Author: 挺
The transformer used for stage coupling is not good, the winding process is complicated, the volume is huge. But it is not questionable inherently.
The figure above, the figure is a grid-plated front-aspect stage of the 3000: 500Ω (UTCA-22) and 15KΩ: 500Ω (UTCA-24) coupled transformer. The supporting power supply circuit is shown below. Its technical parameters UTCA-22 frequency respond to 40Hz ~ 20kHz; input impedance 500Ω, output impedance 500Ω. UTCA-24 frequency response 20Hz ~ 40kHz; input impedance 15kΩ, output impedance 200Ω, 3330, 5000. The above figure is "cathode follower + grid + wcf" stable feedback structure, the middle picture is "common yin + grid + WCF"
There is a loop feedback structure, the main skeleton of the circuit is in a barrier. The gateway AC signal is input from the cathode, and the cathode is also the outflow channel of the screen DC current. If a straight coupling is used, although it can be flattened, the front and rear grades are equipped with each other, design, and adjustment has a considerable difficulty. The transformer is coupled here, which is easily separated from the DC operating point of the input stage to the grid circuit to make the circuit design are quite simple. Although the gateway circuit has excellent frequency characteristics, it is generally only a few hundred Europe.
The coupling transformer can reduce the secondary impedance by N2 (N as a transformer ratio) to the primary, and the impedance matching function of the transformer successfully solves the low input impedance problem of the grid circuit. At the same time, the lunaround function of the coupling transformer is also exhausted here.
The WCF circuit solves the impedance matching problem of the grid circuit and the post-level amplifier.
The cathode follower of the above figure is designed to drive the primary low impedance of the 12: 1 coupled transformer. The cathode follower selects a low μ value, and the 6N10 (12au 7, 5814) of the large dynamic signal can be entered. Here 6N10 output impedance 450Ω, matches the 500Ω primary impedance of the UTCA-22 transformer. UTCA-22 primary adopt capacitance and cathode follower coupling (due to the transformer primary impedance only 500Ω, the cathode follower output is high), and the C1 coupling is really not necessarily, and C1 is pressed by c≥1 / (2πflrl) ) Select, where FL is the minimum frequency of the effective sound, RL is the primary impedance (500Ω) of UTCA-22. The C1 capacity is not too large, otherwise the bass is overfertile, and C1 is 6.6 μ (3.3μF + 3.3μF). Through the test of different types of tubes, the gateway selects the medium μ triode 6N8P in the UG-LA characteristic curve to achieve large dynamics, good output linear; and the 6N8P warm alcohol thick tone can be added to the grid circuit. The WCF is used for 6N8P.
The picture is essentially a step-down transformer due to a 15kΩ: 500Ω coupled transformer, and the input stage with voltage gain should be selected to compensate for its gain loss, which is hereby cathode circuit consisting of 6N3. After calculation, the voltage gain is 9.4 times (19.5 dB), the UTCA-24 transformer ratio is ≈ 5.48: 1, so the voltage gain mapped to the gate circuit input is 9.4 / 5.48 ≈ 1.72 times (4.71 dB). The gateway voltage gain is 14 times, the opening gain of the entire circuit of the entire circuit is 1.72x14 ≈ 24 times (27.6 dB). C2, R10, R2 introduces loop negative feedback, closed loop gain is 11 times (20.83 dB), feedback amount - 6.7 dB.
The coupling transformer determines the passage of the daring stage, which is the soul device of the two circuits, and must pay attention to its quality. Due to the second stage of the above figure, the secondary winding has DC current through the secondary winding. It is advisable to use the core smooth insertion and leave the air magnetic gap to ensure that the number MA DC current does not produce magnetic saturation. Due to the method of the T1 line 1: 1 transformer, the T1 primary obtains a DC current to the secondary size, the opposite direction, and the opposite direction of the DC current to the secondary size, the DC current to the secondary size. Magnetization, thereby using iron cores to cross the stacked transformers to achieve large inductance, to ensure low frequency response. This place uses UTCA-22, UTCA-24 tests to throw bricks, and enthusiasts can replace other high-quality transformers). Other components can be selected as appropriate.
If the medium map circuit is made, do not rush to online, use the multimeter to communicate 100V block test C3 output, if there is an AC voltage output, in addition to the possibility of the circuit welding error, the circuit is connected to positive feedback As long as the two-terminal or secondary two-tier head of the coupling transformer T1 is turned off, the positive feedback can be changed to negative feedback. The above picture is due to the stable feedback circuit, there is no positive and negative feedback.
The above figure, the length of the medium map circuit is to retain the rich orcharge, if the coupling transformer is not discounted, the subjective listening feeling is not absained from many electronic rulers, especially its high frequency flood, enough CoSCODE circuit for the transistor JFET field effect tube.
Our other product:
