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This prototype unit differs slightly from the final version, which now also features a second headphone socket, so that two people can listen at the same time. By PETER SMITH Studio Series Stereo Headphone Amplifier A Top - Cl as s Un i t F or T he A u dio E n t h usias t ! Here’s a top-class headphone amplifier that can drive high or low-impedance ’phones to full power levels, with very low noise and distortion. For best performance, it can be teamed with the Stereo Preamplifier described last month. Alternatively, it can be used as a standalone unit, requiring only a power supply and a volume control pot for use with any line-level signal source (CD/MP3 player, etc). It even includes dual outputs, so you can listen with a friend! 26  Silicon Chip siliconchip.com.au M ANY OF OUR HIGH-POWER audio amplifier designs already provide an output for headphones. The additional circuitry required for headphone support is simple; just two resistors in series with the loudspeaker outputs to limit the drive current and protect the ’phones in the case of amplifier failure. Considering its simplicity, this resistive limiting scheme works well, although it will cause distortion if the load is non-linear – a likely prospect with most headphones. Apart from eliminating this potential source of distortion, there are a number of other reasons why you might consider building a separate headphone amplifier. For a start, not everyone owns a pair of top-rated headphones or even a high-performance power amplifier. After all, an amplifier that equals or betters the performance of this new headphone amplifier will set you back more than a few shekels! Another reason might be for use with the latest “high-tech” audio electronics gear. The headphone outputs in much of this gear cannot drive low-impedance ’phones – or at least not to decent listening levels. In addition, available output power in portable devices is deliberately limited to conserve battery energy. This means that lots of distortion might be present at higher listening levels, even with sensitive headphones. One way around this is to feed the line-level outputs of this gear into your power amplifier and then plug your low-impedance headphones into that. That works but then you’re tethered to an immovable object. Besides, the power required to drive headphones is around 1/1000th of that required to drive loudspeakers, so a large power amplifier could be considered a tad oversized for the job! Features & Performance Main Features • High performance – very low noise & distortion • Drives high and low-impedance headphones • High output power (up to 200mW into 8W and 32W) • Dual headphone sockets – can drive two pairs! • Works with a preamp or any line-level audio source Measured Performance Frequency response.............................flat from 10Hz to 20kHz (see graphs) Rated output power....................200mW into 8W and 32W, 85mW into 600W Max. output power (current or voltage limited)..................... 575mW into 8W, 700mW into 32W, 130mW into 600W Harmonic distortion......................................... typically .0005% (600W load), .001% (32W load) and .005% (8W load) Signal-to-noise ratio (A-weighted).................. -130dB (600W), -120dB (32W) and -111dB (8W) with respect to 100mW output power. Channel crosstalk ..................................better than -68dB from 20Hz-20kHz at 100mW output power (see graphs) Input impedance.......................................................................~47kW || 47pF Output impedance................................................................................... ~5W Note: all tests were performed with the amplifier driven from low source impedance. For crosstalk measurements, the non-driven input was backterminated into 600W. Design outline One of the challenges in designing a general-purpose, low-distortion headphone amplifier is catering for the huge variation in headphone specifications. Models with 8W (nominal) impedance are common, as are 32W, 60W, 120W and 600W versions – and many in between. At the high impedance end of the scale, a large output voltage swing will be necessary to develop full power, whereas at the low end, current limiting is needed to prevent driver siliconchip.com.au Fig.1: these plots of distortion versus frequency at 100mW highlight the impedance non-linearity of a real pair of 8W headphones. When driven directly from the low-impedance headphone amplifier output, performance is very good (bottom curve), as the amplifier’s feedback loop can act to linearise the signal. However, when isolated with a 47W series resistor (top curve), the headphone’s non-linearities are immediately exposed! November 2005  27 Par t s Lis t – Headphone Amplifier 1 PC board, code 01111051, 134 x 103mm 2 PC mount switched RCA sockets (CON1, CON2) 2 6.35mm PC mount switched stereo sockets (CON3, CON4) 1 3-way 5mm/5.08mm terminal block (CON5) 1 8-pin gold-plated IC socket 4 TO-126/TO-220 micro-U heatsinks 4 M205 PC mount fuse clips 2 M205 500mA fast-blow fuses 4 M3 x 10mm tapped spacers 4 M3 x 6mm pan head screws 4 M3 x 10mm pan head screws 4 