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Need another pair of audio inputs? Convert the phono inputs on your amplifier Do you need another pair of line inputs on your stereo amplifier? And do you no longer use the phono inputs? Well, here’s the answer – convert the phono inputs to normal line inputs & presto, another pair of inputs. By LEO SIMPSON Let’s face it, most people these days do not listen to vinyl records and have long since dispensed with their turntable, if indeed they ever had one. So the phono inputs on their stereo system are unused and useless. But these same people often also need another pair of inputs so they can hook in an extra tape deck, VCR or other program source with line outputs. What to do? Until now, there wasn’t much of an answer. If you knew your way inside an amplifier or stereo receiver, you could whip out the phono preamplifier and rewire the inputs as straight line inputs but that is an option few people would care to take. Some people may also still have a turntable which they want to able to use from time to time but want the option of using the phono inputs for line level inputs. The solution is to convert the phono inputs to line inputs via an external adaptor box and for this idea we are indebted to Gary Johnston of Jaycar Electronics. Essentially, all phono inputs provide a great deal of am­ plification (about 34dB or 50 times, at 1kHz), as well as RIAA equalisation for the signal from magnetic cartridges. The RIAA equalisation curve provides lots of bass boost at very low bass frequencies (approaching +20dB at 20Hz), tap­ering down to zero at 1kHz. Beyond 1kHz, the circuit applies treble cut and this in­creases as the frequency rises so that at 20kHz, the cut is almost 20dB. The general slope of the attenuation is -6dB octave right across the frequency range but there are two inflections in the curve at 500Hz and 2kHz (2122Hz, to be precise). So to convert the phono input to a line input with a sen­sitivity of around 100mV at 1kHz, you need a circuit which will provide about 34dB attenuation overall and a filter character­is­ tic which is exactly the reverse of the RIAA equalisation – see Fig.1. This may sound as though a fairly complicated circuit is required but in practice it turns out to be quite simple. Circuit details Our prototype inverse RIAA network was built into a relatively large metal diecast case & fitted with a Dynamark self-adhesive label to dress it up. It enables the phono inputs on your amplifier to be used with line level sources such as tape decks or a VCR. 54  Silicon Chip The suggested circuit is shown in Fig.2. The circuit is passive; ie, it has no active components such as transistors or integrated circuits and no power supply is needed. Actually, while the circuit is quite simple, a good deal of design work has gone into it to make sure that it does the job. As you can see, it involves just three resistors and two Performance We used our prototype converter with a number of commercial amplifiers and also with our latest Studio series remote control preamplifier as described in the September, October +20 +10 DECIBELS capaci­tors for each channel. So how did we go about designing this little network? First of all, we had to consider the type of source which would feed the circuit. Now all program sources, whether they are a CD player, tape deck, AM/FM tuner or whatever, use opera­tional amplifier ICs in their output stages. This is important because it means that the inverse RIAA network can have quite a low impedance and still not upset the performance of the program source. Second, the phono inputs of the amplifier have three re­ quirements if they are to give optimum performance. They must be presented with a source which has the same frequency characteris­ tic as a magnetic phono cartridge and the signal level must be about the same. But most important, the source impedance “seen” by the phono inputs should be as low as possible so that we obtain the optimum signal to noise ratio. If we were to use a high impedance filter and attenuation network, it would give the right overall frequency response and signal levels but the resulting sound quality would be unsatis­factory because the background hiss and noise would be too loud. So we have designed a network which provides a low impedance source for the phono inputs and the result is very low background noise. In fact, when you have the device installed properly, it will be difficult to hear the difference between your regular line