Fullscreen HTML5Med Res (??MB)HTML4

In March, we told you how we rescued an old turntable and fitted it with a self-powered RIAA preamplifier so it could be used with modern hifi amplifiers. Here’s the promised second part of the story: a matching headphone amplifier so you can enjoy the fantastic black plastic in perfect peace and privacy! A Simple Headphone Amp For Your RIAA Turntable (or anything else!) By Ross Tester T we always had this little project in the back of our minds. o briefly recap, the RIAA preamplifier was presented for those who have a modern sound system. About the only change we’d make from the project More often than not these days, modern systems published in March would be the use of larger U-shaped don’t include a “phono” input, which precludes the use heatsinks to cope with increased current (there’s tonnes of of a turntable to listen to records (as distinct from CDs!). room on the PC board for them) and the use of PC-board The preamp not only provides the necessary gain for mounting screw-in terminal blocks on at least the power a magnetic cartridge, it also “tailors” the signal to the supply board (where there is plenty of room) and possibly RIAA standard, allowing you to plug into any “aux” or the RIAA board – where space might be a tad tight. line-level input. Of course, there’s nothing to stop you using this project The idea of our preamplifier was to make it fully as a general-purpose headphone amplifier, as long as it is self-contained, with its own 240V to ±15V DC supply built fed with “line level” signals – such as the output of a CD/ on the PC board. To be honest, this didn’t work out quite DVD player, tuner, etc. as well as we had hoped due to noise being induced into The amplifier circuit the preamp from the mains supply. But we were one step ahead on this one (just in case!), making the two sections This design is a tried-and-true circuit we have used of the PC board separable. before. It’s rather similar (or at least one channel of it is!) The 240V to to the one John power the whole Clarke used in the Performance of Prototype shebang is, of mono headphone course, already monitor amplifier Output level.................................. 90mW (max) into 8Ω headphones available from the Frequency response..................... 0.5dB down at 30Hz and 20kHz (see Fig.2) in the 8-channel turntable supply. Mixer (described in Input sensitivity............................ 0.83V RMS for full power The ±15V, 10VA November and DecDC supply was in ember 1996). We’ve Harmonic distortion..................... see Fig.3 & Fig.4 fact a bit of overmade just a few Signal-to-noise ratio.................... -95dB unweighted (20Hz to 20kHz)     changes which we kill, being capable ..................................................... with respect to 500mV input signal of much more than will go into shortly. demanded by the All components Separation between channels..... -50dB between 20Hz and 10kHz preamp. But then, except a stereo www.siliconchip.com.au May 2002  53 the result of DC flowing in them and to the contact point of the wiper. 5pF RIGHT CHANNEL ONLY SHOWN IC PIN NOS IN BRACKETS ARE FOR LEFT CHANNEL 330k +15V How it works Signal from the RIAA preamp (or other “line level” audio source) is applied to the ganged RIGHT C pots, VR1a and VR1b. AUDIO B Q1(Q3) INPUT BC338 The pot simply forms a voltage divider A 10F 50k E across the preamp output, presenting the 35V LOG (VR1) op amp input with a level of signal de33 2 D1(D3) K 8 pendent on the pot setting. At minimum (6) 68 1N914 0.27F 1 56k IC1a setting, very little (if any) signal gets TL072 (7) A 3 4 through; at maximum most gets through. (5) 33 Presto: a level or volume control. 