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AUDIO OUT AUDIO OUT L R By Jake Rothman Back to the buffers – Part 3 F ollowing the stripboard *Add these parts for 48V single-rail operation R4* 33kΩ 0V VIN R2 620Ω 0.92mA C4 1nF 4.4V R3* 47kΩ C1 470nF R1 100kΩ R7 4.7kΩ R8 390Ω TR2 BC556B ZD1 3.7V 3.9V R9 15Ω + C6 2.2µF 50V TR1 BC550C 44V TR3 MPSA29 1.13V R10 560Ω 0.58V 4.8mA C8 100µF 25V R11 47Ω 10.4mA 0V 47µF 35V 0V R5 10kΩ –0.8V C2 100pF +25V + C9 R14 100kΩ R6 47kΩ C5 + 10µF 10V VO LED1 Red high-efficiency 1.7V R13 47Ω –25V C10 47µF 35V 0V + C3* + 2.2µF 25V R12 47Ω 16.2mA + designs for a discrete buffer in last month’s Audio Out I have produced a PCB version; this board will be available from the PE PCB Service from next month. All the versions of the circuit given in the last issue can be built on this PCB, but it is most likely to be used for the high-voltage Darlington version given originally in Fig.14 (shown again here in Fig.24 in case you missed it). Unlike the discrete op amp PCB, the components are more spaced-out, reflecting the simplicity of the circuit. This helps with experimentation, allows for big audiophile capacitors and is shown in Fig.25. The single-rail version is shown in Fig.26. I’ve also had a few more useful buffering circuit design ideas, which will be the topic of this and next month’s pieces. JFET input buffer I’m sorry, but I just can’t leave anything Fig.24. Buffer circuit repeated from Fig.14 in Part Two (PE, March 2024). alone! A new version of the JFET buffer circuit (originally Fig.22) is shown 0.3% at 6Vrms). Playing with this design also removed the anti-spike components in Fig.27. This ‘re-optimisation’ often reminded me that one of the best things R9 and ZD1. This was done because the happens when I receive a PCB because about discrete circuits is the way the later stages in most audio systems clip it’s easier to change parts and test comresistor values can be optimised for any first. To run the buffer at ±25V, (matchpared to original prototype stripboard given operating condition. ing the discrete op amp power rail) the versions. I found I got better results with I also increased the rails to ±18V, the decoupling resistors R12 and R13 can be the Fairchild/OnSemi J113 JFET than maximum allowable, since the JFET has increased to 470Ω. the BF244A. They are cheaper as well, a voltage limit of 35V Vds (drain-source I was surprised to see noise curves for around 13p each (when buying 100 from the J111 to J113 JFETs on the OnSemi voltage). Of course, I then had to change Mouser). The distortion at 1Vrms into data sheet, I always thought these devices all the resistor values. Note that some were intended for switching rather than components are omitted, such as the 600Ω was 0.0008% with R8 changed audio designs. At an operating current of single-rail biasing parts, R3, R4 and C3. I to 300Ω. (At 4Vrms it was 0.006%, and Fig.25. The assembled discrete buffer amplifier. 62 Fig.26. Single-rail version of discrete buffer amplifier. Extra biasing parts: R3, R4, C3 and SGL supply link installed. C10 is omitted. Practical Electronics | April | 2024 +17.8V C4 270pF 3V 1.4mA VIN R2 1kΩ C1 100nF TR1 J113 R7 2.2kΩ R8 300Ω TR2 BC556B BC143* Ib R1 1MΩ C2 47pF 0V J113 Top view Interchangeable symmetric JFET D S G 2.4V R5 6.8kΩ C6 + 22µF 35V 31.4V IC 8mA 4.8mA +0.8V TR3 BC546B BC141* 0V *TO5 transistor for higher IC R10* 100Ω +25V + C9 0.95V 9.5mA 100µF 25V 0V C7 220nF VO R14 100kΩ R6 2.2kΩ LED1 1.8V Orange Low-bri 1.8V –17.8V 15.3mA *Sets IC C8 100µF 25V R11 33Ω + IS R12 470Ω 15.3mA R13 470Ω Fig.28. JFET buffer amplifier PCB. Note the cross-legged mounting of JFET TR1 and that the output transistors TR2 and TR3 have been upgraded to TO5 devices. –25V C10 100µF 25V 0V + Fig.27. JFET buffer circuit – the supply rails are ±18V and resistors R12 and R13 allow it to be used on ±25V supplies. (Overlay diagram will be provided next month.) 1.35mA, the noise voltage of the JFET is LED1 orange 3mm (or similar with a Vf 2.5nV/√Hz, better than the NE5534 and of 1.8V at 5mA) equal to most expensive audio JFET op ZD1 omitted amps. By increasing the current up to 10mA, the noise can be reduced to just Capacitors 1.3nV/√Hz, not bad at all. C1 100nF 5mm polyester 10% C2 47pF 2.5mm ceramic 10% NP0 C3 omitted JFET buffer assembly C4 270pF 2.5mm ceramic 10% NP0 The JFET buffer assembled on the PCB C5 10µF 10V 2.5mm pitch radial is shown in Fig.28. Note the link for R9 electrolytic and the omitted components. Sadly, the C6 22µF 35V 2.5mm pitch radial pinout of the J113 JFET was different, electrolytic not centre-gate, so a ‘leg wiggle’ was C7 220nF 2.5mm ceramic 20% X7R needed. Note also that the board can C8 100µF 35V 5mm pitch radial accommodate TO5-cased transistors for electrolytic, non-polarised preTR2 and TR3, as shown. This allows a ferred. Nichicon UEP1E101MPD higher output stage current if desired. from Mouser. It’s also much easier to fit heatsink clips C9, C10 100µF 25V 3mm pitch two off if needed. JFET buffer component list Semiconductors TR1 J113 N-channel JFET TR2 BC556 PNP small-signal bipolar TR3 BC546 NPN small-signal bipolar High impedance does not load volume control VIN Resistors R1 1MΩ R2 1kΩ R3, R4 not used R5 6.8kΩ R6, R7 2.2kΩ Low input impedance that also varies with frequency and control setting Baxandall tone control Buffer VO CW Balanced input 3 1 Now that we have a good discrete op amp