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The Currawo 2 x 10W Stereo Valve Ampl The Currawong amplifier is a tried and tested valve amplifier circuit which has been adapted to components which are readily available in 2014. Each channel uses two 12AX7 twin triodes for the preamp and phase splitter stages and two 6L6 beam power tetrodes in the class-AB ultra-linear output stage. It performs very well, with low distortion and noise. 28  Silicon Chip siliconchip.com.au I This progress view of the amplifier shows it sitting in its timber plinth but without the protective Perspex covers in place to protect the PCB and protect the user from high voltages. N DESIGNING this amplifier, we wanted to present a unit which is straightforward to build and which has a good appearance. To satisfy the first requirement, most of the circuitry, with the exception of the power transformers, is mounted on a large double-sided PCB. Hence there is no need for point-to-point wiring from valve sockets, tag-strips, tag-boards or any of that stuff from 60 years ago. Using the large PCB also means that we have avoided the need for an expensive metal chassis. Instead, the PCB slides into a timber plinth stained as rosewood (although you can have any timber finish you desire). As a nice finishing touch, most of the PCB will be covered and protected by a Perspex cover. This will prevent little fingers from touching any part of the circuit and remove any risk of electric shock which would otherwise be possible. We hope you will like the appearance. There are two toroidal power transformers used to power the Currawong and these are concealed underneath the PCB, towards the back of the unit. Control panel By Nicholas Vinen ong ifier, Pt.1 • 10W per channel • Low distortion • Good performance • Easy to build siliconchip.com.au At the front of the timber plinth, there is a small control panel suspended below the main PCB. This carries the volume control, the on/off switch, a bi-colour red/green LED, a blue LED and the headphone socket. And while it might seem like a waste to use the Currawong Stereo Valve Amplifier to drive headphones, we know from long experience that readers will definitely want this feature. By the way, the red/green LED comes into play when you first turn the amplifier on. There is an initial delay while the valves heat up and during this time, no HT (high tension or high voltage) is applied to the plates of the valves which could otherwise suffer damage in the long term. So during this delay, the LED is red. Then, when the HT is applied, the LED changes colour to green, indicating that normal operation is possible. The other LED is lit when the headphones are in use. Plugging into the headphone socket enables a relay which disconnects the loudspeakers and connects the headphones via 220Ω resistors. At the rear of the timber plinth is another panel which accommodates the RCA input sockets, the binding post terminals for the loudspeakers and a fused IEC socket for the mains cord. Both the front and rear panels are made from PCB material to provide a high-quality finish. The overall performance is summarised in an accompanying panel and three graphs. It gives very good performance for a valve amplifier. Circuit concept A major difficulty in the design of the Currawong has involved the out­ put transformers. As valve aficionados will be aware, the output transformer is usually the most expensive component in the circuit, apart from the valves themselves. Similarly, these days the power transformer is also very expensive, simply because there is no locally available off-the-shelf unit which can be pressed into service. Yes, you can purchase imported power and output transformers but if we had specified these, the total cost of the amplifier would have been a great deal higher. Instead, we have taken a very unusual approach in selecting the output transformer by employing a standard off-the-shelf 15W line transformer (Altronics M1115) which would normally be employed with a professional solid-state PA amplifier to drive 100V lines. As a line driver, the transformer’s primary winding is driven by a solidstate amplifier and it steps up the voltage in its multi-tapped secondary winding. In the Currawong though, we drive the transformers back to front, with the push-pull valve output stages driving the 100V windings and the primary windings becoming the lowimpedance drive for the loudspeakers. Conveniently, the 100V winding has a centre-tap, which is necessary for push-pull operation. In addition, we use some of the other taps for the “ultra-linear” connection. Make no mistake though; while these are low-cost transformers (being made in large quantities), they have grain-oriented