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RF Filtering for Audio Amplifier Circuits Summary Today, as more EMC regulations are imposed by governments, or self-imposed by industry, the need for cost effective filtering for audio amplifier circuits is more important than ever. Audio amplifiers are used in everything from car and home stereos to portable CD devices and MP3 players. Any device that outputs sound to a speaker requires an audio amplifier and their use has increased dramatically along with the growing numbers of consumer electronic devices. Changes in the electromagnetic interference (EMI) filter technology, combined with pressures to keep consumer electronics low cost, have altered the way that audio amplifier circuits are filtered. What may have been overlooked as these changes occurred was the DC resistance added to the circuit as designers moved from a standard through-hole type ceramic capacitor to the multilayer chip capacitor (MLCC) feedthrough device. Prohibitive costs for the through-hole type capacitors and the frequency limitations of standard MLCC capacitors have forced engineers to use the newer chip feedthrough components to bridge the price/performance gap. The next generation of audio filtering technology has arrived. This solution will lower overall system cost and boost performance. One single X2Y® component can replace multiple standard filtering devices and offer a higher and broader frequency filtering band with no DC resistance added to the circuit. Introduction to Car Audio In the past, many in the automotive industry used ceramic through-hole capacitors to filter the audio amplifier in car radios. These popular devices provided high insertion loss with no effect on the power output of the audio amplifier because wire was fed through a hole in the center of the device, which was mounted to a grounded plate (Figure 1). As DC current traveling through the wire passed through the filter device, EMI (RF) noise was shunted to a circuit ground reference. Figure 1. Standard “through-hole” feedthrough capacitors. DISCLAIMER: Information and suggestions furnished in this document by X2Y Attenuators, LLC are believed to be reliable and accurate. X2Y Attenuators, LLC assumes no responsibility for its use, nor for any infringements of patents or other rights of third parties which may result from its’ use. X2Y® is a registered trademark. All other brand or product names mentioned in this document are trademark or registered trademarks of their respective holders. These notes are subject to change without notice. Copyright © X2Y Attenuators, LLC all rights reserved. Note# 2003, v3.0, 4/20/05 Page 1 of 9 RF Filtering for Audio Amplifier Circuits Consumer and Original Equipment Manufacturer (OEM) price pressures fuel the demand for alternative solutions. Ongoing research by passive component manufacturers has led to the development of the four terminal surface mount chip feedthrough capacitor (Figure 2). Figure 2. Chip “feedthrough” capacitors in a circuit. The newer type chip feedthrough seemed a natural replacement for its cousin, the through-hole feedthrough capacitor and could offer lower costs in a number of ways. 1. A simplified component design lowers production costs. 2. No need for a mounting plate for the chip feedthrough, which can be surface mounted on a PCB. 3. Labor costs are lowered through the use of automated pick and place machines during production. Although both capacitors are called “feedthrough”, the method by which the DC current is fed through the two devices differs dramatically. The chip feedthrough does not have a hole-through for the wire lead; instead it must carry the current through the internal electrode plates. In doing so, DC resistance is added to the circuit. DC Resistance When current is fed through a chip feedthrough device, the DC resistance associated with the component reduces the power output of the audio amplifier. This is not just true for a chip feedthrough capacitor, but for any filter component that offers resistance to the circuit. The DC resistance of a typical ceramic chip feedthrough is approximately ≤ 0.6 Ω for 0805 to 1206 and ≤ 0.4 Ω for a 1608 size chip ferrite bead device (both rated for 300mA current). The Bridged-output amplifier Power Maximum Equation (National Semiconductor Lm 4862 Boomer) describes the relationship of power to resistance1. PwrMax = Note# 2003, v3.0, 4/20/05 4(Vdd ) 2 2(Vdd ) 2 = 2 2π 2 R L π RL Page 2 of 9 RF Filtering for Audio Amplifier Circuits Car Audio Power Example Given: LM 4862 Amplifier Vdd = 3V RSpeaker = 8Ω RFilter = 0.4Ω per filter (Typical) Using the Power Maximum Equation, first find the maximum power supplied by the audio amplifier without filters (Figure 3). PwrMax Amp w/o Filter represents the power out of the speaker because there is no intermediate circuitry between the amplifier and the speaker. PwrMax Amp w/o Filter = PSpeak w/o Filter = Figure 3. 