Fullscreen HTML5Med Res (??MB)HTML4

AUDIO OUT AUDIO OUT L R By Jake Rothman Analogue Vocoder – Part 6: Universal Audio PSU Fig.1. Why does so much high-quality audio equipment have cheap and nasty £5.00 ‘wall-wart’ power supplies? Our dog quickly dispatches them to landfill. A udio lives or dies by its power supplies, but all too often it runs off Lo-Fi plastic ‘wall warts’ that only last 13 months (ie, just long enough to get past the guarantee). In our house, some even get eaten by the dog! – see Fig.1. Audio is based on transducers, amplifiers, signal processors and oscillators – running off power supplies. All too often the humble power supply is just an afterthought, so I reckon we’re due a high-quality Audio Out power supply. Here’s a big one especially designed for the Analogue Vocoder and microphone pre-amp, but it’s also suitable for mixers, synthesisers and Fig.3. Power supply connectors are non-standardised. This mixer uses an unusual connector – the plug took some searching, and as usual, eBay failed, but Mouser delivered. Fig.4. Careful track routing ensures low hum spikes. Here, PCB designer Mike Grindle has used special star earthing to separate the rectifier ground, quiet power ground and reference 0V. other systems with upwards of 40 op amps, where a higher current than normal (>250mA) is needed. Another common requirement is a +48V rail for phantom-powered microphones. Naturally, we’ll also need the lowest possible noise, and don’t forget hum is the main priority for audio power supplies. This audio power supply provides ±15V dual rails at up to 750mA and +48V at 80mA. There is also a constant-current supply for front panel LEDs and a dethump relay driver. The un-regulated ±21V power rails are also available for use with small power amplifiers driving speakers. Fig.5. There are five main functional blocks in the power supply. I’ve been meaning to design one with all these features on a single PCB for 25 years! – + To de-thumping/ muting relay 3 10mA 5 +55V to +70V L 120V N Fig.2. This excellent mixing desk was rescued, but minus its power supply. 56 – 0V 0V 120V 15V 0V 0V Mains earth + +48V 48V regulator +48V 0V + 15V 1 Main smoothing network Dual PSU regulator 0V 0V – 0V Ph antom power 4 Unregulated + 21V to power amplifer + 15V 0V 0V 0V Main rectifier Mains transformer 15- 0- 15V AC LEDs Driver delay Voltage tripler 2 Constant current source – 15V Unregulated – 21V to power amplifer G round lift resistor and RF byp ass capacitor E Practical Electronics | April | 2022 n Noise This design started when I found a mixer (Fig.2) in a skip. I never found the power supply, which uses a non-standard multi-pin power supply connector (Fig.3). (Remember to always use male/pins for inputs and female/sockets for outputs.) I was shocked when I saw such supplies on eBay were over £200 (second hand), so it made sense to design and build one. Phantom generation The BBC used to use a specially made toroidal transformer with a separate winding for the 48V section supplied by Canford Audio. I avoid expensive bespoke parts, so I generate sufficient voltage using a circuit driven by a standard 15-0-15V transformer. Fig.6. Power supply circuit diagram. R21 D15 + 0.5W D14 – RLY1 (32V holding) 42V D17 D16 R22 D14-D17 1N4002 L N S1 F1 1A A/ S V DR1 275V 15V 0V 0V TR2 BC327 R27 Con5 1W C23 TR3 47µF BC337 10V R26 L De- th ump relay TR4 BC141 R Audio 25mA R14 15V 0V 0V D11 0.5W UF4004 C18 1000µF D10 63V 1N4004 IC3 In TL783 Out Adj + 120V Con1 Screen D9 1N4004 Transformer connector Mains earth connect to metalwork R1 C1 100nF D1 SB350 D3 SB350 R3 C5 1nF + R23 120V E C24 + 470µF D18 63V BZY88 10V R24 T1 15- 0- 15V 50V A (21V holdingl) D19 1N4001 R25 C22 + 47µF 63V IEC filtered mains connector around the main smoothing capacitors (C7 and C8) shown in Fig.4. High-current tracks are also made as wide as possible. It is not often appreciated that output decoupling capacitors can