Hi,
All good here thanks. I hope you are well too.
Here is the text. If you want the file with photos please message me your email address.
Good luck.
Barry
Prologue. The idea of creating this amplifier dates back to the development of the Only Music 3 (OM3) amplifier. Then the plans were to make, in addition to the main version of OM3, also a simplified version of it. I abandoned this idea, but after a while I still decided to return to it. This is due to the excessive complexity of repeating OM3, which caused many difficulties for those wishing to repeat the amplifier. This was the reason for the creation of a new amplifier and a return to the original concept of the series - maximum quality with minimal complexity and dimensions.
Concept. As mentioned above, the new amplifier pursues the old concept, laid down in the "slap in the face of mikruham 2" (OM2). When developing the amplifier, the goal was to achieve maximum repeatability, reliability and stability of the circuit, as well as to minimize the diversity of the used element base and exclude from the list of used radio components, less common names. The qualitative parameters of the amplifier also did not go unnoticed - the new amplifier is not inferior to OM3, and even surpasses it in most parameters. Thanks to the emergence of a huge number of PCB manufacturers and their active competition among themselves, there was a huge number of profitable offers for the manufacture of PCBs in small batches, so I was betting on a double-sided printed circuit board designed for prefabrication. However, I did not refuse a single-sided printed circuit board, so I also made a version of the printed circuit board designed for home production using "laser-ironing technology" (LUT). The dimensions of the factory printed circuit board are exactly the same as those of the author's printed circuit board for OM2 - 100x67mm, and the dimensions of the single-sided board are slightly larger - 100x73mm.
Name. You are probably wondering: why 2.7 and not 3.5 for example? In fact, everything is extremely simple here - in this way I wanted to show that this amplifier is closer in concept to OM2 than to OM3.
Scheme. Once again, I decided not to change the traditions and use the classical Linov topology. The idea of the voltage amplifier circuit (VU), which has undergone significant refinement and improvement in this amplifier, was borrowed from the well-proven BackBen amplifier, which I developed. The differential cascade (DC) was supplemented with a cascode, which made it possible to unload the input pair of transistors in terms of power and voltage. This made it possible to use low-voltage, low-noise, super-beta transistors at the input, which, together with a decrease in the collector-emitter voltage of the input pair and the power dissipated by them, made it possible to significantly reduce the noise level at the amplifier output. The ability to adjust the "zero" that was available in the OM2 amplifier was returned, which allows you to accurately balance the differential stage. Stable current generators (GST) for the differential stage and the voltage amplifier stage (KUN) are independent of each other and have independent reference voltage sources. Capacitor C12 increases the stability of the HTS and significantly reduces the ripple of their reference voltage. A fundamentally different method of frequency correction of the amplifier was used, relative to those used in OM2 and OM3, which made it possible to obtain much greater gain with an open loop of general negative feedback than in OM3 and much greater stability than in OM2. During the development of OM2.7, it was decided to abandon one solution used in the BlackBen amplifier - a composite transistor in the voltage amplifier stage, due to the fact that this solution did not provide any noticeable advantages, but reduced the repeatability of the device due to not the highest prevalence in the sale of compound transistors MPSA13. The output stage, for the first time in the OM series, is a three-stage "triple", which made it possible to improve the buffering of the voltage amplifier from the amplifier load, to reduce the dependence of non-linear and intermodulation distortions on the output power and the nature of the load. This amplifier, like OM3, has protection against short circuits in the load.
Specifications. The circuit parameters were taken using the RMAA 6.2.5 computer program. An ASUS Xonar Essence ST sound card was used as a measuring DAC and ADC. As a load for the amplifier, two powerful 10Ω resistors connected in parallel were used (which gave a resulting resistance of 5Ω). To measure the rise/fall rate of the output voltage of the amplifier, a rectangular pulse generator and a UNI-T UTD2025CL oscilloscope were used. As a laboratory power supply, a classic, non-stabilized power supply was used, consisting of: a transformer, a diode bridge and smoothing capacitors with a capacity of 15000 uF in each arm. The voltage on the amplifier power buses in the absence of an input signal is +/- 42V, the quiescent current of the output transistors is 80mA. Parameters marked with an asterisk (*) - obtained using computer simulation of the operation of the circuit, marked with two asterisks (**) - by recalculating the actually obtained parameters for other measurement conditions (different load, different signal frequency, and so on).
