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Need more line inputs? Build our phono input converter Design: NICHOLAS VINEN Article: BAO SMITH This passive converter circuit lets you use the phono inputs on an amplifier or mixer, normally used for a turntable, as a pair of linelevel inputs. This lets you plug in another CD player, DVD player or other line-level program source. W HILE TURNTABLES may be making a slight resurgence over the last few years, they’re still a rare sight in most people’s homes. Because of this, you may often find the phono inputs on the back of an amplifier go unused. So, what to do if you need another pair of inputs for a CD or DVD player? Our solution is to convert the phono inputs to line inputs via an external adaptor, the original idea being cred- ited to Gary Johnston of Jaycar Electronics. It’s best not to plug a line-level device straight into a phono input for two reasons. One, because they provide approximately 34dB of amplification, which would cause it to overload; and two, because of the RIAA equalisation that is applied by a preamplifier built into the amplifier. 10nF 15nF CON1 INPUT 200k GND 4.7nF 16k 560 CON1 OUTPUT 10nF 10nF 10nF 10nF 200k 16k CON2 INPUT GND CON2 OUTPUT 560 (LEFT CHANNEL SHOWN; RIGHT CHANNEL IDENTICAL) SC 20 1 6 PHONO INPUT CONVERTER Fig.1: the circuits for both versions of the Phono Input Converter. The left circuit is made with through-hole components, while the right circuit is made with SMD components. 46  Silicon Chip RIAA equalisation applies boost for lower frequencies (up to +20dB at 20Hz), approaching zero at 1kHz. Above 1kHz, the circuit applies treble cut that increases as the frequency rises to 20kHz (the cut being almost 20dB). In effect, our converter applies the inverse of RIAA equalisation to the signal before passing it to the amplifier, which after applying the normal equalisation, leaves us with a virtually flat frequency response. Since a typical phono preamplifier applies about 34dB of gain, our passive circuit needs approximately 34dB of attenuation and a filter that is the exact inverse of the RIAA equalisation. In Fig.2 you can see the RIAA equalisation curve (yellow) and the ideal inverse curve (green). The dotted yellow line shows the RIAA equalisation curve with IEC amendment from 1976. The IEC amendment added a bass turnover at 50Hz, used to reduce very low frequency signals from the turntable. The general slope of the RIAA equalisiliconchip.com.au When designing the circuit, we needed to consider the type of source input it would be used with. Most program sources, whether CD or DVD players, AM/FM tuner, etc, use operational amplifiers in their output stages and they have very low output impedances. This means that this passive circuit can have quite a low impedance and not have any adverse effect on the performance of the source signal. Next, as well as applying signal attenuation, the converter must have a characteristic which is the inverse of the RIAA equalisation curve. As a result, there should be minimal difference between connecting a CD player into line inputs compared to feeding it into phono inputs via this converter. Hence the circuit for each channel consists of a 200kΩ resistor shunted by a 15nF capacitor, in series with the combination of a 16kΩ resistor shunted by a 4.7nF capacitor. This describes the through-hole version of the circuit (left side of Fig.1). The SMD version on the righthandside of Fig.1 provides nearly the same shunt capacitance but uses series or series-parallel combinations of 10nF capacitors, allowing us to provide capacitance values closer to the ideal inverse RIAA characteristic. In fact, you can see from Fig.2 the SMD version is much closer to the RIAA equalisation curve for lower frequencies, while the through-hole version is closer at higher frequencies. How accurately you can replicate the curve comes down to the quality of the components used. We used C0G SMD capacitors in our converter as they have better tolerance and a more linear response, providing a closer replication of the inverse curve. siliconchip.com.au Inverse RIAA Frequency Response 14/09/2016 12:04:26 +17.5 +15 +12.5 +10 +7.5 Relative Amplitude (dBr) Circuit details +20 +5 +2.5 0 -2.5 -5 SMD Version -7.5 TH Version -10 Ideal RIAA+IEC -12.5 SMD