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By PETER SMITH Studio Series Stereo Preamplifier A S up er b P r ea m p li f ier F or T h e A u dio E n t h us i as t ! This brand new, easy-to-build preamp features the latest high-performance audio op amps for ultra-low noise and distortion. Its modular design incorporates five switched RCA inputs and support for a headphone amplifier. T HESE DAYS, audio power amplifiers that produce low noise and distortion and cost only a few hundred dollars are relatively easy to find. In fact, they’re built into many of the latest multi-channel home theatre systems. Much of this gear is based around hybrid amplifier modules, which typically produce distortion levels in the 0.02% realm. Those serious about their audio will demand a much higher level of per26  Silicon Chip formance than can be found in these mass-produced units, which explains why the discrete power amplifier projects described in SILICON CHIP are so popular. For example, the 15W Class-A Stereo Amplifier described in July and August 1998 still gets a high ranking, as does the 100W “Ultra-LD” class-AB design described more recently. These amplifiers are expensive to build but offer performance that typically costs many times more in comparable commercial units. Having built one of our high-performance amplifiers, many readers have also asked us for a matching preamplifier design. And so our design brief was simple: a minimalist approach, focused on achieving ultra-low noise and distortion, but with enough gain (with the “wick” wound right up) to overdrive any of our audio power amplifiers, including the big 350W and 500W units. So, what were our options? Discrete versus integrated Initially, we were aiming for a discrete class-A amplifier design, speculating that this would be the best way to achieve the ultra-low distortion figures that were required. Another option was to use boutique op amps siliconchip.com.au specified for hifi audio use, such as those manufactured by Analog Devices and Texas Instruments. High linearity and the lack of crossover distortion are the major reasons for the use of class-A mode in audio applications. However, when compared to an equivalent op amp design, a discrete class-A amplifier would have consumed a large amount of PC board space, making the completed module physically large and more prone to radiated noise. It would also be considerably more difficult to build, containing many more components than an equivalent op amp design. We then looked at the current audio op amp offerings and their implementation. In many of our past designs, we’ve used the industry-standard NE5534 and LM833 devices. These are relatively cheap and easy to obtain, and both typically produce about 0.002% total harmonic distortion (THD) at 1kHz when driving a 2kW load. Don’t get us wrong – this is a very good figure – but it just wasn’t good enough for our new preamp! Our intention was (and is) to produce a preamp which causes virtually no signal degradation when teamed with our benchmark class-A 15W amplifier. From the limited selection of audiospecific op amps available, most were deemed either too expensive or too hard to obtain. However, the BurrBrown (Texas Instruments) OPA134 series is not expensive and easy to obtain and it produces an extremely low 0.00008% harmonic distortion at 1kHz! This is more than an order of magnitude (25 times better!) below the figures for the op amps mentioned earlier and with all things considered, would give superior performance compared to a discrete class-A design. Incidentally, the output stages of these op amps do not run in class-A mode, despite their excellent linearity. The manufacturer’s datasheets do not reveal how they have achieved these impressive results. Extra features We’ve stuck to our minimalist brief and added just two features to the basic preamp. The first of these addresses a common problem faced during preamp construction: how to switch the various signal inputs through to the preamp input while maintaining low noise and crosstalk. Typically, multiple inputs are hansiliconchip.com.au Features & Performance Main Features • High performance design – very low THD+N • Five on-board RCA inputs • Passive-switched inputs maintain signal integrity • Switched headphone amplifier output Measured Performance Frequency response...... flat from 10Hz to 20kHz, -1dB <at> 82kHz (see Fig.5) Maximum input signal..................................... 2.9V RMS (9.5V RMS output) Input impedance...................................................................................~90kW Output impedance..................................................................................100W Harmonic distortion.......................................... typically <.0005% (see Fig.7) Signal-to-noise ratio........................................................ -102dB unweighted Channel crosstalk........................................ -96dB <at> 1kHz, -73dB <at> 10kHz Source crosstalk........................................ -110dB <at> 1kHz, -93dB <at> 10kHz Note: all measurements were performed at the maximum volume setting with the output driving a 50kW load. Input signal amplitude was 600mV RMS (2V RMS output). For crosstalk measurements, non-driven inputs were backterminated into 600W. dled by fitting a bunch of RCA sockets to the rear panel and laboriously wiring these to a rotary switch on the front panel with shielded cable. Alternatively, the RCAs are mounted on a PC board at the rear along with the switch, which is then piped through to the front panel with a long extension shaft. While these methods work, they have their disadvantages. What’s more, they don’t allow for remote control selection! We’ve opted for an electro-mechanical solution, using passive (relay) switching for minimum impact on the audio signal. Each stereo input has its own miniature relay, positioned right next to the RCA socket. This gives absolute minimal source crosstalk and less induced noise, even when compared to some cabling schemes. The second feature is closely related to requests we’ve had for a high-quality headphone amplifier that runs off the preamp (rather than power amp) stage. In support of this idea, we’ve included a relay circuit that can route the preamp’s output to an RCA socket at the rear or a terminal block on the inside, where it would connect to a separate headphone amplifier board. In summary, to operate as a fully functioning unit, the Studio Series Preamplifier module requires only a volume potentiometer, source selection switch and low-noise power supply, all of which are described in detail in this article. In the pipeline Over the coming months, we hope to describe a high-quality headphone amplifier module to suit. We’re also developing a companion control board, which would feature an infrared remote control (motorised) volume pot and remote source selection. In the final article, we’ll show you how to put all of these modules together in a slim rack-mount case. In fact, we’ve heard rumours that Altronics will have a very nice screen-printed and punched case to accept all these goodies. How it works The preamp consists of two identical signal paths from input to output, catering for the left and right stereo channels. Therefore, to avoid duplication and reduce clutter, our circuits show only the left channel. We’ve also divided the preamplifier circuit diagram into two sections, corresponding to the input signal switching (Fig.1) and preamplifier functions (Fig.2). Referring first to the signal inputs (Fig.1), no less than five RCA sockets October 2005  27 28  Silicon Chip siliconchip.com.au Fig.1: the preamp’s input and output switching circuits. Passive (relay) rather than active switching is used to have minimal effect on the audio signal. Any one of five RCA inputs can be selected by bringing the base of the associated relay driver transistor to ground. Fig.2: the amplifier part of the preamp is based on Burr-Brown high-performance OPA2134 audio op amps (IC1 & IC2). To save space, only the left audio channel is shown here – the right channel is identical. (CON8-CON12) are provided for connection to various audio sources. We’ve used labels such as “CD”, “DVD” and “TAPE”, but of course, these inputs will accept any audio signal classed as “line-level”. The sixth socket (CON13) simply loops the selected input pair back out, duplicating the “tape loop” function found in some preamps and control units. Each input pair is connected to the normally-open contacts of a relay, with the poles of all relays connected together. The relays are driven with PNP transistors (Q1-Q5) from the +5V rail, such that when the base of a transistor is pulled to ground it switches on, energising the relay. This closes the relay contacts and connects the signal pair through to the amplifier input. We have not used the normally closed contacts of the relays. With a slightly different switching arrangement we could have used these to short the outputs of the “unselected” program sources to ground. For example, this would stop the audio from a tuner being heard at low levels when a Fig.3: a single-pole 5-position rotary switch can be wired to the 10-way header to provide source selection. siliconchip.com.au CD player was selected. This approach would have ensured low source crosstalk but we felt that shorting some program sources may not be desirable. In any case, we have managed to keep source crosstalk very low, at around -110dB. The bases of Q1-Q5 are connected to a header (CON4) via 3.3kW resistors, so it’s simply a matter of grounding the designated header pin to select a particular signal source. A rotary switch can be used to perform this function, as shown in Fig.3. Note that the relay control circuits operate from completely separate power and ground rails. In fact, we’ve used a different ground symbol for the relay circuits to indicate that this rail is not