Here, I’m presenting a carefully reworked version of an old Russian DIY radio project. I’ve not only translated the text into English but also selected modern equivalents for the old Soviet radio components.
I chose this project because it allows you to achieve that true vintage sound! It uses LC filters, which are quite rare nowadays.
If you like it, we can collaborate to develop a PCB and have it manufactured. The original design was created by engineer Dmitry Starodub.
The schematic diagram of the amplifier is shown in Fig. 1. The amplified signal enters through the input jacks and is fed into a loudness-compensated volume control R2, followed by a source follower stage built on the silicon planar junction field-effect transistor T1 with an isolated gate and an n-channel. The load for this stage is resistor R5. The signal then passes through a resistor to a differential amplifier implemented using transistors T2 and T3. The base of transistor T3 is connected via resistor R13 to the collector circuit of transistor T4, which is configured as a common-emitter resistive amplifier with a collector load. The signal is fed to the base of transistor T4 from the collector circuit of transistor T2, resulting in the differential amplifier being subject to deep negative feedback for both AC and DC signals.
An increase in the collector current of transistor T3 leads to a higher voltage drop across resistor R3, which in turn reduces the collector current of transistor T2, decreasing the base current of transistor T4 and lowering the voltage drop across resistor R18. Consequently, the base current of transistor T3 is also reduced, stabilizing the collector current of T3. A similar effect occurs if the operating conditions of transistors T2 and T4 are disturbed. As a result, the amplifier demonstrates high thermal stability, with ambient temperature variations from plus 5 degrees Celsius to plus 75 degrees Celsius having negligible impact on its parameters.
The above characteristics also apply to the amplification properties of these stages. Due to the deep negative feedback, the amplifier achieves extremely low nonlinear distortion, reaching only 0.05% at an output signal level of 0.5 to 1.0 V.
Resistors R6 and R10 are connected in series with the base circuits of transistors T2 and T3, forming frequency-selective voltage dividers in conjunction with the resonant LC circuits L1C5 to L4C8. The circuit L1C5 is tuned to 80 Hz, L2C6 to 800 Hz, L3C7 to 4500 Hz, and L4C8 to 11,000 Hz. Depending on the positions of the tone control potentiometers R9 to R12, the resonant circuits can be connected either to the base of transistor T2 or the base of transistor T3.
The frequency response curves of the amplifier at the extreme positions of the tone control sliders are shown in Fig. 2. As seen from the graph, each tone control adjusts the gain within a specific frequency band. Overall, the tone control unit enables significant modification of the amplifier’s frequency response, making it possible to compensate for the frequency characteristics of various signal sources.
The final amplification stage based on transistor T5 is required only if the power amplifier has a low input impedance or if the preamplifier is used as a standalone unit connected to the power amplifier via a cable. The output signal to the power amplifier can be taken either from resistor R21, connected in the collector circuit of transistor T5, or from resistor R19, placed in its emitter circuit. In the first case, the maximum output voltage is 1 V with an output impedance of 1.5 kΩ, while in the second case, it is 0.15 V with an output impedance of 200 Ω.
The amplifier is powered by a stabilized rectifier with a voltage range of 10–14 V and ripple amplitude not exceeding 50 µV.
Due to the deep negative feedback, no adjustment is required for the amplifier. It is only necessary to select the capacitances of capacitors C5–C8 and the resistances of resistors R14–R17, as well as the quality factor (Q-factor) of the inductors L1–L4. The quality factor of the inductors determines the steepness of the resonance curve slopes, so the higher the Q-factor of the inductors, the narrower the frequency range that can be adjusted by each potentiometer. The Q-factor can be adjusted by connecting a resistor with a value of 5–100 kΩ in parallel with the inductor. The resistance of this resistor should be experimentally determined by measuring the frequency characteristics of the amplifier at the extreme positions of the corresponding tone control sliders. The circuits of other resonance filters should be disconnected during this process.
Similarly, the resistances of resistors R14–R17 are selected. Increasing the resistance of these resistors reduces the steepness of the resonance curve slopes. However, at the same time, it also reduces the gain and attenuation limits of the frequency response. The optimal resistance for these resistors lies within the range of 240–820 Ω. The resonant circuits are tuned to the corresponding frequencies by adjusting the capacitances of capacitors C5–C8. Since resistors R6 and R13, together with the resonant resistances of the inductive circuits, form voltage dividers, the resistance values of these resistors influence the level of gain or attenuation in specific regions of the amplifier's frequency response. Increasing the resistance of these resistors results in a deeper tone adjustment and simultaneously broadens the range of adjustable frequencies.