steel cores, a wide frequency response and low harmonic distortion. Better still, the taps on the primary winding enable it to be connected for ultra-linear push-pull operation. On the other hand, selection of this transformer is one of the two limiting factors in the maximum output power of the Currawong, at close to 10 watts per channel. The other factor is the power transformer selection. We would have ideNovember 2014  29 Features & Specifications • • • • • • • • • • • • • • Channels: 2 (stereo) • Dimensions: 294 x 304 x 186mm (W x D x H) including protrusions Valve line-up: 4 x 12AX7 twin triodes, 4 x 6L6 beam power tetrodes HT supply: ~310V, actively filtered Tested load impedances: 4Ω, 6Ω, 8Ω Output power: 2 x 10W (8Ω, 6Ω), 2 x 9W (4Ω) (see Fig.3) Operating mode: Class-A (8Ω), Class-A/AB (6Ω, 4Ω) Input sensitivity: ~1V RMS (8Ω, with feedback enabled) Signal-to-noise ratio: 77dB Channel separation: >60dB, 20Hz-20kHz (4Ω, 6Ω & 8Ω) Harmonic distortion: typically <0.1%, 6Ω & 8Ω (see Figs.3&4) Frequency response: ±0.6dB, 30Hz-20kHz (see Fig.5) Damping factor: >20 (8Ω), >10 (4Ω) Mains power draw: typically 120-130W Other features: ultra-linear outputs, remote volume control option, delayed HT, HT soft-start ally liked to use a transformer with much higher secondary voltages but a specially-designed power transformer would be much larger and more expensive, as already noted. Having said that, there is future potential for this amplifier to be upgraded with better (more expensive) transformers to enable it to deliver substantially more output power. The valves can be replaced without any disassembly. Their sockets are mechanically mounted to the thick (2mm) PCB to prevent the solder joints from breaking loose during valve removal or insertion. The thick PCB also helps to support the relatively high weight of the output transformers, which are mounted on the board for ease of construction. Temperature-sensitive components such as electrolytic capacitors have been kept away from the high-dissipation components, primarily the 6L6 valves and associated 5W cathode resistors. However, due to the compact size we have not been 100% successful; one of the large filter capacitors is near the output valves. Checks of its temperature during extended operation show that direct heat transfer is minimal and should not be a problem. Semiconductors There are some semiconductor components in this circuit but not in the audio signal path. Mostly, these 30  Silicon Chip perform power supply filtering, to get rid of ripple and keep the amplifier quiet. The HT delay and soft-start circuit is also built using solid-state components. We should acknowledge considerable input to the design of this amplifier from Allan Linton-Smith, the designer of the Majestic loudspeaker system featured in the June and September 2014 issues. Allan built the first hard-wired prototype and the concept was then considerably refined and transferred to the final PCB featured in these pages. Allan also suggested using the Altronics line transformers, based on a discovery by Grant Wills that they could be used as cheap and effective ultra-linear valve output transformers – see http://home.alphalink. com.au/~cambie/6AN8amp/Grant _ Wills_6CM5amp.htm Circuit description Fig.1 only shows the circuit for the left channel signal path. The right channel is identical and the corresponding component numbers are provided in blue. The line-level input signal from RCA socket CON1 has a 1MΩ DC bias resistor to ground, in case the signal source is floating. The signal then passes through an RF-rejecting lowpass filter comprising a 120Ω series resistor and 100pF ceramic capacitor. The signal is then AC-coupled to (nominally) 20kΩ logarithmic volumecontrol potentiometer VR1 by a 1.5µF MKT capacitor. This gives a -3dB lowend roll-off at 5Hz. Note that depending on part availability, a motorised potentiometer with a value as low as 5kΩ may be used, in which case the -3dB point rises to 21Hz. The wiper terminal of VR1 is connected to ground via a 1MΩ resistor so that if it briefly goes open circuit during volume changes, the grid of V1a does not float. The signal is fed to this grid via a 22kΩ RF stopper resistor. V1a and V1b form the preamplifier, which is very similar to Jim Rowe’s design from the February 2004 issue of SILICON CHIP (“Using The Valve Preamp In A HiFi System”). Essentially, this consists of two common cathode amplifier stages in series, with negative feedback around both. V1’s plates are fed from a filtered HT rail of around 224V DC, somewhat less than the 308V DC main HT rail due to voltage drops across the two RC filter resistors (6.8kΩ and 47kΩ). These filters reduce coupling between channels, reduce coupling