Signal Limiter for Power Amplifiers - THAT Corporation

1 Design Note 03. (formerly Application Note 103). Signal Limiter for Power Amplifiers The circuits within this application note feature THAT4301 Analog Engine to provide the essential elements of voltage-controlled amplifier (VCA) and rms-level detector (RMS). Since writing this note, THAT has introduced several new models of Analog Engines, as well as new VCAs. With minor modifications, these newer ICs are generally applicable to the designs shown herein, and may offer advantages in performance, cost, Power consumption, etc., depending on the design requirements. As well, a standalone RMS is available to complement our standalone VCAs. We encourage readers to consider the following alternatives in addition to the 4301: Low supply voltage and Power consumption: 4320. Low cost, supply voltage, and Power consumption: 4315. Low cost and Power consumption: 4305.

2 High-performance (VCA only): 2180-series, 2181-series Dual (VCA only): 2162. RMS (standalone): 2252. For more information about making these substitutions, please contact THAT Corporation 's technical support group at THAT Corporation ; 45 Sumner Street, Milford, Massachusetts 01757-1656; USA. Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: ; Email: Copyright 2009, THAT Corporation ; All rights reserved. Document 600034 Revision 02. Design Note 03. (Formerly Application Note 103). Signal Limiter for Power Amplifiers Abstract Power Amplifiers , when driven out of their These circuits combine active limiting with a linear range of operation, sound particularly diode-based clipper to provide excellent driver bad, and can produce damage to themselves or protection while avoiding the sonic degradation the transducers to which they are connected.

3 Of simpler designs. An innovative nonlinear capacitor circuit further improves the sonic The design of traditional protection circuits performance of the Limiter . is complicated by the various performance, cost, and sonic tradeoffs involved. There is The design is based on the THAT 4301. certainly no one right answer to the Limiter Analog Engine , and thus requires only a single puzzle. The circuits presented here, however, are designed to maintain a high level of sonic IC, a couple of transistors and diodes, and a integrity, while remaining cost-effective. handful of passive components. Signal Limiter for Power Applications The simplest circuits used to prevent overload Assumptions in Power Amplifiers usually employ diode clippers. These have the advantages of being both fast and We will approach the design of the circuit using inexpensive.

4 They also sound quite unpleasant an approximately real-world example with the when the amplifier is overdriven for more than a following assumptions: few tens of milliseconds. As a result, users may 1. The Power amplifier 's decibel voltage gain is avoid fully exploiting the amplifier 's available 32 dB, a common value;. headroom because they fear the sonic results of 2. The maximum average Power that can be dissi- overload. In the worst case, an amplifier with pated by the 8 load is 600 W;. otherwise admirable performance may gain a reputation for poor sound quality. 3. The maximum peak Power that can be dissi- pated by the 8 load is 6 kW. The significantly improved version shown here With these assumptions, we make the following employs two stages of protection a VCA-based calculations: Limiter which quickly and automatically reduces the 1.

5 The voltage gain of the Power amplifier is input Signal level to just below the overload point, and a conventional diode clipper to handle any 32. A V = 10 20 l 40. short duration excursions while the Limiter stage 2. Assuming a sine wave output, the output voltage at reacts. maximum average Power dissipation is The circuits shown are built around the THAT Vout max avg P = 600W % 8 = 70V RMS. 4301 Analog Engine . The THAT 4301 provides a 3. The output at maximum peak Power dissipation single-chip solution for a variety of analog Signal is: processing applications. It includes a high quality Vin max peak P = 6000W % 8 = 220V RMS. Blackmer gain-cell VCA, an RMS-level detector, and three general purpose op amps, two of which are Knowing these values, we can calculate the appropri- undedicated. The circuits shown are easily adapt- ate Limiter and clipper output voltages as able for use with separate THAT Corporation VCAs 70V RMS.

6 Vin max avg P = = RMS. and RMS-level detectors where even higher 40. performance is required. 220V RMS. And Vin max peak P = 40 = RMS. THAT Corporation ; 45 Sumner Street; Milford, MA 01757-1656; USA. Tel: +1 508 478 9200; Fax: +1 508 478 0990; Email: Web: Copyright 2009, THAT Corporation ; Document 600034 Rev 02. Document 600034 Rev 02 Page 2 of 8 Design Note 03. Signal Limiter for Power Amplifiers Basic Feedback Limiter with Diode Clipper The circuit shown in Figure 1 demonstrates the threshold for long, the Signal 's rms value will basic feedback Limiter with adjustable clipper. The exceed the Limiter 's average Power threshold, input Signal is fed to the Limiter circuitry at the causing the Limiter to quickly reduce the level of node labeled Input . The Limiter 's output is sent Signal being fed to the amplifier . In this way, to the Power amplifier from the point labeled To inaudible (but potentially damaging) peaks of short Power amplifier .

