Created at the end of 1950s and designed by Japanese designer, SRPP circuit instead of being used for audio was originally used for television’s VHF band. Afterwards it was switched to audio circuit.

SRPP (Shunt Regulate Push-Pull) has some distinctions among three kinds of the most basic amplifying circuit with common-cathode, common-anode and common-grid configurations. It has a quite excellent frequency response at high frequency and the higher the frequency is the lower the distortion is. Both output impedance and distortion is low and circuit construction is simple. Therefore SRPP circuit is being widely used in pre-amplifier and input stage of power amp. Also there are some examples that use transformed one in other circuit’s section.

Fig.1 shows a typical SRPP electric circuit. It looks puzzling that two triodes are in series. Virtually it is very simple and now let’s analyse this circuit separately. If we ignore V2, according to output mode, V1 is only an ordinary common-cathode amplifier. With V1 ignored, the operating mode of V2 only shows that it is a common-anode amp. However When we ananlyse this circuit by putting V1 and V2 together, the circuit function comes about interesting change ------V2 becomes not only V1’s active pull-up but also V1’s cathode follower. In other words, V2 has tow actions, constant-current source and common-anode amp.

Figure 1

In general common-cathode amplifier, usually to get sufficient voltage gain, anode load resistance often takes higher value at the sacrifice of frequency response width. Although the use of lower anode load resistance can improve frequency response it is contradictory to circuit’s gain, dynamic and tube’s maximum plate consumption. But SRPP circuit has made a good thing out of the circuit composition, enabling the common-cathode amplifier’s load to use active pull-up to raise gain and reduce distortion ,then making the tube that serves as active pull-up for common-cathode amp become common-anode amplifying. And as common-anode amplifier has higher input impedance and lower output impedance, it not only brings an ordered action but also get lower output impedance. It can really be described as “to kill two birds with one stone”.

In Fig. 2, though the three electric circuits are all SRPP ones they are not selfsame.

Figure 2

Output (Fig.2-a) is from V1’s anode and V2 simply works as V1’s active pull-up. As active pull-up having higher AD impedance is used V1’s distortion rate is improved, but as a common-cathode amp V1’s output impedance is still higher. In this circuit, without by-pass capacitor at cathode, V1 would generate current negative feedback, making V1 output impedance increase, causing bad frequency characteristic. Also because two tubes’ working modes are symmetry the input end becomes sensitive, so the use of by-pass capacitor can relieve the above problems. To ensure this circuit has enough low-frequency lower limits the capacity of the by-pass capacitor must be sufficient and also to avoid unwanted phase drift over capacity is inappropriate. On principle this capacitor’s capacitive reactance should be lower than cathode resistance. The capacity value can be reckoned up by the below formula.

fL represents cut-off frequency of low frequency end and XC represents capacitive reactance. From the worked out result, we learn that all cathode’s by-pass capacitor should use electrolytic capacitor. As electrolytic capacitor mostly fails in high frequency characteristic, to meliorate this characteristic, polypropylene capacitor with lower capacity value can be paralleled with electrolytic capacitor. Below formulas show circuit gain and output impedance(Fig.2-a)

Because output (Fig.2-b) is taken out from V2’s cathode output impedance is very low. Also because the load is not taken out from V1’s anode, V1’s AC impedance is higher than that (Fig.2-a) and distortion rate is much lower than that circuit (Fig.2-a). At present this style of electric circuit is largely adopted by DIY audiophiles and manufacturer. Being a more mature circuit it can offer good performance parameter and sense of listening which has earned admiration of the majority. Regarding its working principle refer to former paragraph. Formulas about circuit gain and output impedance (Fig.2-b) :

The circuit (Fig.2-c), is a type of SRPP circuit modified by a Japanese renowned audiophile. The electric diagram itself shows that this circuit has some difference from typical SRPP circuit, having added three components, Rc, Rg.Cc. Under AC working mode the circuit’s Rk( in Fig.2-b) )became Rk?Rc//Rg(in Fig.2-c). Among them the action of Rc further increases AC load impedance of V1 and this can suppress the load line of tube Ep-lp’s characteristic curve in order to gain much better linearity and lower distortion. Experiment demonstrated that this modified SRPP circuit indeed more improved dynamic distortion and linearity than former two kinds of circuit. But in respect of dynamic performance it seems not as good as the circuit (Fig.2-b). In addition, this circuit’s distortion rate alters along the alteration of load impedance therefore at output end a constant load resistor is joined on and to get rid of affection from external load a cathode follower is added after it. Doubtlessly these raised the circuit’s complexity extent, losing a bit of laconic characteristic of SRPP circuit. With high input impedance, low output impedance, facility for matching and high conversion rate, in actual practice SRPP circuit can easily configure with other circuits. In pre-amplifier SRPP circuit is mostly designed to be the form without negative feedback. Because both tube’s u value and gain in pre-amp are invariable there are some limitations on circuit design. To design a circuit meeting requirement some factors should be synthetically considered, such as µ-value, voltage gain and local negative feedback factor. In other words It should provide not only well behavior parameter but satisfying sense of listening. When it is used in input stage of power amplifier tubes, to ensure overall system to have enough up-dynamic margin we should make much of choosing tubes with wider output range. Some other, as SRPP circuit has a higher rising speed and open-loop frequency response square-wave overshoot will appear when large-loop negative feedback exerts on overall system. Thus fitting value of capacitor should be put in to keep frequency response curve of overall system plane.

Regarding the study of SRPP, it was reported that an American, Chris Panl, had ever done a mathematical simulation manipulation about this circuit and considered the circuit has a big shortcoming that its PSRR (Power Supply Rejection Rate) is quite low. For this reason Japanese designer designed another improved SRPP circuit which also had been done about by Chris Panl. He called the improved one as Mu-follower. In view of second-hand data that author myself has ever read, to remark more about it is harebrained.

Running to these days, as the use of audio circuit, the development of transistor almost closes to extremity on aspect of circuit construction and sound reproduce if the component itself develops without crucial breakout. Turning back to look, people begin to re-know vintage tube and find it has much more advantage on linearity and comprehensive sense of listening than transistor. Between tow transistor and tube amplifier giving analogous sound quality transistor one can be more complicated at several times, essentially due to the natural defect of transistor characteristic. These give us a enlightening that the improvement to electronic component must bring audio industry revolution!

There are many factors which can affect sound reproduction, but once a circuit construction is selected a style of amplifier will bring about. Though you do all your skill to modify circuit data, replace different parts and condition construction, the formed genius will not be changed , you may only improve or adjust a little on sound color. The sound features of SRPP circuit are clear, clarified, sufficient airy sensation of space and large dynamic, and also better sound localization, width and depth on sound field. Unlike some amps giving you a lifeless impression, SRPP’s sound can be relaxed and light-free. Another main feature is that with laconic circuit SRPP circuit can provide uncommon sound.

(Published by CHINA <HIFIAUDIO>Magazine1/1995)