paradoxically increase regulator noise due to resonance with the regulators’ output impedance. Regulators, as in all negative feedback amplifier systems, have an output impedance that rises with frequency; effectively it is ‘an inductance’. This is a result of the open-loop gain declining with frequency. In the case of the LM3XX series of adjustable regulators this is in the order of 40µH. Ringing is also exacerbated by snubber capacitors used to suppress diode switching. Ironically, the use of high-quality low-loss The main source of electrical noise in power supplies is hum caused by capacitor charging currents from the rectifier, which induce voltages into the ground and power rail connections. This can easily be avoided if the layout is ‘starred’ correctly by feeding the rectifier outputs straight into the smoothing capacitor pins and then taking the power rail directly from that pin. It is essential to avoid sharing conductors for charging and power since the finite resistance of the PCB track will lead to charging pulses being superimposed on the output. (My PCB designer) Mike Grindle has taken great care in the PCB track layout to optimise this. Note the diamond track layout C3 100nF R2 C2 100nF + +55 to +70V R4 VR1 . + C9 2200µF 35V + –21V R13 0.5W C17 10nF – 21V Unreg + C10 2200µF 35V + C12 100nF 2.5W Adj –20V + 0.5W 0.5W In IC2 Out LM337 Adj 750mA max R9 R7 R8 C8 2200µF 35V R6 D7 UF4002 IC1 In LM317 Out Adj Clean G nd 10mA L ED power C11 100nF C7 2200µF 35V Dirty G nd star R20 0.5W . Con4 0.5W R5 +20V Con3 TR1 BC143 R18 D5 UF4004 2.5W C6 1nF Practical Electronics | April | 2022 C21 + 100µF 63V Ph antom power R19 D13 BZY88 5.6V 0.5W 4mA +21V C4 100nF 3.5mA R17 D12 UF4004 0.5W + 21V Unreg D4 SB350 R16 R15 C20 + 47µF 100V D2 SB350 +48V 0.5W C19 470µF 80/100V 80mA max + D8 UF4002 C13 22µF 25V C14 + 22µF 25V R10 +15V R11 + D20 1N4001 C15 22µF 25V C16 22µF 25V D21 1N4001 Con2 (x3 ) O p amp power R12 750mA max –15V D6 UF4004 57 snubber capacitors of 10nF to 100nF (C1 to C4) across each diode in a bridge rectifier (D1 to D4). To enhance the snubbing effect and prevent ringing, damping resistors (R1 to R4) are inserted in series with the capacitors. The other rectifiers in the system don’t need these capacitors because the currents involved are much lower. The main rectifier uses Schottky diodes for their low voltage drop, which is needed with a 15-0-15V transformer. Dual-rail op amp supply Fig.7. Toroidal transformers run cooler, radiate less magnetic field and mechanical noise than traditional laminated types. The top transformer is unencapsulated, while the lower transformer from RS has the lower noise with a slight size increase. capacitors makes these problems worse. If cheap ones, having a high ESR (equivalent series resistance) are used, the ringing can be damped out. That said, it is not good design practice to depend on a variable parasitic parameter being ‘bad’. A much better approach is to incorporate a specified damping resistor, – typically 0.6Ω to 10Ω – in series with the capacitor. Circuit description Fig.5. Shows the basic blocks in the power supply. I’ll go through each section describing some of the design aspects. The full circuit diagram is in Fig.6. Transformer A 15-0-15V transformer is sufficient for normal op amp rails of ±15V. If a higher voltage is required then the transformer can be increased up to 18-0-18V or 20-020V. The best transformers for audio are encapsulated magnetically screened toroids with an interwinding screen between the primary and secondary. These are often made by Avel Lindberg in the UK (sometimes re-badged for RS, see Fig.7). I like to buy them at radio rallies where they sometimes turn up for a few pounds. They seem to last forever, so I also salvage them from scrapped equipment. Only audiophiles seem to appreciate their low-noise qualities, so radio hams charge very little for them. The screen must be connected to mains