22k E D2(D4) K As both stereo channels are identical Q2(Q4) 1N914 TO we’ll just look at one (the left) channel. BC328 B LEFT C CHANNEL After passing through a 56kΩ resistor A 10k (which basically sets the input impedance), K 8 E BC -15V the signal is applied to the inverting input of 1 one of the TL072s twin op amps. SC Normally, the feedback loop for the op amp 2002 would go from the output back to the input – Fig.1: each channel of the headphone amplifier uses an op amp driving a but in this circuit, transistors Q1 and Q2 are complementary pair of transistors which are within the overall negative included in the loop. This helps minimise feedback loop. harmonic distortion. (dual ganged) volume control pot mount on a single PC The transistors themselves boost the output current caboard measuring 65 x 65mm. pability of the op amp. To minimise crossover distortion, The biggest change from the earlier circuit was to duplithe transistors are slightly forward-biased by D1 and D2 cate it for stereo. Rather than use two TL071 op amps (one between their bases. for each channel) we used a TL072, which is effectively two The 33Ω emitter resistors on each transistor maintain the TL071s in the one 8-pin DIL package. The other changes bias stability. They, along with the 68Ω resistor between involve the components in the input and feedback circuits their junction and the headphones, limit the output current – these have been adjusted to make them optimum for the to less than 150mA under catastrophic failure conditions, RIAA preamp output. That’s not to say it won’t work with protecting the headphones from damage. other audio sources – it should be pretty-well “universal.” The feedback loop previously mentioned consists of A DC blocking capacitor has also been included in series a 5pF capacitor in parallel with a 330kΩ resistor. This with both inputs. This is not to prevent any DC present in limits the response to about 50kHz and sets the overall the preamplifier output from reaching the op amp inputs; gain of the amplifier (op amp plus output pair) to about rather it is to prevent any DC offset voltage at the op amp six (330kΩ/56kΩ). inputs (albeit tiny) from being impressed back on the Construction potentiometer. DC on pots is a no-no: most “noisy pots” that you come Start by checking your PC board for any defects, then across are not the result of mechanical wear and tear but mount and solder components in the usual way: lowest 10k HEADPHONE AMPLIFIER Fig.2: the frequency response of both channels with an input signal of 250mV RMS. The minor differences between channels is probably due to transistor tolerances. 54  Silicon Chip Fig.3: total harmonic distortion (THD) versus frequency with both channels driven with an input signal of 250mV RMS. www.siliconchip.com.au 12050110 PMA ENOHPDAEH PIHC NOCILIS 5pF 10k D1 D2 10F 56k 0.27 TL072 L IN TO SOURCE (RIAA PREAMP) TO AMP INPUT ( ) VO -15V V51- +15V V51+ 22k 22k 0.27 56k 0V STEREO HEADPHONES 8 1 D3 D4 B C 10k Q4 10k 5pF E 33 L EARTH C B 10k R IN Q1 E 68 R EARTH E Q2 33 C 33 LOOKING AT REAR OF POT 330k B 33 POT WIRING (ONE HALF OF DUAL-GANG POT SHOWN) E 330k Q3 C B 68 Compare the same-size photo and the component overlay (Fig.5) when assembling the PC board. We used 1A diodes in our prototype but ordinary silicon signal diodes are cheaper! profile (resistors) first, the link (using an offcut from a resistor), then the four small capacitors, the electrolytic (watch the polarity) and finally the semiconductors: diodes, transistors and the IC. Again, watch both the placement and the polarity of the semiconductors. The headphone socket mounts on the board – provision is made for sockets with pins either side. The power supply is connected via an on-board screw terminal block while signal is connected via four PC stakes. Checking it out Check your component placement, polarity and soldering before proceeding. If you’re happy with it after a thorough examination, connect the +15V, 0V and –15V from the RIAA preamp supply to this board. Note that the order of the terminals are not the same on both boards: you have been warned! With no headphones nor any inputs connected, turn the power on. Absolutely nothing should happen (at least as far as visual observation is concerned!). Check that you have +15V and –15V on the board and (respectively) on pin 8 and pin 4 of IC1 and (again respectively) on the collectors of Q1 and Q2. All of these voltages, by the way, are measured with respect to 0V. Check that you have 0V (or within a couple of millivolts of 0V) on pins 2, 3, 5 and 6 of the IC. You should also have 0V (or very close to it) on pins 1 and 7 (the outputs). Now check the voltages across the diodes and between base and emitter of the four