The amplifier uses standard components. In the first stage, field-effect transistors such as KG1102, KP103, and KP301, as well as regular transistors like P416B, GT308, and GT310, can be used, configured as emitter followers. In the other stages of the amplifier, any small-power silicon and germanium transistors of the appropriate structure can be used, and there is no need to change the resistor values. This is because the amplifier's deep negative feedback for DC ensures that any transistors with a current gain (hFE) greater than 30 can be used without schematic modifications.
The inductors L1–L4 are wound on toroidal cores with a size of 2000 NH K20X12X5. The inductor L1 contains 2000 turns of PEW-2 wire with a diameter of 0.08 mm; L2 contains 350 turns of PEW-2 wire with a diameter of 0.27 mm; L3 contains 200 turns of PEW-2 wire with a diameter of 0.27 mm; and L4 contains 95 turns of PEW-2 wire with a diameter of 0.27 mm. The entire amplifier should be enclosed in a metal shield to protect it from external electrical and magnetic fields.
Finally, it should be noted that the number of tone controls can be increased to six. In this case, the resonant circuits are tuned to frequencies of 60, 250, 1500, 5000, 12000, and 20000 Hz. However, controlling such an amplifier is less convenient, and without an indication of each potentiometer's position, it becomes difficult to operate. Therefore, linear potentiometers are best suited for use in such a design (see "Radio", 1973, No. 4). The graduation of these potentiometers is directly in decibels.
The panel with the potentiometer knobs can be conveniently designed as a standard frequency response field. In this case, the position of the potentiometer knobs will clearly illustrate the frequency response shape of the amplifier (see Fig. 3).
My comments regarding replacing soviet elements:
The КП305Ж is a silicon diffusion-planar field-effect transistor with an isolated gate and n-type channel. It is designed for use in high and low-frequency amplifiers with high input impedance.
Key Technical Specifications:
When selecting a modern imported equivalent for the Soviet transistor КП305Ж, it is important to consider the specific characteristics of the original transistor. Based on the main parameters of the КП305Ж, the following modern n-channel JFET transistors could be considered as potential substitutes:
1. 2N5484:
The КТ301Д is a silicon epitaxial-planar n-p-n transistor designed for use in amplifiers and generators of general-purpose electronic equipment. It is manufactured in a metal-glass package with flexible leads of type KTY-3-1 and has a weight of no more than 1.0 g.
Key Technical Specifications:
Key Technical Specifications of ГТ308Б:
I chose this project because it allows you to achieve that true vintage sound! It uses LC filters, which are quite rare nowadays.
If you like it, we can collaborate to develop a PCB and have it manufactured. The original design was created by engineer Dmitry Starodub.
Most modern high-quality low-frequency (LF) amplifiers feature separate tone controls for low and high audio frequencies, allowing gain adjustment within a range of ±(10–15) dB while maintaining a constant gain at midrange frequencies.
In practice, such equalization is sufficient when operating with a signal source that has a frequency response close to linear. However, significant deviations from linearity in the frequency response of signal sources—such as those observed when amplifying signals from tape recorders and turntables—make this type of adjustment inadequate, preventing the desired tonal balance from being achieved.
Additionally, there is often a need to suppress noise from vinyl records and magnetic tapes, which have a relatively narrow frequency range (5–8 kHz) and cannot be eliminated using conventional tone control methods. As a result, modern high-fidelity LF amplifiers increasingly incorporate multichannel tone control systems.
This article presents a description of a low-frequency preamplifier with a four-channel tone control system. While it does not aim to cover all aspects of multichannel tone control—such as the optimal selection of control frequencies—it provides valuable insights into the operation of a four-channel tone adjustment module. This information may be useful to radio enthusiasts involved in the design and construction of high-fidelity audio equipment.
The tone control unit presented to the readers is a low-frequency (LF) preamplifier with an adjustable frequency response at 80 Hz, 800 Hz, 4500 Hz, and 11,000 Hz within a plus/minus 22 dB range. The amplifier's operating frequency range spans from 15 Hz to 30,000 Hz, with a frequency response deviation of 1.5 dB and a total harmonic distortion of 0.05% at the midrange positions of the tone controls.In practice, such equalization is sufficient when operating with a signal source that has a frequency response close to linear. However, significant deviations from linearity in the frequency response of signal sources—such as those observed when amplifying signals from tape recorders and turntables—make this type of adjustment inadequate, preventing the desired tonal balance from being achieved.
Additionally, there is often a need to suppress noise from vinyl records and magnetic tapes, which have a relatively narrow frequency range (5–8 kHz) and cannot be eliminated using conventional tone control methods. As a result, modern high-fidelity LF amplifiers increasingly incorporate multichannel tone control systems.
This article presents a description of a low-frequency preamplifier with a four-channel tone control system. While it does not aim to cover all aspects of multichannel tone control—such as the optimal selection of control frequencies—it provides valuable insights into the operation of a four-channel tone adjustment module. This information may be useful to radio enthusiasts involved in the design and construction of high-fidelity audio equipment.