inputs (ie, CD, tuner etc) and your converted phono inputs. As you can see from the circuit of Fig.2, the inverse filter network, consisting of a 200kΩ and 16kΩ resistors shunted by capacitors, is virtually identical to the feedback components of the RIAA version of the universal preamplifier published in the April 1994 issue of SILICON CHIP. Surprised? You should not be, since in an RIAA preamplifier the feedback network is an attenuator and filter, exactly what is needed here. We tried several inverse RIAA filters before settling on this one. 0 -10 -20 20 100 HERTZ 1k 10k 20k Fig.1: this graph shows the frequency characteristic of the filter circuit. RIGHT INPUT .015 .0047 200k 16k RIGHT OUTPUT 560  LEFT INPUT .015 .0047 200k 16k LEFT OUTPUT 560  TO AMPLIFIER CASE INVERSE RIAA NETWORK Fig.2: the circuit of the inverse RIAA network is an attenuator & filter which exactly compensates for the magnetic cartridge equalisation in the amplifier. It provides a low impedance source to the phono inputs to ensure low background noise. LEFT INPUT PARTS LIST 1 diecast case to suit PC board 1 PC board, code 01105941, 39 x 46mm 2 2-way RCA socket panels (Jaycar Cat. PS-0263 or equivalent) 1 solder lug 4 6mm untapped spacers Capacitors 2 .015µF MKT polyester 2 .0047µF MKT polyester Resistors (0.25W, 1%) 2 200kΩ 2 560Ω 2 16kΩ Miscellaneous Screws, nuts, lockwashers, hookup wire, solder. .015 .0047 200k 16k LEFT OUTPUT 560  RIGHT INPUT 200k 560  16k .015 .0047 RIGHT OUTPUT Fig.3: the component overlay of the PC board. Note that the shields of the RCA phono sockets are not connected to the case of the device. June 1994  55 This photo shows the internal details of the inverse RIAA network prototype. No shielded cable is required for wiring the RCA sockets since the metal box provides a total shield for the circuit. & November 1993 issues of SILICON CHIP. With the latter control unit, we were able to obtain an overall frequency response from the con­verted phono inputs within ±0.3dB from 20Hz to 20kHz and a signal to noise ratio of 78dB unweighted with respect to 1kHz and 200mV. These are excellent figures. Just how well the inverse RIAA network performs will depend on the quality of the amplifier it is teamed with but as you can see, the results are more than adequate for program sources such as tape decks, VCRs and tuners. For our listening tests, we used a Yamaha CDX-1110 CD player which has two sets of outputs. One set we connected to the CD inputs on the amplifier in the normal way while the other set of outputs were connected via the inverse RIAA network and then to the phono inputs of the amplifier. We were then able to make direct comparisons between the CD LEFT SILICON CHIP and phono inputs. In practice, unless the CD player was in pause mode, it was very difficult to tell the difference. Construction For this project you can go as basic Fig.4: actual size artwork for the PC board. OUTPUT RIGHT GROUND +20 DECIBELS +10 0 -10 -20 20 100 HERTZ 1k 10k 20k INVERSE RIAA FILTER LEFT INPUT RIGHT Fig.5: this the front panel artwork used for our prototype. It shows the inverse RIAA filter characteristic provided by the circuit. 56  Silicon Chip or as deluxe as you want. You could assemble the circuit on a small piece of Vero­board or you could go for the deluxe approach as we did – put it on a small PC board which is then wired in a diecast metal case. Whichever approach you use, the finished circuit must be mounted in a metal case which is earthed back to the amplifier’s case. If this is not done, you will have problems with hum pickup. We wired up our prototype circuit on a small PC board meas­uring 39 x 46mm and coded 01105941. After the six resistors and four capacitors are mounted, the board was mounted in the diecast box. This needs to be drilled for the four RCA phono sockets and four screws to mount the PC board. Finally, a solder lug and wire lead must be fitted to the case for earthing. When installing the unit, keep the unit away from the power transformer in your amplifier and make sure that the input and output leads do not drape across mains power cords otherwise hum pickup may be a problem. Finally, while we optimised the circuit to suit CD player signal levels and typical audio amplifiers, you may need to increase or reduce the signal level to suit your application. This is easily done. If you want more signal level, increase the 560Ω resistors at the output to say, 1kΩ. Alternatively, if you need less signal level, try reducing the 560Ω resistors to 330Ω SC or 270Ω.