Many years ago, audio equipment came with a tone control, a simple RC filter that would cut or boost the bass to taste. As time passed, this was split into two controls for bass and treble, and then finally into three for bass, mid, and treble. When audiophile fashion shifted towards graphic equalisers, these tone controls were rebranded as “3-band graphic equalisers”, a misleading term if ever we heard one. [Gabriel Dantas] designed one of these circuits, and unlike the simple passive networks found on cheap music centres of old, he’s doing a proper job with active filters.

The write-up is worth a read even if you are not in the market for a fancy tone control, for the basic primer it gives on designing an audio filter. The design contains, as you might expect, a low-pass, a bandpass, and a high-pass filter. These are built around TL072 FET-input op-amps, and an LM386 output stage is added to drive headphones.

The final project is built on a home-made PCB, complete with mains power supply. Audiophiles might demand more exotic parts, but we’re guessing that even with these proletarian components it will still sound pretty good. Probably better than the headphone amplifier featured in a recent project from a Hackaday writer, at least. There’s a build video, below the break.

If you’re the sort who finds beauty in symmetry – and I’m not talking about your latest PCB layout – then you’ll appreciate this clever take on the long-tailed pair. [Kevin]’s video on this topic explores boosting common mode rejection by swapping out the old-school tail resistor for a current mirror. Yes, the humble current mirror – long underestimated in DIY analog circles – steps up here, giving his differential amplifier a much-needed backbone.

So why does this matter? Well, in Kevin’s bench tests, this hack more than doubles the common mode rejection, leaping from a decent 35 dB to a noise-crushing 93 dB. That’s not just tweaking for tweaking’s sake; that’s taking a breadboard standard and making it ready for sensitive, low-level signal work. Instead of wrestling with mismatched transistors or praying to the gods of temperature stability, he opts for a practical approach. A couple of matched NPNs, a pair of emitter resistors, and a back-of-the-envelope resistor calculation – and boom, clean differential gain without the common mode muck.

If you want the nitty-gritty details, schematics of the demo circuits are on his project GitHub. Kevin’s explanation is equal parts history lesson and practical engineering, and it’s worth the watch. Keep tinkering, and do share your thoughts on this.

Want to make your own air knife to cut things with? Unfortunately that’s not what these devices are intended for, but [This Old Tony] will show you how to make your own, while explaining what they are generally intended for.  His version deviates from the commercial version which he got his hands on in that he makes a round version instead of the straight one, but the concept is the same.

In short, an air knife is a laminar pressurized airflow device that provides a very strong and narrow air pattern, using either compressed air or that from a blower. Generally air knives will use the Coandă effect to keep the laminar flow attached to the device for as long as possible to multiply the air pressure above that from the laminar flow from the air knife itself. These are commonly used for cleaning debris and dust off surfaces in e.g. production lines.

As [Tony] shows in the disassembly of a commercial device, they are quite basic, with just two aluminium plates and a thin shim that creates the narrow opening through which the air can escape. The keyword here is ‘thin shim’, as [Tony] discovers that even a paper shim is too thick already. Amusingly, although he makes a working round air knife this way, it turns out that these are generally called an air amplifier, such as those from Exair and are often used for cooling and ventilation, with some having an adjustable opening to adjust the resulting airflow.

Some may recognize this principle for those fancy ‘bladeless’ fans that companies like Dyson sell, as they use essentially the same principle, just with a fan providing the pressure rather than a compressor.

Over the years there have been many different audio amplifier designs which have found favour for a while and then been supplanted by newer ideas. One of them has crossed the bench of [Jazzy Jane], it’s a current dumping amplifier from the mid-1980s. A nicely-done home-made project on stripboard mounted on a wooden base board, it sports a power supply, RIAA pre-amp board, and the amplifier itself.

The current dumping amplifier is one that combines a small class A amplifier with a hefty class B one, and through feedback trickery uses the combination to remove the crossover distortion of the class B stage. It’s a simple yet elegant circuit with fewer parts than an equivalent class AB amplifier, and there was a time back in the day when it was all the rage. This one has an op-amp providing the class A part and a complimentary pair of Darlington pairs as the class B.

The video below the break shows the process of bringing the amp back to life, a process mostly concerned with the power supply. There are a set of tantalum capacitors which have failed, and the replacements she’s using turn out to have problems too. They’re a period part for a project of this age, but we might have been tempted to go for another capacitor type here.

The result is an unusual amplifier, brought back to life. You may have seen [Jane] feature here before, with her 1950s signal generator.

Big-name tube amplifiers often don’t come cheap. Being the preserve of dedicated audiophiles, those delicate hi-fis put their glass components on show to tell you just how pricy they really ought to be. If you just want to dip your toe in the tube world, though, there’s a cheaper and more accessible way to get started. [Jenny List] shows us the way with her neat headphone amp build.

The build starts with an off-the-shelf preamp kit based around two common 6J1 tubes. These Chinese pentode valves come cheap and you can usually get yours hands on this kit for $10 or so. You can use the kit as-is if you just want a pre-amp, but it’s not suitable for headphone use out of the box due to its high-impedance output. That’s where [Jenny] steps in.

You can turn these kits into a pleasing headphone amp with the addition of a few choice components. As per the schematic on Github, a cheap transformer and a handful of passives will get it in the “good enough” range to work. The transformer isn’t perfect, and bass response is a compromise, but it’s a place to start your tinkering journey. Future work from [Jenny] will demonstrate using a MOSFET follower to achieve much the same result.

We’ve seen a great number of headphone amplifiers over the years, including one particularly attractive resin-encased example. Video after the break.

As electronics rely more and more on ICs, subtle details about discrete components get lost because we spend less time designing with them. For example, a relay seems like a simple component, but selecting the contact material optimally has a lot of nuance that people often forget. Another case of this is the Miller effect, explained in a recent video by the aptly named [Old Hack EE].

Put simply, the Miller effect — found in 1919 by [John Milton Miller] — is the change in input impedance of an inverting amplifier due to the gain’s effect on the parasitic capacitance between the amplifier’s input and output terminals. The parasitic capacitance acts like there is an additional capacitor in parallel with the parasitic capacitance that is equivalent to the parasitic capacitance multiplied by the gain. Since capacitors in parallel add, the equation for the Miller capacitance is C-AC where C is the parasitic capacitance, and A is the voltage gain which is always negative, so you might prefer to think of this as C+|A|C.

The example uses tubes, but you get the same effect in any inverting amplification device, even if it is solid state or an op amp circuit. He does make some assumptions about capacitance due to things like tube sockets and wiring.

The effect can be very pronounced. For example, a chart in the video shows that if you had an amplifier with gain of -60 based around a tube, a 10 kΩ input impedance could support 2.5 MHz, in theory. But in practice, the Miller effect will reduce the usable frequency to only 81.5 kHz!

The last part of the video explains why you needed compensation for old op amps, and why modern op amps have compensation capacitors internally. It also shows cases where designs depend on the Miller effect and how the cascade amplifier architecture can negate the effect entirely.

This isn’t our first look at Miller capacitance. If you look at what’s inside a tube, it is a wonder there isn’t more parasitic capacitance.