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AUDIO OUT AUDIO OUT L R By Jake Rothman Analogue Vocoder – Part 4: Driver Amplifier L ast month, I introduced the is dependent on the transformers primary DC resistance and the gain. Fig.12 shows the board wiring. I used this circuit as an excuse to try a new transformer from Vigortronix, the VTX-102-3002, which has a very high saturation level of 37Vpk-pk at 30Hz into 600Ω. It has an upper frequency resonance at 88kHz which requires a Zobel network (Cz and Rz) to damp it. These transformers have a silicon-iron core, and this driver circuit reduces the distortion below that of some expensive nickel-core designs, while maintaining the high saturation. balanced-input Vocoder Driver Amplifier which is used to drive the multiple filters used in the design. This month, we will complete the Driver Amplifier, looking at the design’s output driver circuits and of course components and construction. Transformer driver For our Vocoder, the Driver Amplifier’s output uses a balancing transformer, and hence it is a floating output which prevents ground loops. These can occasionally saturate on low frequency peaks, greatly increasing the current demand from the amplifier, which can result in clipping that may hang on for a few cycles. The additional current capability of this circuit reduces this horrid-sounding effect. Also, transformers give lower distortion with low-source impedance drive. Better still, is to make the output resistance negative to cancel out the transformer’s primary DC resistance by using positive feedback. Fig.11 shows a circuit to do this, using a sensing resistor RSENSE. This resistor value High-impedance speaker driver An additional use for this Driver Amplifier is as a low-power speaker/monitor driver. Most analogue musical products or systems, such as Eurorack synthesisers operate on ±15V rails and often a small speaker is required for monitoring. If a typical power amplifier chip, such as a TDA2030 is used with a standard 8Ω speaker, the output of 10W and the current consumption of over 1A is excessive. Using a high-impedance 50Ω R8 2.2kΩ VTX-101-3002 Bottom view 28x33mm To op amp R5 100kΩ C5 22µF + Gain = 2x Link Link Input Input Link Link – Current boost TR1/TR2 Link out 17Ω coupling capacitors C10/C11 1.8kΩ 13nF For gain = 2x, insert RSENSE = 13Ω R6 in position shown bottom of Fig.17b 100kΩ Use 11Ω for VTX-101-007 with the primaries wired in parallel (shown above) 0V Male XLR output 1+1:2 + VTX-101-007 (Bottom view) 23x28mm Screen To pin 2, XLR To sense resistor 2 33nF 30Ω 1.1kΩ + 1 – 3 Metalwork 17Ω C5 22µF + Input C4 22µF + R7 1.1kΩ Screen Link C12 15pF Fig.12. Assembled transformer output Driver Amplifier – this is basically a summing amplifier with a sensing resistor in the bottom right-hand corner and a tantalum bead coupling capacitor in the R4 position. The negative terminal goes towards the chip. The return from the transformer goes to the terminal pin inserted next to the capacitor To pin 3, XLR positive in the hole for R2. Zobel network RSENSE ‘Dots’ indicate phase Fig.11. Transformer driver circuit with negative resistance output to minimise distortion. Note the amplifier circuit is inverting and correct phase is achieved by flipping the secondary output wires to the XLR socket. If the sense resistor RSENSE is too large the circuit will oscillate at a very low frequency – best optimised by experiment. 