and buffer circuit, we can start combining them into useful audio systems. The classic use of a buffer in Hi-Fi pre-amplifiers is to isolate the low and changing input impedance of a Baxandall tone control from the volume control, as shown in Fig.29. I mentioned in the last issue that another pro audio use is to buffer the inputs of a differential op amp circuit to provide equal high input impedance for good CMRR. This is shown in Fig.30. The op amp resistors have to be kept low for low noise, resulting in a difficult-to-drive low input impedance that needs buffering. Another problem is that the input resistances of the differential op amp circuit are unequal if they are the same value resistors on both inputs. I changed the values to equalise this, giving equal loading to the buffers. This ensures the even-order distortion harmonics produced by the buffers are equal. These harmonics cancel out in 0V Buffer Treble Bass Tone controls Fig.29. A common use of a buffer in audio is driving a filter network such as a Baxandall tone control. Filters need to be driven from a low impedance and log volume potentiometers need to be loaded by a high impedance. Practical Electronics | April | 2024 R is a low value for low noise ≤1kΩ R R1 = R2 = high R2 input impedance, typically 10kΩ to 1MΩ R R – Low impedance Unequal impedance Low output impedance 0V Volume control typically 10-50kΩ Balanced buffer Buffer – + R1 2 0V R8 300Ω R9 link R10 100Ω R11 33Ω R12, R13 470Ω R14 100Ω + VO Differential op amp circuit R 0V 0V Fig.30. Instrumentation amplifier arrangement of a buffered differential amplifier. This gives inputs that are high impedance and equal impedance. The buffers also allow the resistors to have low values, minimising noise. 63 Negative resistance? While making these changes I was surprised to find that the input resistance on the inverting input of the op amp was less than the input resistor of 1kΩ. How could this be? The answer was that the same voltage present on the non-inverting pin of the op amp was also present on the inverting pin, which is always the case with linear op amp circuits with negative feedback. This voltage is 180° out of phase compared to the inverting input terminal voltage. This increases the overall voltage across the resistor, giving it effectively a lower value (the opposite of bootstrapping) as shown in Fig.32. I couldn’t find this effect in differential amplifiers mentioned in any textbook, but like many odd things in analogue design, it’s still just Ohm’s law. Fig.31. Distortion curve of differential amplifier with two JFET buffers on the inputs. Input 1Vrms output 3.2Vrms (9.1Vpk-pk) into 600Ω. Voltage across this resistor is 1.5Vpk-pk Hence, I = 1.5mA Resistance for 1.5mA with 1V = 667Ω not 1kΩ 1Vpk-pk ZIN = 667Ω VIN– VIN+ 1Vpk-pk ZIN = 2kΩ Balanced input 3 1kΩ 1kΩ 1kΩ 0.5Vpk-pk – 0.5Vpk-pk + 1kΩ 1 VO Op amp wired as differential amplifier 0V 0V – + 2 JFET buffer ZIN = 667Ω 3.3kΩ R20 1kΩ – DC-coupled outputs R19 130Ω JFET buffer ZIN = 650Ω (with CMRR preset in middle position) In R8 position VO + 470Ω 100Ω Discrete op amp PCB Offset trim CMRR trim 0V Fig.32. There is an interesting effect related to the input impedance of the differential amplifier; both input signals are equal and anti-phase. The 1kΩ resistor on the inverting input is reduced to an effective value of 667Ω or 2/3 of its value. This assumes all differeerntial amplifer resistors are equal. Fig.33. In comparison with the circuit in Fig.32, the actual resistor values used take into account the resistance-reduction effect mentioned in the text. There is also a gain of 3.2, and the input impedances have been equalised. Note the preset for CMRR adjustment and the resistor values are low for noise minimisation. the differential op amp. I also decided to add a CMRR trimmer (PR3) in the network. Looking at the final distortion curves for the whole system in Fig.31, I think all this fiddly tweaking and experimenting was worth it. The cancellation effect achieved was 8dB reduction on the second harmonic. DC coupling The buffers can be DC coupled to the op amp inputs if desired since their +0.81V offset can be easily be rejected by the differential op amp. The offset adjust preset PR1 will need to be tweaked to bring the output of the whole system to zero. To do this, omit the bipolar DC blocking capacitor C8 from both buffer boards. By omitting these coupling capacitors, the CMRR will be maintained at low frequencies since electrolytics have poor tolerances, resulting in possible mismatching. However, there is the safety issue of a hard DC offset being transmitted to the output if there is a wiring error or fault in one of the buffers. This could then damage what is connected to the output. Of course, the output could be AC coupled, or have a DC detection circuit and muting relay put in. The final circuit is shown in Fig.33. The assembly of the three PCBs is shown in Fig.34. They will go in a nice box one day to make a top-notch balanced headphone amp. Next month Fig.34. Connecting two buffer boards with a differential op amp to make an instrumentation or headphone balanced input amplifier. I hope to tidy this up and build it into a decent enclosure. 64 We will conclude our discrete buffer journey next month by looking at an op amp buffer circuit and an interesting design called the ‘diamond buffer’. Practical Electronics | April | 2024