Output power (1kHz):
Output power (4Ω) = 140W**
Output power (5Ω) = 110W
Output power (8Ω) = 70W**
Noise level (A-weighted) = not worse than -100 dB
The lower limit of the frequency range (at the level of -1.5dB re 1kHz) = 18 Hz
(Note: the blockage on the right side of the graph is due solely to the sound card parameter and does not depend on the presence or absence of the amplifier under test in the signal circuit).
Upper frequency limit (-1.5dB re 1kHz) = 177 kHz*
Rate of change of output voltage = 25 V/µS
Transient response (1, 10, 20 kHz):
The given technical characteristics correspond to the author's version of the amplifier assembled on the factory printed circuit board, they may differ slightly, for better or worse, for each specific instance of the amplifier. The parameters of the amplifier assembled on a homemade board differ by no more than the measurement error. When comparing the parameters of amplifiers assembled on factory and home-made printed circuit boards, absolutely identical radio components and even wires were used. The first amplifier was assembled and measured on the factory board. Then all the radio components were soldered from the factory board and installed on a home-made board, after which the amplifier parameters on this board were measured. Comparative characteristics of the parameters of amplifiers assembled on different printed circuit boards and photos of finished amplifiers:
Element base. In this part of the article, I will simply list the main points related to the element base, which should be taken into account and strictly adhered to in order to successfully repeat and run this amplifier:
- all elements used in the assembly of the amplifier, before installing them on the board, must be checked for operability and compliance with the necessary parameters (resistance, capacitance, ESR, gain, and so on);
- it is not allowed to install elements focusing solely on their marking, without checking for compliance with the actual denomination of the marking;
- it is better to abandon the use of used radio components and use only new radio components during assembly;
- it is preferable to use metal film resistors (MF) and refuse to use carbon resistors (CF) due to their high self-noise;
- the use of metal-film resistors produced in the USSR is allowed;
- when using resistors with a tolerance of 5%, it is highly desirable to select in pairs, resistors R6 and R11, R7 and R12, with an accuracy of no worse than 1%;
- to achieve equal gain and, accordingly, equal volume of both stereo amplifier channels, it is recommended to pre-select two pairs of resistors 15kΩ and 470Ω (for the left and right amplifier channels), with an accuracy of no worse than 1%, in order to use them as - R1 and R19 , R2 and R8;
- when using resistors with a tolerance of 1% - there is no need to select;
- use of wirewound resistors as R35 and R36 is not recommended;
- it is strictly forbidden to arbitrarily replace any resistors with resistors of a different rating that differs from the rating indicated in the circuit;
- as trimmer resistors R9 and R20, it is allowed to use only multi-turn resistors of the 3296W type, it is allowed to use trimmer resistors with a resistance of 200 to 470 ohms;
- when installed on the board, the engine of the tuning resistor R9 should be in the middle position (the resistance between each of the extreme and central terminals should be the same), and the resistance of the tuning resistor R20 should be maximum;
all types of capacitors used in the circuit and their minimum required operating voltage are indicated in the list of radio elements for this article;
it is allowed to use capacitors with a higher operating voltage than indicated in the list of radio elements;
- do not overpay for audiophile series capacitors - using them in an amplifier circuit will not lead to any improvement in the characteristics of the amplifier or its sound;
- each capacitor, before installing it on the board, must be checked for compliance with its marking of the real capacitance value, checked for the ESR value, checked for the absence of excessive leakage. This procedure is performed for EVERY capacitor installed on the amplifier board;
- it is not allowed to use electrolytic capacitors manufactured in the USSR, as well as used electrolytic capacitors, capacitors with visible defects in the form of dents or swelling, electrolyte smudges;
- it is allowed to use zener diodes of other models, but with the same nominal stabilization voltage and power;
- it is allowed to use only those models of transistors that are indicated in the diagram, or their analogues indicated in the list of radio elements;