earth to be effective against radio interference and there is a pin for this on the PCB. If a transformer without a screen is used, placing a couple of capacitors (C5 and C6) between the secondary outputs and mains earth does almost as good a job. They must be kept small (<10nF), or they cause hum spikes. Rectifier It is well known that rectifier diodes make a lot of high-frequency noise when they switch off during their conduction cycle. This noise is usually suppressed by wiring 58 This section uses the standard audio LM317/LM337 regulator pair because they are low-noise and cheap. The only problem is their high-current limit of 1.5A; in practice, they usually shut down thermally before this limit is reached. To ensure reasonable output voltage accuracy the potential divider resistors R7 to R10, must have fairly low-resistance values (under 4.7kΩ). This is to accommodate a small but variable current coming out of the regulator’s adjust pin which may induce a voltage reflected in the output voltage. In the data sheet these resistors are often half the values I have used to ensure the minimum load current which most regulators require to operate. All the normal protection diodes (D5-D8) have been added for input and output shorts. D20 and D21 are included to prevent start-up problems if there is a momentary short between the plus and minus rails. (In audio circuits there are often decoupling capacitors across the rails which can cause this situation.) Phantom power voltage tripler is good news for audiophiles since there were no alternatives apart from complicated LM317 circuits or even more complex discrete circuits. The TL783’s series-pass transistor is a MOSFET, so it is a thermally robust device. To accurately set the output voltage, a preset resistor is needed, rather than a fixed potential divider. This is because the output voltage is such a large multiple of the reference voltage that errors are magnified. The current through presets has to be kept to a minimum – high dissipation here is not good for stability and long life. Often, the potential divider resistor values are reduced to double up as a minimum current load for the regulator. On the datasheet this is specified as 15mA. In view of the relatively low output voltage of the regulator (it can be normally used up to 125V) this can be relaxed a bit to 10mA. LED driver The need for a minimum load current also means wasted energy if just a bleeder resistor is used. Often, this current is used to run an LED. Since the voltage is high it is possible to run lots of LEDs in series, such as bar graph meters. LEDs used for front-panel indication will occasionally have to be turned off (by using switches to short them out) so some means of stabilising the current through the chain is necessary. This is to avoid changes in the brightness of the remaining LEDs that are illuminated. A constant-current source does the trick, as shown in Fig.8. Note that if some LEDs are too bright relative to others, they will have to have a resistor wired in parallel, bypassing some of the current to dim them down. I have set the current to 10mA, which is sufficient for most modern LEDs. If you are using old, low-efficiency TIL209 LEDs from the 70s, you may have to up it to 20mA by reducing R19 to 240Ω. The constan-current circuit is built around TR1 which needs a clip-on heatsink. R20 relieves it of some of its dissipation burden. A voltage reference of 5.6V is provided by Zener diode D13. This supplies 5V across 510Ω resistor R19, setting the constant current to 10mA. Note that if LEDs are To produce a regulated 48V for phantom power, an unregulated voltage of at least 55V is needed to provide sufficient headroom. Obviously, we can’t get this from the 15V transformer output without a bit of circuit trickery. Here I use a standard voltage-doubler circuit consisting of C18, D9, D10 and C19. In fact, since the cathode of D9 is