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Ultra-LD Mk.4 200W RMS Power Amplifier Module, Pt.1 By NICHOLAS VINEN This is our latest and best-performing amplifier module yet. Not only have we reduced the distortion compared to the Mk.3 version but it’s now smaller and has more features – LED indicators for the supply rails and for blown fuses, output offset voltage nulling, flyback diodes for the output stage and a LED clipping indicator. This month we have a detailed description of how it works. 32  Silicon Chip siliconchip.com.au Specifications WARNING! Output power (230VAC mains): 200W RMS into 4Ω, 135W RMS into 8Ω Frequency response (10Hz-20kHz): +0,-0.05dB (8Ω); +0,-0.12dB (4Ω); see Fig.5 Input sensitivity: 1.26V RMS for 135W into 8Ω; 1.08V RMS for 200W into 4Ω Input impedance: 11.85kΩ shunted with 1nF Rated Harmonic Distortion (4Ω, 8Ω): <0.001%, 20Hz-20kHz, 20Hz-30kHz bandwidth; see Figs.3 & 4 Signal-to-Noise Ratio: -124dB unweighted with respect to 135W into 8Ω (20Hz-20kHz) Damping factor: ~250 Stability: unconditionally stable with any nominal speaker load ≥4Ω Music power: 170W (8Ω), 270W (4Ω) Dynamic headroom: 1dB (8Ω), 1.3dB (4Ω) Power supply: ±57V DC from a 45-0-45 transformer Quiescent current: 140mA nominal Quiescent power: 16W nominal Output offset: typically <10mV untrimmed; <1mV trimmed Main Features • Low distortion and noise • Able to produce specified power output on a continuous basis with passive cooling • Compact PCB • Onboard DC fuses • Output offset voltage adjustment • Output protection diodes (for driving 100V line transformers & electrostatic speakers) • Power indicator LEDs • Fuse & power status indicator LEDs • Clipping indicator LED • Clean overload recovery with low ringing • Clean square wave response with low ringing • Tolerant of hum & EMI fields • Survives brief short circuits & overload without blowing fuses • Quiescent current adjustment with temperature compensation A S EXPLAINED in the preview last month, this revised amplifier module has lower distortion than the Mk.3 version. It’s also somewhat smaller and uses more modern parts that are easier to get. We haven’t called this amplifier series “Ultra-LD” for nothing. The Mk.3 version already had extremely low distortion levels of well under 0.001% up to a few kilohertz and just 0.002% at 10kHz. Very few commercial amplifiers would beat that. We’ve really had to work hard to do better but we have – check the performance graphs to see for yourself. In fact, the distortion of this amplifier module is so low that we’ve had to develop new testing techniques just to measure it. We found that the resistive load that we’ve used to test amplifiers for years simply wasn’t linear enough. siliconchip.com.au Even with this module running near maximum power, the distortion level across pretty much all of the audible frequency range is less than 0.001%. That’s fewer than 10 parts per million! Since publishing the preview, we’ve made further improvements to the performance and added a few features. These include onboard LEDs which indicate if the power rails are present and which change colour if the DC fuses blow. We’ve also added a clip indicator circuit which drives a LED to show when the amplifier is being overdriven. This LED can be mounted on the amplifier front panel if desired and can be wired to multiple modules to indicate when any channel is clipping. The power output is the same as before: 135W RMS continuous into 8Ω and 200W into 4Ω, with higher music power (short term) figures of 170W for High DC voltages (ie, ±57V) are present on this amplifier module when power is applied. In particular, note that there is 114V DC between the two supply rails. Do not touch the supply wiring (including the fuseholders) when the amplifier is operating, otherwise you could get a lethal shock. 