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Build a 50W/channel stereo amplifier Looking to upgrade your system with a new amplifier? This new stereo amplifier is easy to build & does not need setting-up adjustments. Most importantly, it will give excellent sound quality & up to 50 watts per channel into 8-ohm loads. By LEO SIMPSON & BOB FLYNN Our last integrated stereo amplifier design was presented in the March & April 1992 issues of SILICON CHIP but it is now obsolete because the power transistors specified are no longer available. This new design is based on the 50W per channel stereo amplifier module presented last month. While this new amplifier offers very similar facilities to the unit referred to above, it is a completely new design with a much wider chassis and all 32  Silicon Chip new PC boards. And while the super­ sed­ ed design had an inbuilt RIAA preamplifier for phono cartridges, in the new amplifier the RIAA preamp is an optional extra board. We took this approach because many people these days do not have any vinyl records or a turntable so they don’t want the RIAA preamp. Leaving the RIAA preamplifier out also has the advantage that you have an extra pair of line level inputs (ie, suitable for CD, tuner or other program source). The overall design approach to this amplifier has been middle of the road. We have not taken the spartan European ap­proach with virtually no controls except for the volume knob and nor have we sought to incorporate every feature found in expen­sive Japanese amplifiers. Still, it does have all the features that most people want and will use. For example, while it does include tone controls, it also has a switch to disable them, to obtain a completely flat frequency response. Let us now talk about the features in some detail. Features The new SILICON CHIP 50W Stereo Amplifier is housed in a low profile case measuring 435mm wide, 95mm high and 320mm deep, including knobs rubber feet and rear projections. Specifications Power Output 47W into 8-ohm loads, both channels driven; 57W into 8-ohm loads with one channel driven. Frequency Response High level inputs: within ±1dB from 10Hz to 50kHz Phono inputs: RIAA/IEC within ±0.5dB from 20Hz to 20kHz. Total Harmonic Distortion Typically less than .05% (see graph). Signal-to-Noise Ratio High level inputs (CD, Tuner, etc): 99dB unweighted (20Hz to 20kHz) with respect to rated output (with volume at maximum) with Tone Defeat switch in or out; 100dB A-weighted under the same conditions. Phono (moving magnet): 83dB unweighted (20Hz to 20kHz) with respect to 10mV input signal at 1kHz & rated output with 1kΩ resistive input termination; 88dB A-weighted under the same conditions. Channel Separation -78dB at 100Hz; -81dB at 1kHz; & -61dB at 10kHz with respect to rated output & with undriven channel input loaded with a 1kΩ resistor. Above: the new SILICON CHIP 50W per channel amplifier offers all the facilities expected on a modern stereo amplifier &, in addition, it has a separate headphone amplifier. Input Sensitivity Phono inputs at 1kHz: 4.3mV High level inputs: 235mV Tone Controls Bass: ±13dB at 50Hz. Treble: ±13dB at 10kHz Damping Factor >56 from 100Hz to 10kHz (for 8Ω loads) Stability Unconditional It has the usual line-up of controls found on most amplifiers: bass, treble, balance, input selector, tape monitor switch, tone defeat switch and volume control. It also has a stereo/mono switch, headphone socket and power switch. Plugging into the headphone socket disables the main power amplifiers and engages a separate high quality low power stereo amplifier to drive the headphones di­rectly. This now only gives better reproduction via headphones but it also simplifies the internal wiring. Block diagram Now let’s have a look at the circuit features which are depicted in the The new 50W Stereo amplifier uses this 50-watt/channel stereo power module, as described in the previous issue. It’s based on two monolithic power ICs to give a rugged, compact design that requires no adjustments. March 1995  33 POWER AMPLIFIER IC4 OPTIONAL RIAA PREAMPLIFIER IC5 AUX 3/ PHONO CD x23 x56 OUT