M3 nuts and flat washers 2 11.8mm I.D. plastic bobbins (Altronics L-5305, Jaycar LF1062) 1 2-metre length of 0.63mm enamelled copper wire 1 120mm length of 0.7mm tinned copper wire (for links) Semiconductors 1 OPA2134PA dual FET-input op amp (IC1) (Farnell 791-039) 2 BC557 PNP transistors (Q1, Q5) 2 BC547 NPN transistors (Q3, Q7) 2 BD139 NPN transistors (Q2, Q6) 2 BD140 PNP transistors (Q4, Q8) 4 3mm red LEDs (LED1-LED4) (Altronics Z-0700) or headphone burnout at abnormally high volume settings. Another consideration is headphone impedance variation with frequency. While distortion due to this effect can be minimised with low amplifier output impedance, this requirement would seem less important than when driving loudspeakers. We’re also aware that some manufacturers are producing models that have virtually flat impedance curves over the audio spectrum and so will be unaffected by an amplifier’s output impedance. In fact, international standard IEC 61938 specifies that headphones should be driven by a 120W source, regardless of headphone impedance. Alas, it seems unlikely that all headphones will exhibit the ideal “flat” (purely resistive) impedance response. To test this theory, we drove a pair of reasonable quality 8W headphones first 28  Silicon Chip 12 1N4148 diodes (D1-D12) Capacitors 2 470mF 25V PC electrolytic 4 100mF 16V PC electrolytic 2 10mF 35V/50V non-polarised PC electrolytic (max. 6.3mm diameter) 6 100nF 50V MKT polyester 2 47nF 100V polyester film (greencap) 2 1.2nF 50V MKT polyester 2 100pF 50V ceramic disc 2 47pF ceramic disc Resistors (0.25W, 1%) 2 47kW 4 1kW 2 7.5kW 8 100W 8 4.7kW 2 47W 1W 5% 2 1.2kW 2 10W 1W 5% 2 2kW 4 4.7W 0.5W 1% Additional items 2 RCA plugs Shielded audio cable 1 50kW dual-gang log pot (for standalone use) 2 panel-mount RCA sockets (for standalone use) For power supply upgrade 2 TO-220 micro-U heatsinks 2 M3 x 6mm pan head screws 2 M3 nuts and flat washers from a low-impedance (5W) source and then added a 47W series resistor. The results are presented in Fig.1. The maximum amplifier output power needed to produce the desired volume level depends on another widely varying parameter: headphone sensitivity. Modern dynamic headphones are very efficient, typically producing 90-100dB SPL (sound pressure level) per milliwatt of input, with many reaching full volume with just a few milliwatts. To cater for varying sensitivity levels, commercial headphone amplifiers are typically rated at between 10mW and 100mW, or more. Unfortunately, the impedance rating of a headphone pair is not necessarily related to its sensitivity, so a general-purpose amplifier design will ultimately be a compromise. It must generate sufficient volume when driving low-sensitivity, low-impedance phones but may well overdrive highsensitivity and/or high-impedance models at high volume settings. It must also remain stable when driving varying impedances and be able to develop full power into 8W units. Updated & uprated A number of headphone amplifier designs have been published in SILICON CHIP over the years. Some are part of larger amplifier or mixer projects, whereas the most recent (described in May 2002) is a standalone module. All are similar in design, using complementary emitter-follower outputs to boost the current-handling capability of an op amp. Although the heart of this design still relies on the old boosted op amp idea, it includes a number of improvements to significantly boost power handling and performance as well. In addition, we’ve carefully designed the PC board layout to minimise distortion and crosstalk. The result is a unit that clearly outperforms our previous designs in all areas, yet is still relatively inexpensive and easy to build. Let’s look at the circuit in more detail. Circuit description The amplifier consists of two identical circuits, labelled “left” and “right” to represent the stereo audio channels. To reduce clutter, the circuit diagram (Fig.2) shows only the left channel. Note that some components are common to both channels, including the power supply input connector (CON5), fuses (F1 & F2), filter capacitors and headphone output sockets (CON3 & CON4). An RCA socket (CON1) accepts the audio signal, which is AC-coupled to the circuit via a 10mF capacitor and terminated with a 47kW resistor. A 100W resistor and 47pF capacitor form a simple RF filter, after which the signal is fed into the non-inverting input of an op amp (IC1a). In common with the Studio Series Stereo Preamplifier, we’ve used an OPA2134 audio op amp here for best performance. These op amps have excellent load-handling characteristics, with the ability to drive loads down to 600W while producing very low distortion. Of course, this falls far short of our 8W load requirement so a current booster stage has been added between the op amp and the amplifier output. As mentioned earlier, the booster siliconchip.com.au Fig.2: the circuit is based on an OPA2134 high-performance audio op amp (IC1), which drives a complementary emitter-follower output stage. This significantly boosts the amplifier’s output current capability and therefore its maximum power output. Only the left channel is shown here – the