Repairing The Questionable £25,000 Tom Evans Audiophile Pre-Amp

November 14, 2024 by Maya Posch 103 Comments

One of the power supply boards in the Tom Evans Mastergroove SR MkIII preamplifier. (Credit: Mend it Mark, YouTube)

It’s not much of a secret that in the world of ‘audiophile gear’ there is a lot of snake oil and deception, including many products that are at best of questionable value. The Tom Evans Mastergroove SR mkIII preamplifier is one example of this, as [Mark] from the Mend it Mark YouTube channel found in a recent video when he got one to repair which the manufacturer claimed ‘could not be fixed’. This marvel of audio engineering provides amplification for record players, for the low-low price of only twenty-five thousand quid, or about 29.000 US bucks. So what’s inside one of these expensive marvels?

Claiming to be a high-end unit, with only ten units produced per year, you’d expect a gold-plated PCB with excellent noise isolation. The unit does come with an absolutely massive external power supply that dwarfs the preamplifier itself, but the real surprise came after opening up the unit itself to take a peek at the damage, some of which was caused by transport.

As it turns out, the inside of the preamplifier consists out of four stacks of rather cheap, home-made looking boards with what looks like improvised RF shielding in the form of bare PCBs and filed-off markings on many parts. In between the rat’s nest of wiring running everywhere, [Mark] had to trace the broken channel’s wiring, creating a full repair manual in the process. Along the way one of the opamp boards was found to be defective, courtesy of a single shorted tantalum capacitor.

With the tantalum capacitor replaced, [Mark] had repaired the unit, but even though the preamplifier isn’t terribly designed, the illusion of its price tag has been shattered worse than the contents of a parcel kicked across the parking lot by the Royal Mail.

Thanks to [Jim] for the tip.

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Low-Cost Electret Microphone Preamplifiers

October 16, 2023 by Bryan Cockfield 8 Comments

Before the invention of microelectromechanical system (MEMS) microphones, almost all microphones in cell phones and other electronics were a type of condenser microphone called the electret microphone. The fact that this type of microphone is cheap and easy enough to place into consumer electronics doesn’t mean they’re all low quality, though. Electret microphones can have a number of qualities that make them desirable for use recording speech or music, so if you have a struggling artist friend like [fvfilippetti] has who needs an inexpensive way to bring one to life, take a look at this electret microphone pre-amp.

The main goal of the project is to enhance the performance of these microphones specifically in high sound pressure level (SPL) scenarios. In these situations issues of saturation and distortion often occur. The preampl design incorporates feedback loops and an AD797 opamp to reduce distortion, increase gain, and maintain low noise levels. It also includes an output voltage limiter using diodes to protect against input overload and can adjust gain. The circuit’s topology is designed to minimize distortion, particularly in these high SPL situations.

Real-world testing of the preamp confirms its ability to handle high SPL and deliver low distortion, making it a cost-effective solution for improving the performance of electret microphones like these. If you want to go even deeper into the weeds of designing and building electret microphones and their supporting circuitry, take a look at this build which discusses some other design considerations for these types of devices.

This Tube Preamp Has A Nixie Volume Display

July 25, 2020 by Jenny List 15 Comments

The pursuit of audiophile hi-fi is one upon which many superlatives and perhaps a little too much money are lavished. But it’s also a field in which the self-builder can produce their own equipment that is as good or often better than that which can be bought, so it provides plenty of interesting projects along the way. [Justin Scott]’s tube preamplifier is a great example, with its novel use of a pair of Nixie tubes to indicate the volume to which it has been set.

The audio side of the preamp comes courtesy of a four-tube kit from tubes 4 hi-fi, in which we notice another tube as power supply rectifier. The case is a beautifully made wooden affair with a professional front panel, but it’s the Nixies which make it a bit special. A high quality motorised potentiometer is used as a volume control, one of its multiple outputs is used as a simple potential divider to provide a voltage. This is read by an Arduino, which in turn drives the Nixies via a BCD-to-decimal decoder. The attention to detail in the whole project is at a very high level, and though he’s not shred any of its audio measurements with us, we’d expect it to sound as good as it looks.

If tube amplifiers interest you, we’ve delved into their design in the past, and it’s worth directing you to Justin’s matching amplifier, as well.