SOURCE MONITOR S2 TONE S5 TAPE PREAMPLIFIER IC1 TUNER VCR AUX 1 VOLUME VR1 SOURCE S1 AUX 2 TAPE OUT TAPE IN S6 x4.2 IN TONE CONTROLS IC2 POWER AMPLIFIER HEADPHONES HEADPHONE AMPLIFIER IC3 x5.7 MONO SPEAKER HEADPHONES MODE S3 STEREO TO OTHER CHANNEL TO OTHER CHANNEL BALANCE S4 Fig.1: the circuit features of the new stereo amplifier are il­lustrated in this block diagram. To keep things simple only one channel is shown. Note the separate amplifier to drive the head­phones. TO OTHER CHANNEL block diagram of Fig.1. This shows only one channel, to keep things simple. All the circuit functions are duplicated in the second channel. S1 is the 6-position selector switch and it feeds the tape output as well as the Tape Monitor switch S2. S2 selects the signal from the input selector S1 or from a cassette deck con­nected to the Tape In inputs. The signal then goes to S3, the stereo/ mono switch which shorts the two channel signals together when in the mono setting. Following S3, the signal is fed to the 11-position balance control switch S4 and the volume control potentiometer VR1. The use of a rotary switch for the balance control is unusual but there are good reasons for this approach. In past designs we have specified a special dual ganged potentio­meter known as an M/N type. This has half the resistance track in each channel shorted out to give a good balance control action and is the same as used in most domestic stereo amplifiers. However, this type of balance control has become difficult to obtain 34  Silicon Chip and so we initially took a different approach, using a single linear potentiometer with the ends connected to the signal in either channel and the wiper connected to signal earth. This approach is cheap but does not work particularly well, for two reasons. First, it has very little apparent effect over most of the middle range of the pot – all the attenuation is cramped into the extreme ends of rotation. Second, because the resistance of the wiper itself is quite high, and this resistance is common to the signal path in both channels, the separation between channels is seriously degraded, to a figure of about 25dB. Now while -25dB separation between channels is adequate to produce a convincing stereo effect, it is far below what the circuit is otherwise capable of. One approach used by some amplifier manufacturers is to use a linear potentio­meter with a centre tap connection. This gets around the problem of the wiper resistance but it still has all its control action con­ centrated at the extremes of rotation. In any case, such poten­tiometers are also difficult to obtain. Our approach was to use an 11-position rotary switch with resistors wired around it. The resistors are arranged to progres­sively reduce the gain of the attenuated channel by about 2dB. So from the centre position, the gain of each channel can be varied by -2dB, -4dB, -6dB, -8dB and then completely off. This works reasonably well and has the advantage of giving good channel separation. Following the volume control, the signal goes to a non-inverting op amp stage with a gain of 4.2. From there, it goes to the unity gain tone control stage which can be switched out of circuit by the Tone defeat switch, S5. After the tone defeat switch, the signal goes to switch S6 which is part of the headphone socket. It normally Fig.2 (right): this diagram shows the circuit of one channel of the new amplifier & the power supply which is common to both channels. The RIAA preamplifier (not shown) is optional & can be omitted, giving another pair of line level inputs. March 1995  35 E 10k C -15V E 5.6k B 33pF C 5.6k D2 1N914 7(1) D1 1N914 47k IC3a 6(2) TLO72 5(3) TO S6a +15V VIEWED FROM BELOW B 10k TAPE IN TAPE OUT AUX2 AUX1 VCR TUNER CD AUX3/ PHONO GIO 7915 A MONO OTHER CHANNEL E N 240VAC A 1.6k F1 1A CASE S7 .01 250VAC OTHER CHANNEL 1.6k 820W 1.6k 4.7k 91k 91k 4.7k 1.6k 820W 1k HEADPHONES OTHER CHANNEL STEREO MODE S3A 82  K 1k TAPE 50W STEREO AMPLIFIER I GO 7815 E Q2 C BC327 B 15  15  E Q1 BC337 C B SOURCE S1a 1k SOURCE MONITOR S2a OPTIONAL PHONO PREAMP T1 1 25V 25V BALANCE S4 VOLUME VR1a 50k LOG 4.7k 