Audio Channel — Amplification & Speakers

[Steve Gibson]

This topic entertains discussion of the challenges presented by the audio channel of a high-power audio generation device.

I started by choosing the smallest, most efficient and highest power high frequency capable "transducer" (tweeter) that I could find, then worked backwards from its maximum power handling capability to design a solution that would be able to push it right up to and past its maximum power handling limit.

This is the BOSS BPT70 300 Watt Super Compression Tweeter.  A pair of them can be purchased from Amazon for just over $50.
http://www.amazon.com/Boss-BPT70-Super-Compression-Tweeter/dp/B0032FOK06

The BOSS website has distressingly little to say about this tweeter, but we do know that its frequency response is flat out past 25k Hz and extends down to 3500 Hz. So it ought to work for our purposes. It is also the right size (though this sucker is surprisingly solid and weighty) for suspending out in front of a parabolic mirror.

As I have mentioned elsewhere (under the root of the battery management discussion topic), driving this sensitive and high-power 4 ohm impedance tweeter to its limits will require a peak-to-peak voltage swing of 110 volts at a current of 12 to 15 amps.

To minimize battery consumption, we don't want to be wasting power heating up audio amplifier heat sinks, which dictates a high-efficiency class-D (digital) switching amplifier design.  I settled upon a n H-bridge output structure so that we only need to provide half the peak-to-peak voltage with each side of the tweeter being switched between ground and 44.4 to 55.5 volts.

(Note that I haven't quite settled upon the operating voltage for the audio [amplification] system. We get 55.5v from having 15, 3.7v lithium-polymer cells connected in series.  This is what I would prefer.  But I found a nice 12-cell battery management chip that could provide charge management and battery cell balancing for up to 12 cells. Twelve cells at 3.7v per cell gives us 44.4v total.  So, obtaining 55.5v would require two of the 12-cell-maximum charge management chips and electronics, effectively doubling the complexity of battery management just to handle and provide the voltage from an additional three cells (11.1v).  The question is... would that difference be audible??  Would/does it really matter?  We'll see.)

I have three favorite high-voltage power-handling class-D switching amplifier chips, all from Texas Instruments.  Their TAS5261, TAS5630 and TAS5631:

http://www.ti.com/ww/en/analog/tas5630/index.shtml

http://focus.ti.com/docs/prod/folders/print/tas5630.html
http://focus.ti.com/docs/prod/folders/print/tas5631.html
http://focus.ti.com/docs/prod/folders/print/tas5261.html

Each of these lovely class-D switching amplifiers is less than $10 in low quantities, making them a good choice for our application. The 5630/1's are stereo amps, but them can be strapped together to double their power handling capability. One takes an analog input whereas the other accepts a pulse-width-modulated (PWM) source.  The TAS5261 is a monophonic 315-watt amplifier which accepts a PWM modulated source.

Since the Cortex-M3 chip has multiple PWM outputs, as well as a single 10-bit DAC output, I plan to experiment with both approaches to see which one makes the most sense.  A high-frequency PWM output can be readily converted into an analog voltage using a simple 2-pole LC filter since the PWM repetition rate can be many times the highest audio frequency we plan to generate.

All three of these devices have very expensive evaluation boards available which, while totally impractical for production use, will make quick prototype decision making much easier.  And they can be used as reference designs for the use of those chips to save on prototype iterations.

[mark moore]

Steve,

   Have you given any consideration to using directional speakers? The cost is VERY high, around $1,000 per speaker, but they would fit in this project quite well. I had the previlage of listening to a keynote by Woody Norris where he demonstrated the technology during the speech. I can attest that it works as advertised. Here is an article discussing the technology and two competing inventors:

http://www.woodynorris.com/Articles/TechnologyReview.htm

Perhaps not practical from a cost perspective, but worth discussing at least.

[Steve Thompson]

Perhaps a separate project could be the home brew creation of directional speakers.  I would love to learn how to build one.

[boile...@gmail.com]

A few things to consider:

You shouldn't need a class "D" amp, just build an H bridge and run it off the PWM output pins. What you will need to do is either run the switching frequency very high, in the 100's of KHz to MHz range or add inductance in series with the speaker. The reason for this is current ripple. When the FETs in the H bridge turn on the only thing that controlls and limits the flow of current (assuming an ideal switch for now) is the resistance and the inductance. The problem with a speaker is that it will likely have very low inductance since it is a coil of wire with an air core. What this means is that if you leave the FETs on for too long the current will increase beyond what you desire and rapidly destroy something. So you have to decide if you run switch slower to save power but add a series inductor which wastes power or switch fast burning more power and loose the inductor.