56 speaker gives a power output of 1.2WRMS with a current consumption of 50mA, which can easily be accommodated by normal op amp power rails. A similar power consumption situation arises when driving spring-lines reverberation units. For these low-power amplifier applications, heatsinks on the output transistor are necessary. For higher rail voltages, an 80Ω speaker should be used. A speaker driver combo is shown in Fig.13. Note that high-impedance speakers are available from the AO Shop, see components below. Bridge-drive output Another useful, non-Vocoder application of this design is to use Practical Electronics | February | 2022 Components list All parts listed here are for the balanced-input Vocoder filter Driver Amplifier, the ones marked with a single * are for the Vocoder output (summing) amplifier only, and the ones with double ** are for the balanced-input Vocoder filter Driver amplifier only. (No * means parts are common to both.) Semiconductors IC1 NE5534P low-noise audio op amp TR1 2N2219 NPN medium-power TO5/39 or equivalent, such as BFX85, Rapid order code 81-0116 TR2 BC143 PNP medium-power TO5/39 or equivalent, such as BC161-16 CPC order code SC15338 Any TO5 type with specification: Vce > 45V (60V for OPA604), Hfe > 50, Ic > 650mA, Ptot > 600mW, Ft > 50MHz. (Multicomp Pro make TO5 and TO18 transistors, they are available at a good price from CPC Farnell Preston UK) D1, D2 Any low voltage small Zener diode – eg, BZY88C2V7 n Resistors All standard audio 0.25W 1% metal-film, such as MSR25. Uncritical positions can be 5% carbon film. Fig.13. Assembled input Driver Amplifier feeding an highimpedance (80Ω) loudspeaker. a pair of Driver Amplifiers to produce a high-voltage bridged power amplifier. (Pads are provided for linking the balanced inputs.) Amplifiers with balanced inputs can be connected in anti-phase to produce a bridged output, as shown in Fig.14. In bridge mode, the real load impedance seen by the whole amplifier is halved because almost double the output voltage swing is achieved. This increases the maximum power by a factor of almost 3.5, so it is essential to have good heatsinks on the output transistors. Since only one bipolar electrolytic output capacitor is required in series with the load, C11 can be linked out of each board. Note that the phase of one of the boards must be reversed. This can be done by connecting one of the board’s balanced inputs in anti-phase (see Fig.15). This gives the benefit of equal input impedance on both input legs, unlike a single differential stage, and this improves interference rejection. Another method is to configure one amplifier as inverting and the other as non-inverting – see Fig.1, last month. Personally, I would always opt for a balanced-input design for lower hum. R1, R2 R3, R4, R5*, R6, R7 R8 R5 R9, R10, R13 R11, R12 R14 R15 R16 100Ω** 2.2kΩ 2.2kΩ 100kΩ* 4.7kΩ 5% 10Ω 22kΩ 5% 33Ω 5% 100kΩ 5% Capacitors C1**, C2**, C3** C6 C7 470pF 5% 5mm ceramic or film C12 15pF (typical) NP0 5mm ceramic, see text C4, C5, C8 22µF 25V C9 10µF 10V C10, C11 150µF 16V C13, C15 470µF 25V C14 100nF X7R 5mm ceramic Stereo It is possible to ‘piggyback’ two Driver Amplifier PCBs together for stereo/dual-channel use by linking the power connector, as shown in Fig.16. This amplifier design will also be used in dual-channel configuration for the Vocoder input and output amplification, which we will explain later in the series. Differential amplifier + XLR Balanced input 1 –3 + Balanced output + 2 Chassis – Load – + Differential amplifier – Fig.14. Balanced-bridged driver circuit – useful for balanced-in balanced-out set-ups. Practical Electronics | February | 2022 Fig.15. Wiring the inputs in parallel anti-phase on the bridged assembly. Notice the ‘flip’ on the wires going from J5 on the top board to J5 on the bottom board. This gives an equal input impedance on the ‘plus’ and ‘minus’ input pins. 57 Heatsinks (2 off) Thermalloy TO5 type 5F 12.5mm 0.5-inch high 45.2 C/W CPC part number SC10803, PR Electronics 300-005. The transformers and many other hard-to-find audio engineering parts are available from the AO Shop – see page 67 in last month’s PE. Assembly When building the circuit, follow the overlay shown in Fig.17. This PCB is a conventional leaded design, so when assembling, just note the points below. The output transistors are medium-power (metal can) TO5 types. These are more reliable than plastic-cased TO92 parts, such as the BC337/327 normally used for this job. They can also be simply fitted with push-on heat sinks. To ensure a solid mount, a nylon mounting pad should