8Ω and 270W for 4Ω. These are measured using the IHF standard of 20ms high-power bursts interspersed with 480ms of -20dB output (ie, two bursts per second). These equate to a dynamic headroom of 1dB for 8-ohm loads and 1.3dB for 4-ohm loads. Circuit description Let’s take a look at the operation of the Ultra-LD Mk.4 Amplifier module circuit now; we’ll go over the changes later. The circuit is shown in Fig.1. A 1MΩ resistor DC biases the input signal at RCA socket CON1 to 0V. The signal ground (ie, RCA socket shield) is connected to power ground via a 10Ω resistor, which improves stereo separation when modules share a power supply; it prevents a ground loop due to the grounds being joined directly both at the power supply module and at the signal source. The signal passes through an RFattenuating RC low-pass filter (100Ω/ 1nF plus ferrite bead) and is then coupled to the base of PNP transistor Q1a via a 47µF DC-blocking non-polarised electrolytic cap­acitor; a 12kΩ resistor provides a path for Q1a’s base current. Low-noise PNP input transistors Q1a and Q1b are in the same SMD package. The input signal goes to the base of Q1a while feedback from the output goes to the base of Q1b. Both transistors have 47Ω emitter degeneration resistors for improved linearity and they are fed with a common 2mA current via trimpot VR2, power indicator LED1 and a 12kΩ voltage dropping resistor. VR2 allows the current split to be shifted slightly between the two transistors, to trim out base-emitter voltage mismatch and thus practically eliminate any output offset, to avoid excessive DC current when driving a line transformer or electrostatic speaker. The 12kΩ resistor reduces dissipation in Q1a/Q1b and also acts as a fail-safe to allow the amplifier to operate more or less normally even if Q3a or an associated component fails. LED1 has no August 2015  33 Fig.1: the complete circuit for the Ultra-LD Mk.4 amplifier module, minus the clip detection circuitry which is shown separately in Fig.2. Q1a & Q1b are the input transistors (housed in a single package) while Q2a/Q2b form the current mirror and Q3a the constant current source. Current drive then flows to a VAS Darlington comprising Q4 & Q6, with a constant current load supplied by Q5. Bias for the output stage is generated by diodes DQ10-DQ13 which are integral to the output transistors, plus VBE multiplier Q9 which is adjusted using trimpot VR1. Driver transistors Q7 & Q8 then supply base current for output transistors Q10-Q13 which are connected to the loadspeaker load via 0.1Ω emitter resistors and an RLC filter consisting of air-cored inductor L1, four parallel 27Ω resistors and a 100nF capacitor. effect on the operation of the circuit except to indicate when it is powered. The currents from Q1a and Q1b go to a current mirror comprising NPN transistors Q2a and Q2b, also in a single SMD package. The 68Ω emitter resistors help ensure that equal current flows through each transistor as the voltage across these resistors is much greater than the base-emitter voltage difference between the two. Since current through Q2a and Q2b is held equal, any difference between the current from Q1a and Q1b must flow to the base of NPN transistor Q4. Thus, Q4’s base current is proportional 34  Silicon Chip to the difference in input and feedback voltages. It forms the first half of a Darlington pair along with Q6, a 250V high-gain transistor. A 2.2kΩ resistor between its base and emitter speeds up switch-off. Q4 and Q6 together form the Voltage Amplification Stage (VAS); Q6 has a constant current collector load and as a result, the current flow to its base is translated linearly to a voltage at its collector which controls the output stage. Output stage The output stage consists of two pairs of power transistors arranged as complementary emitter-followers. NPN transistors Q10 and Q11 are connected in parallel and source current for the load while Q12 and Q13 are PNP types and sink current from the load. 0.1Ω emitter resistors ensure equal current sharing, linearise the output stage and reduce local feedback. They also serve as handy shunts for measuring the quiescent current. Large power transistors require a substantial base current due to limited gain and this is supplied by driver transistors Q7 and Q8. Effectively, this makes the output stage a complementary Darlington. The parallel 220Ω siliconchip.com.au resistor and 1µF capacitor between the driver emitters speed up switch-off when drive is being handed off from one to the other. The four base-emitter junctions in the output stage, plus the voltage across the emitter resistors adds up to around 2.2V and thus a similar DC bias must be maintained between the bases of Q7 and Q8 to keep the output transistors in partial conduction most of the time. Otherwise, there will be substantial crossover distortion each time the signal passes through 0V. However, the base-emitter voltages of these six transistors vary with temperature so a fixed DC bias is not suitable. Since the base-emitter voltages drop with increasing temperature, a fixed bias voltage would lead to increased current as