right channel is identical. stage is based on a pair of mediumpower transistors (Q2 & Q4) connected in a complementary emitter-follower configuration. Let’s look at the positive (uppermost) half of the circuit first. Current source Transistor Q1, a red LED and a 100W resistor form an active current source. With about 1.8V across LED1, close to 10mA flows in Q1’s collector circuit and this is used to drive the base of output transistor Q2. Of note here is the choice of LED type; it must be red in colour and must not be a high-brightness type – just a standard 3mm type. The device we’ve selected (Altronics Z-0700) exhibits the desired forward voltage (1.8V) at the programmed current. Similar types may also be suitable. A current source greatly improves the amplifier’s supply rail rejection when compared to the simpler resistive biasing that could also have been used. Further improvements can be seen in the base circuit of Q1, where we’ve split the usual single bias resistor into two 4.7kW halves and added a 100mF filter capacitor to the centre point, again improving ripple rejection. Note that the use of a LED instead of the more traditional diodes in this instance is really just for convenience, although it does provide a useful visual indication of operation. In the quiescent (no input signal) state, most of the current flows into the op amp’s output terminal (pin 1) via diode D5. This diode compensates for the base-emitter voltage of Q2, to minimise crossover distortion. In practice, the forward voltages of D5 and Q2 will not be equal. Typically, the transistor will have a slightly lower forward voltage, so several milliamps (typically around 15-20mA) will flow in the emitter circuit of Q2 in the quiescent state. A 4.7W resistor adds a measure of stability to the emitterfollower configuration. The other half of the circuit (Q3, siliconchip.com.au November 2005  29 Fig.3: follow this parts layout diagram closely when assembling the headphone amplifier. Be careful not to mix up the different transistor types and double-check the orientation of the diodes, LEDs and polarised electrolytic capacitors before applying power. LED2, D6 & Q4) is powered from the negative supply rail and operates in a complementary fashion, with the output transistor conducting on negative, Fig.4: here’s how to assemble the heatsinks to the output transistors, which must be done before fitting the transistors to the board. Make sure that the metal face of each transistor mates with the heatsink and be sure to smear both mating surfaces with heatsink compound. 30  Silicon Chip rather than positive half-cycles. Diodes D1-D4 add output current limiting and prevent large reverse voltages from appearing across the circuit during a short-circuit condition. All four diodes are installed for operation into 8W headphones, giving a maximum output current of about two diodes drops (2 x 0.7V) divided by the emitter resistance (4.7W). For higher impedance headphones, two of the diodes in each channel must be replaced with wire links, halving the maximum current and therefore reducing the chances of accidental headphone damage. The amplifier’s output signal is picked off at the junction of the two 4.7W emitter resistors and fed back to the inverting input (pin 2) of op amp IC1a via resistor R1. Including the output circuit in the op amp’s feedback loop has two important advantages. First, it allows the op amp to servo the output to near 0V with no input signal, accounting for mismatches in the complementary halves of the circuit. It also results in an overall improvement in linearity and reduces crossover distortion. Resistors R1 and R2 set the amplifier gain in the usual manner, giving a gain of +2 (1+ R1/R2) with the 1kW values shown. This is suitable for use with a preamplifier and/or when driving 8W headphones (see the “Tweaking Your Headphone Amplifier” panel for other options). In conjunction with R1, the 1.2nF capacitor (C1) in the feedback path rolls off amplifier frequency response above the audio spectrum. Finally, an RLC network at the output isolates the amplifier from headphone reactance and ensures stability under all conditions. The low impedance of the inductor (L1) at audio frequencies also allows the amsiliconchip.com.au Here’s what a completed inductor looks like (you need two), prior to scraping off the enamel insulation and tinning the leads. plifier to drive difficult loads (down to 8W) with very good results. We’ve used air-cored inductors to avoid the signal distortion that would be introduced by ferrite and iron-cored alternatives. next. Use wire links for R3 & R6 if you’ll be feeding your amplifier from a preamp. Conversely, install 2kW values in these two locations if you’ll be feeding it from a line-level source via a 50kW volume pot. When inserting the LEDs, make sure that you have the flat (cathode) side of the body oriented as drawn on the overlay. IC1’s socket, the four fuse clips, transistors Q1, Q3, Q5 & Q7, the capacitors and connectors CON1-CON5 can all go in next. Take care not to mix up the two types of transistors (BC547 & BC557), and note that the 100mF and 470mF electrolytic capacitors are polarised