Second thing to consider, the "beaming" pattern of a driver is related to the frequency it is producing and it's diameter. The bigger the diameter of the driver the more it will form a narrow beam at higher frequencies. There is an equation for this out there somewhere, if I find it I will post it. What I know for a recent project is that a 3" driver can start to form a narrow beam as low as 3.5kHz (If I remember correctly). This is relative to 180 degree dispersion, which is ideal for an audio speaker. Perhaps this knowledge can be put to use here to form a narrow beam without a parabolic dish. The military audio weapons do not use a dish, but an array of smaller drivers. I believe (only my own theory right now) that they use a cluster of small drivers to act like one big driver so they get this narrow beam effect.

Third comment, as you stated on the podcast, car audio amps operate at a high internal voltage to overcome the resistance of the speakers and achieve higher wattage. You should build a boost converter and run off a low voltage battery pack for several reasons. A simple boost converter requires a single FET, inductor and a diode. A fully synchronous boost converter requires 2 FETs, inductor and a diode. The boost converter can also be driven off the microcontroller PWM output pin or it could be stand-alone. To save money, run it off another PWM output. To save time buy a synchronous boost converter IC that can handle high voltage output.

[michae...@gmail.com]

I have this book, has all the math you'd want on audio...

http://www.amazon.com/Sound-System-Engineering-Second-Carolyn/dp/0240803051/ref=sr_1_7?ie=UTF8&qid=1315946581&sr=8-7

[michae...@gmail.com]

Could we modify a car amp for this project?

[bohn...@gmail.com]

I have done some audio testing of the Boss BPT 70 tweeter and provide here a brief report on my results.

My plan was to measure the SPL (sound pressure level) of the tweeter as I applied increasing audio power to it.  I worked at 10 kHz, started conservatively, 0.03 watt, and increased power from there.  The tweeter failed while it was subjected to about 17 watts applied for a few seconds while I was capturing SPL data. 

For purposes of applying this tweeter to the Portable Sound Blaster project (or likely any other application) it is clear that the tweeter will perform nowhere near the rated 300 watts and even at 17 watts the input power must be limited to very short bursts.  

How loud is this tweeter at 17 watts?  Subjectively, it is very loud.  I used earplugs during my testing when I was about 3 meters from the tweeter and even with that protection I tried to avoid standing on-axis.  What about a distance that might be used in the Portable Sound Blaster project, say 10 meters? 

Since the tweeter failed before I could get that data, we have to rely on the manufacturer's specifications and on accepted equations for adjusting SPL for power and distance to get more quantitative values of SPL.  This calculation gives about 100 dB which is still pretty loud and may well suit the Portable Sound Blaster project.  At 30 meters distance the SPL would be 91 dB.

You can find online comparisons for the SPL values (e.g. jackhammer at 15 meters is 95 dB) but they may not be very useful since it is unlikely any represent a pure sine wave signal at 10 kHz and it is not clear whether they have been weighted.

------------------------------------------

Here are some details on my measurement method.

Signal source:  

BK Precision 3001 Audio Generator, sine wave function

Amplifier:  

Chipamp based stereo amplifier, 68-watt per channel into 4 ohm.

Voltage/current/phase measurement and quantitative distortion assessment:  

Tektronix 2215 Oscilloscope.  I measured voltage across a calibrated 1/3 ohm series resistor to determine current to the tweeter and measured tweeter terminal voltage directly with the scope.  Current/voltage phase difference was also measured with the scope.

Sound pressure level:

I used an iPhone app that performs spectrum analyses of sound at the phone's microphone.  

Results of the maximum power tested:

Tweeter terminal voltage:  35 volts peak-to-peak

Tweeter terminal current:  8.4 amps peak-to-peak

Phase angle:  current lagging voltage 61 degrees

Power:  17.6 watt

(This power level was just below the level that showed significant distortion on the scope.)

Tweeter specifications given on the manufacturer's web site:

http://www.bossaudio.com/auto/pro-grade-neodymium-super-tweeter-bpt70/

which gives: "Driver Sensitivity: 108 dB."  The Amazon page for the tweeter 

http://www.amazon.com/Boss-BPT70-Super-Compression-Tweeter/dp/B0032FOK06

gives: "SPL (1 watt/1 meter 108dB)" which I assume is the same spec given on the bossaudio.com link.  

(I now have a couple of nice paperweights for sale if anyone is interested.)