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Clean up the clicks & pops on your vinyl records with the Gotta lotta records? Never play ’em because of the dreaded clicks and pops? Well, now you can start enjoying your records again. How? Just play them through the LP Doctor and it will get rid of most of the clicks and pops so you can enjoy them at any time. By LEO SIMPSON & JOHN CLARKE M any people have big collections of vinyl records but they can’t be bothered playing them because of the surface noise, clicks and pops. The truth is, we’ve all been spoilt by the pristine, noise-free sound of CDs. In fact, if you have not listened to some of your old LPs for a while, you might be shocked at just how bad they sound... There used to be two solutions to the problem of clicks, pops and noise on LP (long-play) records. You could (try to!) ignore them – not particularly satisfactory. Or you could download your ! LP to your computer via your ! sound card and use one of the ! many software programs which ! allow you to edit audio files before you dub them to CDs. You ! can then play the new disk on ! your home CD or DVD player, in your car or portable disk player, and never have to play the old vinyl record again. To find out more about this approach, go to our feature article on this subject starting on page 4. For many people, the above approach is the complete answer. They get a CD which sounds better and they can play it any time they like. But as always, there are drawbacks to the computer/software/CD approach. First, you do need a reasonably fast Pentium or equivalent computer 22  Silicon Chip with a late model sound card, a CD Writer and the necessary software, all of which can add up to a pretty big investment. Second, as good as these sound editing programs are, only the more expensive let you listen to the disk as the processing takes place. In other words, they don’t all do this magic processing “on the fly”. Instead, you have to play the record into your computer via an RIAA preamplifier and sound card and store the various Features Stereo RIAA preamplifier included Bypass mode Treble filter for noisy recordings Output level adjustment Signal clipping indicator Indicators for click muting in each channel tracks as .WAV files on your hard disk. Then you set the software to work to process the .WAV files and finally, they can be dubbed onto a CD with your CD Writer. By the way, you are going to need a pretty big hard disk (say with at least 1 or 2 Gigabytes to spare) to store and process these .WAV files. Third, and this is more of a subjective problem than a technical one, even if you do create CDs to replace your old vinyl records, you will not have the satisfaction of looking at the sleeve notes while you are listening or deciding which track to play next. That’s one of the nice features of the old 12-inch disks. Sigh... Finally, even if the process of CD dubbing is entirely satisfactory, who is going to go to the trouble of dubbing their entire LP collection to CD, just for the benefit of being able to listen to any or all of them occasionally? So if you a large collection of records, you need an alternative to CD dubbing, one where you can listen to any of them at any time, substantially free of clicks and pops. You can do this by playing them through the LP Doctor. LP Doctor + CD Writer Maybe you don’t have computer but you may want to consider a CD Recorder such as the Marantz DR6000 pictured elsewhere in this article. These make dubbing all sorts of audio material to a CD a real doddle but they do not include any means of removing noise, clicks and pops from your LP records. This is where the LP Doctor can also play a part. You dub your LPs to a CD Recorder via the LP Doctor and get a very worthwhile sound improvement, especially on jazz and classical music records; ie, those with quiet passages The LP Doctor can be teamed with any turntable having a magnetic cartridge so you can listen to your LPs without clicks and pops or you can feed its signal into a PC sound card for processing and dubbing to CD. which can be really plagued with clicks and pops. Or maybe you do have a computer, CD Writer and so on but you are without a sound editing program. Here again, you can dub your records to CD on your computer via the LP Doctor. LP Doctor – what it does OK, we’ve talked about where and why the LP Doctor might be used but we have not been specific about what it does, apart from the general Fig.1; this block diagram shows the left channel only. The signal from the RIAA preamplifier is fed via a delay so that the click detector has sufficient time to detect and fully mute signal transients. theme of click and pop removal. Let’s describe specifically what it does. The LP Doctor has two independent channels which monitor the left and right signals. When a click or pop is sensed, the signal is muted for a brief interval to greatly reduce its amplitude or completely remove it. At the same time, it can apply a slight degree