Fullscreen HTML5Med Res (??MB)HTML4

AUDIO OUT AUDIO OUT L R By Jake Rothman Balance with Baxandall P eter Baxandall was possibly Britain's greatest discrete analog audio circuit designer. He combined physics, magnetics, acoustics, music and electronics into one with great creative flair. He initially worked at the Royal Signals and Radar Establishment in Malvern. The creator of stereo and the longtailed pair, Alan Blumlein, was also based there, albeit before Baxandall, so they never met. I like to imagine what they would have come up with if they had. Like Blumlein’s work, a Baxandall circuit was always original, backed up with practical research, and costefficient. He is most famous for his *Typically Sowter 10kΩ:10kΩ *Typically Sowter T1* 10kΩ:10kΩ 1:1 T1* 1:1 10kΩ balanced 10kΩ input balanced input 3 – Female 3 – + XLR socket Female 2 + XLR socket 1 2 1 Jinxed! I wanted to interview Baxandall in Malvern for Practical Electronics when Mike Kenward was editor. Sadly, Baxandall died in 1995. Then I thought I’d try to interview John Linsley-Hood in Taunton instead, and he died too. Rf 10kΩ Rf 10kΩ RIN 10kΩ RIN 10kΩ Rf115pF 27kΩ – – + + Output (Non-inverting Output (Non-inverting 2 + 2 1 1 Insert 1000µF non-polarised capacitor for low DC offset Insert 1000µF – non-polarised capacitor for low DC offset – RIN3 27kΩ Rf *600Ω 1:1 mumetal +15V 10kΩ modem transformer R *600Ω 1:1 mumetal +15V f 10kΩ transformer ½Rmodem IN 7 T1* 5kΩIN ½R 1:1 7 T1* 5kΩ – 1:1 – NE5534 NE5534 8 ½RIN + 5kΩ ½RIN + 5 8 4 5 5kΩ 22pF 4 –15V 22pF 0V –15V 0V 3 – 3 – + Output Output Differential virtual earth input Differential virtual earth input 3 3 – Input 1 2 + 2 1kΩ – ½LF353 + 1 Output RIN 10kΩ 0V Metal work Fig.2: a commercial example of a Baxandall balanced line input (not very well optimised). 40 +– Output NE5532A 0V + Output NE5532A 15pF 0V Rf215pF 27kΩ – + +– NE5532A 0V (Signal ground) + NE5532A Fig.3(a): a balanced virtual earth used in the full-spec Baxandall 0V (Signal ground) balanced input in the KEF KM15.6kΩ active monitor Inner speaker. winding has 5.6kΩ Sec lower DC resistance – Inner winding has To differential lower DC resistance virtual earth – input 3 – 180pF 5.6kΩ To differential 5.6kΩ 180pF 5-65pf + + virtual earth Input trimmer input 2 180pF 1 3 – Custom-wound 1:1 transformer 5.6kΩ 5.6kΩ + Mumetal 187 core (Belclare) + Input 25mm wide, 0.72cm² stack, twin bobbin 2 Metal work 1000 turns on each side 1 Custom-wound 1:1 transformer Metal work Mumetal 187 core (Belclare) 25mm wide, 0.72cm² stack, twin bobbin Metal work 1000 turns on each side Metal work 5.6kΩ 180pF Rf 10kΩ Termination resistor – Pri 560pF Sowter 5340 1:2 RIN3 27kΩ Rf1 27kΩ Rf2 27kΩ + Fig.1(b):Metal thework Baxandall balanced input configuration. The Metal work input resistor(s) and input transformer positions are flipped. RIN 10kΩ Normally, a differential op amp 15pF Fig.1(a): an inverting amplifier with balanced input transfomer. Input Input Transformer input Inverting op amp Inverting op amp 0V 0V Metal work Metal work Not to give up, I went to Newport Radio Rally to interview the Reverend George Dobbs, designer of the Ladybird book transistor radio, but he cancelled due to ill health and died. I then went to interview John Birkett in Lincoln, owner of the longest-lived component shop in the UK. You can guess what happened. Am I cursed? You be the judge... Anyway, back to stuff we can control. Following on from the previous investigations of obscure audio circuits, we’ll look at Baxandall’s balanced input circuit. bass-treble tone control, which has adorned almost every Hi-Fi amp since the 1950s. He won a competition with it and never got a royalty, just a $25 watch. His series on power amplifier design in Wireless World in the late 1970s inspired Douglas Self’s series in the 1990s. All engineers, like Newton, build on the shoulders of giants. 5.6kΩ 5-65pf trimmer Pri Sec Fig.3(b): the transformer stage feeding the above amplifier. Most 1:1 600Ω to 2kΩ small audio transformers will work well. Practical Electronics | August | 2025 circuit would be used for a balanced input because it is linear and low in cost. A transformer input, however, is fully isolated from Earth (floating) and has a huge common mode rejection ratio (CMRR). This means it is very useful in areas of high interference, such as broadcast studios and electric transport. The common-mode rejection ratio is the amount by which a common signal to both inputs (such as mains hum) is attenuated at the output of the circuit. So with a 100dB CMRR and a 100mV unwanted common-mode signal, only 1μV of that 100mV will make it through to the output. I remember a situation with a shipbased pirate radio station, where