Amp Section Circuit Description Abstract

Transcendence Circuit Description Abstract The Non Inverting Input Stage the Audio signal is impressed upon the Non inverting input of the amplifier at the gate of Q1 via R1. R1 serves to isolate the input of Q1 from reactive sources and assures real impedance is always seen by the gate of Q1. R1L Serves as a path to ground for Input Bias current of Q1, in the event of an open circuit on Q1’s gate Caused by the wiper losing contact with the Element within the Volume control. The Inverting Input Stage Negative feedback is applied to the Inverting input of the Amplifier at the gate of Q2 via R2. R2 in concert with R5 Form a voltage divider network that divides down the large open loop gain of the Amplifier into more manageable proportions. The cascode topology was chosen for enhanced resistance to Instability. Oscillations in a high gain, common emitter amplifier configurations most often occur because of resistance and capacitance differences between the transistor input and output. The internal capacitance between the collector and the base allow a positive feedback path for the output to flow through the transistor’s internal capacitance back into the input and cause oscillations. The cascode amplifier is a common emitter stage (Q1) directly coupled to a common base stage (Q3). The low impedance common base input of Q3 heavily loads Q1’s output Capacitance which directly enhances stability against oscillations by harmlessly diverting this undesired parasitic to ground or the Power supply rail as is done hear. Q3’s purpose is to provide a constant Vce for Q1 and thus allow Q1 to operate in its most linear manner and eliminates voltage induced parametric changes in Q1. The Differential Gain Block is made up by combining the above described Inverting input Cascode stage with the Non Inverting Stage to form a differential input stage, allowing use as a conventional Op-Amp with both Positive and negative Inputs. DC offset is zeroed out by adjusting potentiometer P1. P1 allows adjustment of the Collector Loads of the Inverting input stage Q3 vs. the Noninverting input stage Q4. This method of DC correction is similar to that employed on IC Op-Amps and works well. Biasing is accomplished by the precision voltage reference formed from Q21, C5, D2, R27 and C13. D2 is an ultra low noise Zener diode that maintains the Base of the Upper cascodes in this stage at precisely 12 volts, Q21 is a constant current source to maintain precision in D2 and C5 provides decoupling of power supply noise. R27 and C13 form a low pass filer and substantially reduce and high frequency noise generated within D2. D5 and D6 provide level shifting of this stage so as to allow the use of a constant current source in the following Voltage amplification stage so as to afford substantial improvement in the Amplifiers Power supply and common mode rejection ratios

The Constant current source for the previously mentioned differential gain block comes from the formation of a constant current generator comprised of Q18 & Q19, D1, R23, and R23L. D1 is a voltage reference for Q19 the Difference between the Vce of Q19 and The reference voltage of D1 is impressed upon R23, This operates Q19 in the constant current mode and thus supplies a constant current to The Differential input stage and eliminate and current induced parametric variations in this Critical stage. By combining Cascoding to maintain a precisely defined operating voltage augmented by this constant current generator supplying an exact operating current the Input differential gain Block operates in it’s most Linear condition with Both a fixed Voltage and Current. The Second Voltage Amplification Stage or (VAS) is comprised of the same cascoded differential gain block topology as the input stage with the Non Inverting stage comprised of Q6 & Q8. This output is directly applied to the base of the positive driver transistor Q14 via base stopper resistor R14 and the inverting stage comprised of Q5 & Q7. This output is impressed upon a Wilder type of current Mirror comprising Q9 & Q10 with emitter degeneration supplied by R11 & R12 the output of this current mirror is delivered to the base of the Negative Driver transistor Q15 via base stopper resistor R15. Biasing is accomplished by the precision voltage reference formed from Q20, D7 and D8, D7 & D8 are series connected LED’s for now noise and High speed, Q20 is a constant current source so as to maintain precision in this reference. This voltage reference is returned to the constant current output of Q4 so as to provide Local feedback and to provide a constant Base collector voltage for Q5 and Q6 in addition to the constant Collector to emitter voltage that cascoding affords. This bootstrapped method was chosen for this stage because of the higher voltage swing of the stage compared to the previous differential input stage. This higher voltage swing requires consideration of the charging of nonlinear capacitances within the transistors from the audio signal, by allowing the voltage reference to float with the signal this concern is no longer valid and substantial gains in low distortion are obtained prior to the application of feedback resulting in a most linear gain block that is already low in distortion and the Negative feedback only serves to reduce distortion to vanishing low levels.

The Constant current source Is unusual for the (VAS) stage in most High End discrete Amplifiers yet is often used at this location in Monolithic Op-Amps. Since the vas stage operates at an extremely high gain the use of a constant current source explodes the Power supply rejection ratio of the complete amplifier. a constant current generator comprised of Q4 & Q20, D6 and R4. D6 is a voltage reference for Q4 the Difference between the Vce of Q4 and The reference voltage of D6 is impressed upon R4, This operates Q4 in the constant current mode and thus supplies a constant current to The Differential input stage and eliminate and current induced parametric variations in this High Amplification stage. By combining Cascoding to maintain a precisely defined operating voltage augmented by this constant current generator supplying an exact operating current the Input differential gain Block operates in it’s most Linear condition with Both a fixed Voltage and Current.

The VBE Multiplier the ICq (idle current Quiescent) often called Bias current is accomplished by a Tunable VBE multiplier Circuit. This circuit is also referred to as an amplified diode; it is used to provide a tunable voltage between the bases of the Driver transistors Q14 and Q15. The purpose of this voltage is to bias the bases of Q14 & Q15, keeping them in a “slightly” ON state. Tuning is obtained through the use of potentiometer P2. This allows the Voltage between the Bases of the Driver transistors Q14 & Q15 to be set high enough to forward bias both pairs of complementary emitter follower junctions comprising the Output stage Q16 & Q17. When P2 is properly adjusted Q16 and Q17 operate in Class A with All Headphone Loads and thus are not subject to the ill effects of crossover distortion typical of Class AB output stages. The Local Voltage Regulation for the Voltage amplification stages are accomplished by the Darlington connected pair of Q12 & Q25 for positive voltages and Q13 & Q22 for negative voltages. The output voltage for each polarity is set by the voltage references D12 & D3. C7 & C8 filter any noise from these Zener diodes while Q23 & Q24 provide a constant current source for D12 & D3 so as to maintain precision and in addition supply drive current to the bases of the two Darlington connected complementary pass transistor pairs Q12, Q25, Q13, and Q22. R9 & R10 are emitter degeneration resistors for the complementary pass transistors and prevent parasitic oscillations. This local regulation augments the substantial regulation upon the Power supply Board in addition to preventing the large currents of the output stage from flowing into the sensitive voltage gain stages via the power supply lines. This makes for much improved stability along with ultra low noise by removing a potential source of contamination of the signal because from output stage currents and the resultant positive feedback these currents create.