connected to the positive op amp power rail rather than ground, the output of the voltage doubler is actually tripled, giving plenty of headroom. (If there is too much voltage (>70V) on C19 because of higher-voltage transformers, D9 can be switched to 0V). The ground conBrigh tness nection of the voltage doubler has match ing to be connected to the ‘dirty’ star 5mA O n O n point (see Fig.6) to avoid charging bypass current pulses. The ground of the 48V regConstant current ulator circuit must go to a different source O ff O ff ground point, the op amp regulator 10mA 5mA circuit’s dual-rail 0V (see Fig.6), +48V because that is the quiet ‘clean’ Blue LED 10mA reference ground. Vf = 3V Production of the versatile TL783 0V high-voltage regulator has resumed recently, courtesy of Texas Instru- Fig.8. LEDs can be placed in series, fed by a ments, and it is the best device for constant-current source. If they need to be phantom-power regulators. This extinguished they are shorted with switches. Practical Electronics | April | 2022 Fig.9. These hermetically sealed 4190 relays from STC have a proven lifetime of around 30 years for audio signal switching. Such relays can be spotted by the glass seals around the pins. not used or needed then the regulator must still be loaded by shorting out the LED connector. Muting relay Another power-related circuit needed is a relay driver. Almost all audio devices require some means of muting switchon and switch-off noises. Active filters built around op amps, such as those used in the Analogue Vocoder, are especially bad, often producing a shriek when turned off. You can imagine the sound of 14 channels squealing at different frequencies simultaneously is pretty horrendous. A double-pole relay or pair of JFETs are often used to mute the audio output lines while these noise sources are active. JFETs have the advantage of being almost a short until biased off, and they consume virtually no power. Relays consume significant power to energise the coil, but they provide the lowest distortion and the best muting. However, Fig.11. The completed power supply board. unless they are hermetically sealed, the contacts can get oxidised, leading to intermittent signals. Normally, such relays are expensive, but I’ve got plenty of ‘old new stock’ inexpensive devices for PE readers, as shown in Fig.9 (see my AOShop ad on p.53 in last month’s PE). The relay control circuit has to generate a delay to turn it on and an instant turn-off the moment the power is removed. To get the muting relay to turn off quickly, it’s necessary for the circuit to be powered by its own bridge rectifier (D14 to D17) followed by a minimally sized smoothing capacitor (C22), just Relay – Relay + NC + 48V 0V sufficient to stop the relay buzzing. The switch-on delay of a couple of seconds is determined by R25 and C23. There is also an 11.2V threshold voltage set by a 10V Zener diode (D18) in conjunction with the 1.2V forward drop of the Darlington driver consisting of TR3 and TR4. I could have used a single MOSFET, but bipolar devices work fine. A circuit comprising TR2, which is normally biased off by potential divider R22 and R23 discharges the timing capacitor C23 quickly at turn-off. R24 limits the current protecting the transistor and tantalum capacitor. This ensures the switch-on delay always happens, even if the power is quickly flipped on and + 15V 0V – 15V off. Finally, a power-saving network, comprising R27 in CO N2 parallel with large electroD20 lytic C24 ensures sufficient D21 current to pull in the relay IC1 CO N2 on turn-on with just enough IN O UT current to then hold it. This ADJ CO N2 almost halves the continuous current consumption. This circuit allows the use of 24V 700Ω and 48V 2500Ω relay coil impedance devices. IC2 L ED+ 0V + C C24 R1 D3 C4 C3 R3 R13 C9 C11 R5 C8 R6 D5 D8 R12 ADJ IN O UT R8 D4 C7 R10 R7 D1 D11 C17 + D2 R26 R23 R24 R21 C14 + C1 + + CO N1 + R4 IN O UT