the transistors heated up and ultimately, to thermal runaway and destruction. siliconchip.com.au So the DC bias is generated by diodes DQ10-DQ13 and transistor Q9. DQ10DQ13 are internal to output transistors Q10-Q13 so their temperatures track well and as a result, their forward voltage drops as the output transistors heat up. These are connected in two parallel pairs – just like the output transistors – for accurate temperature compensation. VBE multiplier Similarly, NPN transistor Q9 is mounted on the heatsink immediately between Q7 and Q8 so it also tracks their temperatures quite well. It forms an adjustable VBE multiplier with a collector-emitter voltage equal to its temperature-dependent base-emitter voltage multiplied by the ratio of the resistive divider across it. Thus, VR1 controls the quiescent current. The bottom end of the bias network is driven directly by VAS transistor Q6 and the voltage swing is coupled to the top of the network by a 47µF capacitor. Operating current for this network is fixed at 10mA by PNP transistor Q5. The 100Ω resistors between either end of the DC bias network and Q7/Q8 act as RF stoppers and also limit current flow under fault conditions (eg, a short circuit). Q5 is able to hold the VAS/bias current constant at around 10mA because its base is driven by Q3b to maintain around 0.6V across its 68Ω emitter resistor. Should this voltage increase, Q3b turns on harder, increasing the current through the two 6.2kΩ resistors and thus reducing the current from Q5’s base, reducing its emitter current. Similarly, if the voltage across the 68Ω resistor drops, Q3b allows Q5 to turn on harder to compensate. The 47µF capacitor at the junction August 2015  35 +57V K LED4 K ZD1 4.7V CATHODE BAND 1k LED4 CLIP 100k Output filter ZD1, ZD2 D5 BAV99 A K1 B ZD2 4.7V 100k A1 100k K (TO A OFF-BOARD CLIPPING K INDICATOR LED) K A A λ collector will not sink much more than 100mA. This is probably still enough to burn out Q8’s 100Ω base resistor but that may be the only damage from an extended short circuit; very brief short circuits will should not cause any lasting damage. However, this resistor will cause Q4’s collector voltage to drop as it is called on to supply more current and the Early effect means its gain will drop when this happens. This can cause local negative feedback and oscillation. A low-value capacitor in parallel with the 150kΩ resistor prevents this while still allowing the current to Q6’s base to quickly drop below 1mA during a short circuit. CON4 A C Q14 BC846 E K2 33k A2 D6 BAV99 100k D7 BAV99 A2 B TP7 K2 E C BC846, BC856, FJV1845E BAV99 C K1/A2 A1 SC 20 1 5 K2 B 68k Q15 BC856 100k B 100k K C E Q16 FJV1845E E A –56V CLIP DETECTOR FOR ULTRA-LD Mk4 AMPLIFIER MODULE Fig.2: the clip detector monitors the waveform at feedback point TP7 relative to the supply rails and pulls ~1mA through LED4 whenever the output voltage comes within approximately 4V of either rail, indicating the onset of clipping. NPN transistor Q14 detects positive excursions while PNP transistor Q15 detects when the output approaches the negative rail and its output is level-shifted by NPN transistor Q16 to light the same LED. of the 6.2kΩ resistors virtually eliminates variations in the current through them with supply voltage, stabilising Q5’s current regulation. Q5’s base bias voltage is also fed to Q3a via an RC low-pass filter (2.2kΩ/47µF), which in combination with the 330Ω emitter resistor, sets the current from Q3a to the input pair to 2mA. Feedback & compensation Feedback goes from the junction of the output emitter resistors to the base of Q1b via a 12kΩ/510Ω resistive divider, setting the closed loop gain to 24.5x (28dB). The bottom end of the feedback network is connected to ground via a 1000µF electrolytic capacitor. This has a negligible effect on low-frequency response but sets the DC gain to unity, so that the input offset is not magnified at the output by the gain factor of 24.5. The compensation network is connected between the collector of Q6 and the base of Q4, ie, it is effectively a Miller capacitor for the VAS Darlington. The junction of the two series 150pF capacitors connects to the nega36  Silicon Chip tive rail via a parallel network comprising a 2.2kΩ resistor and 15pF capacitor. This is a form of two-pole compensation which avoids rolling off the open loop gain until higher frequencies, thus yielding better distortion performance; this was explained in more detail in the July 2011 