I recently had a need to connect the Aria-102 vinyl player to an amplifier. I looked - but Aria comes without a built-in preamplifier - corrector. And this is not observed in my amplifier. So I decided to put together a quick preamp - an RIAA equalizer. So that it’s simple, sounds decent, and can be powered from a regular external power source (unipolar, of course). Until I come up with something more serious with blackjack and whores, bipolar power supply and a passive filter. If you are interested, please see cat.
First, a little theory. Vinyl records are produced with a recording that has an amplitude-frequency response according to the RIAA standard. Accordingly, for correct reproduction it is necessary to adjust the frequency response according to the shape of the inverse RIAA curve.
The frequency response of the recording according to the RIAA standard (blue line) and the required frequency response of the amplifier - corrector to obtain a linear characteristic (red line).
In addition, the output from the MM type pickup head is about 5 mV. That is, it also needs to be strengthened in amplitude.
This is the diagram that came into being:
RIAA preamplifier - RIAA preamplifier - equalizer
Power is supplied to the device through diode D2, which protects against power reverse polarity. You don't have to install it.
LED D1 is a power supply indicator. The low noise operational amplifier TL072 is powered through the RC network R10,C6,C10. Resistors R5-R6 with capacitors C3, C4 form half of the supply voltage to supply bias to the non-inverting input of the operational amplifier, which is necessary when operating from a unipolar power source.
The chain R7, C7-C9, R8, R9, C11 forms the necessary frequency response. These resistors and capacitors must be selected in both channels with maximum mutual accuracy. That is why there are 3 1000pf capacitors in parallel in the circuit, and not one 3000pf capacitor. Also on the board, the 1m2 resistor is made up of 2 resistors, selected for the same values in both channels.
According to the simulation in the Microcap program, the amplifier characteristics are as follows:
The following board was developed:
And the following device was assembled:
It sounded decent enough, but personally I was a little lacking in the high-frequency part of the range. Therefore, after thinking for a while, I decided to add the following change to the diagram:
This capacitor slightly changed the frequency response something like this:
The blue line is the standard characteristic, the red line is after installing a capacitor on the board.
Although it is not kosher to make such changes to a standardized scheme, I personally liked the resulting sound better. Well, no one forbids making this additional correction switchable.
Well, it’s better to power this circuit from a source with a voltage greater than 12 volts, as I have now. This will provide a higher overload capacity of the amplifier.
But in general, enclosed in a copper screen and a plexiglass case, this amplifier perfectly works out the three rubles in change invested in it and is perfect as a simple and inexpensive version of the RIAA preamp for a vinyl player.
Preamplifier-corrector circuits
There are a great many such circuits, but they are all the same in principle and circuit design and differ only in the ratings of the frequency-setting circuits and the element base used. Next, I will give a piece of text from the reference literature of “past” times:
“The quality of playback of a mechanical recording strongly depends on the parameters of the magnetic head of the pickup and the characteristics of the preamplifier-corrector. A corrector designed to work as part of high-quality equipment must have good technical characteristics: low self-noise and harmonic distortion, large dynamic range and amplitude-frequency response (AFC), the inverse frequency response of the recording channel during the “manufacturing” of a vinyl disc. The input and output impedances must also ensure normal matching of the magnetic head and the main 3H amplifier. At least previously, for the majority of magnetic pickup heads produced by domestic and foreign industry, the average output signal level at a frequency of l000 Hz was unified with an amplitude of the needle oscillatory velocity of 10 cm/s within 2.5 mV. The optimal load resistance is 47 kOhm.
With this resistance for most heads, the absence of noticeable electrical resonances in the operating frequency range and the maximum signal-to-noise ratio are guaranteed. The distortion and noise introduced by the pickup head into the general sound reproduction path are small, therefore the degree of distortion and noise in the path is mainly determined by the characteristics of the corrector. Therefore, “standard” circuits are considered to be preamplifier-corrector circuits matched at the input to the output of magnetic pickups operating at a load with a resistance of 47 kOhm. For all correctors, the nominal input signal level is 2.5 mV, output resistance is 1 kOhm.”