connected to the amplifier ground rail on the preamp PC board. Instead, the two ground rails are connected only on the power supply board to minimise noise. Also shown on this circuit (Fig.1) are the coils for relays RLY6 & RLY7 and their control circuit. These form part of the preamplifier’s output signal October 2005  29 routing, which we describe in detail later. Fig.4: we designed a separate low-noise linear supply for the preamp based on common 3-terminal regulators. A regulated +5V output is included for powering the switching circuits and future add-ons. FET op amps 30  Silicon Chip The core function of the preamplifier is performed by a pair of BurrBrown OPA2134 dual audio op amps (IC1 & IC2), as shown in Fig.2. The audio signal from the selected source is AC-coupled to the input of the first op amp (IC1a) via a 47mF capacitor, while a 100kW resistor to ground provides input termination. A simple low-pass filter formed by the 1.2kW resistor and 56pF capacitor attenuates RF frequencies ahead of the op amp input. A relatively large resistor value can be used here because of the OPA2134’s true FET inputs, which present an impressively large 10TW (Teraohms!) impedance. The -3dB point of the filter was chosen to be about 100 times greater than the highest audio frequency, to have minimal effect on the audio signal. The voltage gain of the op amp is set to about 3.3 (10.5dB) by virtue of the 4.7kW and 2kW feedback resistors. The 4.7kW resistor and 220pF capacitor combination roll off the top end frequency response, with a -3dB point at 154kHz. As can be seen from the frequency response graph (Fig.5), this gives a flat response over the audio spectrum while eliminating the possibility of high-frequency instability. The output from IC1a (pin 1) drives one end of a 10kW potentiometer (VR1a) via a 22mF non-polarised coupling capacitor. The pot acts as a simple voltage divider, with more of less of the amplified signal appearing at the input of the second op amp (IC1b), dependent on wiper position. You’ll note that the wiper of the pot is also AC-coupled, again with a non-polarised capacitor. This is done to prevent any DC voltage appearing across the pot, which if present would cause an irritating sound during wiper movement. We’ve used the second op amp in the package (IC1b) as a unity-gain buffer, allowing the preamp to provide a low-impedance output regardless of volume control setting. A 47mF nonpolarised capacitor couples the audio signal to the output via a 100W resistor, which is included to ensure stability when driving cable and amplifier input capacitance. Together with the ferrite bead, it also helps to attenuate siliconchip.com.au Fig.5: a plot of the frequency response for both channels shows a ruler-flat response over the entire audio spectrum, after which the curve gently rolls off, with a –3dB point at 154kHz. RF noise that might otherwise find its way back into the preamp circuit. Impedance matching As mentioned, op amp IC1b is configured for unity gain, so its output (pin 7) must connect back to its inverting input (pin 6). Note, however, that we show a resistor (R1) in the feedback path. Those familiar with op amps will know that a resistor can be included in this loop to impedance match the two inputs. Like many op amps, the OPA2134 shows an increase in distortion in noninverting applications if the impedance seen by its positive and negative inputs is not matched. Unfortunately, the input impedance that the negative input of IC1b “sees” varies with the wiper of the pot. Despite this shortcoming, the distortion levels of the OPA2134 are very low even at the worst case wiper position, where noise far outweighs distortion anyway. Nevertheless, we’ve provided positions on the PC board for two impedance-matching resistors (R1 & R2). If desired, you can install equal value resistors (instead of wire links) in these two locations that approximate the wiper-to-ground resistance of the volume pot at your typical listening levels. This extra little feature allows you to obtain the very best performance from your preamplifier module! Of course, the said wiper resistance can only be determined after you’ve used the preamp with your complete stereo system and favourite siliconchip.com.au Fig.6: crosstalk between channels is also very respectable. Increasing crosstalk at the higher end of the scale indicates electrostatic coupling, due to the physical proximity of the channels and the long PC tracks connecting the relays. music for awhile, so wire links are installed in these locations during construction. We suspect that most constructors won’t bother to replace the wire links! Output switching Finally, provision has been made to allow the preamp output to be switched between the RCA socket pair at the rear (CON14) and a terminal block (CON6). The latter connector is intended for use with an internal high-quality headphone amplifier, presently under development. Two relays (RLY6 & RLY7) are used