The schematic diagram of the amplifier is shown in Fig. 1. The amplified signal enters through the input jacks and is fed into a loudness-compensated volume control R2, followed by a source follower stage built on the silicon planar junction field-effect transistor T1 with an isolated gate and an n-channel. The load for this stage is resistor R5. The signal then passes through a resistor to a differential amplifier implemented using transistors T2 and T3. The base of transistor T3 is connected via resistor R13 to the collector circuit of transistor T4, which is configured as a common-emitter resistive amplifier with a collector load. The signal is fed to the base of transistor T4 from the collector circuit of transistor T2, resulting in the differential amplifier being subject to deep negative feedback for both AC and DC signals.
An increase in the collector current of transistor T3 leads to a higher voltage drop across resistor R3, which in turn reduces the collector current of transistor T2, decreasing the base current of transistor T4 and lowering the voltage drop across resistor R18. Consequently, the base current of transistor T3 is also reduced, stabilizing the collector current of T3. A similar effect occurs if the operating conditions of transistors T2 and T4 are disturbed. As a result, the amplifier demonstrates high thermal stability, with ambient temperature variations from plus 5 degrees Celsius to plus 75 degrees Celsius having negligible impact on its parameters.
The above characteristics also apply to the amplification properties of these stages. Due to the deep negative feedback, the amplifier achieves extremely low nonlinear distortion, reaching only 0.05% at an output signal level of 0.5 to 1.0 V.
Resistors R6 and R10 are connected in series with the base circuits of transistors T2 and T3, forming frequency-selective voltage dividers in conjunction with the resonant LC circuits L1C5 to L4C8. The circuit L1C5 is tuned to 80 Hz, L2C6 to 800 Hz, L3C7 to 4500 Hz, and L4C8 to 11,000 Hz. Depending on the positions of the tone control potentiometers R9 to R12, the resonant circuits can be connected either to the base of transistor T2 or the base of transistor T3.
The frequency response curves of the amplifier at the extreme positions of the tone control sliders are shown in Fig. 2. As seen from the graph, each tone control adjusts the gain within a specific frequency band. Overall, the tone control unit enables significant modification of the amplifier’s frequency response, making it possible to compensate for the frequency characteristics of various signal sources.
The final amplification stage based on transistor T5 is required only if the power amplifier has a low input impedance or if the preamplifier is used as a standalone unit connected to the power amplifier via a cable. The output signal to the power amplifier can be taken either from resistor R21, connected in the collector circuit of transistor T5, or from resistor R19, placed in its emitter circuit. In the first case, the maximum output voltage is 1 V with an output impedance of 1.5 kΩ, while in the second case, it is 0.15 V with an output impedance of 200 Ω.
The amplifier is powered by a stabilized rectifier with a voltage range of 10–14 V and ripple amplitude not exceeding 50 µV.
Due to the deep negative feedback, no adjustment is required for the amplifier. It is only necessary to select the capacitances of capacitors C5–C8 and the resistances of resistors R14–R17, as well as the quality factor (Q-factor) of the inductors L1–L4. The quality factor of the inductors determines the steepness of the resonance curve slopes, so the higher the Q-factor of the inductors, the narrower the frequency range that can be adjusted by each potentiometer. The Q-factor can be adjusted by connecting a resistor with a value of 5–100 kΩ in parallel with the inductor. The resistance of this resistor should be experimentally determined by measuring the frequency characteristics of the amplifier at the extreme positions of the corresponding tone control sliders. The circuits of other resonance filters should be disconnected during this process.
Similarly, the resistances of resistors R14–R17 are selected. Increasing the resistance of these resistors reduces the steepness of the resonance curve slopes. However, at the same time, it also reduces the gain and attenuation limits of the frequency response. The optimal resistance for these resistors lies within the range of 240–820 Ω. The resonant circuits are tuned to the corresponding frequencies by adjusting the capacitances of capacitors C5–C8. Since resistors R6 and R13, together with the resonant resistances of the inductive circuits, form voltage dividers, the resistance values of these resistors influence the level of gain or attenuation in specific regions of the amplifier's frequency response. Increasing the resistance of these resistors results in a deeper tone adjustment and simultaneously broadens the range of adjustable frequencies.
The amplifier uses standard components. In the first stage, field-effect transistors such as KG1102, KP103, and KP301, as well as regular transistors like P416B, GT308, and GT310, can be used, configured as emitter followers. In the other stages of the amplifier, any small-power silicon and germanium transistors of the appropriate structure can be used, and there is no need to change the resistor values. This is because the amplifier's deep negative feedback for DC ensures that any transistors with a current gain (hFE) greater than 30 can be used without schematic modifications.