ADJ 15V AC D12 D10 C2 C13 R14 + 0V C5 0V C19 C16 + 15V AC C6 Earth screen D9 C18 D15 R2 C15 R20 IC3 + Mains earth + D16 D14 Transformer input R27 E C TR1 V R1 + D17 B R15 + D19 C22 R19 + R22 C20 CO N3 + R25 D13 B R18 R17 + TR4 TR3 E B C CO N4 C21 CO N5 E R16 C23 TR2 E B C D19 D6 D7 C10 C12 R11 R9 Unreg Unreg Unreg 0V + 21V – 21V Fig.10. Power supply PCB overlay – note the chunky components, no 64-pin IC packs here! PCB design by Mike Grindle. Practical Electronics | April | 2022 Heatsinks Separate heatsinks are used for each voltage regulator because the insulating washer can then be avoided. These washers cause small TO220 devices to run almost twice as hot because of their additional thermal resistance. When no washers are used, the heatsinks are of course ‘live’ and 59 the upper side of the board. Put Kevlar tape down to provide insulation if necessary. Casing Fig.12. Sometimes, for less arduous applications, smaller pressed heatsinks can be used. If you can keep your finger on the heatsink for a few seconds it will be around 60ºC and within rating. care has to be taken to avoid shorts. REG1 is at +15V, REG2 at the unregulated input voltage of –21V and REG3 is at +48V. Large extruded 6.8ºC/W heatsinks are needed for the op amp power regulators (REG1 and REG2) if you plan to draw big currents. The phantom power regulator (REG3) can work with a small clip-on heatsink for normal use. However, if you plan to use up to 20 microphones for a full band, a bigger bolt-down one is needed. Construction Like all power supplies, this one uses large parts, so it is much less fiddly than most constructions. The PCB is available from the PE PCB Service. The overlay is shown in Fig.10 and the completed board in Fig.11. The main difficulties concern heatsink mounting, which depend to a large extent on what’s available. For lower current use, smaller cheaper types can be used, as shown in Fig.12. Remember to use a smear of heatsink compound. Solidity is key with heatsinking, so use spring washers or Nylock nuts to stop the bolts loosening. Also, do secure the heatsinks to the board. To avoid cracked joints, only solder after all the tightening and mechanical adjustment has taken place. Watch out for shorts from the heatsink to the tracks on Fig.13. When first turning-on the power supply, wire 100Ω current-limiting resistors between the 15-0-15V secondaries and the PCB. Optionally, these can be replaced later with 2A anti-surge fuses. 60 An external power supply always gives lowest hum, and when housed in a steel box provides the best screening of the transformers field’s hum. If you incorporate the power supply alongside audio equipment in the same housing then always place the transformer as far away as possible from audio inputs. An aluminium case is better here, since magnetic fields are less likely to be radiated into sensitive electronics. Always connect any metalwork to mains earth. I have crammed this power supply into some 1U 19-inch rack cases, but it would be better to use a 2U since there is more headroom above the heatsinks. Component list Semiconductors IC1 LM317T adjustable positive regulator IC2 LM337T adjustable negative regulator IC3 TL783 high-voltage regulator, Mouser 595-TL783CKCSE3 D1-D4 SB50 3A 50V (minimum) Schottky diode. D5-D8, D11, D12 UF4001 ultrafast 1A 50V D9, D10 1N4004 1A 200V D15-D17, D19-D21 1N4002 1A 100V D18 BZY88C5V6 5.6V 400mW Zener 5V6 D19 BZY88C10V 400mW Zener 10V TR1 BC143 or other medium-power 60V TO5 600mA PNP TR2 BC327, BC557 or other lowpower PNP TR3 BC337, BC547 low-power NPN TR4 BC141, 2N2219, BFY51 TO5 40V 600mA NPN Capacitors Non-polarised, all 100V ceramic X7R 20% 5mm C1-C4, C11, C12 100nF C5, C6 1nF C17 10nF Electrolytic capacitors Note, these components determine the lifespan of the unit. Use the best you can – eg, Kemet, Nichicon C7-C9 2200µF 35V 16mm diameter radial (Nichicon UPM1V222MHD from Mouser) C13-C16 22µF 35V axial or