2(Vdd ) 2 2(3) 2 = = 228.0mW π 2 R L π 2 (8) LM 4862 Amp without filters connected to 8Ω speaker. Now find the maximum power supplied by the audio amplifier with the filters added. Note by adding a 0.4Ω DC filter to each conductor (Figure 4) will increase the resistance as seen by the amplifier. R OUT_TOTAL = R Speaker + (2) R Filter = 8 + (2) (0.4) = 8.8Ω PwrMax Amp w/ Filter Note# 2003, v3.0, 4/20/05 2(Vdd ) 2 2(3) 2 = 2 = = π R L π 2 (8.8) 207.2mW Page 3 of 9 RF Filtering for Audio Amplifier Circuits Figure 4. LM 4862 Amp with feedthrough filters connected to 8Ω speaker. Now the output power loss and output power loss percentage from the amplifier can be calculated. PwrLOSS = PwrMax Amp w/o Filter - PwrMax Amp w/ Filter = 228mW - 207.2mW = 20.8mW ⎛ PwrMax Amp w/ Filter PwrLOSS % = ⎜1 ⎜ PwrMax Amp w/o Filter ⎝ ⎛ 207.2mW ⎞ <OR> = ⎜1 ⎟ × 100 228mW ⎠ ⎝ = 9.12% ⎞ ⎟ × 100 ⎟ ⎠ The filters are an intermediate between the amplifier and the speaker, so now calculate output power loss and output power loss percentage of the speaker. PSpeaker w/Filter = R Speaker R OUT_TOTAL × PwrMaxAmp w/ Filter (8) × 207.2mW (8.8) = 188.36mW = ⎛ PwrMax Amp w/ Filter ⎞ ⎟ × 100 PwrLOSS % = ⎜1 − ⎜ PwrMax ⎟ Amp w/o Filter ⎠ ⎝ ⎛ 188.36mW ⎞ <OR> = ⎜1 ⎟ × 100 228mW ⎠ ⎝ = 17.5% Another factor to consider by adding DC resistance is damping of the loudspeaker which affects the sound quality of the bass. When resistance is added to R OUT_TOTAL , it decreases the damping factor. A high series resistance and a low damping factor will give a ‘boomy’ bass, in contrast with a "tense" sounding bass for a high damping factor. The Damping Factor is calculated by the following formula: Damping Factor = Note# 2003, v3.0, 4/20/05 R Speaker + R OUT_TOTAL R OUT_TOTAL Page 4 of 9 RF Filtering for Audio Amplifier Circuits X2Y® Bypass Filter An X2Y® component supplies no new DC resistance to the circuit over the trace or wire resistance. A single component is placed in bypass between the two lines and the ground terminals of the component are attached to the circuit ground reference, resulting in no power loss to the speaker circuit (Figure 5). Figure 5. Example showing application of a single X2Y® bypass capacitor. Since the X2Y® chip filter is in bypass (Figure 5), the series DC resistance in the circuit is 0. Hence the bridged-output amplifier power equation is the same as the example shown earlier without a filter in the circuit. PwrMax Amp w/ X2Y Filter Comparing Filter Performance 2(Vdd ) 2 2(3) 2 = 2 = = 228.0mW π R L π 2 (8) A natural question is ‘how can a bypass capacitor attenuate noise in a manner equal to a feedthrough capacitor?’ Consider for a moment a standard two terminal capacitor and the flux field it creates (Figure 6). By placing two standard capacitors in opposite directions some flux cancellation can occur and improve circuit performance2, 3. Figure 6. Two closely placed standard capacitors. A partial flux cancellation occurs on the outside of the components. Note# 2003, v3.0, 4/20/05 Page 5 of 9 RF Filtering for Audio Amplifier Circuits An X2Y® component takes the concept of flux cancellation a step further. The internal electrode design forces opposing current to flow internally in a single component (Figure 7). The spacing from opposing electrodes to a nearby shared ground layer is only a dielectric layer apart. Flux cancellation reduces the internal mutual inductance and is achieved whether X2Y® is applied single ended or differentially. The net result is a bypass capacitor with improved broadband and high frequency performance yielding significant attenuation in common mode noise. Figure 7. Flux cancellation happens internally in an X2Y®. An added benefit to the X2Y® design is that differential mode noise is also filtered