- before installing each of their transistors on the board, you need to make sure that they are working and that the parameters specified in the datasheet for this transistor are correct;
- transistors VT18, VT19, VT20, VT21 and VT12 must be installed on a common radiator. The area of the radiator, very approximately, can be chosen at the rate of 10-15cm2 for each Watt of the output power of the amplifier (1000-1500cm2 for an amplifier with an output power of 100W);
- transistors VT9 and VT13 can be installed on small heat sinks (there are places on printed circuit boards for this), however, the amplifier can be operated without installing transistors VT9 and VT13 on radiators;
- transistors VT2 and VT6, VT3 and VT7, VT4 and VT8, must be matched in pairs by gain with an accuracy of at least 1%;
Printed circuit boards. Photos of "clean" printed circuit boards, as well as photos of these boards in the process of manufacturing, installation and testing:
Outputs on the printed circuit board. The name and purpose of the pins on the single-sided and double-sided boards are the same, so the following information is relevant for both boards.
+ U - power plus;
-U - power minus;
GND - main power ground;
oGND - output ground (negative output terminal);
sGND - signal ground;
IN - signal input;
OUT - signal output (positive output terminal).
Switching amplifier circuits. The correct way to connect the blocks and grounds of a stereo amplifier is:
Operation of the amplifier without a loudspeaker protector is not safe and is strongly discouraged. The operation of the amplifier without protection devices is allowed only for the first start-up and adjustment of the amplifier. As speaker protection for this amplifier, I recommend using a protection device - DEF 2017.
Setting. After a successful first turn on of the amplifier, it is necessary to adjust the "zero" and the quiescent current. To adjust the "zero", it is necessary to close the amplifier input (close the IN and sGND pins on the amplifier board), connect a millivoltmeter or multimeter to the amplifier output (OUT and oGND pins on the board), and then rotate the trimmer resistor R9 to achieve the minimum constant voltage at the output of the amplifier (the result can be considered good when the constant voltage at the output of the amplifier does not exceed +/-5mV). At the next stage of tuning, it is necessary to set some value of the quiescent current in order to warm up the amplifier before the final adjustment of the quiescent current. The probes of the multimeter (millivoltmeter) must be connected to the emitters of the output transistors (VT20 and VT21), as shown in the illustration:
After that, by rotating the engine of the tuning resistor R20, set a small quiescent current (approximately 40-50mA, which corresponds to the readings of an 18-22mV millivoltmeter connected to the output) and leave the amplifier in this state to warm up for about ten minutes. Adjusting the quiescent current without warming up the amplifier is not recommended, because after the amplifier warms up, the value of the quiescent current will change relative to the value set on the cold amplifier. When the amplifier warms up (the temperature of the output transistors stabilizes at one value and stops growing), you can proceed directly to the quiescent current adjustment itself. To do this, in the same way, by rotating the R20 slider, we set the value of the quiescent current you need. I recommend setting the value of the quiescent current in the range from 70 to 100mA (which corresponds to the readings of a millivoltmeter connected to the amplifier output - 30-44mV). A higher value of the quiescent current will not positively affect either the characteristics of the amplifier or its sound, but it will significantly increase the heating of the output transistors and reduce the efficiency. There is no need to connect the load to the amplifier output to adjust the "zero" and quiescent current. At this point, the setup can be considered completed, and the amplifier is ready for use. Now you can start listening.
Thank you for your attention!
P.S. Under the list of used radioelements, you can find and download printed circuit board files: one-sided - designed for home-made LUT (in .lay format), double-sided - designed for factory production (in Gerber format). The archive with Gerber files is already prepared for factory production, it does not need any modifications and can be immediately sent to any PCB manufacturer. Do not feed intermediaries - order factory boards directly from the manufacturer. Now that's all for sure!