radial* C18 1000µF 63V 16mm radial (Nichicon UPM1J102MHD from Mouser) C19 470µF 80V or 100V 16mm radial (Nichicon UPM1K471MHD, Mouser) C20 C21 C22 C23 C24 47µF 50V radial* 100µF 50V radial* 47µF 63V radial* 47µF 16V tantalum axial/ radial 470µF 25V radial* *Some of the smaller ones could be replaced with solid-aluminium or metal-cased tantalum devices for lower noise and longer life. Resistors All 5% tolerance carbon-film, 0.25W unless otherwise stated R1-R4, R11, R12, R17 1Ω (7 off) R5, R6 1Ω 2.5W wire-wound/ metal-oxide R7, R8 3kΩ 0.5W 1% metal-film (Tayda do a 1W version that fits) R9, R10 270Ω 0.25W 1% metal-film R13 1kΩ 0.5W R14 22Ω 0.5W R15 11kΩ 0.5W R16 330Ω R18 12kΩ 0.5W R19 510Ω R20 2.2kΩ 0.5W R21 56Ω 0.5W R22 56kΩ R23 33kΩ R24 10Ω R25 560kΩ R26 47kΩ R27 560Ω 1W metal-oxide VR1 2.2kΩ horizontal cermet preset, 10mm square Bourns 3386, Helitrim 72P or similar. Inductive RLY1 STC 4190 double-pole sealed relay or similar, 24V 700Ω or 48V 2500Ω coil. T1 Transformer 15-0-15V 2A toroidal Cotswold D1012/A or similar. Avel Lindberg or RS 207-425 screened type if available. (I have second-hand ones in stock – see AOShop advert) Heatsinks Redpoint/Avid Thermalloy 436402 6.8ºC/W heatsink RS 402-995 (x2) TO5 clip-on heatsink T0220 Redpoint/Avid Thermalloy TV4 or 400043 21ºC/W heatsink RS 402-954 Miscellaneous PCB from the PE PCB Service (AO1-APR22) M3.5 or 4BA nuts, 10mm bolts and spring washers for mounting the three regulators Self-tappers No.6 x 6.4mm for large heatsinks RS 504-568. Power connectors – eg, JAE SRCN6A13-5S-A534 from Mouser, suitable for Soundcraft mixer 0.2-inch screw connectors ‘Molex’ 0.1 inch 3-way and 2-way crimp headers and plugs Practical Electronics | April | 2022 Testing Since power supplies have many electrolytic capacitors and diodes, there is always the risk of an ‘explosion’ if connection are reversed. It’s a good idea to place 100Ω 0.5W current-limiting resistors in series with the 15V transformer input leads, as shown in Fig.13. There should be no load on the power supply at this stage. If they start to smoke and there’s no voltage on any of the unregulated output pins, something is wrong! The good thing is you won’t have a burnt-out £20 transformer or lost an eye – yes, always wear eye protection for ‘first power up’ of any circuit! ESR Electronic Components Ltd All of our stock is RoHS compliant and CE approved. Visit our well stocked shop for all of your requirements or order on-line. We can help and advise with your enquiry, from design to construction. Full load check It is sensible to test a power supply by drawing the full current expected plus a little extra from all outputs and looking at the output with a ‘scope set to 5mV/div AC. This should also be done to check for heating effects and possible loss of regulation which can happen if the input voltage to the regulators falls too low, allowing ripple to break through. Noise should just be a smooth hiss at around 5mVpk-pk with no hum spikes. Suggested brutal test load resistors are 560Ω 10W for the 48V and 20Ω 10W for the ±15V rails. Outdoor musical events are often powered by generators, prone to ‘mains’ voltage dips. To check fully for this, I feed in the minimum expected mains voltage (say 215V) using a variac or buck transformer. In this case, it will be necessary to use an 18-0-18V transformer and reduce the maximum load current by 20%. 