to allow the non-driven input to be grounded. Relay operation is dictated by the insertion and removal of the headphone jack, which operates a switch inside the jack socket. The jack switch is wired to CON7 (Fig.1), where it controls transistor Q6 to drive the two relay coils. With a jack in the socket, the switch is open and the base of Q6 is pulled high via diode D6 and the two 1.5kW resistors. This turns Q6 on and energises both relay coils, directing the output signal to CON6 and the headphone amplifier. When the jack is unplugged, the socket switch closes, grounding the “SWITCH” input on CON7 and stealing Q6’s base current. After a short delay, the transistor (and the relays) switch off, redirecting the output signal to CON14 and the power amplifier. The diode, capacitor and resistors are included in the base circuit of Q6 to slow the circuit’s response to changes at the switch input. This helps to minimise relay chatter during jack insertion and removal. Power supply To ensure the best possible perfor- Fig.7: all our audio tests were performed in-house on our Audio Precision System One. This graph shows total harmonic distortion & noise versus frequency. The reading is mostly below .0005%. However, this figure is barely above the noise floor of the test instrument, so the real value is probably much lower! October 2005  31 Fig.8: follow this overlay diagram closely when assembling your preamp board. Wire links should be installed for R1 and R2 but these can be replaced with resistors later for a small improvement in performance (see text). As noted, the components within the dotted line aren’t needed in all cases but it does no harm to install them anyway. 32  Silicon Chip mance, we’ve designed a separate, low-noise power supply for the preamp module. It provides regulated ±15V and +5V outputs for the preamp and any future add-ons. The power supply board accepts a 30VAC centre-tapped transformer input, typically formed by joining two 15VAC secondary windings of a toroidal transformer – see Fig.4. Four diodes (D1-D4) and two 2200mF capacitors rectify and filter the input to create ±21V DC (nominal) rails. LM317 and LM337 adjustable regulators generate the complementary positive and negative supply rails. Their outputs are programmed to ±15V by virtue of the 100W and 1.1kW resistors connected to the “OUT” and “ADJ” terminals. We’ve used adjustable regulators in this design because the “ADJ” terminals can be bypassed to ground to improve ripple rejection, which we’ve done using 10mF capacitors. The associated diodes (D5 & D7) provide a discharge path for the capacitors should an output be accidentally shorted to ground. Two reverse-connected diodes (D6 & D8) across the output prevent their respective rails from being driven to the opposite polarity (eg, if a regulator fails), something that should never occur during normal operation. A 7805 regulator (REG3) is used to generate the +5V rail. The 100W resistor in line with REG3 reduces power dissipation in the regulator. While this resistor is not strictly necessary when powering only the preamp module, it will certainly be required for future add-ons, which will demand considerably more current! As the +5V supply draws power from only the positive side of the unregulated DC input, a 330W resistor across the negative input is included to balance the rails so that they decay at similar rates at power off. Preamp assembly Assembly of the preamplifier board is quite straightforward, as all components (except for the volume pot) mount on a single-sided PC board measuring 73 x 192mm. Use the overlay diagram in Fig.8 as a guide to component placement. If you won’t be connecting a headphone amplifier to the board later on, then installation of the associated output switching circuitry is optional. siliconchip.com.au This prototype preamp board varies slightly from the final version shown in Fig.8. The miniature relays switch the selected source to the preamp stages and switch the preamp output between the external power amplifier and an optional internal headphone amplifier (to be described in a coming issue). The components involved are RLY6, RLY7, CON6, CON7, D6-D8, Q6, a 100mF capacitor and a few resistors. Fig.8 shows these items enclosed within a dotted outline, for easy identification. You’ll find assembly much easier if you install the wire links, resistors and diodes first. Note that two of the wire links pass partially beneath the 220pF capacitors and these must be fashioned from 0.7mm tinned copper wire or similar. Zero ohm “resistors” can be used in place of wire links in the remaining 11 positions, if desired. These are shaped just like conventional 0.25W resistors and are identified by their brown body and single black band. Although they impart a neater appearance to the finished work, they have no electrical benefits over ordinary copper wire! For the time being, you should also install wire links in place of resistors R1 & R2. Note that