of treble cut filtering, to reduce surface noise and also make the click attenuation more effective. While it does work well at greatly reducing bad clicks and pops, it does not do well on low level clicks which are difficult to discern from the general program signal. In other words, if you have trouble hearing clicks and pops unless you listen up close to the speakers (or wear headphones) then the LP Doctor will also have trouble. On the other hand, while many pop and rock records might have a fairly serious amount of clicks and pops, the general signal level is usually so consistently high that it does not matter – the music masks the noise. So really it is records that have a wide dynamic range, such as classical music and jazz, with quiet passages among the louder ones, that are more likely to be plagued with clicks and pops. This is where the LP Doctor can work very well. But the LP Doctor does not fix bad surface noise which may be caused by lots of dirt being ingrained into the record grooves or may be the result of fungus attack on records which have been stored for long periods in fairly humid conditions. In fact, if you have a record which has January 2001  23 Fig.2: this is the circuit of the left channel. IC3 is the digital delay which is set to 1ms at switch-on by ICs17, 18, 19 & 20. 24  Silicon Chip January 2001  25 There are also three indicators on the front panel. Two indicate whenever a click is detected in either channel while the third is a clipping indicator. The output level control needs to be adjusted so that there is normally no clipping. As well as providing click suppression, the LP Doctor includes a high performance RIAA preamplifier, making it compatible with virtually any amplifier, PC sound card or free-standing CD Recorder. The preamplifier is designed to suit the majority of moving magnet cartridges but may not have enough gain to suit moving coil cartridges. On the rear panel, there are two pairs of RCA sockets. One pair is for the magnetic cartridge signals while the other pair is for the line level output signals to a stereo amplifier, sound card or CD Recorder. Fig.3: the overall performance of the magnetic cartridge preamplifier, measured by applying an inverse RIAA signal to the input. Our preamplifier is not ideal but it is pretty close – the curve deviates by no more than ±0.3dB over the whole frequency range from 20Hz to 20kHz. been subject to a bad fungus attack, there is nothing you can do because the damage is permanent. And even if you do use the LP Doctor on a regular basis, there is still no substitute for keeping your records as clean as possible and also making sure that your stylus is free of any gunk that may be picked up from the record grooves. In fact, you really need to examine the stylus after every record side has been played, to make sure that it is clean. By the way, if you do lots of critical listening to CDs via headphones, you probably still won’t enjoy vinyl records after they have been processed by the LP Doctor. Let’s be honest: there is still going to be a huge difference in sound quality between an old LP and a pristine CD. Operating features The LP Doctor is housed in rack-mounting case measuring 426mm (W) x 44.5mm (H) x 277mm (D), not including the side-mounting flanges. Of course, you don’t have to mount a rack-mounting case in a rack! There are three knobs on the front panel: an output level control, a click sensitivity control and a selector switch with three positions: Bypass, Process (click suppression) and Filter. The latter setting adds treble cut to the click suppression, as noted above. Operating principles So how does the LP Doctor eliminate record clicks and pops? Essentially, each channel has a comparator which looks for the sudden large signal excursions which produce a click or pop. When the click signal is detected, the audio signal is muted (switched off) to prevent the click from passing through to the output. The signal is shut off for a short period (about seven milliseconds) so that it is more or less unnoticed by the listener. The problem is, by the time the comparator has detected the signal and has muted it, some of the click will have already passed through to the output. So only part of the click or pop will have been muted, resulting in a shorter click but still just as loud and annoying. Specifications Frequency response of RIAA phono preamplifier.... within ±0.3dB from 20Hz to 20kHz (see Fig.3) Signal-to-Noise........................................................... -83dB unweighted (20Hz to 20kHz) with respect to 10mV input at 1kHz and                       560mV; -88dB A-weighted (-84dB and -89dB in Bypass mode, respectively) Total Harmonic Distortion (THD)................................ 0.3% at 1kHz and 1V, 3% <at> 10kHz <at>-20dBV (0.008% <at> 1kHz and .01% <at>                      10kHz in bypass mode) Separation between channels.................................... -67dB at 100Hz; -73dB at 1kHz; -58dB at 10kHz Treble filter................................................................. -3dB at 10kHz, 12dB/octave slope Maximum input signal................................................ 