the RF induced a voltage in every bit of metal, and an op amp balanced input would have been saturated. The ‘oldfashioned’ transformer inputs on the desk worked fine, even with 50V of common mode signal on the positive and negative inputs. There are expensive problems with audio balancing transformers; it typically costs £12-40 for one that has good frequency response and low distortion. The good news is that with Baxandall’s circuit, a cheap transformer can give excellent results. circuit improvements can simply be to swap over the positions of two components. This was the key to the success of Baxandall’s design, which was based on an inverting op amp circuit. Normally, the input balancing transformer feeds the input resistor RIN, as in Fig.1(a). Having the transformer feed the inverting input directly and putting the input resistor first, as in Fig.1(b), makes the transformer more linear. This gives a much better frequency response and lower distortion. This is because the inverting input is a virtual ground, meaning there is almost no voltage across the secondary. Since a transformer reflects what is happening onto the other side, there is no voltage across the primary either; all the input voltage is dropped across RIN. This setup makes the transformer operate in current mode, and because this current is set by the input resistor, which is inherently linear, the currents in the transformer windings are forced to be linear. With a transformer operating in normal voltage mode, saturation and hysteresis effects are much more pronounced. Inverting the inverted Practical circuits Sometimes one of the simplest To optimise the CMRR, RIN is split Photo 1: a typical expensive input transformer in a Mumetal can. into two individual resistors, which positions the transformer’s capacitance symmetrically, helping the balance at high frequencies. The first example of Baxandall's circuit I saw was in a line input for a Sonifex call box used in broadcast systems, shown in Fig.2. I had to service one in the early eighties. This circuit strangely used an expensive Sowter transformer (similar to the one in Photo 1) with a noisy JFET op amp. It also had an odd termination resistor, RT, to ensure stability, which increased the noise gain. It would have worked better if it had had a small capacitor in series with RT, making it into a Zobel network. The circuit reached a new level of sophistication in the input stage for KEF’s KM1 active studio monitor, for which Baxandall designed the electronics. It was made for the BBC and Abbey Road studios. A differential virtual earth amplifier was used, as in Fig.3(a), which gave another layer of common-mode signal rejection. The transformer input stage shown in Fig.3(b) also split the input resistors to include an HF trimming network. A big electrolytic capacitor was included to minimise the DC offset, which was otherwise around 300mV because of the high input bias currents of the NE5532 op amps. This caused a low-frequency (LF) hump of 5dB at 0.8Hz by resonating with the secondary inductance. This could be fixed by putting in an LF step network in series with the capacitor, or with the offset removed entirely using a servo. There is another variation of the circuit (by Barry Blesser, on p297 of the Journal of the Audio Engineering Society, May 1972) where the feedback resistor is fed into a third winding to cancel the flux, which shrinks the transformer even more. Winding your own Baxandall was highly adept at designing signal transformers, a lost skill today. He used to give full winding details for many of his transformerbased designs, such as his Wireless World March 1957 Inexpensive HighQuality Amplifier. This input stage was no exception, and the details are given in Fig.3(b). Since the primary has series resistors, it can be wound with very thin wire. The secondary should be wound with thicker wire on the inside winding for lower resistance, giving lower noise. Sadly, one can’t get Belclere transformer kits any more, which included the laminations and bobbin. But I have found there are Mumetal 600Ω 1:1 modem isolation transformers available inexpensively on the surplus Practical Electronics | August | 2025 41 Rf 10kΩ AC negative feedback Rf 10kΩ 470kΩ AC negative feedback DC negative feedback 1:1 600Ω modem transformer CIN 2 1:1 600Ω R RIN IN modem 1.1kΩ 1.1kΩ transformer 3 1 4 2 + 2 1 RIN 1.1kΩ 3 – Input Metal work + 2 1 3 0V + C VBiasIN V+ input Cascode 8Ω 2200µF – power amplifier + 3 – Input V+ 470kΩ 2200µF DC negative feedback – 4 + RIN 1.1kΩ (see last month) + VBias 1 Zobel network Cascode input power amplifier (see last month) 8Ω 0V Fig.4(a): two weird circuits joined into one. Adding the