The simplest corrector can be assembled with just two transistors, but this does not mean that it is “bad” - such an amplifier, with well-selected low-noise transistors with a high gain, provides quite decent sound. According to subjective assessments, the “transistor” sound is much more pleasant and “softer” than the microcircuit sound. Technical characteristics of this amplifier:
- Maximum input voltage......40 mV
- Maximum output voltage....... 4 V
- Overload capacity, not less........ 24 dB
- Gain at 1 kHz....... 100
- Signal-to-noise ratio (unweighted)....... 65 dB
- Harmonic coefficient, no more......... 0.1%
- Supply voltage............ 15 V
- Current consumption............... 1.5 mA
The circuit below is given as an example and taken from reference literature on amplifier circuitry:
However, corrector circuits on modern op-amp microcircuits also have high technical parameters and, at the same time, a smaller number of passive elements, do not require careful adjustment of individual stages, that is, they are easier to manufacture. In addition, transistor circuits tend to increase nonlinear distortion with a decrease in the frequency of the reproduced signal, and although this is eliminated by introducing deep feedback, it significantly reduces the level of the output signal and requires the use of additional intermediate amplification stages. Therefore, the preamplifiers-correctors I made were based on a standard classical op-amp circuit:
The microcircuit is connected according to a non-inverting amplifier circuit with a correction circuit R3C3R4C4 in the OOS circuit. The input resistance of the op-amp itself is high, and the resistance of the input stage is practically determined by resistor R1. Input capacitor C1 provides DC isolation and. In addition, together with resistor R1, it forms a low-pass filter that attenuates unwanted ultra-low frequency signals created by the mechanical moving parts of the electrophone. Resistor R2 determines the gain of the cascade and allows it to be adjusted if necessary. When using parts with the ratings indicated in the diagram, the equalizer gain at a frequency of 1000 Hz is 80 (38 dB).
The “native” original circuit was assembled at one time on the “ancient” op-amps K153UD2, K.140UD7, K140UD8, K140UD6, K153UD1, K153UDZ, now you can successfully use any modern microcircuits When connecting the corrector to a power source (bipolar, stabilized, voltage ±12...18 V) it, as a rule, begins to work normally provided that all elements are in good working order and there are no installation errors. No adjustment is required, but you can adjust the transmission coefficient of the amplifier by selecting the resistance of resistor R2. And by selecting capacitors C3, C4, you can regulate the rise or suppression of high and low components of the frequency response. The harmonic coefficient of the corrector at a frequency of 1 kHz does not exceed 0.03%.
A similar circuit was also assembled in a version with unipolar power supply (+15...18 volts):
But still, the option with bipolar power supply is preferable for normal operation of the microcircuit.
For those who want to experiment, I propose another circuit from the reference literature - a corrector using one op-amp with a low-noise transistor stage at the input. In this corrector, to reduce noise at the input, a differential stage is installed on low-noise transistors, which makes it possible to combine the simplicity of an on-chip corrector with the ability to obtain low noise through the use of such an input stage. The corrector has the following main technical characteristics:
- Maximum input voltage....... 120 mV
- Maximum output voltage. . . ... ... 9.5 V
- Overload capacity, not less........ 33 dB
- Gain at 1 kHz....... 80
- Deviation of frequency response from standard......... ± 1 dB
- Signal-to-noise ratio (unweighted)....... 66 dB
- Harmonic coefficient, no more. . . ..... 0.08%
- Supply voltage............ ±15 V
- Consumption current............. . 10 mA
To obtain minimal noise in the input stage, the collector current of transistors VT1 and VT2 is also set to a minimum - about 50 μA. Capacitor C2 ensures the stability of the RF corrector. The corrector has no other features and can be assembled on a modern element base without any changes:
Main difficulties
Players from “reputable” and well-known companies, as a rule, do not contain any pre-amplifiers, at least I come across just such ones. They simply have direct output from the pickup. Some explain this by saying that all preliminary amplification stages should be located as far as possible from sources of strong interference and interference, such as, for example, electric motors and transformers. Good turntables often use small, low-voltage DC motors with a power supply in the form of a remote “adapter.”