The inductors L1–L4 are wound on toroidal cores with a size of 2000 NH K20X12X5. The inductor L1 contains 2000 turns of PEW-2 wire with a diameter of 0.08 mm; L2 contains 350 turns of PEW-2 wire with a diameter of 0.27 mm; L3 contains 200 turns of PEW-2 wire with a diameter of 0.27 mm; and L4 contains 95 turns of PEW-2 wire with a diameter of 0.27 mm. The entire amplifier should be enclosed in a metal shield to protect it from external electrical and magnetic fields.
Finally, it should be noted that the number of tone controls can be increased to six. In this case, the resonant circuits are tuned to frequencies of 60, 250, 1500, 5000, 12000, and 20000 Hz. However, controlling such an amplifier is less convenient, and without an indication of each potentiometer's position, it becomes difficult to operate. Therefore, linear potentiometers are best suited for use in such a design (see "Radio", 1973, No. 4). The graduation of these potentiometers is directly in decibels.
The panel with the potentiometer knobs can be conveniently designed as a standard frequency response field. In this case, the position of the potentiometer knobs will clearly illustrate the frequency response shape of the amplifier (see Fig. 3).
My comments regarding replacing soviet elements:
The КП305Ж is a silicon diffusion-planar field-effect transistor with an isolated gate and n-type channel. It is designed for use in high and low-frequency amplifiers with high input impedance.
Key Technical Specifications:
- Maximum Power Dissipation (Psi max): 150 mW
- Maximum Drain-Source Voltage (Usi max): 15 V
- Maximum Gate-Source Voltage (Uzs max): 15 V
- Maximum Gate-Drain Voltage (Uzi max): 15 V
- Drain Current (Is): 15 mA
- Cutoff Voltage (Uzi cutoff): At least 6 V
- Transconductance (S): 5.2 to 10.5 mA/V
- Input Capacitance (Cin): No more than 5 pF
- Feedback Capacitance (Cfb): No more than 0.8 pF
- Power Gain (Ku.p): At least 13 dB at a frequency of 250 MHz
- Noise Factor (Ksh): No more than 7.5 dB at a frequency of 250 MHz
When selecting a modern imported equivalent for the Soviet transistor КП305Ж, it is important to consider the specific characteristics of the original transistor. Based on the main parameters of the КП305Ж, the following modern n-channel JFET transistors could be considered as potential substitutes:
1. 2N5484:
- Maximum Drain-Source Voltage (VDS): 25 V
- Maximum Drain Current (ID): 10 mA
- Transconductance (gfs): 1.5–5 mA/V
- Package: TO-92
- Maximum Drain-Source Voltage (VDS): 30 V
- Maximum Drain Current (ID): 25 mA
- Transconductance (gfs): 1.5–6.5 mA/V
- Package: TO-92
- Maximum Drain-Source Voltage (VDS): 40 V
- Maximum Drain Current (ID): 1 mA
- Transconductance (gfs): 0.5–1 mA/V
- Package: TO-92
The КТ301Д is a silicon epitaxial-planar n-p-n transistor designed for use in amplifiers and generators of general-purpose electronic equipment. It is manufactured in a metal-glass package with flexible leads of type KTY-3-1 and has a weight of no more than 1.0 g.
Key Technical Specifications:
- Maximum Power Dissipation (Pk max): 150 mW
- Cutoff Frequency of Current Gain (fgr): At least 30 MHz
- Maximum Collector-Base Voltage (Ukbo max): 30 V
- Maximum Emitter-Base Voltage (Uebo max): 3 V
- Maximum Continuous Collector Current (Ik max): 10 mA
- Reverse Collector Current (Ikbo): No more than 10 µA
- Static Current Gain (h21e): 20–60
- Collector-Base Capacitance (Ck): No more than 10 pF
- Saturation Resistance between Collector and Emitter (Rke sat): No more than 300 Ω
- 2N842
- SE8001
- BC120
- MPSH07A
Key Technical Specifications of ГТ308Б:
- Maximum Power Dissipation (Collector): 150 mW
- Cutoff Frequency of Current Gain (for common emitter configuration): At least 120 MHz
- Collector-Base Breakdown Voltage: 20 V
- Emitter-Base Breakdown Voltage: 3 V
- Maximum Continuous Collector Current: 50 mA
- Reverse Collector Current at 5 V: No more than 2 µA
- Static Current Gain (h21e) at 1 V and 10 mA: 80–200
- Collector-Base Capacitance at 5 V: No more than 8 pF
- Saturation Resistance between Collector and Emitter: No more than 24 Ω
- 2N796 — n-p-n transistor with similar characteristics.
- 2N797 — p-n-p transistor with similar characteristics.