the two 100W resistors require special treatment. Slip a 5mm ferrite bead over one lead before bending and inserting each resistor into its PC board holes. The relays (RLY1-RLY7) can go in next, taking care to insert them the right way around. The white line on the top of the package must match the corresponding marking on the overlay diagram. Remember that RLY6 & RLY7 can be left out if headphone amplifier switching isn’t needed, as explained earlier. However, you must fit two wire siliconchip.com.au links in place of the relays, as shown in Fig.8. Install the two 8-pin IC sockets and the 10-way header (CON4) next. Note that one side of the header housing is keyed and this must be oriented towards the centre of the board. Likewise, the notched (pin 1) end of the IC sockets must be correctly oriented. Follow with the screw terminal blocks, all of the capacitors and the transistors. Five of the electrolytic capacitors (100mF & 10mF values) are polarised and must be installed with their positive leads aligned as shown. The remaining electrolytics are nonpolarised (marked “NP” or “BP”) and can go in either way. The RCA connectors (CON8-CON14) go in last of all. Be sure to push each connector all the way home and check that it’s sitting perpendicular to the board surface before soldering. be mounted about 2mm proud of the board surface. Take care with the orientation of the electrolytic capacitors, as all on this board are polarised. Also, be sure not to interchange the two adjustable regulators (REG1 & REG2) and note that they face in opposite directions! Unlike REG1 & REG2, regulator REG3 mounts horizontally. Bend its leads at 90° about 5mm from its body and trial fit it in position to verify that Power supply assembly The power supply PC board is a relatively simple design and should not present any particular assembly problems. Apart from the mains transformer and wiring, all components mount on a single-sided PC board measuring 54.6 x 80mm, including the bridge rectifier, filters and voltage regulators. As before, install all of the lowprofile components first, starting with the single wire link, resistors and diodes (see Fig.10). To aid heat dissipation, the two 5W resistors should Fig.9: here’s how to wire both halves of the dual-gang volume pot. We plan to present a motorised volume control in a future instalment. If you can’t wait, then check out the Infrared Remote Volume Control published in June 2002. October 2005  33 board in the holes provided using M3 x 6mm screws. Mains wiring The power supply board should only take a few minutes to assemble. All connections are made via screw terminal blocks. Fig.10: follow this diagram to assemble the power supply board. Take care not to get the 3-terminal regulators mixed up. the hole in the tab lines up with its hole in the board. Adjust as necessary, then slide a TO-220 heatsink between the regulator and the PC board after applying a thin smear of heatsink compound to the mating surfaces. Secure the assembly to the board with an M3 x 10mm screw, flat washer & nut. Don’t solder the regulator’s leads until after the screw has been tightened, otherwise the PC board or regulator package could be damaged. Before moving on to the wiring, attach four threaded standoffs to each It’s very important that the power supply is checked out before it’s connected to the preamplifier module. To do this, first assemble the transformer into your metal project case. For best results, the mains transformer should be located as far away from the preamp board as possible to minimise induced noise. A toroidal model is recommended for its low radiated field and low physical profile. Important: a full metal case is recommended for this project. Plastic will not provide the necessary electrical screening! Connect the mains (primary) side of the transformer, using the basic diagram in Fig.4 as a guide. Be sure to adhere to any instructions provided with the transformer, particularly with regard to mounting, fuse ratings and wire colour coding. All work must be carried out professionally and in accordance with mains wiring practices. In particular, ensure that all live connections are properly insulated, which includes the use of rubber boots (or equivalent) over the rear of all switches and mains sockets. The mains wiring is not complete until the mains earth is secured to the metal chassis using the scheme shown in Fig.11. That done, use your multimeter to verify that a good electrical connection exists between the earth pin of the mains plug and all panels of the metal chassis. Power supply test The power supply test is uncompli- Electrolytic vs Polypropylene Capacitors H IGH-CAPACITANCE non-polarised electrolytics are used for signal coupling throughout this design. The results are excellent, as reflected in the various performance measurements. However, some hifi proponents will be unhappy with this choice, instead insisting that polypropylene capacitors somehow “sound” better