Hi-Fi Phono Preamp (RIAA Equalisation)

Audio and Amplifier | By: Diagram mania

Introduction

The phono preamp shown has an accurate RIAA equalisation curve, is very quiet, and offers far better sonic performance than the vast majority of those seen in magazines and application notes. Like most other ESP projects, it is very tolerant of opamps but the NE5532 dual op-amp is a good choice. This is a low noise, high speed device with excellent characteristics, and is inexpensive. It is ideally suited to this sort of application. Noise is extremely low, since any amplifier stage noise is rolled off above 2kHz with the passive filter. Another excellent opamp is the OPA2134, and these are used in my own units.

One factor often overlooked with phono preamps is the capacitive loading on the opamp output at high frequencies. This is all but eliminated in this design, and since the NE5532 and OPA2134 can both drive a 600 ohm load with ease, the 820 ohm output resistor isolates the output stage from any capacitive loading at all. The first stage has 10k in series with the cap, so capacitive loading is not an issue.

Note that if a moving-coil phono cartridge is to be used, a step-up transformer or ultra low-noise preamplifier circuit is needed before the phono preamp. This circuit is intended for use with the standard moving magnet type phono pickup.

Phono Preamp

Figure 1 – Phono Preamplifier (RIAA Equalisation)

The cartridge loading capacitor marked * (CLL, and its equivalent on the right channel – CLR) is entirely optional. In almost all cases, it isn’t needed, because the cable capacitance between the phono cartridge headshell and the preamp will be (more than) sufficient. Some manufacturers specify a loading capacitance, but many do not. The vast majority of phono cartridges perform at their best with the lowest possible capacitance, and adding more rarely makes things better. Few people have the ability to measure the capacitance of their interconnects or the internal tonearm cables, but it’s usually in the vicinity of 100pF with typical cables. Should the cartridge maker suggest a higher capacitance, feel free to experiment with the value of CLL/R. It’s best to locate these caps (if used) directly across the RIAA input sockets, rather than on the PCB, and for this reason there is no provision for the loading caps on the board. The caps also should be matched so the Left and Right channels remain properly balanced.

There is some debate on the Net about cartridge loading impedance, with various suggestions that
reducing the standard load from 47k to something lower (even as low as 10k) provides benefits. While this is plausible, I’ve not run any tests
so cannot confirm or deny that there may be an advantage. If this is something you wist to try, I suggest that it be done at the phono RCA input
socket (along with the capacitor if you want to try that too).

In general, I’d like to think that after all these years, the cartridge manufacturers would have a pretty fair idea of what they are doing.
On that basis, I suggest that if you wish to experiment, do so by all means, but don’t expect to get any ‘magic’ results. Bear in mind that
any additional parts will also increase the input capacitance of the circuit, so you can easily end up much worse off than if you just left the
circuit alone.

The high value capacitors should preferably be non-polarised electrolytic types, since they will have (virtually) no DC voltage across them. However, these are quite large, and standard electrolytics or even tantalum capacitors may be used instead. Tantalum caps will function more or less normally without DC bias, but are one of my least favourite capacitor types. Standard aluminium electrolytics are actually perfectly alright with no bias (despite what you may read elsewhere), and if sufficiently large (in value) will contribute virtually no measurable distortion.

The low value capacitors should be 2.5% tolerance if obtainable, otherwise you may be able to measure a selection of standard tolerance caps to find those which are closest to the required value. Some deviation from the ideal RIAA equalisation curve will occur if these caps are too far from the designated values. More important is matching between channels – this should be as accurate as possible.

Resistors (as always) should be 1% metal film for close tolerance and low noise. This design differs from most in that the low and high frequency equalisation are performed separately, with the LF being active and the HF passive. Because of the low value of the output resistor, a following stage input impedance down to 22k will cause little degradation of the EQ curve.

The customary “flattening” of the curve at 50Hz has not been fully incorporated, since many listeners found that the bass sounded far more natural without this. In this respect it can be said that accuracy is lacking, but I am still using this arrangement, and have not found rumble or other low frequency “noise” to be a problem.

Based on the RIAA specification, the table shows the performance with frequency – below 50Hz there is a marked deviation,
and “accuracy” figures are not quoted.

Note that there is no provision for a “rumble” filter, and the circuit as shown has a low frequency -3dB point of about 3Hz. A low rumble turntable is essential – especially if you use a subwoofer. A well damped and isolated turntable platform is an excellent idea, and I have had great success with a large concrete paving slab, neatly covered with speaker carpet or other material, and isolated using foam rubber. Some experimentation will be needed to get this exactly right. Usually, good results will be obtained when the foam support is compressed to 70% of its normal thickness with the weight of the concrete slab and turntable.


Freq – Hz Gain – dB Ideal – dB Error – dB
20 62.25 N/A N/A
50 59.16 58.42 0.74
500 43.87 44.42 -0.55
1000 41.42 Reference  
2100 38.88 38.42 0.46
21 k 22.17 21.42 0.75

As can be seen from the table, accuracy is better than 1dB, and gain at 1kHz is about 40dB (100) so a nominal 5mV cartridge output will give 500mV output. This may be increased if necessary, by increasing the value of the 100k resistor in the second stage. Care is needed to ensure that the gain is not increased so far as to cause clipping of the signal – allowing for the worst possible case. As it stands, stage 2 has a gain of 38 (31dB).

If the 100k resistor were to be increased to 220k, the gain will be slightly more than doubled, at 38dB, so an input
signal to stage 2 at 100mV (50mV phono cartridge output) above 1kHz stage 2 is on the verge of clipping. This is a highly unlikely possibility, due to the nature of music, since there are very few fundamental frequencies of any instrument (other than a synthesiser) above 1kHz, and most harmonics roll off naturally at approximately 3 to 6dB per octave above about 2kHz.

Only one channel is shown, the other channel uses the remaining half of each op-amp, the pinouts of which are shown
on the diagram. Remember that the +ve supply connects to pin 8, and the -ve supply to pin 4.

Op-amps should be bypassed from each supply line to ground with a 10uF electrolytic and a 100nF polyester or ceramic
capacitor to ensure stability.

Photo of completed preamp

Photo of Completed Unit

The photograph above shows a complete phono preamp using the PCB. This is as shown in Figure 1, but is an early version of the board (those now available look much nicer).


Update 02 Oct 00   From Tom Windelinckx … I’ve adapted one of your projects, Project 06: the Phono Preamp with RIAA Equalisation. Since the amp it’s going to be put in works on a single power supply, I adapted the preamp for a single power supply.

Figure 2

Figure 2 – Single Supply Version

19 Sept 2008 – I have modified Tom’s original circuit modification to make it closer to the original, and to reduce the possibility of supply noise getting into the signal path. The amended drawing is shown. The components RX1, RX2 and CX1 are external to the PCB – these components can be shared with left and right channels. You will need to make the appropriate changes to the PCB to use this version. If possible, the dual supply version (as designed) is preferable and should be used unless there is no alternative. As noted on the circuit diagram, I really don’t recommend this, because of the difficulty of modifying the PCB connections to ensure that the artificial earth (ground) is sufficiently stable to prevent low frequency disturbances.