This project is a departure from that of my normal sort. It is based around an operational amplifier (op-amp)… Yup I’ve transgressed to the dark side. Actually I have nothing against integrated circuits (IC) or solid state devices and in fact use a number of solid state components in my designs. This project is for a relatively simple high performance phono preamplifier that can handle both moving magnet (MM) and low output moving coil (LOMC) cartridges.
The basis for the project was a temporary need to have a LOMC preamp in addition to the valve (vacuum tube) ones I have to study low noise performance. Without trying to start any arguments, I have found that it is a lot easier to make a low noise high gain solid state circuit than it is to do the same for valve circuit of similar capabilities. The circuit I came up with is pretty much like many common op-amp based phono preamplifier circuits. I did go through the process of doing the RIAA calculations and then some final tweaking with both my test equipment and use in my reference system.
The values were initially fairly close, but tweaking made the performance significantly better. I used quality components throughout the build. I have always sought ways to do things better and easier so the power supply is a great example of this concept. I won’t say I cheated, but I did use a solid state DC to DC converter to get the symmetrical voltages needed (available from Jameco for $10.95 each). For those of you unfamiliar with the Meanwell DCW03A-12 converter – (PDF 78kB) it is about the size of an ice cube and takes anything from about 9 volts to 18 volts DC as input and puts out symmetrical regulated dual supply at plus and negative 12 volts DC. This particular one is limited in current to about 125 mA on each rail.
Other members of the family can deliver more and there are numerous voltage combinations available. Another one I like can take 120 or 240 volts AC input and deliver a regulated 12 volts DC at nearly half an amp. It is not much bigger in size. Line to load isolation is excellent. Since the output from the converter is regulated and symmetrical around “zero” volts there is no provision for restricting DC gain or offset. None was needed as constructed. Note however that DC offset adjustment might be necessary if you use a different method for the power supply.
Design Considerations – Op-amp based Phono Preamp
The circuit is a non-inverting IC based filter with significant gain. Four of the components in the feedback loop determine the proper RIAA compensation. Slight variation in any of them is unlikely to be audible. The remaining components determine the gain and are for stability. Using a 330 ohm resistor from the inverting input to ground will result in sufficient gain for MM cartridges (about 43 dB). Putting a 51 ohm in parallel with that resistor will increase the gain sufficiently to permit the use of LOMC (about 61 dB). Calculated values were different from these values and the gain was determined by measuring it. The values are approximate as the input signal levels are approaching the noise floor of my test gear. I used a 4 pole double throw mini-switch to change both the gain and input loading resistors to match my cartridges. Compliance with the RIAA curve was excellent. When I measured it, I was limited to the tolerance of the components (hand selected to be under 1%) in my reverse RIAA filter. The values on a Bode plot (done with a PC based DSO) of the response were within 0.2 dBV from 20 Hz to past 30 kHz (yes I know the RIAA standard doesn’t go that high, but I can test for that response). Signal to noise was better than -90 dBV for either type of gain at any frequency. Throughout the majority of the audio band it was very close to -100 dBV. The input loading resistors should be changed to match your needs. I suggest using dip switches and having several values available. This is particularly the case for LOMC cartridges. Suggested values would be from about 40 to 500 ohms. Since the resistors are shorted to ground it is possible to get several combinations in parallel as well as individual values. For MM cartridges the normal value is 47 k-ohms. This can vary with some cartridges and you should use whatever value works best for your cartridge. No input capacitance was used, but it is sometimes needed with MM cartridges. Values in the range of 100 to 220 pF are typical. The need for them depends on the cartridge and the cabling between it and the preamp. Often the cable will have sufficient capacitance to negate the need for additional capacitance at the preamp input. I didn’t measure distortion as the results were likely to be inconsistent because of the measuring method needed. However since this is a high gain feedback controlled IC amplifier I would expect the distortion values to be very low.
Read More: DIY OPA2134 RIAA Phono Preamplifier