than electrolytics when used in the audio signal chain. To explain, polypropylene capacitors have a much lower dissipation factor (DF) and lower dielectric absorption 34  Silicon Chip (DA) than electrolytics; a major reason for their use in sample-and-hold circuits, high-frequency filter networks and speaker crossovers, for example. However, their benefits in low-level audio frequency circuits are much harder to quantify, especially considering their proportionally larger size, higher cost and limited local availability in appropriately large values. Those with a personal preference for polypropylene capacitors can of course substitute these for the specified non-polarised electrolytics, given sufficient board space and part availability. Smaller capacitance values will need to be used for polypropylene substitutes due to the sheer size and cost differences. To minimise impact on bass response, a minimum of 2.2mF should be substituted for the 47mF and 2.2mF electrolytics and a 4.7mF value for the 22mF electrolytic. It would also be preferable to use a 20kW log pot for VR1. Note that the use of physically larger coupling capacitors is likely to increase noise and crosstalk. siliconchip.com.au Par t s Lis t Fig.11: the mains earth terminal is secured to the case as shown here. The top nut serves as a lock-nut, so that the assembly cannot possibly come loose. cated and involves simply measuring the unloaded output voltage of the three supply rails. To do this, first connect the two secondary (15VAC) windings to the transformer input (CON1) of the board. Apply power and use your multimeter to measure the three rails at the supply outputs (CON2 & CON3). Assuming all is well, the +15V, -15V and +5V rails should all be within ±5% of the rated values. Low-voltage wiring Once you’re satisfied that the power supply is working properly, disconnect input power and wire up the ±15V and 5V outputs to the preamp supply inputs. Note that these supplies must be cabled separately, meaning that two wires are required for the 5V supply (+5V & GND) and three for the ±15V supply (+15V, -15V & GND). Use medium-duty, multi-strand hook-up wire for the job and twist the wires tightly together to reduce noise and improve appearance. Again, run the cable for the 5V supply separately; do not twist it together with the ±15V wiring. For most installations, the preamp’s common (GND) rail should be connected to chassis earth. This is achieved by running a wire from the pad marked “EARTH” on the preamp board to the main chassis earth point. Do not connect any other part of the preamp circuit or power supply to chassis earth (except the volume pot, see below). The volume pot must be a dual-gang logarithmic type, preferably 10kW in value. If using a motorised pot, a 20kW value may have to suffice. Don’t use a larger value, as this will affect the preamp’s noise performance. Do use twin-core shielded cable for each siliconchip.com.au 1 PC board coded 01109051, 73mm x 192mm 7 DPDT 5V DIL relays (RLY1RLY7) (Altronics S 4128B) 7 PC-mount gold-plated dual RCA sockets (CON8–CON14) (Altronics P 0212) 1 10-way 2.54mm PC mount shrouded header (CON4) (Altronics P 5010) 4 3-way 5mm/5.08mm terminal blocks (CON1-CON3, CON6) 2 2-way 5mm/5.08mm terminal block (CON5, CON7) 2 5mm ferrite beads (L1, L2) (Altronics L 5250A) 2 8-pin gold-plated IC sockets 4 M3 x 10mm tapped spacers 4 M3 x 6mm pan head screws 7 self-tapping screws (for RCA sockets) Semiconductors 2 OPA2134PA dual FET-input op amps (IC1, IC2) (Farnell 791-039) 5 BC327 PNP transistors (Q1-Q5) 1 PN100 NPN transistor (Q6) 8 1N4148 diodes (D1-D8) Capacitors 3 100mF 16V PC electrolytic 2 10mF 16V PC electrolytic 4 47mF 35V/50V non-polarised PC electrolytic (max. 8mm diameter) 2 22mF 35V/50V non-polarised PC electrolytic (max. 8mm diameter) 2 2.2mF 35V/50V non-polarised PC electrolytic (max. 5mm diameter) 5 100nF 50V metallised polyester (MKT) 2 220pF 50V ceramic disc 2 56pF 50V ceramic disc Resistors (0.25W 1%) 2 1MW 2 2kW 6 100kW 2 1.5kW 6 10kW 2 1.2kW 2 4.7kW 2 100W 5 3.3kW 13 0W (for links) section of the pot, wired as shown in Fig.9! The metal body of the pot must be connected to chassis earth to reduce Additional items 1 dual-gang 10kW log potentiometer 1 single-pole 5-position rotary switch (eg, Altronics S 3021) 1 10-way IDC cable-mount socket 10-way IDC ribbon cable 2-core shielded audio cable for volume pot connections Medium-duty hook-up wire for low-voltage wiring Power Supply 1 PC board coded 01109052, 54.6 x 80mm 1 Micro-U 19°C/W TO-220 heatsink (Altronics H 0637) 2 3-way 5mm/5.08mm terminal block (CON1, CON2) 1 2-way 5mm/5.08mm terminal block (CON3) 4 M3 x 10mm tapped spacers 5 M3 x 6mm pan head screws 1 M3 nut & flat washer Semiconductors 1 LM317T adjustable positive regulator (REG1) 1 LM337T adjustable negative regulator (REG2) 1 7805 +5V regulator (REG3) 8 1N4004 diodes (D1–D8) Resistors 2 1.1kW 0.25W 1% 2 100W 0.25W 1% 1 330W 5W 5% 1 100W 5W 5% Capacitors 2 2200mF 25V PC