be used, as shown in Fig.18. This needs to be slid over the leads the right way up with the smallest end of the tapered holes towards the bottom. For a solid flush mount, solder one lead, then remelt it while at the same time pushing the device flat. If it’s tilting, then strain to the leads may result in the glass seals cracking. Remember the PNP transistor TR1 goes towards the corner of the board. To ease heatsink fitting, twisting a flat blade screwdriver in the slot while pushing down helps. This technique requires practice and is a bit risky. The professional technique is to use circlip pliers, as shown in Fig.19. These must be the external type that move outwards when squeezed. The emitter resistors R11 and R12 should be mounted a few millimetres above the board to prevent heat damage to the board in the advent of overcurrent, as shown in Fig.20. This is an important point with all power circuitry where certain resistors can burn under fault conditions. The Zener diodes D1 and D2 need to be mounted around 1cm high to make contact with the heatsinks. One of the good things about PCBs with plated-trough-holes is that components can be mounted up in the air without the risk of broken solder pads. Fig.16. Piggy-backing the power supply connections for stereo. Note: all electrolytic capacitor positions on the PCB are dual-outline, giving the option of using cheap radial wet aluminium types, or expensive bead/axial hermetic solid-tantalum types for long life. The main decoupling capacitors can also be upgraded to long-life solid aluminium. Inductors L1 1mH (only needed for long bus, >150mm) Sigma SC30 or other small axial ferrite type. Epcos B82144A2105J 1000µH ±5% 200mA RF Axial LBC Choke Rapid Order Code: 51-7618 L2 6µH to 15µH low resistance suppressor choke axial. Epcos B82141A1103K 10µH ±10% 410mA RF Axial SBC Choke Rapid Order Code 51-7613 T1 output transformers. Vigortronix VTX-101-3002 or VTX101-007 Rapid order code 88-2124. Note these need a few associated passive components, see text. Miscellaneous Molex 0.1-inch PCB interconnectors Transistor mounting pads (2 off) Nylon/PET TO5 RS 402175, Rapid 38-0278 R11 R15 R11 R15 TR2 D2 L2 TR1 R12 D1 D1 TR2 D2 L2 R12 Fig.17a. (left) Component overlay for the Driver Amplifier module. TR1 C14 C11 C8 C9 + C4 + C5 + R9 + + + C9 + + + C8 R10 C10 C5 C4 R10 Link R9 C14 nstall lue components if using transformer IC1 + IC1 C13 C15 C 7 C 6 Power R 6 R 8 R 4 R 1 R 2 L1 Sum input C 1 C 2 C 3 Diff input C 12 Chassis metalwork R 14 R 13 C13 R 16 C15 Output C 7 C 6 Power R 5 R 7 R 6 R 8 10µ F 25V Tant Insert vero pin L1* Sum input RSENSE 0V Input 0V 0V –15V +15V Output 0V Input (XLR-1) + (XLR-2) – (XLR-3) 0V Input 0V 0V –15V +15V Output 0V 58 can e linked out Transformer* Primary Output R 16 R 3 + R 13 R 7 + R 14 R 5 + C 12 Output Note: the blue components are only for the Vocoder output Driver Amplifier; the red components are only for the Vocoder balancedinput filter Driver Amplifier. All other components are common to both. Fig.17b. (right) Overlay for the low-distortion transformer-output circuit shown in Fig.11. PCB design: Mike Grindle Practical Electronics | February | 2022 Fig.18. Plastic mounting pad for supporting a TO5 transistor. Fig.19. Fixing the heatsink of a TO5 with circlip pliers. Testing Distortion Once you are happy with assembly and have double checked all parts for value, location, orientation and solder bridges, it’s time for testing. Turn on for a short period, make sure the current on each rail is below 30mA and nothing gets hot. The emitter resistors R11 and R12 will smoke if something is amiss, such as a polarised device inserted incorrectly, causing excess current. Left for more than a few seconds in this state and the transistors will join the resistors in the component graveyard. Since the op amp can withstand shorts to either power rail for a short period, it usually survives. Check the output offset voltage at the junction of the emitter resistors. Ideally, it should be less than 