some, this may be old information. But for others, it may be the “so that’s why my speakers blew!” moment.

Join us next month when we discuss tracking down noise and spurious signals in high-speed DACs.


About the Author

Dafydd Roche is the home audio strategic marketing and systems engineer for the Audio Converter group at Texas Instruments. An avid musician in his spare time, Dafydd pours his passion and knowledge of audio and music-making into his work.

Editor's note : Liked this? Want more?

If you are interested in “analog” issues such as signal input/output (sensors and transducer, real-world I/O); interfacing (level shifting, drivers/receivers); the signal chain; signal processing (op amps, filters, ADCs and DACs); and signal integrity, then go to the Planet Analog home page here for the latest in design, technology, trends, products, and news. Also, sign up for our weekly Planet Analog Newsletter here.

3 comments on “SIGNAL CHAIN BASICS #64: How a poorly designed 20W amplifier can destroy a speaker system

  1. DA_100
    May 16, 2013

    I tried to follow the liks under “here” but the links displayed do not work.

    (Editor's note : Signal Chain Basics is an ongoing and popular series; click here for a complete, linked list of all installments.)

  2. Brad Albing
    May 16, 2013

    We apologize for that. Sadly, some of this older material was not archived or archived properly. We'll see if we can track down the correct links.

  3. lkjsdfoijdljv
    August 17, 2015

    Real estate salespeople, and other licensees who are required to work for and under the umbrella of a designated broker, are often referred to as real estate agents. In legal language, the term “agent” refers to the strictly defined relationship between a real estate salesperson and the buyers and sellers with whom he or she conducts business. An agent is an individual who is authorized and consents to represent the interests of another person. As an agent, you act to assist people through the process of buying, selling and renting land, homes and other properties.

    In addition to meeting licensing requirements and having a comprehensive understanding of pertinent real estate laws, real estate agents have a long list of responsibilities. Real estate agents typically do the following:

    Advise clients on mortgages, market conditions and pricing
    Advise sellers on how they can make their homes more attractive to buyers
    Compare properties to determine fair and competitive market prices
    Generate lists of appropriate properties for buyers based on their price range and needs
    Guide buyers and sellers through the transaction process
    Manage lists of contacts
    Mediate negotiations between buyers and sellers
    Prepare and submit all required paperwork, including various contracts
    Present all purchase offers to sellers for consideration
    Promote properties through advertising, listing services (such as MLS) and open houses
    Show properties to prospective buyers
    Solicit potential clients to buy, sell and rent properties
    Stay current with real estate laws and trends
    Work nights and/or weekends to accommodate clients' needs
    Here, we take a look at a few considerations that will help you fulfill your agent responsibilities, while advancing your career as a real estate agent.

    Sphere of Influence
    One way to build contacts and generate leads is through a real estate sphere of influence (SOI) business model. This networking strategy focuses on generating real estate leads through the people you already know, including your family, friends, classmates, business associates, sports team members, and even vendors (like your doctor and hair stylist). The best way to keep track of all the names and contact information is in one place, such as in a spreadsheet program (such as Excel), your email's contacts database or with commercial software.

    Once you have established your SOI database, it is easy to add contacts as you meet people – at community events, thru mutual acquaintances or by any other means. The more people you have on your list, the larger your SOI and the better your chances for a referral. Plan on sending out a “new agent” announcement – through the mail, phone, email, text or any combination catered to each type of contact – when you begin your career, as well as periodic updates and shout-outs to keep your SOI thinking about you and your real estate services.

    Developing a Professional Image
    Developing and maintaining a professional presence is vital to your success as a real estate agent. Your image is projected through your:

    Appearance – tidy clothing, hair, accessories, makeup. Should be appropriate for your office and local market. Avoid perfume/cologne and revealing clothing (i.e., nothing too tight, too short or too low cut).
    Face-to-face meetings – good eye contact, positive body language. Strive to be attentive, engaging and courteous.
    Marketing and advertising materials –quality photos and accurate, compelling text. Should be free of grammatical and spelling mistakes.
    Phone calls – articulate, engaging and courteous.
    Web presence – carefully planned Web site, engaging social media. Should be free of grammatical errors and spelling mistakes.
    Written communications – well-written letters, emails and texts free of grammatical errors and spelling mistakes.
    It is important to note that any presence you have on the Internet – whether for your business or personal life – lasts forever and can easily find its way to your potential clients. Therefore, it is judicious to work toward a professional image both in and out of your real estate career.

    Utilizing Tools and Technology
    Going hand-in-hand with developing a professional image is utilizing tools and technology. Today's agents can use a number of tools to help organize and promote their real estate businesses, including:

    Contact Management
    Keeping track of your clients is a must. You can go basic with a spreadsheet program or an email database program such as Outlook's, or you can invest in database software designed specifically for real estate professionals. Commercial products offer a number of useful features, including automated contact synchronization to your smart phone. No matter how you keep track of your contacts, keep the list current – updating, adding and deleting contacts as appropriate.

    Agent Web Sites
    Consider setting up your own domain name and Web site. While this may seem daunting to some, it is easier than ever to build a professional looking Web site. While large corporations may spend hundreds of thousands, or even millions of dollars, developing and maintaining their Web sites, you can do it for a very small financial investment.

    A Web site provides you with a “landing page” to direct your existing and potential clients to, while creating a professional, searchable Web presence. It is possible to have individual property Web sites to promote your listings and keep your sellers happy. You can also take advantage of social media by linking to your Facebook, Twitter and LinkedIn profiles (and any other social media platforms).

    Marketing
    Real estate agent marketing software can help you manage your marketing efforts. These packages typically include templates for business cards, door hangars, postcards, property flyers, brochures, email campaigns and animated home tours to help you efficiently reach out to existing and potential clients.

    Apps
    A number of apps for iPhone and Android-based devices are available to help you stay connected while you are outside the office. The House Hunter app, for example, allows agents to track and compare an unlimited number of homes, using a proprietary scoring method to identify houses that best match your clients' requirements. Open Home Pro allows you to run an open house on your iPad, follow up with leads, create listing pages and export collected data to Excel or other software.

    The Bottom Line
    Working as a real estate agent has its challenges: you don't get paid unless you sell, you can work long hours and still have no paycheck, and you have to adapt to changing market conditions. That said, it can be a rewarding career, both financially and professionally. Calling on your sphere of influence, projecting a professional image and utilizing today's real estate tools and technology can help you develop a successful career in real estate.

     

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