simultaneously along with the common mode noise. This is due to the internal “X” capacitor connection between the power and return lines. Feedthrough capacitors can only filter the common mode noise and requires an additional “X” capacitor for differential mode filtering. Other advantages of a single X2Y® component solution include: 1. Using one bypass component versus two or more resistive devices resulting in reduced component count on a PCB. 2. Better balance between power and return lines to ground: capacitance tolerance between the internal line to ground capacitors is typically 13% or less. This means matched suppression of common mode noise4. 3. The capacitor maintains a balance over time; which means equal aging and temperature tracking from side to side. Note# 2003, v3.0, 4/20/05 Page 6 of 9 RF Filtering for Audio Amplifier Circuits Portable Audio Devices In consumer electronic devices, where smaller, better, and cheaper is desired, chip feedthrough devices are used extensively. A typical MP3 portable music device may use as many as 10 components for filtering the different internal components such as audio amplifiers, ASIC devices, digital to analog signal converters, etc. (Figure 8). Figure 8. Components contained in a typical MP3 device 5. Stereo headphone jacks are another ideal application for X2Y® bypass filter devices. Standard components would require one component per line (Figure 9). Although there is a new device available to filter all three lines at once, it still adds series resistance to the circuit. Figure 9. Standard resistive components applied to the headphone jack. A single X2Y® component can be used in the same application by placing the end terminals of the X2Y® device on the right and left speaker outputs and the center ground terminals on the ground of the jack (Figure 10). In this manner the full intended power goes to the headphone. Note# 2003, v3.0, 4/20/05 Page 7 of 9 RF Filtering for Audio Amplifier Circuits Figure 10. A single X2Y® bypass capacitor applied to the headphone jack. Figure 11 compares the different attributes of selected components used for filtering. Filter Devices EIA Size DC Rated Current DC Resistance Per Line/Device Circuit Power Loss ? Devices Required Cost O603-2220 Bypass, No Limit None None One Low/Medium O805 Feedthru, 300mA Yes, 0.6 Ohms Yes Mono two, Stereo three Low/Medium Feedthru, 300mA Yes, 0.6 Ohms Yes Mono two, Stereo three Low/Medium 1806 Feedthru, 2A Yes, 0.04 Ohms Yes Mono two, Stereo three Medium 2220 Feedthru, 6A Yes, 0.01 Ohms Yes Mono two, Stereo three High 2506 Feedthru, 2A Yes, 0.04 Ohms Yes Mono two, Stereo three High 1206 Feedthru, 6A Yes, 0.01 Ohms Yes Mono two, Stereo three High 1206 Feedthru, 100mA Yes, 0.40 Ohms Yes One High 1206 Figure 11. Comparison Chart. Note# 2003, v3.0, 4/20/05 Page 8 of 9 RF Filtering for Audio Amplifier Circuits Conclusion Using the X2Y® bypass capacitor for filtering audio amplifiers will result in no power loss because of DC resistance. Power loss is inherent in the design of series feedthrough chip components and can degrade circuit performance. In addition, X2Y® will function equally well in amplifier circuits where the speakers are fed power differentially. Note: Performance results reported in this and other application notes can only be achieved with patented X2Y® components sourced from X2Y® licensed manufacturers or their authorized distribution channels. References 1 Data Sheet, HNational Semiconductor LM4862 BoomerH. 2 Decoupling Strategies for Printed Circuit Boards Without Power Planes, Hwan W. Shim,Theodore M. Zeef, Todd Hubing, EMC Labrotory, University Missouri-Rolla, Presented at the August 2002 IEEE EMC Symposium, Minneapolis, MN - TU-PMG-5, Volume 1, page258. 3 Dell Patent #6,337,798 4 Differential-to-common-mode conversion, By Howard Johnson, PhD, Hhttp://signalintegrity.com/Pubs/edn/DifftoCommonMode.htmH ,(Originally published in EDN Magazine, October 17, 2002) 5 Source, “How Stuff Works”, Hhttp://static.howstuffworks.com/gif/mp3player.gifH. Contact Information Direct inquiries and questions about this application note or X2Y® products to [email protected] or telephone: X2Y Attenuators, LLC st 2730B West 21 Street Erie, PA 16506-2972 Phone: 814.835.8180 Fax: 814.835.9047 To visit us on the web, go to http://www.x2y.com. Note# 2003, v3.0, 4/20/05 Page 9 of 9