issue, on page 34. We’ve added the 15pF capacitor since it improves overall stability, by providing a small “third-pole” compensation characteristic. The 1nF capacitor across Q4’s collector similarly improves stability, for reasons explained below. The 150kΩ resistor limits the current through Q6 under fault conditions. Should the amplifier outputs be shorted, it will try to pull the output either up or down as hard as possible, depending on the offset voltage polarity. If it tries to pull it up, the output current is inherently limited by the ~10mA current source driving Q7 from Q5. However, if it tries to pull down, Q6 is capable of sinking much more current. The 150kΩ resistor limits Q6’s base current to around 150μA and thus Q6’s The emitter resistors of output transistors Q10-Q13 are connected to the output at CON2 via an RLC filter comprising a 2.2µH series inductor in parallel with a 6.8Ω resistance (4 x 27Ω in parallel), with a 100nF capacitor across the output terminals. The inductor isolates any added capacitance at the output (eg, from the cables or the speaker’s crossover network) from the amplifier at high frequencies, which could otherwise cause oscillation. The resistor reduces the inductor’s Q, to damp ringing and also forms a Zobel network in combination with the 100nF capacitor, which also aids stability. Driving a line transformer While a very low output offset voltage gives slight benefits when driving normal speakers, it’s absolutely critical when driving a 100V line transformer or electrostatic speaker (which will typically have an internal transformer). That’s because the DC resistance of the primary winding will be much lower than that of a loudspeaker’s voice coil, so a lot of DC current can flow with an offset voltage of just a few millivolts. The other requirement for driving a transformer is to have protection diodes on the amplifier output to clamp inductive voltage spikes which occur when the amplifier is driven into clipping (overload). These would otherwise reverse-bias the output transistor collector-emitter junctions, possibly causing damage. D3 and D4 are 3A ultrafast, soft-recovery diodes with low junction capacitance for their size and we have checked that they do not have any impact on performance. siliconchip.com.au U-LD Mk4 THD+N vs Frequency, 100W Total Harmonic Distortion + Noise (%) 0.05 16/07/2015 12:05:59 0.01 0.005 0.002 0.001 0.0005 0.0002 14/07/2015 15:19:51 8Ω 4Ω 0.02 0.01 0.005 0.002 0.001 0.0005 0.0002 50 100 200 500 1k Frequency (Hertz) 2k 5k 10k Fig.3: THD+N when driving resistive loads at 100W. It’s so low, it’s really pushing our ability to measure distortion with the equipment that we have. 4Ω performance is usually worse than 8Ω but in this case, not by much! So there should be no changes necessary to use this module in a PA amplifier or to drive electrostatic speakers, as long as the output offset voltage is trimmed out during set-up. Indicator LEDs While producing the final PCB design, we decided to use some of the spare real estate to add indicator LEDs. LED1 (blue) is connected in series with the input pair current source and is on while ever the board has power applied. Since there is an ~50V drop required from Q3a’s collector to VR2’s wiper, the power to operate this LED is effectively free. We’ve also added red/green LEDs LED2 & LED3 to indicate the status of the output stage power rails. It isn’t always obvious that a fuse has blown without careful inspection. In the case of LED2, assuming F1 has not blown, the voltage at either end of the fuseholder is the same so no current will flow through the red junction. However, the green junction is connected between the collectors of Q10/Q11 and ground via a 47kΩ current-limiting resistor, so it will light up. Should the fuse blow, the collector voltages will drop to near 0V, so the green LED will turn off but the full rail voltage will be across the fuseholder and so the red junction will switch on. Similarly, LED3 indicates green/red when F2 is OK/blown. These LEDs will also indicate if one of the two supply rails is missing (eg, due to a wiring fault); in this case, LED1 will probably siliconchip.com.au 0.0001 20k Fig.5: frequency response is very flat for 4-8Ω loads, with no detectable roll-off at the lowfrequency end and only about one tenth of a decibel by 20kHz at the high-frequency end. Most of the high-frequency roll-off is due to the necessary output filter. 