from the output stage to the preamp stages and minimise supply ripple reaching the preamp. The preamp is the most noisesensitive section as the signal level is lowest here. In fact, because hum can be picked up from AC-powered heater filaments, we are running the 12AX7 filaments from regulated 12V DC. Self biasing All valves in the circuit are selfbiased. V1a’s anode runs at around 120V, ie, 224V minus the drop across the 270kΩ resistor. With zero bias, a 12AX7 will conduct around 3mA at this voltage, dropping to near-zero with a grid-cathode bias of around -2.2V. With a 3.3kΩ cathode resistor, V1a’s operating point tends to settle at about 0.3mA and thus the cathode is 1.2V above ground. The output signal from V1a’s anode is coupled to V1b’s grid by a 220nF capacitor and this grid is DC biased using a 1MΩ resistor to ground. V1b runs at a higher power than V1a, with a 680Ω cathode resistor giving an operating current of around 1mA. Therefore, its anode load resistance is lower at 100kΩ. The output at V1b’s plate is coupled back to V1a’s cathode via a pair of parallel 470nF polyester capacitors siliconchip.com.au siliconchip.com.au November 2014  31 Fig.1: the left channel circuit of the Currawong Stereo Valve Amplifier (the right channel is identical). The incoming signal passes through a low-pass RC filter and volume pot VR1 and is then fed into V1 (a 12AX7 twin triode) which provides signal preamplification in two stages. Its output is then fed to V2 (another 12AX7 twin triode) which operates as a phase splitter and gain stage to drive push-pull output pair V3 & V4 (both 6L6 or KT66 tetrodes). Output transformer T3 has tapped connections to the output valve screens for ultra-linear operation. The transformer output is switched to either the speaker terminals (via CON3) or to the headphone socket by relay RLY1. Components with their text in red are late changes to fix a relay switching problem. 32  Silicon Chip siliconchip.com.au E FUSE FUSED IEC MAINS MALE SOCKET N 12V AC 12V AC A 80VA TOROID 230V AC T2 116V AC 1N4007 A LK6 12.2V AC ~ BR1 1A SLOW F1 D2 1N5408 K 10k F3 5A SLOW 3A SLOW F2 A K K – + W04 VEE ~ 400V 470 µF 400V 470 µF +310V K A CURRAWONG STEREO VALVE AMPLIFIER 1 2 3 4 5 CON8 1 2 3 CON7 A D1 1N5408 6 5 K A LEDS 3-6 560Ω MKT 1W 1M 1M E B C λ λ LED1 VEE K LK2 VEE 1k 470Ω 10k 1W 120Ω 16V 1 14 B C E STX0560 OUT ADJ 3 1k C E E C IC1c 10 IN B E B C 13 12 E 1M E +308V 7 IC1d +12V K C ~ + VEE 11 A D4 1N4007 1M B KSC5603DTU Q8 B B Q7 OUT LM1084/LT1084 IC1: 4093B 9 8 C * OR BUJ303A B E Q3 STX0560 C Q5, Q7: BC547 Q6, Q8: BC557 C E E C B A D5 1N4007 100 µF 2 IC1a 150k Q6 B B Q5 E C Q4 STX0560 BC547, BC557 100nF 16V 100 µF +12V 630V 470nF 120Ω (POWER SUPPLY SECTION) K LK1 4 470Ω TAB OUT ADJ IC1b 25V IN 1N5 40 8 A K λ LED6 BLUE BLUE λ LED5 K A A REG1 LM/LT1084-ADJ 2200 µF K BLUE λ LED4 A BLUE λ LED3 1W 47k 1W 47k Q2 STX0560 C Q1 KSC5603DTU* – ~ 1 1 W04 4 3 2 TO REMOTE PCB CON10 2 DC OUT CON9 400V 39 µF +HT Fig.2: the secondaries of toroidal power transformers T1 and T2 are connected in series and rectified using a voltage doubler to produce a 310V HT rail. Most of the ripple is filtered out by a capacitance multiplier comprising high-voltage transistors Q1-Q3 and a 470nF polyester capacitor. T2’s remaining 12VAC secondary drives the 6L6 filaments directly in a series/parallel configuration, while the 12AX7 filaments run from a regulated 12V rail produced by bridge rectifier BR1, a 2200μF filter capacitor and linear regulator REG1. IC1 provides an HT turn-on delay and soft start. SC 20 1 4 WARNING: LETHAL VOLTAGES ARE PRESENT ON THIS CIRCUIT WHILE IT IS OPERATING! S1 15V AC 15V AC 37V AC 37V AC 160VA TOROID 230V AC T1 Phase splitter The phase splitter is another 12AX7 twin triode, V2. The phase splitter provides some gain but its main job is to produce two similar drive signals with opposite phase for the grids of the push-pull output stage valves Signal is coupled to this phase splitter from V1b’s anode via another 220nF polyester capacitor. V2a operates as an inverter, to generate the out-of-phase drive signal. Like V1a and V1b, it is configured as a common-cathode amplifier. It runs from a higher HT rail of around 288V DC which comes from the first HT RC filter stage (6.8kΩ/39µF). Its grid is tied to ground by a 1MΩ resistor, with the voltage across the shared 6.8kΩ cathode resistor providing the required bias potential. This resistor is shared with V2b (and both cathode currents flow through it). V2b’s grid is connected straight to ground so when its cathode voltage increases, the grid-cathode bias voltage decreases. As such, when V2a’s cathode current increases and its anode voltage drops, V2b’s bias increases and thus V2b’s anode/cathode current decreases, causing