electrolytic 2 100mF 16V PC electrolytic 1 47mF 25V PC electrolytic 3 10mF 16V PC electrolytic 2 100nF 50V metallised polyester (MKT) Additional items: 1 15V+15V 30VA toroidal transformer 1 Mains switch and connection hardware noise pickup. Do not connect the body to either of the shielded cables! Normally, the front panel will provide the necessary earth connection. If October 2005  35 OPA134 Series High-Performance Audio Op Amps Fig.12: at unity gain, the THD+N performance for these op amps is almost invisible and certainly inaudible! This graph is reproduced from the datasheets, which can be obtained from the Texas Instruments website at www.ti.com. T HE OPA134 series op amps include single (OPA134), dual (OPA2134) and quad (OPA4134) versions. The series is fully specified for audio applications, boasting ultra-low distortion and low noise. They include true FET input stages to provide superior sound quality and speed for exceptional audio performance. This in combination with high output drive capability and excellent DC performance allows use in a wide variety of demanding applications. In addition, the OPA134’s wide output swing, to within 1V of the rails, allows increased headroom, making it ideal for use in any audio circuit. OPA134 op amps are easy to use and free from the phase inversion and overload problems often found in common FET-input op amps. They can be operated from ±2.5V to ±18V power supplies. Input cascode circuitry provides excellent common-mode rejection and maintains low input bias current over its wide input voltage range, minimising distortion. These op amps are unity-gain stable and provide excellent dynamic behaviour over a wide range of load condi- it doesn’t, then connect the pot to a convenient chassis earth point using hook-up wire. Note that solder won’t adhere to the plating on the pot, so remove a small area of the plating with an ink rubber or scouring pad prior to tinning. If you’ve installed the headphone 36  Silicon Chip Fig.13: our preamp uses the OPA2134 (dual) version, which follows the industry-standard package configuration. tions, including high load capacitance. The dual and quad versions feature completely independent circuitry for lowest crosstalk and freedom from interaction, even when overdriven or overloaded. Another strong characteristic of this series is its extremely low signal distortion. Total harmonic distortion plus noise (THD+N) is below 0.0004% throughout the audio frequency spectrum (20Hz to 20kHz) with a 2kW load. In fact, the THD+N produced by these op amps is below the normal measurement limit of all known commercially available test instruments! amplifier switching circuitry (RLY6, RLY7, etc) and have a suitable amplifier board, then connect the headphone audio output (CON6) to the input of your headphone amplifier using twincore shielded cable. In addition, the switch contacts of the headphone jack socket must be wired to CON7. Many jack sockets have isolated switches built in, so all you need to do is connect across the normally-closed terminals of one of the switches. However, the switch contacts in some sockets share a ground connection with the audio signal. If you have this type of socket, then find the contact that is disconnected from ground when the jack is inserted and connect this back to the “SWITCH” input of CON7, leaving the “GND” input disconnected. This avoids creating a certain earth loop in your system! Important: if the headphone jack switch isn’t connected to the preamp board, then you must insert a shorting link between the two terminals of CON7; otherwise, you’ll get no signal from the RCA output (CON14)! Source selection As mentioned earlier, each RCA input pair is individually selectable via one control line on the 10-way header (CON4). To select a particular input, simply connect the associated control line to ground (GND). While we intend to describe a means of remotely controlling the preamplifier’s source switching (and volume!) in a future issue of SILICON CHIP, a far cheaper and simpler method is to use a mechanical switch. All that’s required is a single-pole 5-way rotary switch, a 10-way IDC cable-mount socket and a length of IDC cable. As the cable doesn’t carry lowlevel audio signals, it can be routed anywhere you like within your case without regard to length. The equivalent electrical circuit for the switch wiring is given in Fig.3. Final checks Before connecting inputs and outputs, power up and with your negative meter probe touching a handy ground point, measure the voltage on the power supply pins of the two op amps. Obviously, pin 8 should measure +15V and pin 4 should measure -15V. In addition, the outputs (pins 1 & 7) of both op amps should be within a few mV of ground. Finally, if you’ve connected a source selection switch, you should be able to hear the relays clicking when you rotate the knob. OK, that’s it – you should now have a working hifi preamp! Happy SC listening! siliconchip.com.au