and must be installed with their positive leads oriented as indicated by the “+” marking in Fig.3. All that now remains to be installed are the 1W resistors, the output transistors and their heatsinks and the two in- Table 1: Capacitor Codes Value 100nF 47nF 1.2nF 100pF 47pF μF Code 0.1µF .047µF .0012µF   NA   NA EIA Code   104   473   122   100    47 IEC Code   100n   47n   1n2    100p    47p ductors. The transistors and inductors require special attention, so fit the 1W resistors first. The two 47W units are positioned in the inductor “centres” and therefore must be mounted vertically, rather than horizontally. Transistor installation The four output transistors (Q2, Q4, Q6 & Q8) are fitted with “micro-U” style heatsinks before installation. To Assembly Assembly is quite straightforward, with all parts mounting on a 134 x 103mm single-sided PC board (code 01111051). Fig.3 shows the details. Begin by installing the 10 wire links, then install the 1N4148 diodes (D1D12). Note that D2, D4, D8 & D10 are only installed if you intend using the amplifier with 8W headphones. For all higher impedance phones, install wire links in these four locations instead (see the “Tweaking Your Headphone Amplifier” panel). Make sure that the cathode (banded) ends of the diodes are oriented as shown on Fig.3. The 0.25W and 0.5W resistors and LEDs (LED1-LED4) can be installed This is the prototype Headphone Amplifier. The final version includes a second headphone socket and has a few other minor changes. Table 2: Resistor Colour Codes o o o o o o o o o o o siliconchip.com.au No.   2   2   8   2   2   4   8   2   2   4 Value 47kW 7.5kW 4.7kW 1.2kW 2kW 1kW 100W 47W 10W 4.7W 4-Band Code (1%) yellow violet orange brown violet green red brown yellow violet red brown brown red red brown red black red brown brown black red brown brown black brown brown yellow violet black gold brown black black gold yellow violet gold brown 5-Band Code (1%) yellow violet black red brown violet green black brown brown yellow violet black brown brown brown red black brown brown red black black brown brown brown black black brown brown brown black black black brown not applicable not applicable yellow violet black silver brown November 2005  31 Tweaking Your Headphone Amplifier F OR THE BEST listening experience, the headphone amplifier can be fed from the Studio Series Stereo Preamplifier described last month. With this combination, a pair of top-quality 32W (or higher) impedance headphones will provide superb performance. Good results can also be obtained with 8W headphones or even two pairs of 32W (or higher) units, if your want to share the experience. In addition, the headphone amplifier can be operated “standalone”, where it connects directly to a line-level signal source (no preamp required). Let’s see how to get the best performance in each case. Using 8-ohm headphones Considerable efforts were made to ensure that the amplifier drives 8W headphones with low distortion. To ensure you get the same results, all eight limiting diodes (D1-D4 & D7D10) must be installed when driving 8W headphones! For higher impedance ’phones, wire links are used in place of D2, D4, D8 & D10 only. What if you own both 8W and 32W (or higher) impedance phones and you want to use all of them with the headphone amplifier – without making changes to the board? Well, while 32W (or higher) headphones can be plugged into an amplifier that’s configured for 8W use, you need to be aware of the potential risks. The amplifier is capable of delivering over 1W into 32W in this case, which is potentially destructive for headphones, your hearing and ultimately the amplifier as well! By the way, we do not recommend increasing the amplifier gain (see “Boosting volume” below) when driving 8W headphones, as this will cause an unavoidable increase in harmonic distortion. With the default signal gain of 6dB, only about 630mV RMS is required at the input to develop the full 200mW into 8W, hence increas- do this, apply a thin smear of heatsink compound to the rear (metal) face of each transistor as well as the mating surface on each heatsink (do not use 32  Silicon Chip ing gain for typical line-level signals is pointless. Boosting volume Using the component values shown on the circuit and overlay (Figs.2 & 3), the headphone amplifier operates with a voltage gain of 2 (6dB), which is more than adequate when the unit is fed from a preamplifier. It should also work fine when driving 8W headphones, regardless of the audio source. However, if you want to connect the unit directly to a line-level source via a volume pot (see “Standalone use” below) and you’ll be using 32W or higher impedance ‘phones, then you may find that the volume is not loud enough, even with the controls wound right up. If after building and testing the amplifier you find that more volume is required, then amplifier gain can be increased to 7.2 (17dB) to allow the full rated output power to be realised in all cases with a 1V RMS input signal. To increase the gain, use the following component values in place of those shown on the circuit and overlay diagrams: R1 & R4 = 7.5kW, R2 & R5 = 1.2kW and C1 & C2 = 100pF. One negative aspect of increasing amplifier gain is an accompanying increase in harmonic distortion. Nevertheless, performance is still excellent, with .0004% THD when driving 600W and .004% when driving 32W headphones, measured at the full rated output power. Standalone use (no preamp) When feeding the amplifier directly from a line-level source, some method of volume control will usually be required. This is easily provided with a 50kW dual-gang log potentiometer, inserted in series with the inputs to the amplifier (see Fig.5). One disadvantage of this scheme is that op amp source impedance var- insulating washers). Affix each transistor to its heatsink using an M3 x 10mm screw, nut and flat washer (see Fig.4), allowing just enough slack so that ies with changes in volume, resulting in higher signal distortion. To offset this effect somewhat, 2kW values can be used for resistors R3 & R6. Accounting for feedback resistance, the inverting input will then see about 2.5kW (R1||R2 + R3), assuming the default 1kW values were used for the feedback resistors. The result is improved matching at the non-inverting input at nominal volume settings. Note that the same 2kW values can be used for R3 & R6 when the amplifier is configured for the higher 17db gain option (see “Boosting volume” above). In this case, the inverting input will see about 3kW. We acknowledge that the 2.5kW 3kW values are only a rough estimation, as the real source impedance can vary anywhere from about 100W to 10kW. Considering headphone sensitivity variation, it would appear to be impossible to establish a “typical” volume setting. Important: when feeding your headphone amplifier from a preamplifier or other low-impedance source, resistors R3 & R6 must be 0W in value (use wire links)! Dual outputs The headphone amplifier includes dual 6.35mm output sockets, allowing simultaneous connection of two pairs of headphones. Two important rules must be followed when using both sockets at once: (1) the headphones must be of the same nominal impedance rating; and (2) the impedance ratings must be 32W or higher. Many listeners will prefer to set their own volume levels and this can be catered for by using headphones with in-line volume controls. Separate volume control boxes are also available from specialist audio outlets. Note that although the sockets are connected in parallel, the jack switch output connects to the first (primary) socket only, so this socket will control the headphone/power amplifier signal routing relay on the Studio Series Preamp. transistor and mounting screw can move up and down in the heatsink slot. Insert a transistor into its holes in the PC board (don’t mix up the two siliconchip.com.au Fig.5: a potentiometer can be inserted in series with the input signals to function as a simple volume control. The metal body of the pot must be connected to chassis earth, otherwise mains hum will be introduced into the amplifier inputs. Fig.6: amplifier total harmonic distortion & noise versus output power into 8W, 32W and 600W resistive loads. When driving 8W and 32W loads, the current-limiting diodes begin to conduct around the 200mW mark, causing a gradual increase in distortion. Once the diodes are fully forward-biased, the output current is aggressively clamped, resulting in an almost vertical rise in distortion. For the 600W case, the amplifier abruptly runs out of voltage headroom at about 130mW and hard clipping is the result. types), pushing it all the way home, so that the mounting screw is all the way down in the heatsink slot and the edge of the heatsink is in full contact with the board surface. If you can’t achieve this, then you’ve fitted the heatsink upside down! Without disturbing the transistor/heatsink assembly, turn the board over and solder the transistor leads. The mounting screw can now be carefully tightened. Don’t overdo it; too much torque will disturb the package/ heatsink position! Winding the inductors The two inductors (L1 & L2) are hand-wound. Each requires a plastic bobbin, about 1m of 0.62mm enamelled copper wire and some electrical insulation tape. Some kit suppliers might provide these items preassembled, in which case you can skip the following instructions. The insulation tape is needed to hold the windings in place while the assembly is fitted to the PC board. General-purpose tape will be wider than the bobbin, but can easily be made to fit by slicing off the unneeded width with a razor blade. Stick the tape down on a smooth, clean surface first to make the job easier. Play out the wire before beginning and remove any kinks. Starting at one of the slots, wind on one complete layer, keeping the wire taut as you go. With one complete layer in place, start winding back over the first layer. In all, 21 turns are required but you’ll need an extra half-turn so that the wire exists at the opposite slot to the starting end (see photo). Wind on two or three turns of insulation tape to hold the windings in place. Finally, scrape the enamel insulation siliconchip.com.au Fig.7: amplifier total harmonic distortion & noise versus frequency, measured with an output power level of 100mW. As is clear from these