200 Watt Amplifier Based on Quad 405

The scheme we offer you here is a variant of the QUAD 405 amplifier.

Why did we turn to this famous scheme? First of all, it achieves a very low level of distortion. At the same time, the QUAD scheme has stable operation and trouble-free "revival". The last advantage is very important, as the difficulties in reviving most quality amplifiers often sway many young enthusiasts. From all implemented QUAD-amplifiers, including the published scheme, we came to the conclusion that with proper installation and working elements, the amplifier always works and does not need adjustment. What's more, due to the deep feedback, it is not sensitive to the gain of the transistors used, and as long as you do not make mistakes during installation, the circuit does not need any adjustments (note that there is no trimmer on the amplifier board). .

In addition to the traditional CAB (negative feedback), this amplifier has the so-called. direct connection, which essentially compensates for the distortions of the final stage operating in class B. Compensating for high harmonics with direct connection allows this amplifier to be heard in most cases without a tone corrector, which is typical of expensive high-quality amplifiers.

Those of you who implement the scheme we have proposed (Fig. 1) will notice that the amplifier sounds as soft as the QUAD 405, but more detailed, clearer and more natural. What caused this? The input stage in this circuit is made of the cascade circuit OE, OB with T1 and T2, loaded with current generator T3. T4 is a particularly favorable repeater, matching the high output resistance of the input stage with the next stage. A voltage amplifier (practically a triple Darlington transistor) is built with T5 and T6, which supplies the complementary output stage according to the OK circuit with T11 and T12.

T6 is loaded with T9 and T10 - a generator of stable current. However, T12 in R24 works together with T6 according to the "PUSH-PULL" scheme, or self-inverting stage. In practice, the distortions at low signal levels are compensated by T6 with its dynamic load T12. (In the original version of the amplifier, we used a value of 22 for R24, which we later changed to 47, which made the amplifier sound better at high frequencies. At this value of the resistor, the voltage at the control point (at the top of R14) rises to 1.5V.)

The operational amplifier OU1 (cheap and popular 741) unlike the QUAD 405 together with R15 and C7 is included as an integrator, ie it sets only the DC mode of the amplifier. The voltage of the bases of T2 and T9 is stabilized by means of zener diodes D1 and D2, thus ensuring the independence of the DC mode in case of "failure" of the power supply.

Another improvement is the diodes D4 and D5, which reduce the step when unlocking T11.

Transistors T9 and T10 build a circuit, which in the literature is often called "current mirror". If for some reason the current through T9 changes (eg increases), the drop on R17 increases, T10 is unlocked and the voltage based on T9 decreases. The latter leads to a reduction in current through T9.

The elements C5, C9, C10, C11, C12, C15, R23 and L2 are for frequency correction and eliminate self-excitation. R4 and C4 build an RF input filter that does not pass frequencies above the sound range. R25 and C16 compensate for the potentially highly inductive load at higher sound frequencies.

Capacitors C2, C6, C8, C13 and C14 build filters for the various RF interferences through the power supply, thus protecting the amplifier from self-excitation. It is recommended that at ± 50V power supply, C13 and C14 have a breakdown of at least 63V.

Capacitor C3 is the only electrolytic capacitor in the circuit. It does not participate in the AC circuit, but connects the base of T3 to AC ground. The lack of electrolytic capacitors in the useful signal circuit, which is achieved by connecting IC 1 as a DC amplifier, increases the sound quality and is a circuit solution that is increasingly used in modern amplifiers.

Here is the place to say that those who already have QUAD 405 can improve it by including OU 1 according to this scheme on the original board and adding T2. Even with this change, they will find a significant improvement in the sound of the circuit. In order to achieve a sufficiently large gain without feedback, stable current generators equipped with transistors T3 and T9 are used.

The negative feedback is built with the elements R11 and R5. The gain of the amplifier is determined by the ratio (R11 + R5) / R5 and in our scheme is about 50 times. If it is necessary to increase the sensitivity of the input, reduce the value of resistor R5. (For example, at R5 = 6.8, Ku = 75 times.) If the input level is high (as with most modern CD players), it is better to keep the value of R5 or even increase it to 18, where Ku = 29 times. In the latter case, the feedback is deeper and hence the distortions will be even smaller. Resistor R6 determines the input resistance of the amplifier. C1 is an input isolating capacitor that transmits only audio frequencies and "disconnects" the DC input. R12 is designed to divert part of the emitter current of T5 and protects it from operating in modes where the gain of the transistor is low. Resistors R3 and R17 set the current of transistors T3 and T9 so that the stages made with them operate in class A.

The protection of the amplifier is made with the transistors T7 and T8. Resistors R20 and R22 record a current signal for protection. When the output current rises above the allowable value, the voltage drop across R20 and R22 increases to a value sufficient to unlock T7 and T8. The latter bypass the bases of T11 and T12, which in turn limits the output current. R19 and R21 limit the activation threshold of the protection to reach a peak output power of 200W, when using load 4. At load 8, to adjust the trigger threshold, the two additional resistors R13 and R18 are mounted on the board.

The amplifier was tested with improved speakers from the English company "TANNOY" and gave excellent results. In a comparative audition with a transistor amplifier of another English company - "NAD", the scheme proposed here reproduced a significantly more realistic and natural sound.

Here again we ask the question: What is most important for an amplifier - the parameters it has, or its sound? When it comes to a music amplifier, not a measuring amplifier, the answer is definitely good sound. This is the main quality that must be taken into account when developing a scheme. We hope, and you will be convinced that in this scheme the good sound of the amplifier has been achieved successfully. i have attached the pcb file you can check on bom file