3D Printing • Cable • CCTV • Connectors • Components • Enclosures • Fans • Fuses • Hardware • Lamps • LED’s • Leads • Loudspeakers • Panel Meters • PCB Production • Power Supplies • Relays • Resistors • Semiconductors • Soldering Irons • Switches • Test Equipment • Transformers and so much more… Monday to Friday 08:30 - 17.00, Saturday 08:30 - 15:30 Station Road Cullercoats North Shields Tyne & Wear NE30 4PQ That’s all folks We haven’t got space or time this month to integrate this power supply into the Analogue Vocoder so that will have to be covered next time. Tel: 0191 2514363 sales<at>esr.co.uk www.esr.co.uk STEWART OF READING 17A King Street, Mortimer, near Reading, RG7 3RS Telephone: 0118 933 1111 Fax: 0118 933 2375 USED ELECTRONIC TEST EQUIPMENT Check website www.stewart-of-reading.co.uk Fluke/Philips PM3092 Oscilloscope 2+2 Channel 200MHz Delay TB, Autoset etc – £250 LAMBDA GENESYS LAMBDA GENESYS IFR 2025 IFR 2948B IFR 6843 R&S APN62 Agilent 8712ET HP8903A/B HP8757D HP3325A HP3561A HP6032A HP6622A HP6624A HP6632B HP6644A HP6654A HP8341A HP83630A HP83624A HP8484A HP8560E HP8563A HP8566B HP8662A Marconi 2022E Marconi 2024 Marconi 2030 Marconi 2023A (ALL PRICES PLUS CARRIAGE & VAT) Please check availability before ordering or calling in PSU GEN100-15 100V 15A Boxed As New £400 PSU GEN50-30 50V 30A £400 Signal Generator 9kHz – 2.51GHz Opt 04/11 £900 Communication Service Monitor Opts 03/25 Avionics POA Microwave Systems Analyser 10MHz – 20GHz POA Syn Function Generator 1Hz – 260kHz £295 RF Network Analyser 300kHz – 1300MHz POA Audio Analyser £750 – £950 Scaler Network Analyser POA Synthesised Function Generator £195 Dynamic Signal Analyser £650 PSU 0-60V 0-50A 1000W £750 PSU 0-20V 4A Twice or 0-50V 2A Twice £350 PSU 4 Outputs £400 PSU 0-20V 0-5A £195 PSU 0-60V 3.5A £400 PSU 0-60V 0-9A £500 Synthesised Sweep Generator 10MHz – 20GHz £2,000 Synthesised Sweeper 10MHz – 26.5 GHz POA Synthesised Sweeper 2 – 20GHz POA Power Sensor 0.01-18GHz 3nW-10µW £75 Spectrum Analyser Synthesised 30Hz – 2.9GHz £1,750 Spectrum Analyser Synthesised 9kHz – 22GHz £2,250 Spectrum Analsyer 100Hz – 22GHz £1,200 RF Generator 10kHz – 1280MHz £750 Synthesised AM/FM Signal Generator 10kHz – 1.01GHz £325 Synthesised Signal Generator 9kHz – 2.4GHz £800 Synthesised Signal Generator 10kHz – 1.35GHz £750 Signal Generator 9kHz – 1.2GHz £700 HP/Agilent HP 34401A Digital Multimeter 6½ Digit £325 – £375 HP33120A HP53131A HP53131A Audio Precision Datron 4708 Druck DPI 515 Datron 1081 ENI 325LA Keithley 228 Time 9818 Practical Electronics | April | 2022 HP 54600B Oscilloscope Analogue/Digital Dual Trace 100MHz Only £75, with accessories £125 Marconi 2305 Modulation Meter £250 Marconi 2440 Counter 20GHz £295 Marconi 2945/A/B Communications Test Set Various Options POA Marconi 2955 Radio Communications Test Set £595 Marconi 2955A Radio Communications Test Set £725 Marconi 2955B Radio Communications Test Set £800 Marconi 6200 Microwave Test Set £1,500 Marconi 6200A Microwave Test Set 10MHz – 20GHz £1,950 Marconi 6200B Microwave Test Set £2,300 Marconi 6960B Power Meter with 6910 sensor £295 Tektronix TDS3052B Oscilloscope 500MHz 2.5GS/s £1,250 Tektronix TDS3032 Oscilloscope 300MHz 2.5GS/s £995 Tektronix TDS3012 Oscilloscope 2 Channel 100MHz 1.25GS/s £450 Tektronix 2430A Oscilloscope Dual Trace 150MHz 100MS/s £350 Tektronix 2465B Oscilloscope 4 Channel 400MHz £600 Farnell AP60/50 PSU 0-60V 0-50A 1kW Switch Mode £300 Farnell XA35/2T PSU 0-35V 0-2A Twice Digital £75 Farnell AP100-90 Power Supply 100V 90A £900 Farnell LF1 Sine/Sq Oscillator 10Hz – 1MHz £45 Racal 1991 Counter/Timer 160MHz 9 Digit £150 Racal 2101 Counter 20GHz LED £295 Racal 9300 True RMS Millivoltmeter 5Hz – 20MHz etc £45 Racal 9300B As 9300 £75 Solartron 7150/PLUS 6½ Digit DMM True RMS IEEE £65/£75 Solatron 1253 Gain Phase Analyser 1mHz – 20kHz £600 Solartron SI 1255 HF Frequency Response Analyser POA Tasakago TM035-2 PSU 0-35V 0-2A 2 Meters £30 Thurlby PL320QMD PSU 0-30V 0-2A Twice £160 – £200 Thurlby TG210 Function Generator 0.002-2MHz TTL etc Kenwood Badged £65 Function Generator 100 microHz – 15MHz Universal Counter 3GHz Boxed unused Universal Counter 225MHz SYS2712 Audio Analyser – in original box Autocal Multifunction Standard Pressure Calibrator/Controller Autocal Standards Multimeter RF Power Amplifier 250kHz – 150MHz 25W 50dB Voltage/Current Source DC Current & Voltage Calibrator £350 £600 £350 POA POA £400 POA POA POA POA Marconi 2955B Radio Communications Test Set – £800 61