0.06 0.1 0.2 0.5 1 2 5 Power(W) 10 20 50 100 200 Fig.4: THD+N, this time showing how it varies with power at a fixed frequency. It’s dominated by noise below 10W and is very low until the amplifier starts to run into clipping at 135W for 8Ω loads and 200W for 4Ω loads. +3 U-LD Mk4 Frequency Response, 4Ω & 8Ω, 10W 14/07/2015 15:21:28 8Ω 4Ω +2 +1 0 Amplitude Variation (dBr) 0.0001 20 U-LD Mk4 THD+N vs Power, 1kHz, 20kHz BW 0.05 8Ω, 20Hz-30kHz BW 8Ω, 20Hz-80kHz BW 4Ω, 20Hz-30kHz BW 4Ω, 20Hz-80kHz BW 0.02 0.1 Total Harmonic Distortion + Noise (%) 0.1 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 10 20 50 still light up so it might not otherwise be obvious. Clipping indicators We’ve also added an on-board clipping detector/indicator circuit. This involves just a few components and allows you to quickly see if the amplifier is overloaded; sometimes moderate clipping is not obviously audible. It can drive an external LED mounted on the front panel of the amplifier. These components may be omitted if they are not required. The clip detector circuit is shown in Fig.2. Zener diode ZD1 derives a reference voltage 4.7V below the nominally 57V positive rail, ie, at about 52V. This is connected to the emitter of NPN transistor Q14. Its base is connected to the amplifier output via a 100kΩ 100 200 500 1k 2k Frequency (Hertz) 5k 10k 20k 50k 100k current-limiting resistor, with diode D6 preventing its base-emitter junction from being reverse-biased. At the onset of clipping, the speaker voltage will rise above the reference voltage plus Q14’s base-emitter voltage, ie, to about 53V. Q14 will switch on and sink current via LED4, a 4.7kΩ current-limiting resistor and isolating diode D5, lighting up the clip indicator LED. As the reference voltage is relative to the positive rail, any variations in supply voltage will be accounted for. ZD2, PNP transistor Q15 and diode D7 work in an identical manner for negative excursions. However, Q15 drives LED4 via high-voltage NPN transistor Q16 which acts as a level shifter. The 100kΩ resistor in series with its collector limits the LED current to a similar level (1mA) despite the much August 2015  37 Parts List: Ultra-LD Mk.4 Power Amplifier 1 double-sided PCB, code 01107151, 135 x 93mm 1 black anodised aluminium heatsink, 200 x 75 x 45mm (L x H x D) 2 SMD M205 fuse clip assemblies (F1, F2) (Digi-Key F4546-ND) 2 6.5A M205 fast-blow fuses (F1, F2) 2 blown M205 fuses (for testing) 1 SMD 3216/1206 ferrite bead (L1) 1 2.2µH air-cored inductor (L2) (or 1 20mm OD x 10mm ID x 8mm bobbin and 1m of 1.25mm diameter enamelled copper wire, plus 10mm length of 20mm diameter heatshrink tubing) 1 1kΩ vertical multi-turn trimpot (VR1) 1 100Ω SMD single-turn trimpot, EVM1D type (VR2) (Digi-Key P1D101TR-ND) 4 TO-264 or TOP-3 silicone insulating washers 2 TO-220 silicone insulating washers 1 TO-126/TO-225 silicone insulating washer (or a TO-220 washer cut down) 2 transistor insulating bushes 7 PC stakes (optional) Connectors 1 vertical mounting RCA socket (CON1) 1 4-way vertical pluggable terminal block with matching socket (CON2) 1 3-way vertical pluggable terminal block with matching socket (CON3) 1 2-pin polarised header (CON4) (optional, for off-board clipping indicator LED) 1 FZT696B high-voltage NPN transistor, SOT-223 (Q6) (Digi-Key FZT696BCT-ND) 1 MJE15030* NPN driver transistor, TO-220AB (Q7) (Digi-Key MJE15030GOS-ND) 1 MJE15031* PNP driver transistor, TO-220AB (Q8) (Digi-Key MJE15031GOS-ND) 1 BD139* NPN transistor, TO-225AA (Q9) (Digi-Key BD139GOS-ND) 2 NJL3281D* NPN ThermalTrak transistors, TO264-5 (Q10, Q11) 2 NJL1302D* PNP ThermalTrak transistors, TO264-5 (Q12, Q13) 1 BC856C NPN transistor, SOT-23 (Q15) (Digi-Key BC856CMTFCT-ND) 1 FJV1845E 120V 50mA NPN transistor, SOT-23 (Q16) (Digi-Key FJV1845EMTFCT-ND) 1 wide viewing angle blue LED, SMD 3216/1206 (LED1) (Digi-Key 754-1439-1-ND) 2 red/green dual SMD LEDs, 3226/1210 (LED2,LED3) (Digi-Key 350-2081-1-ND) 1 yellow high brightness LED, SMD 3216/1206 (LED4) (Digi-Key 350-2050-1-ND) 4 BAV99 high-speed series double diodes, SOT-23 (D1,D5-D7) (Digi-Key 568-1624-1-ND) 1 MMBD1401A high-voltage diode, SOT-23 (D2) (Digi-Key MMBD1401ACT-ND) 2 VS-3EJH02 hyperfast soft recovery 3A diodes, DO221-AC (D2,D4) (Digi-Key VS-3EJH02-M3/6BGICT-ND) 2 4.7V Zener diodes, SOT-23 (ZD1,ZD2) (Digi-Key BZX84B4V7-7-FDICT-ND) * Use On Semiconductor branded genuine parts Semiconductors 2 HN3A51F dual PNP low-noise transistors, SC-74 (Q1,Q3) (DigiKey HN3A51F(TE85LF)CT-ND) 1 HN3C51F dual NPN low-noise transistors, SC-74 (Q2) (Digi-Key HN3C51F-GR(TE85LFCT-ND) 2 BC846C NPN transistors, SOT-23 (Q4,Q14) (Digi-Key BC846CMTFCT-ND) 1 FZT796A high-voltage PNP transistor, SOT-223 (Q5) (Digi-Key FZT796ACT-ND) Capacitors (SMD 3216/1206 or 2012/0805 ceramic unless specified) 1 1000µF 6.3V SMD electrolytic, 8mm diameter (Digi-Key 493-6341-1-ND) 1 47µF 63V SMD (8mm) or throughhole electrolytic capacitor (eg, Digi-Key 493-6401-1-ND) 1 47µF 35V SMD electrolytic, 6mm diameter (Digi-Key 493-9433-1-ND) 1 47µF 16V non-polarised SMD electrolytic, 8mm diameter (Digi-Key 493-9818-1-ND) Screws, nuts, spacers & washers 4 M3 x 9mm tapped spacers 7 M3 x 20mm machine screws 8 M3 x 6mm machine screws 7 M3 nuts 7 M3 flat washers 38  Silicon Chip 2 47µF 6.3V X5R (Digi-Key 1276-1167-1-ND) 7 1µF 100V X7R (Digi-Key 1276-2747-1-ND) 1 100nF 250V NP0/C0G ceramic capacitor, SMD 1812 or 2022 package (Digi-Key 445-15480-1-ND) OR 1 100nF 250VAC Polypropylene capacitor, 15mm lead spacing (EPCOS B32652A6104J) (Digi-Key 495-1333-ND) 2 1nF 100V NP0/C0G (Digi-Key 445-5759-1-ND) 2 150pF 200V NP0/C0G (Digi-Key 399-9174-1-ND) 1 15pF 100V NP0/C0G (Digi-Key 311-1838-1-ND) Resistors (0.5W 1% Thin Film, 3216/1206) 3 12kΩ or 12.1kΩ (Digi-Key RNCP1206FTD12K1CT-ND) 2 6.2kΩ or 6.49kΩ (Digi-Key RNCP1206FTD6K49CT-ND) 4 2.2kΩ or 2.21kΩ (Digi-Key RNCP1206FTD2K21CT-ND) 1 510Ω or 511Ω (Digi-Key RNCP1206FTD511RCT-ND) 2 330Ω or 332Ω (Digi-Key RNCP1206FTD332RCT-ND) 1 220Ω or 221Ω (Digi-Key RNCP1206FTD221RCT-ND) 1 120Ω or 121Ω (Digi-Key RNCP1206FTD121RCT-ND) 3 100Ω (Digi-Key RNCP1206FTD100RCT-ND) 3 68Ω or 68.1Ω (Digi-Key RNCP1206FTD68R1CT-ND) 2 47Ω or 47.5Ω (Digi-Key RNCP1206FTD47R5CT-ND) 1 10Ω (Digi-Key RNCP1206FTD10R0CT-ND) Resistors (other) 1 1MΩ 0.25W 1% 3216/1206 SMD 1 150kΩ 0.25W 1% 3216/1206 SMD 6 100kΩ 0.25W 1% 3216/1206 SMD 1 68kΩ 0.25W 1% 3216/1206 SMD 4 47kΩ 0.25W 1% 3216/1206 SMD 1 33kΩ 0.25W 1% 3216/1206 SMD 1 1kΩ 0.25W 1% 3216/1206 SMD 1 390Ω 1W 5% (Digi-Key RHM390BCCT-ND) 1 100Ω 1W 5% (Digi-Key A102496CT-ND) 2 68Ω 5W wirewound (for testing) 4 27Ω 1W 1% (Digi-Key 541-27.0AFCT-ND) 4 0.1Ω 3W 1% Metal Film/Element (Digi-Key CRA2512-FZ-R100ELF) siliconchip.com.au higher rail voltage differential. This is not the simplest clip detector circuit but it presents an almost completely linear load to the amplifier output, to minimise the possibility of any distortion due to its input load current. It’s connected to the driven end of L2, to give the amplifier the best chance to cancel out any non-linearities in the load it introduces. Summary of improvements The obvious changes to the circuit are the additions: the power indicator LED, fuse status LEDs, clipping indicator LEDs and clip detection circuitry, offset adjustment trimpot and output protection diodes. However, some of the changes compared to the Mk.3 version are more subtle. First, the input RF filter capacitor has been reduced to 1nF to make the amplifier less sensitive to source impedance, as it was decided this is more than enough capacitance for good RF filtering. In addition, the input pair operating current has been reduced from 6.5mA to 2mA. This change was originally suggested by Alan Wilson for lowering noise although we were only able to measure an improvement of one decibel as a result. But the circuit also seems more stable with the new arrangement so it was a worthwhile change. Two additional changes were made to improve stability in the front end, which have already been mentioned: the 1nF capacitor across Q4’s collector resistor and the 15pF capacitor across the 2.2kΩ resistor in the two-pole compensation network. These changes and the improved layout have allowed us to reduce the value of the two main compensation capacitors from 180pF to 150pF while should improved distortion cancellation. It also worked reasonably well with 120pF capacitors but recovery from positive clipping was no longer clean so we went back to 150pF. Since Q6 has a much higher gain than the BF469 used previously, we’ve had to increase Q4’s collector resistor from 22kΩ to 150kΩ to limit currents to a safe level under fault conditions. We’ve also increased the capacitance across the bias network (for the output stage) from 100nF to 47µF and also changed the front end negative rail RC filter from 10Ω/470uF to 100Ω/47uF to make clipping more symmetrical and provide slightly better fault tolerance. Also, we found that the large bypass siliconchip.com.au You Must Use Good-Quality Transistors To ensure published performance, be sure to use