6(2) 1k 5(3) 4700 50VW 4700 50VW BR1 KBPC10-4 11 TO HEADPHONE AMPLIFIER 100k 1 15k 7(1) 22k 1 4.7k 2x330  1W -35V 47 63VW 47 63VW IN REG2 7915 GND GND OUT 100 16VW 100 16VW +35V REG1 2x330  7815 1W OUT IN L1 : 16T 0.5mm DIAMETR ENAMELLED COPPER WIRE WOUND ON 10  1W RESISTOR 100pF IC1a LM833 22 BP 22k TREBLE VR3a 25k LIN -15V GND 8 100 16VW 4 *0.1 5.6 1W F3 2A 100 16VW 100 16VW 100 16VW 100 16VW *SEE TEXT 100 63VW 3 100 63VW 10  1W LED1 3.9k 0.5W  8W +35V 7(1) -35V L1 0.7uH F2 2A IC2a 5(3) LM833 6(2) CONTROL BOARD FILTERING 0.1 0.1 5 0.1 100 16VW 39k 22k 7 IC4 LM3886 1 22k 6.8 S6a BP 0.1 33pF TONE CONTROLS S5a IN 4.7k 22k OUT .0047 22 16VW 9 10 AMP +15V 47 16VW 1k 220pF 1k 100W PHONES .0047 22k BASS VR2a 100k LIN .01 Fig.3: this graph shows the frequency response of the tone controls at their maximum boost & cut settings & also at the flat setting. AUDIO PRECISION FREQRESP AMPL(dBr) & AMPL(dBr) vs FREQ(Hz) 5.0000 14 JAN 95 20:39:02 5.000 4.0000 4.000 3.0000 3.000 2.0000 2.000 1.0000 1.000 0.0 0.0 -1.000 -1.00 -2.000 -2.00 -3.000 -3.00 -4.000 -4.00 -5.000 -5.00 20 100 1k 10k 50k Fig.4: the frequency response of the headphone amplifier, with the right channel dotted. feeds the audio signal through to the following power amplifier but when the headphone jack is inserted, the signal is diverted to the headphone amplifier. Circuit description The complete circuit diagram, except for the optional RIAA preamplifier, is shown in Fig.2. The three 36  Silicon Chip op amps are shown as IC1a, IC2a and IC3a and each is half of a dual low noise op amp. The pin numbers for the other halves which are in the second channel, IC1b, IC2b and IC3b, are shown in brackets on the circuit. For example, the non-inverting (+) input for IC1a is pin 5 while the corresponding input for IC1b is pin 3 (shown in brackets). IC1a is the non-inverting op amp with a gain of 4.2, as set by the feedback resistors connected to pin 6. Besides providing gain and a high impedance load for the volume control pot, IC1a acts as a low impedance source for the tone control stage, IC2a. This has the tone controls connected in the negative feedback network. When the bass and treble controls are centred (ie, in their flat settings), the gain of stage is unity, up to at least 50kHz. Winding the bass or treble controls towards the input side of IC2a (ie, applying boost) increases the gain for frequencies above 2kHz for the treble control and below 300Hz for the bass control. When the tone controls are rotated in the opposite direction (applying tone cut), the gain is reduced above 2kHz and below 300Hz. This is because the negative feedback has been increased, giving a reduction in gain at these frequencies. The amount of treble boost and cut provided by IC2a is limited by the 4.7kΩ resistors on either side of the 25kΩ treble pot, VR3a. Similarly the maximum bass boost and cut is limited by the 22kΩ resistors on either side of the bass pot, VR2a. Fig.3 shows the action of the tone controls at their maximum boost and cut settings and also at the flat setting. Note how S5a, the Tone Defeat switch, bypasses the tone control circuitry. Its output feeds a 6.8µF bipolar capacitor which is there to block DC from the tone control stage from getting into the input of the headphone amplifier. Headphone amplifier Following the 6.8µF capacitor and headphone switch S6a is the head­ This prototype amplifier uses five PC boards, including the optional RIAA preamplifier which is adjacent to the selector switch. No setting up adjustments are required for the power amplifiers. phone amplifier which consists of op amp IC3a in combina­ tion with transistors Q1 and Q2. The transistors are there to boost the output current capability of the TL072 op amp. They are slightly forward-biased (to keep crossover distortion to a mini­mum) by the two diodes connected between the bases. Any distor­tion produced by the transistors is also minimised by incorporat­ing them inside the feedback network for the op amp. The output current of the head­ phone amplifier is limited by the 15Ω emitter resistors and the 82Ω output resistor. This provides short