Negative inverter made with AD827/OPA2604/NE5532

　　There are two types of audio circuits: RC attenuation and feedback. There is also the Class AA tone circuit introduced on this site (actually it is also
RC attenuation, but the pre-stage uses Class AA amplification). Both circuits have their own advantages and disadvantages. Since the RC circuit is a passive component, the circuit works stably and has good phase characteristics, but the signal-to-noise ratio is poor, and the requirements for the input and output impedance of the pre- and post-stage amplifier circuits are high. It is easily disturbed by external magnetic fields. Another problem is that the control range of high and low bass is small. The negative feedback tone circuit has a certain gain, high signal-to-noise ratio, small nonlinear distortion, and a large dynamic range of the circuit. However, since the circuit is in a deep negative feedback state, it is easy to produce self-excitation if the wiring design is unreasonable. Combining the advantages and disadvantages of the above two circuits, this site decided to use the feedback tone circuit to match the SSE01/SSE02 of this site. The reason is that its shortcomings can be overcome through careful and reasonable wiring. At the same time, phase compensation capacitors are added to the output and inverting input of the op amp to prevent self-excitation, and power supply decoupling capacitors are added to the nearest position of the power supply pins 4 and 8 of the op amp. This also creates conditions for using the audiophile AD827/OPA2604 with a higher conversion rate and higher requirements for circuit design and wiring for tone control. Another reason for not using the RC circuit is that the AA tone board that this site once made was not customized as a finished product. In fact, when it is used with the SSE01/SSE02 board, the signal-to-noise ratio is not ideal and it is easily disturbed by the magnetic field of the power transformer. Therefore, it was abandoned and redesigned as the SSE06 introduced below. The HIFI tuning board, when actually used with our SSE01/SSE02 board, can achieve quite satisfactory listening effects by replacing different op amps.

　　The tone control circuit is shown in the figure above. W1, W2, W3, and W4 respectively realize the treble, midrange, bass, and balance control circuits. As the volume circuit is already equipped with a volume potentiometer on the SSE01/SSE02 board of this site, no more volume potentiometers are added. The op amp U1 is used as a buffer amplifier for the pre-stage signal. The value of R3/R2 is between 1-5 times. This site sets it to 2 times amplification. The value of R2 can be changed according to the actual sound source. The capacitors C2, C4, and C7 in the signal channel have a greater impact on the sound quality. High-quality audiophile capacitors such as German red WIMA are used. Capacitors and operational amplifier U2 can be selected from high-quality audiophile op amps AD827/OPA2604/NE5532. The timbre performance is different, and audiophiles can choose according to their own preferences. For the timbre characteristics of the pre-stage audiophile op amp, please see the detailed description in Contact Us . The tone control in the above figure is different from the common power amplifier in that only the treble and bass control is added. The mid-range control circuit is added. In listening, the mid-frequency part is closely related to the presence of music. If the mid-frequency is too bright or thin, it will cause presence distortion. The relevant peripheral components of W2 can enhance or attenuate the 1000HZ-2000HZ intermediate frequency signal by 6-10dB to achieve the purpose of intermediate frequency control.

　In the design of the power supply, an active servo voltage-stabilized power supply composed of LM317/LM337 is used here. Compared with the active servo power supply composed of the 78/79 series, the internal resistance and noise of the power supply are one order of magnitude lower, the ripple suppression is stronger, and it has the advantages of simple power supply structure and stable performance compared to some relatively complex Kubota-type. Although the cost is slightly increased, the improvement in sound quality in actual use is also quite obvious. The circuit diagram is as shown above. The input voltage can be AC ​​dual 12V-dual 18V. The parameters of the resistors R14/Re14 and R15/Re15 that affect the voltage accuracy must be consistent, so as to achieve good symmetry between the positive and negative power supplies. The output voltage value is not necessarily dual 15V, but the positive and negative voltages are consistent (generally, the op amp voltage is DC dual 12V-dual 15V and can work normally)

　　　　　　　　　　　　　　The PCB board design diagram is as follows

　　The PCB design fully considers the strict requirements of using high-speed op amps, adds WIMA CBB decoupling capacitors at the position closest to the IC power pin, keeps the circuit power away from the small signal processing part, increases ground isolation measures, and strictly
　　implements one-point grounding wiring measures to get the best signal-to-noise ratio using this board. The following is a real picture of the tone control board with high, medium, low, and balanced audio frequency op amps made by this site recently from the manufacturer.