the voltage at its anode to rise. So the signal at V2b’s anode has the opposite phase to that at V2a’s anode, ie, it is in phase with the signal from the preamp. The 220kΩ anode resistor value has been selected so that the two output signals have a similar swing and so that V2a and V2b both operate with as high an anode voltage as possible, to give maximum drive amplitude for the following stage. These drive signals are applied to the grids of 6L6 output valves V3 & V4 via 220nF polyester capacitors. These grids are again tied to ground by 1MΩ resistors and there are 10kΩ series stopper resistors to prevent parasitic oscillation. Output stage V3 & V4 are self-biased using 330Ω 5W cathode resistors, with around 22V across each. This gives an operating siliconchip.com.au 09/10/14 14:40:21 Currawong THD+N vs Power 10 Filter: 240VAC AP AUX-0025 mains, 1kHz + 20Hz-80kHz signal, 20Hz-20kHz bandpass BW w/AUX-0025, both channels driven 5 Total Harmonic Distortion + Noise (%) (ie, around 1µF) in series with a 9.1kΩ resistor. This sets the closed-loop gain of the preamp section at around 2.75, so that the following phase splitter receives around 3V RMS at maximum volume. Note, however, that there is also a feedback path from the amplifier’s output, which we will cover later. 2 1 0.5 0.2 4Ω 6Ω 0.1 8Ω 0.05 0.02 0.01 0.1 0.2 0.5 1 2 5 10 20 Power ( W atts) Fig.3: distortion versus power for a 1kHz sinewave into 4Ω, 6Ω and 8Ω load impedances. Again, both channels are driven for a realistic test. As you can see, distortion remains low at under 2W and then rises slowly until the onset of clipping at around 8-10W, depending on load impedance. The best power delivery is actually for 8Ω loads, with 6Ω being virtually identical and 4Ω being a little lower, clipping at around 9.5W/channel. This is partly due to output transformer drive impedance mismatch. current of about 65mA. Each output valve is powered from the main HT rail of around 308V, via the primary windings of T3, for a quiescent power of around 20W each. Note that DC and AC currents flow in the two halves of the push-pull winding since both plates of the tetrodes are fed from the transformer centre-tap connection. However, the magnetic fields produced by these direct currents are cancelled, as they flow in opposite directions. This is important because otherwise the transformer would be magnetically saturated. As the current split between V3 & V4 changes in response to the input signal however, an AC magnetic field is induced which is coupled into T3’s secondary. The resultant voltage drives the speakers or headphones. Since the output valve quiescent power of 20W is around twice the amplifier’s power output of 10W per channel into 8Ω, this gives Class-A operation. With lower load impedances (for example, 4Ω), V3 or V4 may be fully cut off during signal peaks, giving Class-AB operation. When the input signal swing is positive, pin 1 of V2a has a negative swing and so the current through V3 drops. At the same time, pin 6 of V2b has a positive swing and thus the current through V4 increases. This causes an increase in current flow from the top (dotted) side of T3’s primary to the other, resulting in a positive swing at the dotted side of the secondary. Thus, the output of the amplifier is in phase with the input. T3 also has taps approximately halfway between each end and the centre (HT) tap. These are connected to the screens of V3 & V4 via 47Ω stopper resistors, providing the ultra-linear connection mentioned earlier. This negative feedback from the transformer to V3 & V4 cancels out some of the transformer distortion. Note that while the feedback signals are high amplitude, the screen gain is much less than for signals applied to the grid, so the feedback doesn’t overpower the drive signals. Because the signal levels in the output stage are much higher and since 6L6 valves require much more filament November 2014  33 Parts List Chassis/power supply 1 timber plinth with base (details to come) 1 top cover cut from 3mm clear acrylic (details to come) 1 small tube acrylic glue 1 front panel, code 01111142, 249 x 30mm 1 rear panel, code 01111143, 248 x 53mm 1 160VA 37+37+15+15V toroidal transformer (Altronics MC5337) 1 80VA 12+12V toroidal transformer (Altronics M5112) 4 screw-on 30mm equipment feet (Jaycar HP0830, Altronics H0890) 4 M4 x 15mm machine screws and nuts (for feet) 1 15mm anodised aluminium knob to suit VR1 1 snap-in fused IEC mains male socket for 1.6mm panels (Altronics P8325) 2 M205 250VAC 1A slow-blow fuses (one spare) 1 red chassis-mount RCA/RCA socket 1 white chassis-mount RCA/RCA socket 2 red RCA line plugs 2 white RCA line plugs 2 red binding posts (Jaycar PT0453, Altronics P9252) 2 black binding posts (Jaycar PT0461, Altronics P9254) 1 SPST ultra-mini rocker switch, 250VAC rated (Altronics S3202, Jaycar SK0975) 1 1m length 2-core mains flex 1 1m length 3-core mains flex 1 200mm length 3mm diameter black heatshrink tubing 1 200mm length 8mm diameter black heatshrink tubing 1 200mm length 20mm diameter black heatshrink tubing 1 1m length heavy duty red hookup wire 1 1m length heavy duty black hookup wire 1 1m length single-core shielded cable 1 1m length medium duty black hook-up wire 1 12-way screw terminal strip (Jaycar HM3194, Altronics P2135A) 6 M3 x 25mm Nylon screws and nuts 1 M4 x 6mm machine screw 2 M4 nuts 2 4mm ID shakeproof washers 1 4mm ID eyelet crimp connector 3 red 6.4mm crimp spade connectors 12 4G x 9mm self-tapping screws 10 small Nylon cable ties current than 12AX7s, we run the filaments of V3 & V4 (and V7/V8) from 6.1V AC, slightly shy of the nominal 6.3V, due to compromises made in power transformer selection. It still works fine; it just takes a little longer for the valves to reach full emission after switch-on. speaker terminals via the normally closed contacts of RLY1 and pluggable terminal block CON3. RLY1 is energised if headphones are plugged into the front panel socket, disconnecting the speaker and re­ directing the signal to headphone socket CON5 via a 220Ω resistor. If LK4 is fitted (and we recommend that it is), feedback is applied from T3’s secondary to V1a’s cathode via a 470nF capacitor and 22kΩ resistor. Since the output is in phase with the input, by applying some of the output signal to V1a’s cathode, we effectively reduce the drive for V1a, giving about 14dB of negative feedback. There is a limit to how much feedback can be applied in this manner due Speaker connections & feedback A 470Ω 1W resistor across T3’s secondary ensures that there is some load even if there is no speaker connected. This is necessary because operating a push-pull transformer-coupled amplifier with no load can lead to very high AC voltages at the valve plates and subsequent flash-over in the valve sockets. T3’s secondary connects to the 34  Silicon Chip Main board 1 double-sided PCB, code 01111141, 272 x 255mm 2 15W 100V line transformers (T1,T2) (Altronics M1115 – do not substitute) 2 5VDC coil 3A contact SPDT micro relays (RLY1,RLY2) (Altronics S4141B) 6 M205 fuse clips (F1-F3) 1 1A M205 slow-blow fuse (F1) 1 3A M205 slow-blow fuse (F2) 1 6A M205 slow-blow fuse (F3) 1 white vertical RCA socket (Altronics P0131) (CON1) 1 red vertical RCA socket (Altronics P0132) (CON2) 2 2-way vertical pluggable terminal blocks (CON3,CON4) (Jaycar HM3112+HM3122, Altronics P2512+P2532) 1 PCB-mount switched 6.35mm stereo jack socket with long pins (CON5) (Jaycar PS0190) 1 3-way vertical pluggable terminal block (CON7) (Jaycar HM3113+HM3123, Altronics P2513+P2533) 1 5-way vertical pluggable terminal block (CON8) (Altronics P2515+P2535) 4 chassis-mount phenolic 9-pin valve sockets with bracket (V1,V2,V5,V6) (Jaycar PS2082) 4 chassis-mount ceramic 8-pin valve sockets with bracket (V3,V4,V7,V8) (Altronics P8501) 6 2-way pin headers, 2.54mm pitch (LK1-LK6) 2 shorting blocks (LK4-LK5) 1 5-50kΩ 16mm dual gang log pot* (VR1) 2 6073B-type mini flag heatsinks 4 M4 x 10mm machine screws 4 M4 shakeproof washers 4 M4 nuts 8 M3 x 15-16mm machine screws 10 M3 x 10mm machine screws 12 M3 shakeproof washers 12 M3 nuts to the phase shift created by the inductance of T3. We have set the feedback to give as much distortion cancellation as possible, while keeping it stable with capacitive loads. The circuit as presented is stable with several microfarads across the load, even when driving it with a square wave. By the way, the 470nF capacitor in the feedback path is important as it damps shifts in valve bias in response to changes in mains voltages and valve temperatures. With feedback enabled, input sensitivity is around 1V RMS. Typical CD/ DVD/Blu-ray players produce around 2V RMS so this should be plenty in most circumstances. With LK4 resiliconchip.com.au 14 M3 Nylon nuts 22 3mm inner diameter Nylon flat washers 8 6.3mm M3 Nylon tapped spacers 2 TO-220 insulating washers and bushes 1 1m length medium duty blue hookup wire (250VAC rated) 1 1m length shielded audio cable 1 200mm length 3mm diameter blue heatshrink tubing 6 small green Nylon cable ties (maximum 2mm wide) 2 small blue Nylon cable ties * ≥ 20kΩ recommended; substitute motorised pot for remote control option (see details in part two next month) Valves 4 12AX7 dual triodes (V1,V2, V5, V6) 4 6L6 beam tetrodes – matched pairs if possible (V3,V4, V7,V8) Semiconductors 1 4093B quad NAND Schmitt trigger IC (IC1) 1 LM/LT1084-ADJ 5A adjustable low-dropout regulator (REG1) 1 KSC5603D 800V 3A high-gain NPN transistor (Q1) 3 STX0560 