Versatile Hi-Fi preamplifier

The circuit is shown in Figure 3-1. It is mainly composed of an electromagnetic phono equalizer amplifier, a high-level line amplifier and a tone control
circuit . It can be used as a preamp for electromagnetic phono players, radio tuners, ∞ phono players, tape playback and line inputs. The amplifier
circuit is assembled using NE5532 integrated operational amplifier, which has the advantages of high signal-to-noise ratio, low transient distortion, and high technical
performance.
    The electromagnetic phono equalization amplifier uses an attenuation-feedback equalization amplifier because it has high technical performance. The general
negative feedback type phono equalization amplifier relies on the negative feedback of the large loop to achieve the standards specified by the RIAA. Deep
negative feedback is beneficial to reducing harmonic distortion and improving signal-to-noise ratio, but sometimes it can also increase transient distortion, making it difficult to guarantee the high fidelity of the circuit
.
    The resistor-capacitor attenuation balanced amplifier connects the resistor-capacitor network in parallel in the signal channel, and adjusts the signal through the attenuation network
to make it comply with the standards stipulated by the RIAA. The disadvantage of this equalization method is that the attenuation of the signal after passing through the equalization network
is relatively large, so it is necessary to add an amplifier after the equalization network to make up for the loss of signal gain caused by the attenuation network.
    This circuit combines the characteristics of the two circuits and uses a feedback-attenuation phono equalizer amplifier. The negative feedback equalization method is used in the low frequency section
(20H ~ lkH). Since the frequency of this section is low and an appropriate amount of negative feedback is used,
the distortion and signal-to-noise ratio of the circuit in the low frequency section are greatly improved. For the high frequency band of 1kffi-20kHz,
the attenuation network composed of R6.C6 performs attenuation processing to make it finally comply with the RIAA standard. At the end of the Cui circuit, Ilcz performs a total amplification
to achieve the required output level of 35~40dB. .
    The line amplifier consists of Ic3 and related components. It appropriately amplifies the input signal, including phono, tuner, phono, and
line input, to meet the input level required by the subsequent stage power amplifier. At the same time, it also ensures that
the output and input impedance between the front and rear stages are matched Reasonable match. The amplification of this level is about 25dB.
    IC4, arsenic, RP3 and related components form an attenuated tone control circuit to achieve high and bass control. If
tone , the tone circuit can be cut off by switching the switch I, and IC3 can directly input the signal to the subsequent stage. Amplifier circuit.
    This circuit should be assembled using high-quality components, and the resistors should use metal film resistors with Class I accuracy. Capacitors should be polyol capacitors
. The power supply should use a positive and negative symmetrical regulated power supply.