curves, the amplifier performs much better when driving 32W and higher impedance headphones. Most headphones will reach full output well below 100mW, so you can expect even better performance than these already excellent curves reveal! off the ends of the two leads and tin them before mounting the inductor on the PC board. Hookup For best results, the amplifier should be powered from the low-noise power supply described last month as part of the Studio Series Preamplifier (SILICON CHIP, October 2005). Even if you decide to use a different supply, the guidelines in that article regarding mains wiring, housing and general layout also apply here. An additional step when using the low-noise supply with November 2005  33 Fig.8: this graph plots the amplifier output voltage versus frequency when driven at 200mW into 8W, 32W and 600W loads and with 6dB of gain. As can be seen, the response is ruler flat over the audio spectrum, gently rolling off at the top end at a rate dependent on the feedback network and output loading. this amplifier is to fit small heatsinks to the ±15V regulators (see parts list). Apply a thin smear of heatsink compound to the mating surfaces during assembly, to aid heat transfer. We’ll assume that you’ve already assembled and tested the power supply. All that remains then is to hook up the amplifier’s power and signal inputs. Connect the +15V, -15V & GND outputs of the supply to the headphone amplifier inputs at CON5 using medium-duty, multi-strand hook-up wire. Twist the wires tightly together to reduce noise and improve appearance. Take great care to ensure that you have all of the connections correct – a mistake here will destroy many components on the amplifier board! When installing the unit in a case with a preamp module, the headphone amplifier must not be separately earthed – only the preamp board should be earthed. However, if you’re building a standalone unit (no preamp), then the headphone amplifier’s common (GND) rail should be connected to chassis earth. This is achieved by running a wire from the pad marked “EARTH” on the amplifier board to the main chassis earth point. Do not connect any other part of the circuit or power supply to chassis earth (except the volume pot, see below). For a standalone unit, the volume pot can be wired up next. Use a dualgang, 50kW logarithmic type, connected with audio-quality shielded 34  Silicon Chip Fig.9: this is the crosstalk, again measured for 8W, 32W and 600W loads. Some of the coupling is due to the commoning of the headphone left and right return (ground) leads at the jack plug. The results (although good) would be better if headphones used 4-contact jacks, thus allowing separate grounds for the left and right channels. cable (see Fig.5). The cable can be terminated with panel-mounted RCA sockets on the signal input side and RCA plugs on the output side, which are then plugged into the RCA inputs on the amplifier board. The metal body of the pot must be connected to chassis earth to reduce noise pickup. Do not connect the body to either of the shielded cables! Normally, the front panel will provide the necessary earth connection. If 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 nickel plating on the pot, so remove a small area of the plating with an ink rubber or scouring pad prior to tinning. When used with a preamp, the additional volume pot is not needed. Instead, you simply wire the switched headphone outputs on the preamp to RCA plugs using audio-quality shielded cable. These then plug into the RCA sockets on the headphone amplifier. In addition, the “JACKSW” output of headphone amplifier must be wired to the “SWITCH” input on the preamp Caution! Continual exposure to very high noise levels (including loud music) will cause hearing loss and can cause tinnitus. Hearing loss is cumulative, gradual and almost symptomless! board. This connection will allow the preamp to reroute the audio signal from the power amplifier output to the internal headphone output when a headphone jack is inserted in its socket. Leave the “GND” terminal on CON7 of the preamp disconnected. Testing To check out your completed amplifier, install the fuses and power up. The four LEDs should immediately light up – it not, switch off quickly and check for serious cabling or board assembly problems. If only one LED doesn’t light, then the problem is at least restricted to the associated current source/sink part of the circuit. If all LEDs light as expected, then use your multimeter to measure the voltage between each output and ground. These points are conveniently accessible at one end of the 10W 1W resistors. If all is well, your meter should read within ±2mV of 0V. Next, measure the voltage drop across each of the 4.7W emitter resistors (situated adjacent to the heatsinks). All should measure between about 0V and 100mV, representing a maximum emitter current of about 21mA. Note that this measurement assumes the transistors are idling at room temperature. The reading may be higher if the amplifier has been in recent use and the output transistors have warmed up. OK, we’re done. Now for the best part – the listening test! Enjoy! SC siliconchip.com.au