Baxandall balanced input 0V 0V Metal work to last months complementary cascode amplifier. Unfortunately, the inductance of 10kΩ Zobel network the transformer resonates with the capacitor CIN and the AC feedback path. V+ – 2kΩ 3 – Input 1 + 2 3 – Input Metal work + 2 1 2kΩ 2kΩ VBias + V+ 0V – VBias Output 15Ω capacitors e.g. LS3/5A omitted 0V V– 15Ω e.g. LS3/5A 10kΩ + +23V – Metal work +23V 10kΩ – VBias Output capacitors omitted + VBias 2kΩ +23V 10kΩ V– 0V + +23V 0V Fig.4(b): using the Baxandall circuit with two complementary cascode amplifiers to make a more powerful bridge set-up. The distortion should, in theory, be lower than one amplifier. market that do the job (see Photo 2). I bought 200 of them just so you can have some as well! Note that the inner winding is pins 1 and 2. Ferrite pot cores are not normally used for audio signals, but the Baxandall circuit compensates well for their shortcomings. If I was designing a balanced input for, say, a railway public address system, where maximum fidelity was less important, I would use them. They are wellshielded with the winding enclosed, and don’t need a screening can like laminated transformers. It is well worth reading Baxandall’s chapter, Microphone amplifiers and transformers, in the Microphone Engineering Handbook by Michael Gayford, published by Focal Press. Performance Using the circuit shown in Fig.1(b) with Rf = RIN =10kΩ, using a NE5534 op amp and the modem transformer at 0dB (line level), the total harmonic distortion plus noise (THD+N) measured 0.0018% between 100Hz and 20kHz. It rose gently to 0.028% at 20Hz and then was flat down to 5Hz. Using the transformer in the conventional way gave over 5% THD+N at 20Hz due to saturation, falling to 0.01% at 1kHz. The Baxandall circuit basically reduced the core flux density to a minimum at LF. The >100Hz THD+N reduced to 0.001% when the resistors were reduced to 5.1kΩ because of the lower Johnson (thermal) noise – see Fig.5. Note that to ensure HF stability, Rf needs a 47pF phase-lead capacitor across it, and the NE5534 needs a 22pF compensation capacitor between pins 5 and 8. Putting it all together Photo 2: this modem phone-line isolation transformer (25D8310MO1) is ideal for the Baxandall circuit. It is unscreened and must be kept away from hum sources, like mains transformers. 42 The complementary cascode power amp described last month, being an inverting configuration, could combine nicely with the Baxandall balanced input, as shown in Fig.4(a). There is an interaction between the transformer’s inductance and the loudspeaker output and input capacitor (Cin), causing a hump in the LF response that needs resolving, though. Another interesting project would be to make a bridged version to more than double the power output, like in Fig.4(b), using the balanced drive from the transformer. Since the complementary cascode amplifier is asymmetrical, the even harmonic distortion components should cancel out, dropping the THD. In theory, the output capacitors could be left off (avoiding the LF hump Practical Electronics | August | 2025 problem) since both output pins are sitting at the same voltage, but I would be worried about transient DC currents flowing through the speaker. There’s only one way to find out, and that’s to build it. I’ll do that as soon as I have some proper PCB designs ready to go. Analog audio design is like cooking; it’s how you combine the elements and their interactions that matter. Very few circuits are truly new; just different combinations. Total Harmonic Distortion (%) 0.5 0.2 0.1 0.05 0.02 0.01 .005 .002 .001 .0005 .0002 .0001 Minor erratum 20 50 100 200 500 1k Frequency (Hz) 2k 5k 10k 20k Fig.5: the Baxandall balanced input distortion plot with a modem transformer (768 25D8310MO1) and NE5534 op amp at full output (27V peak-to-peak into 600Ω). In last month’s article, I mentioned that the Bailey and Quad 303 amplifiers used rare Motorola transistors that had minimal Early effect. I should have written that they were RCA transistors PE instead. Techno Talk puzzle This question was posed on the bottom of page 5, asking readers to spot the difference between two images of circuit boards, one a reference and the other a productiton part, which was used as a test for an image difference analysis system. The system detected an extra part, which was added as a test (upper red box). It also detected foreign object debris, which could become a problem at some point in the future (lower red box). FIND ALL YOUR ELECTRONIC COMPONENTS IN ONE PLACE BASIC MICRO E L E CT R O N I C S C O M P O N E N T S U P P L I E R w w w . basicmicro . co . u k High-quality, genuine parts Practical Electronics | August | 2025 43