The ESP based DIY preamp Part 1.

I love restoring vintage HiFi and I got several preamps and integrated that I used to swap between. Each of them got their own pros and cons, but no one is perfect from a functional point of view. There is always something from one of them, I would like to have on the others and so on. I have therefor been playing with the idea of building my own preamp with exactly the functions I want, no more no less.

Having a lot of options is not always a good thing. Take tone controls, for example, endless opportunities to get things wrong since of all possible settings only one is the right one and one that keeps on changing with source and material. My first design goal was therefore to add only the functions I need and prioritize a few obvious and usable settings over great flexibility with a majority of wrong settings.

I mostly listen to vinyl and Spotify, these days. An input for CD with the opportunity to attenuate the input signal to level it with the phono input would be nice if I ever decide to add a CD player. A phono input for MM is a must but I can do without MC support because I mostly use MM cartridges. Another A/V input for the DAC streaming Spotify and sound from an Apple TV would be enough, but I will add an extra AUX input just in case. I don't need tape input and output, and I will start out with unbalanced outputs, but leave some space for balanced if I ever need them.

Volume control is a given, but I would also like balance control and a mono switch (especially for vinyl). Rumble filter on the phono input and a NOOLOM filter instead of tone and loudness control. More about the NOOLOM filter later.

I often listen through headphones so it would be nice with a high quality headphone amp that could drive even the most demanding cans integrated into the preamp. I would like to reuse volume and balance control, mono switch and filter for the headphone output.

I personally find it much easier to design and build a power amp than a preamp full of different functions. I therefor turned to Elliott Sound Products (ESP) for as many ready made PCBs as possible. I really like all of Rod Elliotts projects, great design, lot of options, verified and well documented. What else can you ask for?

I wanted to use P88 as the core of the preamp. It offers great flexibility to configure gain, etc. P06 as RIAA preamp and P99 as a rumble filter was an easy pick. I also picked P113 as headphone amp and P05C as power supply. I added another P05-Mini as a separate power supply for the headphone amp. A bit overkill maybe, but better safe than sorry.

Thus far a set of ready made PCBs from ESP but I would need additional boards to be able to integrate them and additional functions like NOOLOM into the preamp I envisioned.

Next step was to draw a block diagram in order to figure out what additional PCBs I needed to design and etch? The diagram finally turned out like the one below (click images for higher resolution).

I also made a decision to use micro relays on the PCBs instead of signal wires to switches on the front panel. The P05C got an an auxiliary supply complete with a 'loss of AC' detector for a muting circuit. It is designed to drive one muting relay, but I would like to use it for more than one. I also wanted a separate power supply and ground for all relays. So I came up with the following concept for powering the circuits.

The plan was to use a toroidal transformer shared between the power supplies for the preamp and the headphone amp. The power supply for the preamp has the auxiliary supply that will feed a common aux relay switching power from a separate 12V aux transformer and power supply powering all other relays. If the main auxiliary supply fails the aux relay will cut the power for all other relays muting all outputs. The auxiliary supply has a delay at power on that will make sure the selected output stays muted for a while like when P05C is used with one muting relay as intended.

I also made a mock-up of what I would like the front panel to look like.

This marks the end of this first post about my ESP based DIY preamp. The next post will be about the additional PCBs needed for this build.