This apparently makes sense. In any case, the preamplifier-corrector must be placed in a shielding metal case connected to the “Common” wire of the circuit (!) and it is also advisable to take the power supply for it outside (make it in the form of an “adapter”), or also carefully shield it.
Examples of finished structures
All metal parts should be connected to the "Common" wire (GND) at one point, usually at the input of the preamp board.
A couple of years ago, once again going through the rubble in the attic, I came across a small box of records. As a child, I somehow didn’t pay much attention and showed no interest in the vinyl player (Vega-106), which complemented the then classic set of the Odyssey amplifier and the Olympus reel-to-reel tape recorder. And it so happened that my love for music began to appear and develop at the peak of the popularity of compact cassettes, and then came CDs, the Internet, mp3 and so on.
It’s hard to say what stopped me from continuing to clean out the attic... or the unbearable heat, or a feeling of laziness... or a great sense of nostalgia, because on the records I began to find artists I loved that I had not listened to for a long time. In general, without hesitation, I took the record player and records under my arm and went home, not expecting anything fantastic from my find... I just had a pleasant feeling of nostalgia and the feeling that I had found a “family heirloom” of sorts.
At home, I still did not know what kind of information flow I would face in the near future. And since the player already had a modified phono preamplifier, which my father made with his own hands using diagrams from magazines of that time... Without thinking twice, I soldered 2 RCA cables, connected them to the amplifier and started the player. Surprisingly, it was in good condition; the grease on the bushing had not even dried out. I knew absolutely nothing about the very structure of the turntable, about the needles, about the tonearm, about tracking force... nothing at all. The first records I listened to were The Doors and Creedence Clearwater Revival. And at that moment I realized that my very budget sound configuration sounded completely different. The sound was engaging, jump-starting, and simply “sausaging.” It was at that moment that a “spark of love” flashed through me and I realized that I couldn’t quit this business so easily. In the evenings I enjoyed the sound of my favorite bands and thought about buying a couple more used records.
Pitch hell began after I started studying the materiel. One point began to bother me - the dependence of the needle clamping force on the wear of the plates. First of all, I felt sorry for the records... most of them were simply in perfect condition and I would like to preserve them for as long as possible. And one fine evening I started setting up the player... and ultimately, after setting it up “according to the instructions,” I got a dry, lifeless and banal sound, not very different from flac and even mp3 played on a computer. It was precisely that moment that I still hate the most... because it became the basis for purchasing a new player, a new cartridge, a brush, scales, washing liquid and other crap that didn’t make my soul feel any warmer. I just wanted to hear again the engaging and exciting sound that was there at first. By and large, if I had the desire and straight arms... I could put my old turntable in order, buy a new stylus for it, finish the tonearm and it would continue to please me. But no... the path of a vinyl lover is never short... it all starts with a banal desire to listen to music and ends with “sex” with tonearms, heads, specks of dust on records... and this is unfortunately the reality! Everything turns into illness, obsession and sometimes sleepless nights. Thank the eggs, but I was able to stop myself and finally be happy with what I have and... start listening and enjoying music... because in the end, this is the most important thing - to listen and experience emotions, and no matter what you do it on.
This phono stage was taken as a gift for my father... who really likes to listen to music and does not breathe evenly towards the “lamp”, but there is not enough time for all this “insanity” :) Having looked at the prices for custom phono stages - somehow the desire completely disappeared, not to mention branded... but our Chinese, under the Little Bear brand, as it turned out... assembles very worthy equipment with the possibility of subsequent upgrades.
The parcel arrived in a relatively small box. To be honest, it was packed like a 4... the cat cried out of the polystyrene foam, but everything was intact.
There are many reviews on specialized forums for phono stages of this brand. In short - cheap and decent, with the possibility of finishing. 
Complete set:
- Preamp
- 1 lamp 6Z4
- 2 lamps 6N2
- 3 plexiglass flasks
- Network cable
- Grounding cable 
The cables are standard, the length of the network cable is 1 meter, the length of the ground cable is 3 meters. Grounding is absolutely necessary... without it there is a constant low-frequency hum in large quantities 
Plexiglas flasks. Made with high quality. Each has 3 screws for fastening to the case 
Well, our handsome phono stage with the following characteristics:
Output voltage: 1.5V
Gain = 48dB
Input RIAA ≤ 5 mV
THD ≤ 0.5%
Frequency Response: Standard Specifications
Signal-to-noise ratio ≥ 65 dB(A) 1 kHz (input: 10 mV, output: 0.7 V)
Power: AC110V (115V) / 220V (235V) 60Hz / 50Hz (the correct version will be sent according to your address)
Weight: 1.7 kg
Also worth adding:
- Works only with MM cartridges, MC - not supported
- Does not support Class D amplifiers
- Supports 6N2/ECC83 tubes, which can be replaced with 6H2,6H2n (for 6N2 mode) and 2AX7B,6N4(9PIN),ECC83,5751 (for ECC83 mode) 
There is a transformer at the back, which is protected from the lamps by a metal plate. Designed for 220V input and 6.3V output. 
On the front there are gold-plated RCA connectors and a spring-loaded latch for grounding 
At the top there are connectors for lamps, as well as a switch between different types of lamps 
As for the lamps, unfortunately nothing special can be highlighted. These are ordinary standard lamps that are not in great demand among vinyl lovers. But I’m glad that there is room for freedom and you can install other lamps without finishing anything... this is undoubtedly a huge plus. I also want to draw your attention to the fact that the 6Z4 lamp is not involved in any way in the audio path, but is used as an anode voltage rectifier 
And of course, what’s a review without internals? There will be a lot of photos, I tried to show in detail every detail for those who are interested. For those who want to go deeper, the datasheets are available online. I’ll add on my own behalf - the details are decent, correctly selected and not “turd”. Overall, excellent switching and wiring, everything was done wisely. Silicone pads are glued to the large brown capacitors to avoid contact with the metal case. 
The lamps become very tight and you need to use a lot of force to install or remove them. The flasks are screwed on just as tightly, do not dangle or turn, everything fits well. 
Well, now about the main thing - about the sound! Over time, I came to the conclusion that all these pseudo-scientific and occult descriptions of sound, which are replete with almost all reviews, are mostly complete pumpkin and spanked crucian carp. The sound is either there or it’s not... you either like it or you don’t. All! At the output we should get a pleasant and balanced sound. Everything else is hype, nagging, the desire to have better and more, having read everything on the forums, etc. You need to clearly understand for yourself that you are either listening to music and enjoying it, or you are trying to catch the light buzz of a mosquito flying past the microphone and the accidental fart of the drummer while a guitar solo is playing. Of course, detailed sound is very good... but it’s even better when it’s just high-quality, involves and rocks you, in common people it’s called “rushing.” So, I liked the sound on this phono stage... it’s nice to listen to, it’s attractive, and I feel the difference with my cheap transistor one. Therefore, I can safely recommend it to everyone who wants to touch the “lamp” and who is joining the world of vinyl, but does not want to spend a lot of money on it and is aware of it :)
You can prove something to someone for a long time, explain something... give graphs, call on spirits... the main thing is that each of us receives pleasure and joy. Your ears are the most faithful “professor” in the music world... only you will understand what is pleasant and good for you, and what is not... and of course, don’t forget about your brains! They should also be friends with your ears... and at the right moment tell your ears when they are starting to get bullied :) And of course, tell you when you need to stop being fooled by all sorts of stories, spending crazy money in search of the very best. What am I talking about anyway? - So that disputes and condemnations do not begin, although they will happen anyway.
