Updated with new material 7th January 2019
This set of c. 50 slides is the culmination of my experience over a period of about 25 years building numerous power amplifiers and preamplifiers. I first started out in audio around 1975 or 76 a teenager. Some of my creations were reasonably quiet – through pure luck – and others hummed and hissed horribly. Later, my skills improved dramatically, and especially so after reading one of Henry Ott’s books back in about 1988/89 whilst developing a very high resolution Digital Panel Indicator for industrial applications at the company I worked for. I then left DIY audio for about 15 years (career, family etc), returning to the subject again about 15 years ago, having forgotten a lot of my practical skills. The path from electromagnetic theory expounded on numerous websites, application notes and posts on various web forums to building quiet amplifiers every time is not easy and requires a bit of practice. The underlying theory can be extremely complex, however, with some effort and focus you can quickly master the basics. This set of slides focuses on unbalanced interconnects (aka ‘single-ended’) that use the standard RCA phono connectors, since this is where problems mostly arise.
When it comes to humming and hissing amplifiers, good practical advice is scarce, though often misguided opinions on the subject seem easy to find.
This presentation (which will remain a work in progress and be expanded from time to time) is designed to get audio constructors up and running quickly both in the construction/planning phase, but also debugging. It will hopefully also serve as a useful reference for anyone wanting to know a bit more about EMC as applied to amplifiers. One important thing about EMC: you will never stop learning, and finding new problems to solve.
Here is some additional material
For some practical examples of low noise amplifiers using these techniques, see the nx-Amplifier and sx-Amplifier on www.hifisonix.com
here are some commercial products that use the techniques described in the presentation: www.ovationhifidelity.com
The picture below shows the internal wiring of one of the two nx-Amplifiers I built with zero noise or hum problems. The power wiring is tightly bundled, and small signal wiring is kept well away from the transformer and other power wiring. On the PSU PCB, strict attention to the capacitor ‘T’ connection and ‘star’ ground return result in an exceptionally quiet amplifer.
Here are some pictures dealing with input wiring (included in the updated presentation above). Common sense says that when you run a screened cable (or twisted pair) from the input connectors to the power amplifier module on your amplifier, the shortest route is best, as depicted below (you may have the right hand channel amplifier module flipped in your specific layout, but the principle still applies)
This happens to be quite wrong and the reason why is you need to be thinking about the total loop area between the central power supply and both amplifier channels – its a huge area prone to noise pickup and will ultimately cause cross channel ground loop problems.
Shown below is the correct way to do it. The total area now available for noise pickup is a fraction of previous approach. I measured this on a practical amplifier and got a 15 dB reduction in mains noise as measured at the amplifier output from c. -93 dB to close to 110 dB
Finally here is a quick tip that can help you gain an additional 8-10 dB reduction in noise (unless of course you happen to have fortuitously positioned your toroidal transformer for minimum noise). Typically, you only have to rotate the transformer about 60 degrees either way to find the null. This is also included in the updated presentation.