7 In addition, the output from the duration will be clipped, while longer duration Power amplifier is fed back to the Limiter circuit by peaks will be handled by the Limiter , and little way of the node marked From Power amplifier audible impairment should occur. Output . The Clipper Under normal operation, the input Signal is below the Limiter 's threshold and so the VCA is at Figure 2 shows the clipper circuit used in this unity gain, its lowest distortion region. design. A trans-impedance amplifier , OA3, converts the output current from the VCA to a For peak output levels of short duration which voltage which drives the actual clipper circuitry. exceed the predetermined clip level, the clipper When OA3's output voltage exceeds the threshold circuit hard limits the output to this level, set by VR1, the transistor pair Q1 and Q2 combine performing very much like the (adjustable) diode to bypass R2 and clip the output to a fixed level.

8 Clipper that it is. If the output level remains above +15. R3. 10k Q2. 2N3906. D3. 1N4148. +15 Symmetry Adjustment VR1. +15 20k VR4 R6. 50k 220u C11. 300k R5 C6 To Power Amp Input 51R 47p D4. -15 R2 1N4148. 15. 14 20k EC+. 11 Q1. C3 SYM 2N3904. R1 17 V+ 13. Input IN OUT 12 R4. 20k V- OA3 10k 22u GND. 10. EC- U1A. 9 Clipper Circuit 4301P. 16 -15. C7. 100n VCA -15. R10. 10k Zero dB Reference Level D1. R15 R16 From Power Amp RMS Output 1N4148 Detector 820k 75k R13 C4. C13 D2 4 1 R7 VR2. R12 7 OUT IN R11. OA2 6 4k99 5 2 91k 50k 10k 10k CT IT 22u 22n 1N4148 8. Make-up Gain Adjustment U1C U1B. R14 4301P R9. +15 4301P 2M. 100k +20dB Threshold 19 R8 amplifier -15. 18 VR3 Stereo OA1 20 1M2 50k Connection U1D -20dB. 4301P C2. -15 220u Control Port Buffer Figure 1. Schematic of basic Signal Limiter THAT Corporation ; 45 Sumner Street; Milford, MA 01757-1656; USA.

9 Tel: +1 508 478 9200; Fax: +1 508 478 0990; Email: Web: Copyright 2009, THAT Corporation Document 600034 Rev 02 Page 3 of 8 Design Note 03. Signal Limiter for Power Amplifiers 1. Vin dB is the input level in decibels and Vout dB is the output level in decibels, +15 2. GdB is the VCA gain in decibels, and R3. 10k Q2 3. A is the gain between the detector and the 2N3906. control port of the VCA. D3. 1N4148. VR1 The minus sign in the side-chain gain equation 20k C11. To Power Amp Input comes from the fact that this is a compressor 47p D4 circuit, and the gain of the VCA moves in the 1N4148. R2. Q1 opposite direction of output Signal amplitude. 20k R4 2N3904. From VCA OA3. 12 10k Combining these equations yields U1A -15. V out dB = V in dB A % V out dB. 4301P. which can be rearranged to form Figure 2. Detail of clipper circuit V in dB.

10 Using our design example, the peak allowable V out dB =1+A. Power is specified as 220 VRMS, and since we are This is the compression ratio of the ultimately clipping the Signal to a square wave, this compressor. To get the compressor to act as a is equivalent to 220 Vpeak. Given the Power ampli- Limiter , we need to set the compression value to a fier's gain of 40, the Limiter must clip at Vpeak. high value. A suitable and convenient value is 21, The two 1N4148 diodes prevent base-emitter and we can calculate the gain required to achieve breakdown in Q1 and Q2. The addition of these this compression ratio as diodes means that the clipping voltage will be two V in dB. A= V out dB 1 = 21 1 = 20. diode drops (approximately V) greater than the voltage at the bases of Q1 and Q2. VR1 adjusts the voltage at the base of Q1 between 0 and V, and A side-chain gain of 20 will, therefore, yield a at the base of Q2 between 0 and + V.