Well, at the end of the day, for relatively sane (or insane) money, we get a well-made device that pleases with both the build quality and sound quality... or we buy a Hi-Res player, download our favorite music on the ball and don’t look at all the sick people who need to be taken care of nothing :) Thank you all for your attention, I hope I didn’t offend anyone and don’t forget to listen to the music and not disassemble it into atoms! Happy upcoming year everyone!
I'm planning to buy +4 Add to favorites I liked the review +54 +79On the rear panel you can see a DIN-5 (SG-5) type connector. I left it as a reserve for the future, not knowing exactly how I could use it. This year, clarity finally came - I used it to connect a homemade phono preamplifier, placed in a separate unit.
Sometimes a phono stage is confused with a tone block - a unit that allows you to change the balance of high and low frequencies to give the sound the desired color. The phono stage has a completely different purpose: you cannot do without it when playing music from vinyl records if you use a magnetic pickup head. The fact is that the signal is recorded on plates with a change in the spectrum: the amplitude of low-frequency oscillations decreases significantly, and the amplitude of high-frequency oscillations increases. This is done to reduce losses; The frequency response of such a conversion is called the RIAA (Recording Industry Association of America) curve and looks like this:
To restore the signal to its original form, the audio track must be “unpacked” and then amplified to a level sufficient to feed the main amplifier circuit. This is what a phono stage does. Essentially, it is a small preamplifier with a built-in set of filters.
Most factory amplifiers have a built-in phono stage, but my homemade one did not have one for obvious reasons. The phono stage could be housed in the existing housing, but then either the internal layout would have to be radically compacted to accommodate the tubes in the “basement”, or the beautiful green indicators on the front would have to be abandoned. These options did not suit me, so I decided to make a phono stage as a separate device. And when I started thinking about its design, I realized that you can do without a separate power supply if you use the one built into the amplifier. The power reserve made it possible to do this, and the technical implementation of such a trick was not difficult. Thus, it was decided to make a phono stage in the format of an attachment to the main device. Personally, I have not seen such devices - although, perhaps, they simply did not catch my eye.
I connected the filament and anode voltages, as well as the ground wire, to the DIN-5 connector. I assigned the contacts so that if any other audio equipment were mistakenly connected, no closed circuits would form and nothing would burn out.
An aluminum box from Gainta (from the same series as housings for amplifier transformers) was chosen as the housing for the phono preamplifier. I figured that it would be possible to place in it such a simple but time-tested circuit using Soviet 6N2P lamps:
I found it on the Internet and modified it a little after reading discussions on forums. Then I redrew the circuit in sPlan 7.0, creating my own design template based on illustrations in old books on radio electronics.
I assembled a working prototype of the phono preamplifier on the same stand where I assembled the amplifier three years ago. I secured the lamp sockets at the same distance at which I was going to place them in the case, and thanks to this, in the future I was able to simply rearrange the finished circuit from the breadboard without resoldering anything.
Classic “tube” mounted mounting with maximum use of the leads of the radio components themselves not only simplifies assembly, but also reduces the level of interference.
The diagram above shows only one phono channel; for stereo sound you need two of them. The 6N2P lamp is a double triode, that is, it would be possible to assemble each channel on its own lamp, but for many reasons it is better to use halves of different lamps, as I did.
In order not to make a mistake with the body layout, I made a simplified model of the future device in Inventor:
I took Soviet ceramic sockets of the PLC-9 type with a belt that allows you to install shielding caps. The phono stage adds two stages to the amplification circuit, so additional noise protection will not be superfluous. The panels did not fit completely in height, so they had to be moved outside a little. To make the belts look better, I planned to polish them. Well, for now we are marking the holes for the lamps...
And we drill many, many holes along the contour. Probably, it was possible to use a large-diameter step drill, but I decided to play it safe so as not to spoil the workpiece.
I was helped to align the holes by cutting-off wheels for the Dremel that were precisely matched to the diameter.
One of the most critical stages of work is making holes for the posts to which the filling will be attached. You can’t make a mistake even by 0.5 mm, otherwise the panels simply won’t match the windows in the lid, and it will be very difficult to change anything. But everything worked out the first time.
The racks do not form a rectangle in plan - this was done so that the internal layout would be more symmetrical, and the lamps would face me.
After adding filter capacitors for power supply, we got this densely packed filling:
In the photo, alas, the editing looks rather messy, although I tried to do it as neatly as possible. Perhaps the fact is that the photograph does not convey volume well, and elements from different levels overlap each other. In fact, they are spaced at a sufficient distance, and in some places, for safety, insulating tubes are put on their terminals.
In the back of the cover I sawed arches with a diameter slightly smaller than that of the cables. Together with the lip running along the edge of the base of the case, this ensures reliable fixation of cables, and the cover remains easily removable.
In the next photo, the left cable is used to connect to the amplifier's power supply, the middle one transmits the output signal, and the right one is the input for the record player.
The Radiotehnika “Aria-102-stereo” player I inherited has an output connector of the same type DIN-5. Of course, it can be replaced with modern “tulips”, but I decided to leave the thing in its original form. If I get another player, it will be easier to resolder the connector on the phono preamplifier cable.
Four legs, cut from sheet rubber with good “grip”, are glued to the bottom of the body.
This is what the assembled system looks like:
The phono stage works cleanly, adding virtually no noise, so you don’t even have to put shielding caps on the lamps.
I can’t be called a vinyl lover, and Aria-102 is, frankly speaking, not the kind of player from which you should expect unprecedented depths of sound. I did not make my phono stage with the goal of surpassing serial solutions. Rather, I was interested in creating a tube device in an unusual form factor - and, of course, being able to listen to records through my amplifier. In this regard, the idea was a complete success.
Introduction
The RIAA curve is the generally accepted standard for vinyl discs. It has been in use for a long time since 1954. By 1956, the new standard, which became known as the “RIAA curve,” had supplanted competing formats and captured the markets of the United States and Western Europe. The RIAA curve was approved in 1959 and standardized in 1964 by the International Electrotechnical Commission. In 1976, the IEC modified the standard RIAA bass response curve; the innovation met with fierce criticism and was not accepted by the industry. In the 21st century, the vast majority of preamplifier manufacturers follow the original RIAA curve standard without the changes introduced by the IEC in 1976.
Frequency correction according to the RIAA standard can be implemented by both active and passive filters, and combinations of two types of filters. Many people use equalizers built entirely on passive filters in the belief that they sound “better,” but the circuit shown here is implemented by combining two types of filters. This concept was developed by me long before the Internet, and the circuit shown (with a few minor modifications) was first published on the ESP website in 1999.
The graph above shows the theoretical and actual RIAA frequency response normalized to 0 dB at 1 kHz. Most RIAA phono stages have an extra (and unwanted) null at some frequency above 20 kHz. This extra zero is missing from this design because the circuit uses a passive low-pass filter that extends the frequency response curve above 20 kHz, with the final limit well above 10 MHz (depending on the capacitor's own inductance).
The terms "pole" and "zero" require some (in this case simplified) explanation. A single pole causes the signal to fall at a rate of 6 dB/octave (20 dB/decade), and a single zero causes the signal to rise at the same rate. If a zero is introduced after the pole (as shown above), the effect is to force the frequency response into a horizontal form. Horizontal frequency response is observed at frequencies from 500 Hz to 2100 Hz. The next pole (2,100 Hz) will cause the signal to decrease again. The "undefined" zero above 20 kHz is caused by the fact that many preamps cannot reduce their gain below some fixed value determined by the circuit. However, not all correctors have this problem, and it is not present in the given diagram.