RIAA+Inverse -15 TH RIAA+Inverse -17.5 -20 20 50 100 200 500 1k 2k 5k 10k 20k Frequency (Hz) Fig.2: the red and blue line in the centre show the frequency response of the Phono Input Converter hooked up to our LP Doctor. For the most part it is fairly flat until it starts to deviate at 100Hz due to the IEC amended RIAA equalisation curve, shown in the dotted yellow line. +5 RIAA+Inverse Frequency Response 27/09/2016 10:42:02 +4 +3 +2 Relative Amplitude (dBr) sation curve is 6dB/octave, and there are two inflections at approximately 500Hz and 2kHz. Now look at Fig.1. There are two versions of the converter circuit; one using conventional through-hole components while the other uses surfacemount components (SMD). Note that both versions are virtually identical electrically. Both are passive circuits, meaning that no semiconductors or integrated circuits are used, and no power supply is needed. +1 0 -1 SMD Version -2 TH Version -3 -4 -5 20 50 100 200 500 1k 2k 5k 10k 20k Frequency (Hz) Fig.3: a close-up of both frequency response curves for the SMD and throughhole versions. Variations in the response can occur due to component tolerances and the quality of the amplifier used. Whether your amplifier uses an IEC amended equalisation curve will also affect the response below 50Hz. November 2016  47 Above, you can see the rear view of the SMD version of the Phono Input Converter, while to the right is the front of the converter with optional label. At lower right is the rear of the through-hole version. Performance Parts List SMD version Capacitors (SMD 3216/1206) 10 10nF 50V C0G 5% We tested both circuits with the LP Doctor (Silicon Chip, January 2001) which incorporates a high-performance RIAA preamplifier. The superposition of both curves is shown in Fig.3, the dotted lines showing the response with the IEC amended equalisation curve. Both provide an overall frequency response within ±0.5dB from 20Hz to 20kHz. However, due to the bass turnover, from the IEC amendment, you end up with a slight cut to bass below 50Hz, culminating in about 3.5dB of cut at 20Hz (if your preamplifier applies the IEC amendment). Both circuits had a signal-to-noise ratio of 96dB unweighted with respect to 1kHz using a 2.2VRMS input. Which is in the range of what you would expect from your average CD player. Resistors (SMD 3216/1206, 1/4W, 1%) 2 560Ω* 2 16kΩ 2 200kΩ Build the SMD or throughhole version — ­ or both * change values to vary attenuation, see text For this project, we provide a PCB measuring 53 x 40.5mm and coded 01111161 which has both versions for 1 PCB coded 01111161, 53 x 40.5mm 1 4-way RCA socket (Altronics P0211 or equivalent) 1 UB5 jiffy box (Jaycar HB6015, Altronics H0205 or equivalent) 1 laser-cut lid (optional) 1 panel label to fit lid (optional) Through-hole version Capacitors 2 4.7nF MKT polyester, 63/100V 2 15nF MKT polyester, 63/100V Resistors (1/4W metal film, 1%) 2 560Ω* 2 16kΩ 2 200kΩ CON2 Lin 560 16k 560 16k 200k 200k 4.7nF 15nF 4.7nF 15nF GND Rout 16k 10nF 2x 560 Lout Rin 16k 200k Lin 200k 4 x 10nF 2x 10nF 10nF 10nF GND 01111161 Rin Inverse RIAA Lout RevA CON1 Rout a complete converter. You can build either one, or both if you need two converters. Either way, you will need to break the board in two and populate the one you want with surface-mount of through-hole components. To house the finished converter, we used a small plastic case from Jaycar. Five holes will need to be drilled in the lid for the four RCA phono sockets and screw to hold the PCB/socket in place, or purchase a laser-cut lid. The laser-cutting diagram and panel label can be downloaded from www. siliconchip.com.au When using the unit, keep it away from the power transformer in the amplifier and make sure the input and output leads do not run across mains power cords, otherwise hum pickup can become a problem. Depending on the signal levels from your CD player, or other input source, you may need to increase or decrease the degree of attenuation provided. You can provide greater attenuation by reducing the 560Ω resistor at the output, eg, using a 330Ω resistor. Alternatively, a 1kΩ resistor will provide less attenuation. SC 10nF Fig.4: complete PCB overlays for the through-hole (left) and SMD (right) versions of the Phono Input Converter. 48  Silicon Chip siliconchip.com.au