transistors. In every case it should be around 0.6V, give or take. Obviously, in half the cases the polarity should be reversed compared to the other half. Parts List – Headphone Amp 1 PC board, 65 x 65mm, coded 01105021 1 PC mount stereo headphone socket 1 PC mount 3-way terminal block 4 PC stakes 4 mounting screws (as required) Suitable lengths red, black and white hookup wire Semiconductors 1 TL072 dual op amp (IC1) 2 BC338 NPN transistors (Q1, Q3) 2 BC328 PNP transistors (Q2, Q4) 4 1N914 or similar silicon signal diodes (D1-D4) Capacitors 1 10µF 35VW PC electrolytic 2 0.27µF MKT polyester (code 274 or 270n) 2 5pF ceramic (code 5 or 5p) Fig.4: total harmonic distortion (THD) versus output level; measured at the junction of 33Ω emitter resistors when driving 8Ω headphones. www.siliconchip.com.au Resistors (0.25W, 5%) 2 330kΩ 2 56kΩ    2 22kΩ 4 10kΩ 2 68Ω 4 33Ω 1 50kΩ dual-gang logarithmic pot (VR1) May 2002  55 Finally, check the voltage bescrape off some of the passivation SILICON CHIP HEADPHONE AMPLIFIER 010502011 tween the outputs and 0V – again, to bare metal) to the earth braids. it should be pretty close to zero. The pot should be a “logarithIf all this passes muster, it’s time mic” type to match the sensitivity to plug in a pair of headphones. characteristics of the human ear. This done, apply the “blurt” test: But if all you have (or can get) RIGHT touch your finger on the input pins is a linear pot, you could use it (not the earth pins!). You should – it won’t do any harm but there + hear a healthy “blurt” out of each won’t be much control: you’ll of the headphones in turn. find that minimum to maximum is squeezed into a tiny section of If you do, you can reasonably LEFT the pot’s travel. assume the system is working (what you’re doing is simply If you are fitting the amplifier connecting a big source of hum board underneath your turntable OV and noise – you!) If you don’t as with the power supply and -15V get any sound, check that (a) the preamp, you’re going to have to headphones are turned on or up, mount the board so the headphone +15V if they have any controls; (b) they socket emerges from an appropriare plugged into the socket; and ately-drilled hole in the plinth. c 2002 (c) you still have power to the And the volume control will board. also need a mounting hole drilled Fig.6: actual size artwork for the PC board. There’s not much that can be – make sure that neither the board that the inputs to the amplifier go to wrong given the tests you have already nor the pot foul any under-deck methe wipers of the pots and that both the chanical operations. undertaken but if you have sound in earths (preamp and amplifier) connect one channel only, it’s nice to know Just remember to keep the input there is a “reference” right alongside together at the bottom end. side of the PC board as far away as If when using your amplifier later possible from the mains transformer with which to compare voltages. you find that a lot of hum is generated and the motor. The pot when you touch the volume control, Being a line level input, it’s not as it might pay you to solder a wire from critical as the RIAA preamp input – but Wire the 50kΩ dual-ganged pothe pot body (you’ll probably need to tent-iometer as shown. It’s important it’s better to play safe. SC Protect Your Valuable Issues Silicon Chip Binders REAL VALUE AT  Heavy board covers with 2-tone green vinyl covering $12.95 PLUS P &P  Each binder holds up to 14 issues  SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price: $A12.95 plus $A5 p&p each (Aust. only). Just fill in & mail the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. 56  Silicon Chip Subscribe & Get this FREE!* THAT’S RIGHT – buy a 1or 2-year subscription to SILICON CHIP magazine and we’ll mail you a free copy of “Computer Omnibus”. Includes articles on troubleshooting your PC, installing and setting up computer networks, hard disk drive upgrades, clean installing Windows 98, CPU upgrades, a basic introduction to Linux plus much more. *Australia only. Offer valid only while stocks last. Subscribe now by using the handy order form in this issue or call (02) 9979 5644, 8.30-5.30 Mon-Fri with your credit card details. www.siliconchip.com.au