It should be noted that striving for "perfect" accuracy is pointless, since a lot depends on the stylus, tonearm and (of course) the recording. When you buy vinyl, no one will tell you what EQ was applied during mastering, and the frequency response deteriorates after repeated playbacks. So ultimately, you have to let your ears be the final judge of what is best for you.
The presented phono preamplifier follows the RIAA curve, is very quiet and provides much better sound efficiency than the vast majority of devices that are cited in various magazines. As with the rest of the preamplifier stages, the phono circuit uses an NE5532 op amp. It has low noise, high speed and reasonable price. It is ideal for this type of application. Another great op amp is the OPA2134.
Rice. 1. Phono stage circuit
The input capacitor is marked * (C LL, and its equivalent on the right channel is C LR) and is optional. In almost all cases it is not needed as the cable capacitance between the pickup and preamp will be (more than) sufficient. Some manufacturers specify the required load capacity, but many do not. The vast majority of pickups are designed with the lowest capacitance possible, and adding an additional capacitor is unlikely to improve the situation. Few people have the ability to measure the capacitance of the interconnects or internal cables of a tonearm, but it is typically within 100 pF with standard cables. If the pickup manufacturer claims a higher capacity, feel free to experiment with the C L value. It is best to connect these capacitors directly to the input connectors rather than placing them on the PCB. The capacitors must be adjusted to within 1% so that the left and right channels remain properly balanced.
Capacitors with high capacitances can be non-polar electrolytic, since there will be (virtually) no direct current flowing through them. However, they are quite large in size and standard electrolytic or even tantalum capacitors can be used instead. Polarized capacitors will function fine without being affected by DC voltage, but tantalum is my least favorite type of capacitor and is therefore not recommended. The AC voltage flowing through C2L/R and C3R/L will never exceed ~5 mV at any frequency up to 10 Hz, and these capacitors play no role in constructing the RIAA curve. Don't be afraid to increase the value if you want (100uF is not a problem).
Capacitors with low capacitances should be accurate to within 2.5%, otherwise it will be difficult to select those that are closest to the required value. There will be some deviation from the ideal RIAA curve if the values of these capacitors are too far from the specified values. The most important thing is the correspondence between the channels - it should be as precise as possible.
Resistors are metal film with 1% accuracy and low noise level. This design differs from most others in that the formation of low and high frequencies is performed independently - by an active low-pass filter and a passive high-pass filter. Due to the low value of the output resistor, the input impedance of the next stage will drop to 22 kΩ and cause slight distortion of the RIAA curve.
In Fig. 1 shows only one channel, and the other uses the remaining half of each op amp. Remember that the + power supply is connected to pin 8 and the – power supply is connected to pin 4.
The generally accepted curve flattening at 50 Hz has not been fully implemented, as most listeners find the bass to sound much more natural without it. In this regard, it can be said that the accuracy is lacking, but I still use this inaccuracy and have not identified any problems with low-frequency noise.
Note that there is no need to use an LPF filter. The circuit provides -3 dB at around 3 Hz. LPF plays an important role, especially if you are using a subwoofer. A well-damped and insulated turntable platform is an excellent option. I have had success using a large concrete slab covered with carpet and cushioned with foam rubber. Getting it right will require some experimentation. Typically, good results are obtained by compressing the foam to 70% of its normal thickness under the weight of the concrete slab and turntable. A shelf attached to the wall is another good method of providing infrasonic insulation.
If low-frequency noise does occur, you will see vigorous movement of the cone even if there is no bass. In this case, I recommend including an infrasonic filter (Project 99) in the circuit. The standard configuration is 36 dB per octave with -3 dB attenuation at 17 Hz. This will generally eliminate even the worst low-frequency noise caused by warped drives. This will usually also help eliminate low-frequency feedback problems, but they should be below the filter's cutoff frequency.