600V 1A NPN highgain transistors (Q2-Q4) 3 BC547 100mA NPN transistors (Q5,Q7,Q9) 2 BC557 100mA PNP transistors (Q6,Q8) 1 red/green 2-lead bi-colour 3mm LED with diffused lens (LED1) moved, the overall gain is much higher and the input sensitivity is around 350mV RMS for full power. However, distortion rises to around 0.5% at 1kHz and >1% at lower frequencies. Note that the 470nF series capacitors in the feedback network are important. These form high-pass filters in combination with the feedback resistors, with a -3dB point of around 15Hz. If DC feedback is used, the bias time constants in the circuit form a type of relaxation oscillator and the bias voltages never quite settle down, leading to asymmetric clipping and other undesirable behaviour. Power supply The separate power supply circuit siliconchip.com.au 5 blue diffused lens 3mm LEDs (LED2-LED6) 1 W04 1.5A bridge rectifier (BR1) 2 1N5408 3A 1000V diodes (D1,D2) 3 1N4007 1A 1000V diodes (D4-D6) 1 1N4004 diode (D9) SIGNAL HOUND USB-based spectrum analyzers and RF recorders. Capacitors 1 2200µF 25V electrolytic 2 470µF 400V snap-in electrolytic 4 100µF 50V electrolytic 3 100µF 16V electrolytic 5 39µF 400V low-profile snapin electrolytic (Nichicon LGJ2G390MELZ15) (Mouser) 2 1.5µF 63V MKT 5 470nF 630V polyester 2 470nF 63V MKT 8 220nF 630V polyester 1 100nF 63V MKT or 50V multi-layer ceramic 2 100pF ceramic disc SA44B: $1,320 inc GST Resistors (1W, 5%) 9 1MΩ 2 9.1kΩ 2 270kΩ 4 6.8kΩ 2 220kΩ 2 3.3kΩ 2 120kΩ 2 680Ω 2 100kΩ 2 470Ω 6 47kΩ 2 220Ω 2 22kΩ 1 82Ω 5 10kΩ 4 47Ω 4 330Ω (5W, 10%) The BB60C supercedes the BB60A, with new specifications: Resistors (0.25W, 1%) 7 1MΩ 1 560Ω 1 150kΩ 3 470Ω 1 10kΩ 1 330Ω 2 1kΩ 4 120Ω • is shown in Fig.2. All components, except the two power transformers T1 & T2, power switch S1 and the fused IEC mains socket, are on the main board. There are three main power requirements for this circuit: the 310V HT rail, the ~12V DC filament supply for the 12AX7s (at around 1A) and ~6VAC for the 6L6 filaments, at around 4A. We also use the 12V DC rail to power various ancillary circuits, as described below. All of T1’s secondaries are connected in series, along with one of T2’s secondaries, to produce 114VAC. T2’s other secondary provides a little over 12VAC, to run the 6L6 filaments at around 6.1VAC each, in series pairs. The 12VAC is also rectified, filtered • • • • • Up to 4.4GHz Preamp for improved sensitivity and reduced LO leakage. Thermometer for temperature correction and improved accuracy AM/FM/SSB/CW demod USB 2.0 interface SA12B: $2,948 inc GST • • • • • • • Up to 12.4GHz plus all the advanced features of the SA44B AM/FM/SSB/CW demod USB 2.0 interface The BB60C streams 140 MB/sec of digitized RF to your PC utilizing USB 3.0. An instantaneous bandwidth of 27 MHz. Sweep speeds of 24 GHz/sec. The BB60C also adds new functionality in the form of configurable I/Q. Streaming bandwidths which will be retroactively available on the BB60A. Vendor and Third-Party Software Available. Ideal tool for lab and test bench use, engineering students, ham radio enthusiasts and hobbyists. Tracking generators also available. Silvertone Electronics 1/8 Fitzhardinge St Wagga Wagga NSW 2650 Ph: (02) 6931 8252 contact<at>silvertone.com.au November 2014  35 Most of the parts except mainly the power transformers are mounted on a single large PCB to make the assembly easy. The optional remote volume control is built on a separate PCB. and regulated to provide the 12V DC rail (actually about 12.3V DC), for the 12AX7 filaments and DC-powered circuitry. The 114VAC from CON7 is rectified in a half-wave voltage doubler consisting of 1000V 3A diodes D1 & D2 and two 470µF 400V capacitors, giving about 310V across both capacitors with several volts of ripple. Fuse F1 provides some protection against faults. There are two 47kΩ series-connected bleeder resistors to discharge the 470µF capacitors when power is removed. Four blue LEDs are connected in series with the two 47kΩ 1W resistors. The blue LEDs indicate the presence of HT and also illuminate output transformers T3 and T4 (very effective in a room with subdued lighting). The output stage has no HT lowpass filter, unlike the preamplifier and phase splitters. So to prevent HT ripple in the output stage from affecting the signal, we are using an active ripple 36  Silicon Chip filter. This is a capacitance multiplier filter built around high-voltage, highcurrent transistor Q1, configured as an emitter-follower. The traditional HT filter is a large iron-cored choke but these are heavy and expensive, not to mention hard to find these days. Our transistor-based method is more effective and cheaper. Q1 is driven by Q2 and Q3 which are high-voltage high-gain signal transistors, in