Assembling a tube preamplifier. High Quality EL2125 Preamplifier IC Preamplifier Circuits
High quality preamplifier NATALY
Schematic diagram, description, printed circuit board
This preamplifier is used for timbre correction and loudness compensation when adjusting the volume. Can be used to connect headphones.
For a high-quality path that includes an UMZCH with nonlinear and intermodulation distortions of the order of 0.001%, the remaining stages become important, which should allow the full potential to be realized. Currently, there are many known options for implementing high parameters, including using op-amps. The reasons for developing our own version of the preamplifier were the following factors:
When assembling a preamplifier on an op-amp, the threshold of its output voltage, and therefore the overload capacity, is entirely determined by the supply voltage of the op-amp, and in the case of power supply from +\-15V it cannot be higher than this voltage.
The results of subjective examinations of preamplifiers based on op-amps in their pure form (without output repeaters) and with those, for example, based on a parallel amplifier, show listeners’ preference for the op-amp + repeater circuit, with almost identical parameters “from the point of view of Kg”, this is explained by the narrowing of the spectrum of op-amp distortion when working with a high-resistance load and operating its output stage without entering the AB mode, which produces switching distortions that are practically below the level of sensitivity of the devices (Kg OU ORA134, for example - 0.00008%), but clearly noticeable when listening. This is why, as well as for a number of other reasons, listeners clearly distinguish a preamplifier with a transistor output stage.
The well-known circuit solution containing an integrated repeater based on the BUF634 parallel amplifier is quite expensive (buffer price is at least 500 rubles), although the internal buffer circuit can be easily implemented in discrete form - for a much more reasonable amount.
Amplifiers in which the op-amp operates in a small-signal mode show high performance, but lose in audition results. In addition, they are very critical to set up and require, at a minimum, a square wave generator and a wideband oscilloscope. And all this with clearly worse subjective results.
The lack of output voltage in the PU circuit (op-amp + buffer) can be eliminated by implementing voltage amplification in the buffer, and deep local feedback eliminates distortion. A sufficiently high initial quiescent current in the output transistors of the buffer guarantees its operation without distortions characteristic of push-pull structures in the AV mode. The presence of only a twofold voltage amplification allows one to achieve an increase in overload capacity by 6 dB, and with a threefold amplification, this figure becomes equal to 9 dB. When the buffer operates from a +\-30V power source, its output voltage range is 58 volts peak to peak. If the buffer is powered from +\-45V, then the output voltage from peak to peak can be about 87V. This margin will be beneficial when listening to vinyl discs that have characteristic features in the form of clicks from dust.
The two-stage implementation of the preamplifier is due to the fact that the timbre block introduces attenuation into the signal up to 10...12 dB. Of course, you can compensate for this by increasing the gain of the second stage, but, as practice shows, it is better to apply as much voltage as possible to the tone block - this increases the signal to noise ratio.
Matyushkin’s well-proven tone block was used as a tone block. It has a 4-stage low-frequency adjustment and smooth high-frequency adjustment, and its frequency response corresponds well to auditory perception; in any case, the classic bridge TB (which can also be used) is rated lower by listeners. The relay allows, if necessary, to disable any frequency correction in the path; the output signal level is adjusted by a trimming resistor to equalize the gain at a frequency of 1000 Hz in the TB mode and when bypassing.
The balance regulator is built into the OOS of the second stage and has no special features.
The low bias voltage of the OPA134 (in the author’s practice, at the output of the second stage is no more than 1 mV) makes it possible to exclude transition capacitors in the path, leaving only one at the input of the control unit, because the level of constant voltage at the output of the signal source is unknown. And, although at the output of the second stage the diagram shows capacitors of 4.7 μF + 2200 pF - with a zero offset level of about a millivolt or less - they can be safely eliminated by short-circuiting them. This will put an end to the debate about the effect of capacitors in the path on sound - the most radical method.
Design characteristics:
Kg in the frequency range from 20 Hz to 20 kHz - less than 0.001% (typical value about 0.0005%)
Rated input voltage, V 0.775
The overload capacity in the tone block bypass mode is at least 20 dB.
The minimum load resistance at which operation of the output stage is guaranteed in mode A is with a maximum peak-to-peak output voltage swing of 58V 1.5 kOhm.
When using a pre-amplifier only with CD players, it is permissible to reduce the buffer supply voltage to +\-15V because the output voltage range of such signal sources is obviously limited from above, this will not affect the parameters.
Setting up a pre-amplifier should begin by checking the DC modes of the output buffer transistors. Based on the voltage drop in the circuits of their emitters, the quiescent current is set - for the first stage it is about 20 mA, for the second - 20..25 mA. When using small heat sinks, which become mandatory at +\-30V, it is possible, depending on the temperature situation, to increase the quiescent current a little more.
It is best to select the quiescent current using resistors in the emitters of the first two buffer transistors. If the current is low, increase the resistance; if the current is high, decrease it. Both resistors need to be changed equally.
With the quiescent current set, we then set the TB regulators to the position corresponding to the flattest frequency response, and, by applying a 1000 Hz signal with a rated voltage of 0.775V to the input, we measure the voltage at the output of the second buffer. Then we turn on the bypass mode and use a trimming resistor to achieve the same amplitude as with the TB.
At the final stage, we connect the stereo balance control, check for the absence of various forms of instability (the author did not encounter such a problem) and conduct a listening session. Setting up Matyushkin's TB is well covered in the author's article and is not discussed here.
To power the preamplifier, a stabilized power supply is recommended, with independent windings for control panel and relay switching. Technically, the power requirements are nothing new.
The main thing is the low level of midrange and high-frequency noise, the suppression of which by power supply is known for the op-amp. About the ripple level - it should not exceed 0.5 - 1 mV.
A complete set of boards consists of two PU channels, Matyushkin RT (one board for both channels) and a power supply. Printed circuit boards were designed by Vladimir Lepekhin.
Double Sided Pre-Amplifier PCB:
INCREASE
Printed circuit board for TB Matyushkin with relay switching:
Double Sided Pre-Amplifier PCB:
ENLARGE The circuit is stable. There is no noticeable voltage ripple at the output; measurements were taken on an oscilloscope in the 0.01 division/volt mode (for mine this is the minimum limit).
Measurement results:
On OPA134 (only the first link of two), the power supply is single-stage, +\-15V:
Kni(1kHz)........................ -98dB (about 0.0003%)
Kim(50Hz+7kHz)................less than -98dB (about 0.0003%)
On OPA132 (both links), full version, two-stage power supply:
Kni (1kHz)........................ -100dB (about 0.00025%)
Kim (19kHz+20kHz)................... -96dB (about 0.0003%)
In the case of self-excitation of HF cascades, mica correction capacitors with a capacity of 100 to 470 pF should be soldered in parallel with resistors R28, R88 and their complementary ones in another channel.
Preamplifier circuits are numerous and, provided a few simple precautions are taken and modern op-amps are used, they are very easy to design and provide high performance. I appeal to those for whom the educational institution is “banned”: Please skip this section, but ONLY after reading the next two paragraphs.
While op-amps are considered a bad thing in audiophile circles, it must be remembered that the sound from the musician's instrument to the listener's ears passes through anywhere from 10 to 100 op-amps - in the mixer (usually more than once ), in external effects devices, in the recording device (analog or digital), and finally in the CD player itself. Many of them are not as good as those used in this design.
This doesn't mean that a good tube preamp won't sound better (or maybe just different), but don't buy into the myths about bad "chip sound" that are quite popular. This is the opinion of those who have used tube preamps as well. and preamplifiers based on op-amps of my design.
Description
The preamplifier has optional tone and balance controls, which can be omitted if desired. The input selector can be expanded if necessary to provide more signal sources.
The tone control is based on passive controls, but does not include the traditional Baxandal feedback circuit. It provides ±6 dB of control at maximum, which may not seem like much (most tone controls offer between 12 and 20 dB), but in reality, this is usually sufficient for the adjustments that are typically needed.
Note: The tone control has been modified slightly since this diagram was originally published. The RF regulator should ideally use a 1 nF capacitor (10 nF was used previously). The above circuit provides adjustment of ±3 dB at frequencies of 6 kHz and 55 Hz in the extreme positions of the potentiometers. If the change in timbre is too small, increasing the capacitance of the capacitors in the bass and treble control circuits (100 nF and 1 nF, respectively) will lower the frequency, and vice versa. When using small speaker systems, it is better to use a 47 nF capacitor in the bass control circuit.
The circuit provides an optional recording output. It can be excluded if it is not needed. Needless to say, any recording device can be used and it does not have to be a tape recorder.
Rice. 1. Input selector and circuit switching
There are no special design features here, but care should be taken during installation to ensure that the left and right channel wires are separated wherever possible to prevent crosstalk. It is recommended to use a rotary switch with an extended shaft as an input selector. This will allow you to place all inputs and the switch within one section and reliably shield them.
Input controls for CD and DVD inputs allow you to balance levels with other sources. After conducting a small number of experiments, it is necessary to ensure the ability to switch from one input to another while maintaining the volume level.
Rice. 2. Input buffer and tone control
The diagram shows only the left channel. The right channel is identical, and uses the second half of the NE5532 op amp. Pay attention to how power is connected to the op-amp:
+V - Pin 8, –V - Pin 4
If connected incorrectly, the op amps will fail!
The input stage has a gain of 2 (6 dB) and acts as a buffer for the tone block. The buffer stage at the output of the tone block also has a 2-fold gain to compensate for losses at the tone control stage (6 dB). Thus, the total gain after the tone controls is 4 (for those frequencies that are boosted to maximum). Considering a standard 2V RMS signal from a CD player, the output will be 8V RMS or 11.3V peak amplitude (assuming the input level control is at maximum).
To prevent signal clipping at peaks, the op-amp supply voltage must be at least ±15 V. The signal level of other sources will be significantly lower than the 2 V RMS of the CD player. Therefore, all possible possibilities of clipping are eliminated.
Please note that the tone controls in the central position provide an almost flat frequency response. Any deviation will most likely be due to mechanical rather than electrical reasons.
When switching S2, all elements of the tone block and the output buffer are excluded from the circuit.
Rice. 3. Balance, volume, output gain stage
The output stage provides the bulk of the gain (12.6 dB), and includes volume and balance controls. The balance control introduces 2.3 dB of attenuation in the center position and has a semi-logarithmic response. Therefore, precise control is easily achieved in the area of the central position of the engine. When the control is rotated to its extreme position, the opposite channel receives 1 dB of signal. Using step gain control can reduce noise levels
If your amplifier has unusually high sensitivity, you will need to increase the value of R19. The gain of this cascade is determined by the formula:
Ku = 20log((R18 + R17) / R17) - 2.3 dB (2.3 dB is lost in balance control)
The total gain of the system with all controls (except the tone controls) at maximum is 18.5 dB, so 230 mV will drive an amplifier with a 2 V input sensitivity to full power.
If more gain is required (which is highly unlikely), then this can be achieved by derating R17 in the final output stage (currently 22k ohms). If, for example, a total gain of 24 dB is desired, then the value of R17 should be reduced to 12 kOhm. In this case, the intrinsic noise increases in proportion to the increase in gain.
To drive normal sensitivity power amplifiers (27 dB gain), a total preamp gain of 10 dB is sufficient for most sources. This value can be achieved by increasing R17 to 82 kOhm, so that the total gain will be
6 dB + 7 dB – 2.3 dB = 10.7 dB
If desired, the values of R17 and R18 can be divided by 10 (up to 10 kΩ and 2.2 kΩ as shown in the diagram). This can reduce noise due to lower impedances. I haven't measured the noise levels in both configurations, but they will be very low either way.
All potentiometers are used with a linear characteristic.
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 (see Fig. 4). The latter should be located as close as possible to the op-amp power terminals; the location of the 10 µF electrolytes is not critical. Failure to bypass will result in high-frequency oscillations that will significantly distort the sound of the preamplifier.
Rice. 4. Op-amp power supply shunt circuit
These op amps are very common and will not be difficult to find. There are undoubtedly better devices out there, but the overall quality of the NE5532s used in this design should satisfy the most discerning listener. These devices have an internal stabilizer and no external stabilization is required.
Note that all op amps (except the tone buffer) operate with DC gain. This leads to the appearance of a constant voltage within a few millivolts at the outputs of the op-amp. To eliminate this would have required the use of electrolytic capacitors in the signal path, which was something I wanted to avoid.
Using a 2.2uF output capacitor prevents DC voltage from entering downstream devices. It is strictly not recommended to remove these capacitors, because DC voltage (even in small quantities) is not allowed to be transmitted to the amplifier! Parallel connection of two 2.2 μF capacitors provides a signal at a level of -3 dB at a frequency of up to 5 Hz and a load of 10 kOhm. This should be acceptable for most amplifiers
The 100 ohm resistor on the output is designed to prevent any oscillation of the op amp when connected to a coaxial cable.
It is advisable to use an external transformer as a suitable power source to eliminate any possibility of interference, especially if a phono stage is used.
A suitable power supply is provided in Project 05 (see Project 05). In this case, a transformer is used to provide 16 VAC, and rectification, filtering and stabilization are mounted within the preamplifier chassis.
If you want to include a transformer in the chassis, use a toroidal type transformer (20 VA is more than enough) to reduce magnetic fields to a minimum.
When connecting to the mains, be careful and follow safety precautions, mains voltage is dangerous to life! In this case, use a standard IEC type power connector. To connect to a 12V AC voltage source, I recommend using XLR connectors. They are significantly more durable than tubular power connectors and will never fall out. XLR connections are described on the power supply project page
Often, when building an audio system, the task arises of matching an audio signal source that has a low level or low load capacity with the main amplification stage. An amplifier that converts a weak (in voltage or load capacity) electrical signal into a more powerful one is called a preamplifier, or preamplifier. Such a preamplifier is usually placed as close as possible to the signal source in order to transmit this signal without significant distortion and noise for subsequent processing (for example, via cable). The preamplifier can also act as an isolating device, protecting the signal source from the unstable input impedance of the next path.
An ideal preamplifier should be linear (that is, have a constant gain over the entire operating frequency range), typically have a high input impedance (require a minimum current to sense the input signal) and a low output impedance (provide a minimum output voltage drop across the payload) , and also have a low level of noise. For specific applications of the preamplifier and its circuit design, some variations of these requirements are possible.
In high-end audio systems (Hi-Fi, Hi-End), the preamplifier is used as a hub for connecting other components of the audio system (for example, CD and record players, microphones, power amplifiers). Preamplifiers can be either integrated into mixing consoles or sound cards, or stand-alone devices. As a rule, controls and adjustments are located on the front panel of a stand-alone preamplifier, and on the rear panel there is a set of connectors for connecting audio components.
If we talk about the numerical values of signal levels, then the preamplifier must provide amplification of a weak signal of about 10 mV to the level of 250 mV (the so-called linear output) or units of volts required for further processing. The low level signal can come from, for example, pickups and microphones.
A typical audio preamplifier consists of an input switch, a volume control, and an output amplifier that provides sufficient voltage to the output. Audio preamplifiers are often equipped with a tone control, as well as a switchable loudness circuit to correct the amplitude-frequency response of certain signal sources, such as the piezoelectric pickup used when playing vinyl records, or the pickup of an electric guitar.
Often, a pre-amplifier, in its circuit design, is not inferior in complexity to the main amplification path and power amplifier.
In this material we will look at two simple pre-amplifiers from the Master Kit company. These modules do not pretend to be Hi-End, but with their simplicity and compactness they can provide a significant improvement in sound in home and car amplifiers.
The kit is intended for self-assembly of a tube preamplifier using two 6ZH1P pentodes and is housed in an original transparent case. The kit includes a complete set of parts for self-assembly of a high-quality pre-power amplifier: printed circuit board, electronic components, plastic parts made of transparent plexiglass for assembling the housing. Once assembled, you will have a classic pre-amplifier designed to convert a weak audio signal into a more powerful signal for further amplification or processing.
The design of the pre-amplifier is based on the Soviet radio tube 6Zh1P, which was quite common in its time - “a high-frequency pentode with a short characteristic.” Its detailed characteristics and application features can be easily found on the Internet. The main feature of this lamp is its ability to work with low anode voltage. This feature of this pentode made it possible to develop a device without a heavy and expensive high-voltage anode transformer, powered by a safe voltage of 12 V.
The module performs the main function of a pre-amplifier - matching the level and output impedance of the signal source with the load, and also introduces small specific distortions into the signal, characteristic of electronic amplifier tubes in the audio range.
The source of the stereo signal for it can be a player, a digital-to-analog converter (possibly as part of a sound card) or an electronic musical instrument (including one with a high output impedance). The output from the preamplifier is fed directly to the final amplifier, or any device with a line input.
In the photo: Natalie preamplifier in the housing of a satellite receiver
The article will discuss my version of assembling the Natalie pre-amplifier with a successful solution to the housing problem.
This project became another long-term construction project on my list and beat all deadlines for completion. The fact is that the idea of assembling a preamplifier appeared more than a year ago, and along with the idea, almost all the components necessary for this circuit settled in my parts drawer.
And, as often happens, all the enthusiasm suddenly evaporated somewhere, so we had to stop everything we started for an indefinite period of time. Although why is it indefinite... very definite - before the onset of autumn cold, when all the summer tasks, of which there were a lot this year, will be completed and there will be free time for soldering.
About the diagram and details
I chose the scheme for a long, very long time! The path to this pre-amplifier began with the use of specialized microcircuits like LM1036 or TDA1524 as a control unit with a tone control, but local forum users successfully dissuaded me from this sin. Next was a circuit taken from some foreign site on three op-amps of the TL072 type with HF and LF adjustment. I even etched out the PP and collected it, and listened to this pred for a while, but my soul did not fall in love with it.
Then I paid attention to the circuit of the famous Solntsev preamplifier, and already while searching for information on Solntsev’s PU I came across a circuit reminiscent of Solntsev’s in conjunction with Matyushkin’s passive RT. It was . This was exactly what I needed!
Having slightly simplified the preamplifier circuit and modified it to suit myself, I got this result. The transition to a single-story power supply and the removal of “extra” parts made it possible to somewhat simplify the board layout, make it one-sided and, most importantly, slightly reduce the size of the PCB. I didn’t change anything significant in the circuit that could worsen the sound quality, I just removed the functions of bypassing the tone control, balance and loudness compensation unit that I didn’t need.
To the tone control circuit I didn’t contribute anything of my own, but I still needed to reset the board because... I couldn’t find a ready-made single-sided seal of the size I needed on the Internet. Switching of the tone block modes is done using domestic relays RES-47.
In order to make the control I needed for the tone control and preamplifier, I immersed myself for several days in the theory of the principles of operation of counters and triggers of domestic microcircuits. For the preamplifier, I chose a case from an outdated satellite receiver, which had a rather large window, and it needed to be filled with something beautiful and useful. So, I wanted to make sure that there was visual information about the modes of the tone control, and it would be better if these were not LEDs, but numbers familiar to the eye and brain. As a result, such a diagram of three MSs was drawn.
K561LE5 sets pulses that arrive at the inputs of K174IE4 and K561IE9A. The counter on IE9 controls 4 keys that switch relays on Matyushkin’s RT. At the same time, the counter on IE4 changes the readings on the seven-segment indicator ALS335B1, indicating what mode the tone control is in at the moment. The number “0” corresponds to the mode with the minimum level of low frequencies, the number “3” – to the maximum. Another simple electronic switch is made on the MS K155TM2. One half of the microcircuit controls the switch that switches the modes of the signal level indicator, the second half is responsible for the input selector relay. Well, and a typical circuit of the signal level indicator on the LM3915 MS separately for each channel.
power unit made on the basis of the TP-30 transformer, of course with the secondary winding rewound to the required voltages.
All voltages are stabilized:
+/- 15V - on / LM337 to power the preamp board
+9V at 7805 to power the relay and control unit
+5V is again on to power the USB sound card
About setup and possible problems
Despite all the apparent complexity of the circuit and the multitude of parts, with proper assembly and use of known-good components recommended for this circuit, you can most likely protect yourself from unpleasant surprises that may arise when assembling this control unit. The only part of this entire circuit that needs adjustment is the preamp board itself. It is necessary to set the quiescent current, check the constant level at the output, and the shape of the signal.The recommended quiescent current for this control unit is 20-22 mA, and it is calculated by the voltage drop across 15 ohm resistors R20, R21, R40, R42. For a current of 20-22 mA, 300-350 mV should drop across these resistors (300:15=20, 350:15=22). The voltage drop, and accordingly the current, can be adjusted in one direction or another by changing the value of resistors R9, R10, R30, R31 (in the original circuit, 51 Ohms). A higher quiescent current corresponds to a higher resistance of the resistor and vice versa. In my version, instead of constant 51 Ohm resistors, I soldered in multi-turn trimmers with a nominal value of 100 Ohms, which made it possible to set the required quiescent current without any extra effort and with high accuracy.
Two troubles that a person who decides to repeat this preamplifier may encounter is excitement, and a constant output. Moreover, as a rule, the first problem gives rise to the second. First you need to make sure that there is a DC component at the output of each buffer and each op-amp. A small amount of constant is allowed, but just a small one, roughly speaking no more than a few mV.
If there is no permanent residence, I congratulate you! If there is, we look for the reason, but there are not so many reasons. This is either an installation error, or the “wrong” part, or there is an excitment somewhere. The first thing you need to do is to carefully inspect the board for missing connections or, on the contrary, stuck together tracks, double-check whether you are using all the parts of the required value, and if everything is correct, the third option remains, i.e. excited To find it you will need an oscilloscope.
I myself encountered this problem. All four buffers had a constant output of 100-150 mV. And the reason for its occurrence turned out to be precisely the “wrong” detail. The fact is that instead of OPA134 operational amplifiers, I installed NE5534, which are not entirely suitable for use in this circuit. I struggled with this problem for a long time and unsuccessfully, and the problem disappeared by itself after replacing the op-amp with OPA134.
About location and connection
Due to the fact that the existing case was not very large, we had to draw all the boards again in order to make them at least a couple of centimeters more compact. The placement of the boards in the case turned out to be very tight, but fortunately everything fit. Everything is a preamplifier board, a tone control board, a dual control and display unit board, a USB sound card, a power supply transformer and a rectifier-stabilizer board, and two small boards for an input selector and a volume and HF control.
I connected all the common wires at one point, on the volume and treble control board. This got rid of the problem of hum and background that frightened me, which are possible with incorrectly diluted ground.
Again, due to cramped conditions, the control and display board had to be made composite, consisting of one large and one small board. They are connected to each other via a pin connector.
I attached all the boards to the chassis of the case through these plastic insulating spacers. This made it possible to completely isolate the boards from contact, both with the metal case and from each other, in places where this is not needed.
Convenient housing
I'll tell you a little about the case itself. As I already mentioned, the housing from the satellite receiver is used as a housing for the preamplifier. The old man served faithfully for many years, was repaired several times, and after another trip to the workshop was sent to me with a diagnosis of “dead.”The buildings used to be good, big ones! It was precisely because of its size and large window that I chose this case. There was nothing superfluous on the front panel except for the inscriptions. Of course, there are 3 unused buttons left, but that’s not a big deal. I painted over the inscriptions with matte paint from a spray can purchased at a car dealership. The paint matched 98 percent of the color with the one the body was originally painted with. The difference can only be noticed if you look closely.
I installed them as handles for these regulators, which by the way. They fit perfectly (in my opinion) into the overall design of the preamplifier, which is designed in silver and black.
About sound and impressions
And the time has come to talk about the most interesting thing, about what happened in the end. And in the end it turned out to be another good toy in my collection of sound-reproducing equipment.The scheme undoubtedly deserves attention and to be repeated. I liked the sound of the finished device; it adds some color to the music. Despite only 4 steps in the Matyushkin tone control, I can’t say that there are not enough low-frequency adjustments. Four positions of the bass control are enough to select the desired level of low frequencies for a specific style of music and your preferences.
Do you like explosive bass? Switch the tone block to the fourth position and let the speakers explode! The range of adjustments for highs is also abundantly sufficient; when the knob is positioned as far to the right as possible, the amount of highs begins to hurt the ear.
At the turn of 2004 and 2005, a natural desire arises to build amplifiers on a modern element base, taking advantage of the advanced achievements of global electronic technology.
I bring to your attention a high-quality preamplifier based on the EL2125.
The basic materials are FREE and DIYers are free to use them to replicate them in their own designs.
WHY EL2125?
An excellent chip, in terms of its characteristics it almost ranks 2nd in the top ten op amps according to model reviews in 2004.
This is, of course, not the AD8099 (first place in the world, award from Intel “Innovation of 2004”), but the EL2125 has already appeared on the CIS market and it is quite possible to get it, especially for those who live in capitals and large cities.
For those who find it very difficult to get an EL2125, I will try to help as much as possible.
JUDGE FOR YOURSELF HOW GOOD THE EL2125'S CHARACTERISTICS ARE:
Ability to operate on loads up to - 500 Ohm
Operating frequency range up to - 180 MHz
Supply voltage - ±4.5 ... ±16.5 V.
Nonlinear distortion coefficient - less than 0.001%
Output slew rate - 190 V/µs
Noise level - 0.86 nV/vHz (better than AD8099!!!)
The EL2125 retail price is usually $3 each, not very cheap, but worth it.
Most often, EL2125 is found in SO-8 type housing (prepare micro-tips for soldering irons).
I should note that I would add “amazing musicality” to the list of characteristics.
This indicator cannot be measured with instruments and expressed in numbers; it is felt only by ear.
1. As an amplifier for phones with a wide range of impedances:
2. As a high-quality preamplifier for power amplifiers with bipolar power supply (ranging from ± 22 to ± 35 V.) and sensitivity 20 ... 26 dB:
This op-amp involuntarily suggests itself as a more serious pre-amplifier, created on the basis of the Solntsev amplifier and described on the Soldering Iron website:
The described preamplifier is connected to an AF power amplifier with an input impedance of at least 10 kOhm. With a significant increase in Kg, this control unit can also be loaded onto an UMZCH with Rin up to 2 kOhm (which is extremely undesirable), in such cases (if the Rin of your UMZCH is less than 10 kOhm), you just need to once again power up the output stage (a copy of the circuit section VT1-VT2- VT3-VT4-R4-R5-R6-R7, connect to output DA2), connect resistors R23 and R24 in the same way as resistors R2 and R3, although in this case the noise level may increase. And if Rin of your UMZCH is greater than or equal to 100 kOhm, then it is recommended to use K574UD1A(B) as an operational amplifier DA2, this will reduce the level of distortion and noise.
Possible changes in the scheme (improving):
- To exclude P2K switches (very unreliable in operation) from the audio signal path, it is recommended to exclude switch SA1 from the circuit (together with resistors R8, R9), and move switch SA2 to the last stage by short-circuiting resistor R23 (resistors R13, R14 are excluded in this case from the diagram).
Preamp circuit:
It would also not be useless to use this op-amp in a universal pre-amplifier that can also serve as a headphone amplifier. The circuit diagram is shown below:
Emitter followers VT1-VT2 unload the output of the op-amp, and then follows a circuit with local feedback, which further reduces non-linear distortions.
Resistors R19 and R20 set the quiescent current of the window stage of the preamplifier, similar to power amplifiers, within 7-12 mA. In this regard, the last stage must be installed on a small heat sink