circuit protection and protects the headphones against damage in the unlikely event of the amplifier being damaged. Fig.4 shows the frequency response of the headphone amplifier, with the right channel dotted. Power amplifiers As noted above, the power amplifiers are the stereo 50W module described last month. For the sake of completeness and for those who did not see the previous article, we repeat the circuit description. IC4 is an LM3886 monolithic power amplifier module with balanced supply rails and direct coupling to the loudspeaker load. It is very similar to the LM3876 50W module featured in the March 1994 issue of SILICON CHIP. The input signal which comes via the headphone switch S6a is coupled via a 1µF MKT polyester capacitor and then via an RC network consisting of a March 1995  37 distortion versus frequency at 30 watts into 8Ω loads. Phono preamplifier Fig.5: total harmonic distortion versus frequency at 30W into 8Ω loads, for both channels. 1kΩ series resistor and a shunt 220pF capacitor. This is an RF suppression capacitor. The voltage gain of the power amplifier is set 23 by the 22kΩ negative feedback resistor from pin 3 to pin 9, in conjunc­tion with the 1kΩ resistor and 47µF capacitor. The output from IC4 drives the loudspeaker via an RL network consisting of a 10Ω resistor in parallel with an inductance of 0.7µH. This acts in conjunction with the Zobel network comprising the 5.6Ω resistor and 0.1µF capacitor to ensure that the LEFT INPUT amplifier is stable under varying load conditions. Power supply The power supply uses a 50V centre-tapped 160VA transformer feeding a bridge rectifier and two 4700µF 50VW capacitors. Posi­ tive and negative 3-terminal regulators fed by paralleled pairs of 330Ω resistors provide the ±15V supply rails to the preamplifier boards (ie, tone control board and optional RIAA preamplifier). Fig.5 shows another aspect of the amplifier’s performance: harmonic +15V L1 150  100k 47 BP 100k 8 3(5) IC5a 2(6) LM833 4 100pF L1 : 4T, ENCU WIRE ON PHILIPS 4330 030 3218 FERRITE BEAD 16k IC PIN NUMBERS IN BRACKETS ARE FOR RIGHT CHANNEL .0047 390 -15V 1(7) 100  10 BP LEFT OUTPUT 1M 200k .015 +15V +15V 22 BP 0.1 0V 0.1 RIAA PREAMPLIFIER (OPTIONAL) -15V RIAA/IEC equalisation -15V Fig.6: the circuit of the optional RIAA preamplifier is based on an LM833 dual low noise operational amplifier. 38  Silicon Chip As noted above, this phono preamplifier is optional. The circuit is depicted in Fig.6 and again, only one channel is shown. IC5a is one half of an LM833 low noise op amp. It takes the low level signal from a moving magnet cartridge and applies a gain of 56 at the median frequency of 1kHz. Higher frequencies get less gain while lower frequencies get considerably more, as called for in the RIAA equalisation. The preamplifier board is the same as the universal preamplifier board presented in the April 1994 issue of SILICON CHIP. The phono signal is fed directly from the input socket via inductor L1, a 150Ω resistor and a 47µF bipolar capacitor to the non-inverting input, pin 3, of IC5a. The inductor, series resis­ tor and 100pF shunt capacitor form a filter circuit to remove RF interference signals which might be picked up by the phono leads. The 100pF capacitor is also important in capacitive loading of the magnetic cartridge. Most moving magnet (MM) cartridges operate best with about 200-400pF of shunt capacitance. The 100pF capacitance in the preamp input circuit plus the usual 200pF or so of cable capacitance for the pickup leads will therefore provide about the right shunt capacitance. For its part, the 47µF bipolar cap­ acitor is far larger than it needs to be as far as bass signal coupling is concerned. If we were merely concerned with maximising the bass signal from the cartridge, then an input coupling capacitor of 0.47µF would be quite adequate. At 20Hz, a capacitor of this value would have an impedance of around 15kΩ which is considerably less than the nominal 50kΩ input impedance of the preamp. However, having a large input cap­ acitor means that the op amp “sees” a very low impedance source (ie, essentially the DC