a “Triplington” configuration; it’s like a Darlington but with an extra stage. The higher the gain in this buffer, the more effective the filter is. Base bias comes from an RC low-pass filter across the incoming HT rail, consisting of a 1MΩ resistor and 470nF polyester capacitor. Q2 and Q3 have a gain of around 70100 each while Q1 has a gain of around 30. So the overall gain is about 70 x 70 x 30 = 147,000 which multiplies the effect of the 470nF capacitor to act as if it is 69,000µF! In practice, it isn’t quite as good as this as the 470nF capacitor discharges slightly through the three base-emitter junctions at the trough of each ripple cycle but despite this, the ripple at Q1’s emitter is just a few hundred millivolts. Q1 has an integral emitter-collector diode so that when the unit is switched off, the output filter capacitors can safely discharge back into the input filter capacitors without doing any damage. D4 protects Q2 while D5 provides similar protection for Q3 but also has a role in the start-up delay, which we’ll explain later. Note that this arrangement also results in HT “soft-start” as it takes a few hundred milliseconds for the 470nF capacitor to charge and the HT rail tracks this voltage. Turn-on delay We have also incorporated a 20-second (or so) turn-on delay, to allow the valve filaments to heat up before HT siliconchip.com.au Low-voltage supplies 5-pin pluggable terminal block CON8 provides separate low-voltage AC connections for the 6L6 filaments (pins 1 & 3) and the regulated supply (pins 4 & 5). Each is fused on the board. siliconchip.com.au The PCB is slid into a slot that runs around the top inside edge of the timber plinth. Perspex covers will be used to protect the PCB and speaker transformers. 09/10/14 14:35:26 Currawong THD+N vs Frequency 10 240VAC mains, output level 1W, 20Hz-80kHz bandwidth, both channels driven 5 Total Harmonic Distortion + Noise (%) is applied. Part of the rationale for this is to prevent “cathode stripping” which can occur with cold cathodes, although the existence of this phenomenon is somewhat controversial. But since the valves aren’t “ready” to operate immediately anyway, it certainly doesn’t hurt to delay the application of HT. IC1 is a quad Schmitt-input NAND gate which runs from the 12V rail and provides the turn-on delay. Note that ground for the 12V rail is labelled VEE and will be close to, but not necessarily at, GND (0V). IC1a is connected as an inverter with a 100µF capacitor from its input to ground. A 150kΩ resistor charges this capacitor from the 12V rail while a 1MΩ resistor discharges it when power is switched off. It takes about 20 seconds for this capacitor to charge to a sufficient voltage for the output of IC1a to go low. During this time, IC1a’s output is high. This is inverted by IC1c and then again by IC1d, so Q4 (another 600V transistor) is switched on initially. This keeps the 470nF capacitor in the HT filter from charging until the delay has ended. Diode D5 in the HT filter prevents the base of Q3 from being pulled below GND when VEE is (slightly) negative. IC1a and IC1c also drive LED1 via two pairs of complementary emitterfollowers (Q5-Q8). LED1 is a bi-colour device and consists of a red LED and green LED on the same die, connected in inverse parallel. Since inverter IC1c is between them, one inverter is always driving one end of LED1 high and the other is driving it low. Thus LED1 is red initially at turn-on and switches to green once the time-out period has expired and the HT rail is powered up. A 1kΩ resistor sets the LED current to about 10mA while another 1kΩ resistor partially isolates the bases of Q5 & Q6 from IC1a’s output. This allows the optional remote control board to independently drive LED1, in order to flash it to acknowledge infrared command reception. The remote control board connects via CON10 and will be described next month. 2 1 0.5 4Ω 0.2 6Ω 8Ω 0.1 0.05 0.02 0.01 20 50 100 200 500 1k 2k 5k 10k 20k Frequency (Hz) Fig.4: distortion versus frequency, with both channels driven at 1W into three different resistive loads. As you can see, the distortion is pretty low for a valve amplifier, especially between 100Hz and 10kHz. Below 100Hz, distortion rises steeply due primarily to the output transformer’s non-linear response. Distortion into lower impedances is only slightly worse than that for 8Ω. Note the 80kHz bandwidth used, to ensure that higher frequency harmonics are included in the measurements. However, we ultimately decided to use one transformer winding to power both, hence they are wired in parallel despite the separate connections. The 12VAC from pins 4 & 5 of CON8 is rectified by 1.5A bridge rectifier BR1 and filtered with a 2200µF capacitor to produce