190mV RMS at 1kHz Signal delay time....................................................... 1ms Click muting time....................................................... 7ms 26  Silicon Chip Inside the LP Doctor. It uses a digital delay in each channel for effective muting of clicks and pops. Therefore, we need to incorporate a delay circuit. This allows for the response time of the comparator and muting circuit such that the delayed signal has the full muting applied to it. In this way, the comparator/muting circuit can attenuate the whole of the click or pop instead of just the latter part of it. Block diagram Fig.1 shows the block diagram of the LP Doctor. For simplicity, only the left channel is depicted; both channels are completely separate and identical. First, the signal from the magnetic cartridge is fed to the RIAA (Recording Industry Association of America) preamplifier. From there, the amplifier signal is fed via four separate paths: (1) straight through to the output via the bypass position on switch S2; (2) to the one-millisecond digital delay unit (IC3) which converts the analog signal to a digital signal, feeds it into memory, shuffles it out again with the required delay and converts it back to analog; (3) to the click detector which operates switch IC4 where click muting takes place and (4) to the clipping detector which drives a front panel LED to indicate that the signal level is too high. Circuit description The full circuit of the LP Doctor, apart from its power supply, is shown in Fig.2. As with the block diagram, we are only showing the left channel. The equivalent IC and other device numbers for the right channel are shown in brackets. The phono (magnetic cartridge) 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 op amp IC1a. The inductor, series resistor and shunt 100pF capacitor form a filter to attenuate RF signals which may be picked up by the phono leads. The RIAA equalisation is provided by the feedback network comprising 16kΩ and 200kΩ resistors and the .0047µF and .015µF capacitors, connected between pins 1 & 2. This network provides the standard equalisation time constants of 3180µs (50Hz), 318µs (500Hz) and 75µs (2122Hz). The circuit also includes the IEC recommendation for rolloff below 20Hz. This is provided by the 1kΩ resistor in series with the 10µF capacitor and by the 10µF coupling capacitor to the 10kΩ potentiometer VR1. There is also rolloff in following stages to ensure that the signal below 20Hz is attenuated. This rolloff prevents amplification of record warp and rumble which occur at sub-audible frequencies but could possibly overload an amplifier and loudspeakers. The gain of IC1a is 22.45 (27dB) at 1kHz with a boost of +13.1dB at 100Hz and a cut of -13.75dB at 10kHz. With a typical magnetic cartridge output, the gain provides us with a nominal 100mV of signal. This is further boosted by op amp IC2a, by a factor of 11, to produce a nominal signal level of 1.1V. The frequency response graph of Fig.3 shows the overall performance of the RIAA preamplifier. It was measured by applying an inverse RIAA signal to the preamplifier. The response should be a dead-flat line if the RIAA preamplifier is ideal. Our preamplifier is not ideal but it is pretty close – the curve deviates by no more than ±0.3dB over the whole frequency range from 20Hz to 20kHz. IC6a & IC6b are comparators which form the clipping detector and they monitor the signals from IC2a (left channel) and IC2b (right channel) via 10kΩ resistors and a 10µF coupling January 2001  27 Fig.4: this is how the click detection works. The top trace is the averaged signal at pin 5 of IC12a, while the lower trace is the rectified signal at pin 6. When a transient occurs, pin 6 goes above pin 5 and the comparator output goes low to trigger the monostable timer IC13. Fig.5: these scope waveforms show only a short click being detected and muted but they can last a lot longer than this so we have settled for a compromise muting time of 7ms. capacitor. The non-inverting input of IC6a is held at +1.14V while the inverting input of IC6b is held at -1.14V. Thus, IC6a monitors positive swings of the signal while IC6b monitors negative swings. So if the audio signal to the comparators exceeds 2.3V peak-to-peak (800mV RMS), LED1 will light. 