resistance of the cartridge) at low frequencies and this helps keep low fre­quency noise, generated by the input loading resistors, to a minimum. The RIAA equalisation is provided by the RC feedback compon­ents between pins 1 and 2 of IC5a. These PARTS LIST 1 steel case with aluminium front panel, 435 x 90 x 265mm 1 2-pole, 6-position rotary switch, Altronics S-3022 (S1 3 2-pole 2-position pushbutton switches, Altronics S-1410 (S2,S3,S5) 1 single pole 12-position rotary switch, Altronics S-3021 (S4) 1 SPST 250VAC rocker switch, Altronics S-3210 (S7) 1 dual-gang 50kΩ log potentiometer (VR1) 1 dual-gang 100kΩ linear potentiometer (VR2) 1 dual-gang 25kΩ linear potentiometer (VR3) 1 PC mounting 6.5mm switching stereo socket, Altronics P-0076 3 3 x 2-way RCA socket panels, Altronics P-0213 1 black binding post terminal, Altronics P-0264 3 22mm diameter black aluminium knobs, Altronics H-6213 2 30mm diameter black aluminium knobs, Altronics H-6224 1 3-way mains terminal strip 2 solder lugs 1 toroidal power transformer, 2 x 25V, 160VA 1 M205 panel mount fuse holder 1 2A M205 20mm fuse 8 20mm fuse clips 4 2.5A M205 20mm fuses 2 single sided heatsinks, 72mm high, Altronics H-0522 2 TA11B IC mounting kits 3 3-way PC terminal blocks, Altronics P-2035 23 PC pins 7 15mm tapped standoffs 4 3mm x 6mm untapped standoffs 4 4M x 10mm screws 11 3M x 10mm screws 2 3M x 15mm screws 10 3M x 6mm screws 9 3M nuts 6 black No.6 x 10mm self-tapping screws 1 1-metre length 0.5mm enamelled copper wire 1 1-metre length twin shielded audio cable 3 1-metre lengths 32 x 0.2mm hookup wire (three different colours) 1 3-core mains cord & moulded 3-pin plug 1 cordgrip grommet (to suit mains cord) 4 rubber feet 1 6.4mm shaft coupler 1 6.4mm dia. x 144mm long extension shaft 1 LED bezel equalisation components provide the standard time constants of 3180µs (50Hz), 318µs (500Hz) and 75µs (2122Hz). The preamplifier also adds in the IEC recommendation for a rolloff below 20Hz (7950µs). This is pro- vided by the 22µF bipolar capacitor in series with the 390Ω resis­tor. The 390Ω resistor sets the maximum AC gain at very low frequencies while the 22µF capacitor ensures that the gain for DC is unity. This means that any input offset voltages are not ampli­ PC boards 1 power amplifier board, code 01102951, 247 x 58.5mm 1 input selector board, code 01103951, 132 x 58mm 1 selector switch board, code 01103952, 55 x 37mm 1 tone control board, code 01103953, 277 x 86mm 1 RIAA preamp board (optional), code 01103954, 76 x 78mm Semiconductors 2 LM833 operational amplifiers (IC1,IC2) 1 TLO72 operational amplifier (IC3) 2 LM3886 audio power amplifiers (IC4) 4 1N914 signal diodes (D1,D2) 2 BC337 NPN transistors (Q1) 2 BC327 PNP transistors (Q2) 1 KBPC10-04 bridge rectifier (BR1) 1 LM7815T 3-terminal regulator (REG1) 1 LM7915T 3-terminal regulator (REG2) 1 red LED (LED1) Capacitors 2 4700µF 50VW electrolytics 4 100µF 63VW electrolytics 2 47µF 63VW electrolytics 8 100µF 16VW electrolytics 2 47µF 16VW electrolytics 2 22µF 16VW electrolytics 2 22µF 50VW bipolar electrolytics 2 6.8µF 50VW bipolar electrolytics 4 1µF 63V MKT polyester 10 0.1µF 63V MKT polyester 1 .01µF 250VAC metallised paper 2 .01µF 63V MKT polyester 4 .0047µF 63V MKT polyester 2 220pF 50V ceramic 2 100pF 50V ceramic 4 33pF 50V ceramic Resistors (0.25W, 1%) 2 100kΩ 4 1.6kΩ 2 91kΩ 12 1kΩ 2 47kΩ 2 820Ω 2 39kΩ 4 330Ω 1W 12 22kΩ 2 100Ω 2 15kΩ 2 82Ω 4 10kΩ 4 15Ω 4 5.6kΩ 2 10Ω 1W 8 4.7kΩ 2 5.6Ω 1W 1 3.9kΩ 0.5W Optional RIAA Preamplifier 1 RIAA preamp board, code 01103954, 76 x 78mm 11 PC pins 1 LM833 operational amplifier (IC5) 2 Philips ferrite beads, 4330 030 3218 Capacitors 2 47µF 50VW bipolar electrolytics 2 22µF 50VW bipolar electrolytics 2 10µF 50VW bipolar electrolytics 2 0.1µF 63V MKT polyester 2 .015µF 63V MKT polyester 2 .0047µF 63V MKT polyester 2 100pF 50V ceramic Resistors (0.25W, 1%) 2 1MΩ 2 390Ω 2 200kΩ 2 150Ω 4 100kΩ 2 100Ω 2 16kΩ fied, which would inevitably cause trouble with asymmetrical clipping and premature overload in the preamplifier. Next month, we shall continue with the construction details for the new SC 50W Stereo Amplifier. March 1995  39