around 15-16V DC with November 2014  37 09/10/14 14:58:07 Currawong Frequency Response +3 Note: parts of this circuit operate at over 300V DC. Do not touch any components or any part of the PCB while the unit is operating or immediately after switch off. The blue LEDs in the circuit indicate when dangerous voltages are present. +2.5 +2 +1.5 Amplitude Variation (dBr) Warning! +1 +0.5 +0.0 4Ω 6Ω -0.5 8Ω -1 -1.5 -2 -2.5 -3 10 20 50 100 200 500 1k 2k 5k 10k 20k 50k 100k Frequency (Hz) Fig.5: the frequency response is pretty flat in the audible range (note: the vertical scale is only ±3dB for the entire diagram). Roll-off at the high frequency end is -3dB at around 50kHz while the low-end -3dB point is below 10Hz. The peak at around 20Hz is partly due to the AC-coupled global feedback and partly due to greatly increased waveform distortion below about 30Hz due to the output transformers. However, the peak amplitude is only around +1.5dB and 20Hz signals are barely audible. about 1V ripple. This is regulated to provide a nice smooth rail by REG1, a low-dropout, high-current equivalent to the LM317. Pins 1 & 3 of CON8 connect straight to the series/parallel-connected 6L6 filaments and as a result, they get about 6.1VAC each. One end of this AC supply is grounded for noise immunity. Now because of this ground connection and the fact that the same transformer secondary is used to feed BR1, the negative end of BR1 actually floats between about +0.7V and -15V. Hence, the need to disconnect VEE from GND. If two separate 12V transformers or windings were used, LK6 could be fitted and thus VEE would be at the same potential as GND. LK6 must not be fitted with the supply arrangement shown here! The circuit will work the same regardless as to whether VEE is connected to GND, as Q4 is the only connection between the two supply “domains”. The DC supply is also used to power relays RLY1 and RLY2 when headphones are plugged in. These are 5V relays, so an 82Ω series resistor drops 38  Silicon Chip the 12V DC to an appropriate voltage. LED2 is also connected across the relay coils, in series with a 330Ω currentlimiting resistor, to indicate when the speakers are disconnected. Unused linking options Note that the supply was also designed to operate with the regulated rail at 6V DC rather than 12V. This would require a different transformer (ie, 6VAC rather than 12VAC) and the option was provided as there are some 12AX7-compatible valves with 6.3V-only filaments (rather than the typical arrangement with a 12.6V centre-tapped filament). However, given the relative rarity of these valves, we aren’t going to go into details as to how to reconfigure the supply except to say that LK1-LK3 are fitted for this purpose. Normally, they are left out. PCB layout We wanted to put as many parts on the PCB as possible to make this amplifier easy to build. Soldering parts to a PCB is certainly a lot easier than point-to-point wiring! It minimises the chances of mistakes and also means that performance will be consistent between amplifiers. The PCB layout was a bit tricky though, due to the voltages involved. We have kept tracks with voltages that may differ by over 60V apart by 2.54mm to prevent arcing, while in other areas low-voltage tracks need to be closer together so they can fit. We also used “star” earthing as much as possible to avoid hum and ripple injection into the preamp stages. Most of the grounds on the board converge on the main power supply filter capacitor negative pin. The board has been designed with plated slots for the valve socket pins so that they fit snugly and neatly. All connectors have been placed along the back of the board, on the underside, to keep the chassis wiring neat. The input signals run from the back of the board to the front (where the volume pot is mounted) through shielded cables that are strapped to the underside of the board, to prevent the low-level input signals from picking up ripple, hum and buzz. We have also used low-profile components where possible, so that a clear perspex shield can be fitted over the top, to prevent prying fingers from getting a shock, as mentioned earlier. The valves, main filter capacitors and output transformers will pass through cut-outs in this shield, with perspex boxes around the transformers. The rest of the components will be safely underneath. Next month That’s all we have room for this month. Over the next couple of months we will present the main PCB layout diagram, describe the assembly procedure, explain how to build the plinth and finish the wiring. We’ll also go through the testing and troubleshooting procedure and describe the optional infrared remote control which SC uses a motorised potentiometer. siliconchip.com.au