16 provides the demodulation of the digital back to analog after the delay within the IC. The 22kΩ and 11kΩ resistors plus the 560pF and 150pF capacitors at pins 13 & 14 form another low-pass filter which removes the 500kHz digital switching artifacts from the now-delayed analog signal. Digital delay The output signal from IC3 passes through a final filter comprising a 270Ω resistor and .001µF capacitor and is then fed through a 10µF DC blocking capacitor to the analog switch, IC4. This is an optically controlled FET which has a low impedance when the internal LED is lit and a very high impedance when the LED is off. IC4 is controlled by the click detection circuitry involving IC11, IC12 & IC13. When switch IC4 isclosed, the signal passes through unaffected to pin 3 of op amp IC5a which is connected as a unity gain buffer. However, there is a wrinkle here because the 10kΩ resistor and .01µF capacitor following IC4 form a sample-and-hold circuit and the capacitor is constantly following the audio level as it is charged and discharged via the 10kΩ resistor. When switch IC4 is turned off, in response to a click, the voltage at pin IC3 provides the digital delay and this is set to provide a time delay of one millisecond by data fed into its pins 4, 5 & 6 at the moment of switchon. This initialising data is provided by ICs 17, 18, 19 & 20. We’ll briefly describe their operation later in this article. For the moment, though, all we need to know is that IC3 (IC7 in the right channel) are set to provide a one-millisecond delay. The input signal from IC2a is coupled into IC3 via some low-pass filter components comprising the 22kΩ and 11kΩ resistors at pins 23 & 22 of IC3. This filter rolls off at 12dB/octave above 36kHz to prevent quantisation errors in the analog-to-digital conversion process. The .068µF capacitor series 27Ω resistor between pins 20 and 21 form the integrator used in the analog to digital (delta-sigma type) conversion. The .068µF capacitor at pins 15 and 28  Silicon Chip Muting switch 3 of IC5a remains roughly where it was until the switch closes again. In this way, the signal is not muted down to zero but to its average level. This results in a less-audible muting effect and it duplicates the muting action of the best software packages in removing clicks. The signal voltage from the sampleand-hold circuit is applied to pin 3 of IC5a via a 10µF bipolar capacitor. This is included to avoid having the input bias voltage for IC5a from being applied to the output side of IC4. If it was, then an audible click would be produced each time IC4 switched on or off. IC5a’s output is applied direct to switch S2a and becomes the “Processed” output. It is also fed to op amp IC5b which provides the “Filter” output to switch S2a. IC5b provides a gentle treble cut at 12dB/octave above 10kHz. Click detection circuitry Apart from the delay circuit just described, the click detection circuit is really the heart of the LP Doctor. It takes the signal from IC2a and further amplifies by 4.7 in IC10a. It is then AC-coupled via a 1uF bipolar capacitor to a precision full-wave rectifier comprising op amps IC11a and IC11b, diodes D6 & D7 and associated Fig.6: the loading process for the delay codes which are fed into IC3 (and IC7) at switch-on. Serial data (lower trace) is transferred on the negative edge of SCK (centre trace). The REQ line (top trace) must be low before the following 12 clock pulses. The positive edge of REQ signals the end of the serial data stream. On the first clock pulse, the sleep data is input and this must be a low. The following six pulses are the delay codes while next are the low mute and ID1 and ID2 (identification codes). resistors. When the input signal goes positive, the output of IC11a goes low, biasing on D7 so that the gain is set by the 10kΩ input resistor R1 and the 10kΩ feedback resistor R2; thus gain is -1. This signal is coupled to the inverting input of IC11 via the 5.1kΩ resistor. Gain for IC11b is set by the 5.1kΩ input resistor and 10kΩ feedback resistor between pins 6 and 7 and is therefore -2. Overall gain of the input signal for positive signals is therefore -1 x -2 = + 2. However, there is another path for the input signal via the 10kΩ resistor R3 to pin 6 of IC11b. This signal gives a negative signal at the output of IC11b with a gain of -1. Adding the two gains Fig.7: this is the effect of the delay through IC3 and IC7. The top trace is the input signal and the lower trace is the delayed signal. gives us +1. For negative input signals the output of IC11a is clamped high, due to conduction of diode D6 and the cathode of D6 is held at ground, effectively switching the output of IC11a out of circuit. Signal then passes via the 10kΩ resistor R3 to pin 6 of IC11b. IC11b inverts the signal and provides gain at -1. Since the input signal is negative, the signal at pin 7 of IC11b is positive. Thus pin 7 of IC11b always goes positive, for both positive and negative swings of the input signal and so we have a precision full-wave rectifier. Trimpots VR2 and VR3 provide offset trimming for IC11a and IC11b respectively. These are set so that pin 1 and pin 7 of IC11 are at ground (0V) when no signal is applied. Comparator has floating threshold The rectified signal from IC11b is applied via the 27kΩ resistor and potentiometer VR4a to pin 6, the non-inverting input of comparator IC12a. It is also applied to pin 5 via a filter network comprising a 4.7kΩ resistor and 1µF bipolar capacitor, before being applied to the inverting input of IC12a. So IC12a has a slowly varying DC level at pin 5 and the rapidly moving signal level at pin 6 and it is looking for a sudden transient, ie, a click or pop, which will cause its output at pin 7 to go low. The Marantz CD Recorder is an attractive alternative to dubbing your LPs to CD via a computer. You don’t need a computer or to learn about new software and the result is much the same, when you process your LPs via the LP Doctor. January 2001  29 The oscilloscope waveforms of Fig.4 show how the click detection works. The top trace is the averaged signal at pin 5 of IC12a, while the lower trace is the rectified signal at pin 6. When a transient occurs, pin 6 goes above pin 5 and the comparator output goes low to trigger the monostable timer IC13. IC13 is a CMOS 555 connected as a monostable. It works in two ways. Normally, when pin 7 of IC12a is high IC13 is untriggered (as pin 2 is high) and the .0068µF at pins 6 & 7 of IC13 will be discharged and the output at pin 3 will be low. When pin 7 of IC12a goes low, it turns on transistor Q1 to maintain discharge the .0068µF capacitor and low at pin 2 of IC13 triggers the beginning the timing period. The result is a seven-millisecond positive pulse from pin 3 of IC13 and this turns on transistor Q2 which turns off IC4. IC13 also drives LED2 which gives a visible indication of the muting action. This is shown in the scope waveforms of Fig.5. This shows only a short click being detected and muted but they can last a lot longer than this so we have settled for a compromise muting time of 7ms. If IC12a detects a longer transient, it will hold Q1 on and keep the .0068µF capacitor discharged for longer and this will extend the muting period beyond the nominal seven-millisecond period. Comparator IC12b is there to provide power-on muting via IC13 and IC4. Initially, the 10µF capacitor at pin 3 of IC12b is discharged and pin 1 is low. When the 10µF capacitor charges via the 100kΩ resistor, its voltage goes above pin 2 and so pin 1 of IC12b goes high, to let IC4 unmute via IC13 and Q2. The scope waveforms of Fig.7 show the effect of the delay through IC3 and IC7. The top trace is the input signal and the lower trace is the delayed signal. Delay Control Power IC3 (and IC7 in the right channel) are set to the 1ms delay time via their REQ, SCK and DATA inputs at pins 4, 5 & 6 at switch-on. IC17 to IC20 are used to provide these clock and data codes and after they have done this, they have no more function in the circuit, until it is next switched on. They function in exactly the same way as in the Digital Reverberation circuit described in last month’s issue so we won’t go into the detail here, except to say that IC20, the 74HC165 serial shift register, is responsible (can we say that about a lowly IC?) for loading in the eight bits of data at switch-on. It is clocked by IC17, the 4060 oscillator/ counter while IC18 & IC19 perform related house-keeping tasks. The scope waveforms of Fig.6 show the loading process for the delay codes. The serial data (lower trace) is transferred on the negative edge of SCK (centre trace). The REQ line (top trace) must be low before the following 12 clock pulses. The positive edge of REQ signals the end of the serial data stream. On the first clock pulse, the sleep data is input and this must be a low. The following six pulses are the delay codes while next are the low mute and ID1 and ID2 (identification codes). The power supply circuit is shown separately on Fig.8. It uses a transformer with two 9V windings connected in series to give an 18V centre-tapped supply. This feeds a full-wave rectifier (diodes D1-D4) and the two 470µF 25VW capacitors. The resulting DC voltage is around ±12V. This feeds adjustable 3-terminal regulators set to give ±7.5V supply rails. The +12V supply is also applied to a 5V regulator, REG3, via diode D5. The diode isolates the supply to the input capacitors of REG3 when power is switched off. The idea is to maintain the +5V supply to the delay ICs (IC3 & IC7) to avoid switch-off thumps. The mains power switch is bypassed with a 250VAC-rated .001µF capacitor. This prevents arcing across the switch when it is switched off. The mains earth connects to the circuit earth via a 0.47µF capacitor to prevent hum in the signal where there is no connection to mains earth in any accompanying amplifier. Should the power amplifier be earthed, then the capacitor will minimise any resulting hum loop. Next month we will complete the presentation of the LP Doctor with all the constructional information and the parts list. SC Fig.8: the power supply of the LP Doctor provides +5V and ±7.5V rails. The 5V rail powers to the two delay chips. IN 0.001F 250VAC F1 150mA A SLOW BLOW 2200F 16V D1 - D4 4 x 1N4004 D5 1N4004 S1 250VAC 0V 470F 25V SC LP DOCTOR POWER SUPPLY 30  Silicon Chip +5V 10F 16V +7.5V 1k 4.7k 100F 16V 4.7k 100F 16V 470F 25V IN 2001 OUT ADJ 0.47F CHASSIS 2200F 16V OUT GND REG1 LM317 IN 9V N E T1 20VA 9V 0V 240VAC IN REG3 LM2940-5 ADJ OUT REG2 LM337 10F 16V 10F 16V 1k -7.5V