A Basic Audio Workbench

To work on audio electronics, you need some test equipment. In this article I discuss what you need and how much you need to spend on it.

My old (about 2015) workbench for audio testing – working here on a Soundcraft mixer module. Quite inexpensive gear, but it worked well.

Old electronic test equipment can now be bought at a very sharp price. Gear which would have cost thousands 15 years ago, is now available for often less than 100€. This is a real bargain – the quality of equipment made by companies like Hewlett Packard and Tektronix back in the 70s and 80s was very high indeed, and it can be more robust than almost anything made today. When you buy it in good condition, it often works as well as it did when new.

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ART MPA Gold Dual Preamp Repair

A friend asked me to fix his ART MPA Gold Dual Preamp. It’s really nice ti be able to dish out a fat spoonful of audio-praise to ART, who were ultra-helpful in helping me get this repair done.

ART MPA Gold Dual Preamp

The ART MPA Gold dual preamp is a hybrid valve/transistor affair with very good specs and some nice extra features such as variable input impedance and a tunable low cut filter.

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Universal Audio 4110 and Millenia HV-3D Mic Preamp Repairs

I was recently contacted by the owner of a local studio who had two high end microphone preamps (Universal Audio 4110, and Millennia HV-3D), both of which had developed noisy channels. In both cases the channel in question was producing a constant low level crackling sound, making it unusable. Unfortunately I found both manufacturers to be less than helpful in supporting the repair.

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KEF PSW2000/2010 Active Sub Woofers

UPDATE

Because of the number of issues I have seen with KEF PSU2000 sub-woofers, I am no longer accepting repair jobs on them. The mechanical construction makes them awkward to work on, and they seem to have just too many almost insoluble problems.

However, I leave this page up as it may be of interest or useful to some.

SUMMARY

These boxes, made by British manufacturer KEF, are floor standing cubes that are about 35 cm square. They contain a single bass speaker, some preamp electronics and a power amplifier.

In this article I describe some modifications which makes them run cooler. This should help their long term reliability. Two are described : one is a redesign of the low voltage power supply. The other is a simple mechanical change which allows air to flow in and out of the enclosure that houses the electronics. I also give some measurements I made of the internal air temperature, with and without the mods.

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Dynacord “Eminent” Valve Amplifier Restoration And Conversion

Dynacord Eminent Valve Amp

The Dynacord “Eminent” is a nice old German made valve amp that I was asked to restore. It was originally designed as a small mono PA. It has two instrument and two mic inputs, simple bass and treble tone controls, and 4, 8 and 16 ohm output taps. The owner wanted to repurpose this lovely amp for recording guitar. This is an account of the work involved.

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Rebuild Of A Soundcraft CPS150 Power Supply

Soundcraft CPS150 PSU

This PSU belongs to a Soundcraft Delta 200 desk which I restored. It is pretty old. On first inspection the output voltages were correct, the only problem was that the mains transformer made an annoying buzzing sound. Looking more closely, it had other issues.

This post shows the kind of things I pay attention to when restoring or rebuilding old equipment. The devil really is in the details with this kind of work.

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8 ohm load for testing power amplifiers

This is a very simple project that takes only a few hours to make. It is a pair of 8 ohm resistive loads which can be used to test a mono or stereo power amplifier.

Dummy 8 ohm load built on an old baking tray

If you are working on an audio amp, you often need to see how it will behave when it is delivering power to a load. Using real loudspeakers is not practical, especially if you want to test at high levels and with sine waves. The noise would be unbearable, also you might damage a speaker. So you need a load to use instead of a loudspeaker.

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