１

２

３

　　Component selection : fever components are used on the key components that affect the sound quality. Decoupling capacitors are all WIMA CBB 0.1U capacitors, audio cross-coupling capacitors are WIMA 1U capacitors, C14/Ce14 uses genuine ELNA (S) high-speed supplement electrolytic capacitors, and the PCB board is the same process as the SSE01/SSE02 board, which is a blue thickened 2MM gold-plated board.
　　Power input: AC dual 12V-dual 18V can share the small voltage output group on the ring transformer.
Installation wiring precautions:
The input and output sockets of the signal are both white pin sockets on the board. The socket close to the large electrolytic filter capacitor is the signal output socket, and the other one is the input socket. It is best to use a dedicated shielded signal line for the signal line to avoid interference from the AC magnetic field. The power input socket is the blue wiring socket on the board. Note that the middle one marked as GND is the ground wire, so don't make a mistake.　　

　　　　　　　　　　　　　　　　　　　　　
　　　　　　　　　　　　　Note when using with our SSE01/SSE02 board:
　 1. In order to reduce unnecessary noise, improve signal-to-noise ratio, and save costs, users who have purchased our SSE01/SSE02 board should make some small changes to the SSE01/SSE02 board when using this board as a pre-stage tone control, that is, disconnect or remove the pre-amplifier part of the board, as shown in the figure below

After the modification, the wiring can use the audio input socket on the original SSE01/SSE02 board to directly connect to the signal output line of the SSE06 board introduced on this webpage, that is, the wiring process is: signal source--》SSE06 tone board signal input socket, SSE06 tone board signal output socket--》SSE01/SSE02 audio input socket, connect the power supply, speaker output, and power on to work. It should be pointed out that in order to prevent the signal line from being affected by the AC power frequency magnetic field, the signal line is best to use a three-core shielded wire.
2. Regarding the selection of audiophile op amps, this site provides several types of audiophile op amps, including NE5532, OPA2604, AD827, and AD812. They are all in DIP8 package form and can be directly replaced with each other on the front stage of the power amplifier board. However, when used on this feedback tone board, due to their different conversion rates and gain bandwidths, the capacitance values ​​of the phase compensation capacitors C6/Ce6 will be different. In the high-frequency 10KHZ-80KHZ frequency band square wave characteristic test, it was found that when U1 and U2 are both AD812 or AD827, no overshoot was found in this frequency band without adding the phase compensation capacitor. The output pin voltage of AD827/AD812 did not have zero drift. Instead, the square wave characteristics became worse when the compensation capacitor was added. When using NE5532/OPA2604, the compensation capacitor must be added, and the value should be 30-50P (33P is used in this site) of high-frequency ceramic chip capacitors. Capacity, used to eliminate the slight ringing phenomenon of the square wave peak. In terms of listening, through the above strict test values, when used with the post-stage power amplifier board SSE01/SSE02 of this site, the high-pitched area is much clearer and more delicate, and the transparency is improved, which makes good use of the superior performance of each fever IC. Simple substitution usually does not achieve the expected effect. In the test, except for the buffer amplifier U1 and the tone control U2, the same type of amplifier is used, that is, the code name of this site is SSE06A (using NE5532x2), SSE06B (AD827X2), SSE06C (OPA2604X2), SSE06D (AD812X2). We also tried to make several other types of combinations, all of which can have better mid- and high-frequency square wave characteristics. Figure
　　 A: NE5532 (U1, used as buffer amplifier) ​​+ OPA2604 (U2, tone control), C6/Ce6 uses 33P, Codename: SSE06E
　　 B. Use AD827 (U1) + AD812 (U2), without phase compensation capacitor (cancel C6, Ce6), codename: SSE06F
Other combinations such as NE5532/OPA2604 + AD827 or NE5532/OPA2604 + AD812 have ringing phenomenon in the frequency range of 10KHZ to 80KHZ, so it is recommended not to adopt them. The above two solutions can provide reference suggestions for netizens who have purchased SSE01/SSE02 boards from this site. Another point is that a good performance op amp is used as U2, and the output directly drives the power amplifier board of the later stage to give full play to the excellent performance of the op amp.