0.1V. Check the quiescent current by measuring the voltage across R11 and R12. It should be 0.1V. If it is much higher than this, then the quiescent current is too high. Occasionally, with transistors having an exceptionally low turn-on voltage or Vce, the emitter resistors will have to be increased. TO5 silicon planar devices typically range from 0.65 to 0.72V, so there should be no problems. Another useful check is to look for high-frequency (hundreds of kilohertz upwards to megahertz) output instability with an oscilloscope. This is an essential check if unusual reactive loads, different op amps or slow (Ft < 20MHz) output transistors are used. Distortion tests require expensive test gear, but it is not a worry for Vocoder use since the distortion from the VCAs is in the order of a few percent and swamps any amplifier-related distortion. However, if you want to use this amplifier with Hi-Fi then it’s vital to get the distortion as low as possible because op amp high-order, odd-harmonic distortion is quite unpleasant. All of the following commentary is related to HiFi, not Vocoder applications. If you are lucky enough to have a Marconi, Wayne Kerr AMS1, Radford or Audio Precision analyser you should be able to get some pretty graphs. This is a simple clean amplifier, so the distortion and frequency response curves can be rather boring. I’m not going to bother printing the frequency response because it’s just a flat line from 20Hz to 20kHz! Op amp selection Let’s first look at op amp choice – I tried several audio-quality devices. The ever-popular NE5534 gave the lowest distortion with the input balanced filter Driver Amplifier, at 0.001% – despite being a 1970s design. The OPA604 and the venerable LM318 gave slightly more distortion at 0.0015%. The LM318 was the op amp for low-distortion audio before the revolutionary TDA1034/5534 came out. However, it was noisy, at 50nV/ Hz. (True to form, the distortion was lower 0.1 0.05 0.02 0.01 0.005 % 0.002 0.001 0.0005 0.0002 0.0001 20Hz Fig.20. Emitter resistors R11/R12 mounted off the board to avoid heat damage to PCB laminate. Note the transistors used experimentally as bias diodes. These must be insulated since TO5 transistor cases are connected to their collectors. in the inverting amplifier configuration, by about half.) Here, the OPA604 was the winner with 0.0004% distortion and noise (THD+N). Ouput capacitor If you’re using our Driver Amplifier to supply an input to a Hi-Fi amplifier, then it must deliver zero DC offset. A good way to do this is by adding biased capacitors to the Driver Amplifier’s output (C10/C11). In this circuit, tests showed that the addition of this tantalum output capacitor network only doubled the very low distortion over 20-30Hz. The effect of load Next, Let’s look at loading effects. Reducing the Driver Amplifier’s load from 600Ω to 50Ω to simulate headphones or a speaker increases distortion a small amount, but it was still below the audibility threshold. In the same 50Ω loading situation, the TL071 op amp had insufficient drive capability and was therefore rejected. When adding the output balancing transformers, the distortion increases, especially at low frequencies. Fig.21 compares distortion levels with and without the negative-output-resistance, distortion-reduction technique discussed above. This graph used the Vigortronix VTX-101-3002 transformer. I found that a cheaper transformer, the VTX-101-007, can give very good distortion, but only at low levels, around 0dBm. This cheaper transformer is perfect for the Vocoder. Next month 50Hz 100Hz 200Hz 500Hz 1kHz 2kHz 5kHz 10kHz 20kHz Fig.21. (above) Distortion with the VTX-101-3002 transformer and (below) the result of negative output resistance using circuit in Fig.11. Here RSENSE = 18Ω, gain = 4×. Practical Electronics | February | 2022 We are roughly halfway through our all-analogue Vocoder project. Up next, we will discuss and build the two types of the all-important Vocoder filter boards – high/low pass and bandpass. 59