List of radio elements for Only Music 2.7
Designation | Type | Rating | Value | Qty | Note |
R3, R31, R32 | Resistor | 0.25W | 1 ohm | 3 | |
R23, R24, R25, R26 | Resistor | 0.25W | 22 ohm | 4 | |
R6, R7, R11, R12, R16, R17, R30 | Resistor | 0.25W | 100 ohm | 7 | |
R5 | Resistor | 0.25W | 200 ohm | 1 | |
R2, R8, R21 | Resistor | 0.25W | 470 ohm | 3 | |
R14, R29, R33, R34 | Resistor | 0.25W | 1K Ohm
| 4 | |
R13, R18 | Resistor | 0.25W | 2.4k Ohm | 2 | |
R1, R4, R10, R15, R19, R22, R27, R28 | Resistor | 0.25W | 15K Ohm
| 8 | |
R38, R39 | Resistor | 1W | 3.6 ohm | 2 | SMD 2512 |
R37 | Resistor | 1W | 10 ohm | 1 | SMD 2512 |
R35, R36 | Resistor | 2W | 0.22 ohm | 2 | |
R9, R20 | Resistor | Trimmer | 200 ohm | 2 | 3296W |
C17, C18 | Capacitor | Ceramic | 47 pF 50V | 2 | MLCC (NP0) 50V or CT81 (Y5P) 1000V |
C11 | Capacitor | Ceramic | 4.7nF 1000V | 1 | CT81 (Y5P) 1000V or MLCC (X7R) 50V |
C15 | Capacitor | Ceramic | 0.1uF 50V | 1 | MLCC (X7R) 50V |
C7, C19, C20 | Capacitor | Film | 1 nF 100V | 3 | CL11 100V or MLCC (X7R) 50V |
C26 | Capacitor | Film | 47 nF 100V | 1 | CL11 100V |
C1, C3, C9, C16, C25, C27, C28, C29 | Capacitor | Film | 0.1uF 100V | 8 | CL11 100V |
C2, C13, C21, C22 | Capacitor | Film | 1uF 63V | 4 | CL21 63V |
C8 | Capacitor | Electrolytic | 100uF 16V | 1 | Non-polar (NP) 16V |
C12 | Capacitor | Electrolytic | 100uF 50V | 1 | |
C4 | Capacitor | Electrolytic | 220uF 25V | 1 | |
C5, C6 | Capacitor | Electrolytic | 470uF 50V | 2 | |
C23, C24 | Capacitor | Electrolytic | 1000uF 50V | 2 | |
VD1 | Zener Diode | 0.5W | BZX55-C15 | 1 | 15V |
VD2 | Zener Diode | 0.5W | BZX55-C3V3 | 1 | 3.3V |
VD3, VD4, VD5, VD6 | Rectifier Diode | | 1N4148 | 4 | |
VT10 | Transistor | Bipolar | BC550
| 1 | BC550B or BC550C |
VT2, VT3, VT6, VT7 | Transistor | Bipolar | BC560 | 4 | BC560B or BC560C |
VT4, VT8, VT14, VT16 | Transistor | Bipolar | 2N5551 | 4 | |
VT1, VT5, VT11, VT15, VT17 | Transistor | Bipolar | 2N5401
| 5 | |
VT12 | Transistor | Bipolar | BD135 | 1 | or BD137, BD139, KT815, KT817 |
VT9, VT19 | Transistor | Bipolar | 2SD669 | 2 | or HSD669 |
VT13, VT18 | Transistor | Bipolar | 2SB649 | 2 | or HSB649 |
VT20 | Transistor | Bipolar | NJW0281 | 1 | or NJW3281, NJW21194, 2SC5198, 2SC5200 |
VT21 | Transistor | Bipolar | NJW0302 | 1 | or NJW1302, NJW21193, 2SA1941, 2SA1943 |
L1 | Inductor | | 1 μH | 1 | 12-15 turns, wire d1.0-1.2mm, diameter 8.0-8.5mm |