RIAA Curve Characteristics
As you can see from the table, the deviation from the standard is less than 1 dB, and the gain at 1 kHz is about 40 dB (100), so a nominal 5 mV from the cartridge output will give 500 mV. This value can be increased if necessary by increasing the value of the 100 kΩ resistor in the second stage. Care must be taken not to increase the gain too much and cause clipping. As you can see, the second stage has a gain of 38 (31 dB).
If the 100 kOhm resistor is increased to 220 kOhm, the total gain will be slightly more than doubled, by 38 dB. A 2nd stage input of 17 mV (5 mV from the cartridge output) gives a normal output at 1 kHz (before the passive filter) of 1.12 V RMS. The theoretical output at 20 kHz exceeds 9.75 V RMS, but this never happens because at 20 kHz all recordings will be 15-20 dB below the level at 1 kHz (see frequency response in Fig. 2).
This means that the actual output level at 20 kHz is typically around 1 V RMS at best. However, if the gain of the second stage is increased too much, there is a risk of clipping. This possibility is unlikely due to the nature of the music - very little fundamental frequency of any instrument (other than a synthesizer) is above 1 kHz, and most harmonics naturally roll off 3-6 dB per octave above 2 kHz - but it should definitely be considered.
One factor that is often overlooked in phono preamps is the capacitive load at the op amp output at high frequencies. This is eliminated in this design, and since the NE5532 and OPA2134 can easily drive a 600 ohm load, the 820/750 ohm resistor isolates the output stage from any capacitive load. The first stage has 10k ohms combined with a capacitor, so capacitive loading is not an issue.
Each op-amp must be shunted with 10 µF x 25 V electrolytic capacitors from each power leg to ground and 100 nF capacitors between the power pins.
Note that when using a moving coil cartridge, a step-up transformer or ultra-low noise preamp must be used. This circuit is designed for use with a standard moving magnet.
Dependence of signal level on frequency
There is very little information online and elsewhere to give anyone an idea of what level they should expect sound at any given frequency. Image in Fig. 2 was captured using Visual Analyzer, one of many fast Fourier transform based computer programs available. The signal was taken from an FM tuner - you can see the sharp roll-off above 15 kHz and the pilot tone at 19 kHz used to decode the 38 kHz FM subcarrier. The capture was filmed from an Australian "alternative" radio station, so includes several different genres of music as well as speech.
Rice. 2. Typical frequency response
The capture was configured to hold the maximum level detected over the sampling period (more than 2 hours), so that it represented the highest level recorded across the entire frequency band. No correction was used on the received signal; the broadcast signal was captured directly. Although everything above 15 kHz is removed, the overall trend is clearly visible. While there will always be deviations and exceptions with different musical styles, the general trend operates across a wide range of musical styles.
The “reference” level is -9 dB at 1 kHz. Maximum peak levels are observed between 30 Hz and 100 Hz, and the level between 200 Hz and 2 kHz is quite “flat”, showing approximately 3 dB of drop within this frequency range. There is a roll-off of 6 dB per octave in the 2-4 kHz range, followed by a 10 dB roll-off in the 4-8 kHz range.
Of greater interest is the amplitude of the highest peaks because the overload will occur at the peaks rather than the average levels. At 10 kHz and just above, there are peaks at -18 dB and some additional peaks (-24 dB) at just below 15 kHz.
Based on this, it is reasonable to expect that the worst-case signal level at frequencies above 15 kHz will not exceed -30 dB, and this is 21 dB below the level at 1 Hz (slightly less than 1/10). Therefore, a cartridge with an output of 5mV at a 1kHz reference frequency will not have more than 5mV at any frequency around 20kHz - which is the highest level we can expect.
Using the recommended component values for the RIAA equalizer, the maximum possible signal level at the second stage output is about 1 V RMS - quite well within the capabilities of the proposed op amps. Even if the maximum level is 50 mV (the same result at 20 kHz as at 1 kHz), the second stage will still be below the overload level.