the low-noise transistors specified in the parts list. Be wary of counterfeit parts. We recommend that all other transistors be from reputable manufacturers, such as NXP Semiconductors, On Semiconductor, ST Microelectronics and Toshiba. This applies particularly to the MJE15030 & MJE15031 output driver transistors. capacitors for the output stage are not necessary if the power supply leads are short and thick. Basically, their only benefit is to reduce the voltage drop in that wiring and thus maintain full power output at lower frequencies if that drop is significant. As such, they can be regarded as optional. The 1µF high-frequency bypass capacitors for each output transistor are sufficient to ensure stability and guarantee good performance. Component selection Even though the circuit retains considerable similarity to the Ultra-LD Mk.3, almost all the components besides the output and driver transistors have changed. This is mainly because, as we explained last month, we are using SMDs extensively in an attempt to keep signal paths as short as possible and provide a ground plane covering the entire front end. This also allows us to improve magnetic cancellation. So we’ve had to be very careful to ensure that each new component provides equal or better performance to the through-hole part it replaces. The resistors and capacitors must have excellent linearity. For active components like transistors and diodes, we’ve chosen components with similar or better gain, bandwidth, lower parasitic capacitance, etc. All the low-wattage resistors are thin-film types. Many SMD resistors have thick-film construction and have a worse performance than through-hole thin-film resistors; for an explanation, see www.davehilldesigns.com/smt_ resistror_distortion_rev1.pdf [sic]. So you need to be careful to use the types we specify. The higher-power resistors in the circuit (1W and 3W) are thick film or bulk metal types but their values are low enough that the linearity is acceptable. Some new components have been chosen for their physical size or configuration. For example, trimpot VR2 goes right in the middle of a critical part of the front-end circuit so we’re using a tiny SMD type to make the layout in that section better. Having all components in the front-end being SMD types (besides CON1) allows a single unbroken analog ground plane under that section for maximum hum/ EMI rejection. Similarly, the SMD fuses and 0.1Ω emitter resistors mean that we can place them directly on opposite sizes of the PCB for maximum magnetic loop cancellation. With the through-hole parts in the Mk.3 amplifier, the sideby-side arrangement did not have as effective magnetic cancellation. And with the emitter resistors on the other side of the board, it should be easy to replace the fuses if necessary. Capacitors Many of the capacitors in this circuit must be almost perfectly linear to obtain the desired performance. We extensively tested C0G/NP0 ceramic “chip” capacitors in comparison to polypropylene types, which are generally regarded as among the best available. There was no measurable difference. Many of the C0G/NP0 capacitors need to be rated at 100V or 200V as they may be exposed to voltage swings close to the full rail-to-rail supply voltage. Note that “C0G” and “NP0” mean the same thing. They refer to a type of low-K ceramic dielectric which has an effectively zero temperature coefficient. For bypassing, multi-layer SMD ceramics with X5R or X7R dielectrics are used. These have extremely low ESR and work very well in this role. Where larger-value bypass capacitors were called for than are practical for ceramic types, we used SMD electrolytics to ensure the ground plane “shield” is unbroken. Our attempts to use X5R/X7R ceramic capacitors for signal coupling failed miserably so we went back to a non-polarised electrolytic type; plastic film types are too bulky and tantalums too unreliable. The problem is that all multi-layer ceramic capacitors, with . . . continued on page 112 August 2015  39 Notes & Errata Driveway Monitor (July 2015): IC1 is incorrectly listed as an AD723AN in the parts list. It should be an AD623AN as shown on the circuit. This error has been corrected in the on-line edition of the magazine. Next Issue The September 2015 issue of SILICON CHIP is due on sale in newsagents by Thursday 27th August. Expect postal delivery of subscription copies in Australia between August 24th and September 4th. Ultra-LD Mk.4 Amplifier Module, Pt.1 – continued from p39 the exception of C0G/NP0 types, have very high voltage coefficients. As the voltage across the capacitor increases, its capacitance drops. While electrolytics have a reputation for non-linearity, they are nowhere near as bad as these multi-layer ceramics in this respect. It’s so bad that with just 10mV RMS across the coupling capacitor, we were measuring distortion levels as high as 0.1% at 10kHz. Luckily, the same attribute that gives C0G/NP0 a near-zero temperature coefficient means they also have a very low voltage coefficient and so are free of this problem. The output filter capacitor can either be a high-voltage SMD NP0 ceramic or through-hole polypropylene. Its linearity is absolutely critical to performance. Both types are acceptable. However, the NP0 ceramic may be a better bet as we’ve found several different 250VAC polypropylene capacitors with less-than-ideal linearity. We tested several suitably-rated polypropylene capacitors, some of which were X2 types, intended for mains applications. Of these, two introduced measurable distortion of around 0.001% in a simple RC filter (with a 6.8Ω resistor) at just 12V RMS. One X2 capacitor, and the a 400V DC/250VAC type from Epcos/TDK, measured much lower at around 0.0004%. So if you are going to use a polypropylene capacitor we highly recommend sticking to the type we have specified in the parts list. Others may have similarly low distortion but without a high-performance distortion analyser, there’s no way of telling. We do not recommend you use an X2-rated polypropylene as a consequence. Semiconductors In the preview last month, we explained the rational behind changing the small-signal transistors and the advantages of the new parts. Besides replacing the obsolete parts, one of the biggest benefits is that with the input pair in a single package, there will be very little drift in the output offset voltage with temperature as they will track closely. The output transistors, driver transistors and VBE multiplier are identical to those used in the Mk.3 amplifier as these all need to be mounted on the heatsink. The driver and output transistors are among the best available so we didn’t see any point in changing those. By the way, the heatsink mounting arrangement is identical, so it’s easy to replace a Mk.2 or Mk.3 module with the Mk.4 version, by simply replacing the PCB assembly. Next month That’s all we have space for now. Advertising Index Altronics.................................. 80-83 Aust. Exhibitions & Events.............. 5 Av-Comm Pty Ltd........................... 7 Emona Instruments...................... 63 Gooligum Electronics................... 12 Hare & Forbes.......................... OBC High Profile Communications..... 111 HK Wentworth Pty Ltd.................. 64 Icom Australia.............................. 13 Jaycar .............................. IFC,53-60 KCS Trade Pty Ltd........................ 75 Keith Rippon .............................. 111 Keysight Technologies.................. 65 LD Electronics............................ 111 LEDsales.................................... 111 Master Instruments........................ 3 Microchip Technology................... 11 Mikroelektronika......................... IBC Ocean Controls.............................. 8 Premier Batteries Pty Ltd............... 9 Qualieco Circuits Pty Ltd.............. 63 Questronix.................................. 111 Radio, TV & Hobbies DVD............ 25 Sesame Electronics................... 111 Silicon Chip Online Shop.... 104-105 Silicon Chip Subscriptions......... 103 Silvertone Electronics.................. 15 Trend Lighting............................. 111 Tronixlabs................................... 111 Worldwide Elect. Components... 111 Next month we will present the power supply, PCB overlay and photos of the final prototype, along with construction details. We’ll also describe a slightly cheaper, cut-down version of the amplifier for lower power applications, without compromising its SC excellent performance. WARNING! SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of SILICON CHIP magazine. Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. SILICON CHIP also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws. Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Competition & Consumer Act 2010 or as subsequently amended and to any governmental regulations which are applicable. 112  Silicon Chip siliconchip.com.au