RIAA Preamp Project

This RIAA phono preamplifier project has Baxandall bass and treble controls and sounds great when connected to a record player. This is one of my best RIAA equalisation circuit designs that will sound amazing if you build it. This is a multipart article where the previous parts show how to design and build the circuits involved in this project. It shows a Baxandall tone and bass control circuit, and a split-rail power supply for the operational amplifiers. This project started because I wrote a formula for the RIAA equalisation network within the feedback loop and I wanted to see if the mathematics behind it pans out. The circuit works very well and produces a wonderful treble making your records sound almost new! The sound quality is vibrant, crisp, clear and unique to this design. As you can see, I have also added pre and post filters to further fine tune the RIAA equalisation curve. The mathematics for this is beyond the scope of this article; however, it takes a roll of wallpaper to write it in entirety. I wrote some of the proofs on my bedroom mirror, which I will transfer, to computer when I have the time, otherwise, I will likely wipe it all off!

I was looking for a metal box to house the circuits and could not believe how expensive "project boxes" are to purchase these days. After an hour of trawling the internet, the doorbell rang and it was My Friend Father Clarke... He had a stack of broken Sky+ boxes he was throwing away and he wanted to see if I would be interested in them, otherwise they were heading for the recycle centre. Of course, I jumped at the chance because I was interested in the metal box. This Sky+ box is the Thomson one, which has a metal chassis. The good thing about this one was that the front metal panel is removable, and you can re-attach the plastic front, which already has plenty of holes for the bass and treble potentiometers.

Before dismantling the box, I had a go at some "educational experimentation" and managed to extract the firmware from the flash ROM. I also managed to change the tuner chip so it could receive digital terrestrial reception. It is not much use to me since I do not watch television, but figured there must be an article in this that someone might be interested in. Still I am not sure it is publishable, since the firmware will likely be copyright protected.

Regrets / Modifications

I did the best with what little I had using some recycled components, and some bought components on eBay.

Ideally, you need to add some decoupling on the power planes, which I totally forgot about, but you can always add them on the soldering side directly on the IC socket pins. These should be ceramic disc types.

Once the circuits are totally to your satisfaction you should varnish the copper trackside so it does not oxide and perish. This way, your pride and joy will be around for all time to enjoy!

I did not add any power LEDs; however, you can easily add your own embellishments. This Sky+ box has a circular LED assembly, and in the future, I might use that. If you use a decade counter IC to drive it, you can make a circular animation display, which would be fitting for this project.

Conclusions: This circuit works extremely well and I had no problems constructing it.

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