The ESP based DIY preamp Part 2.

The boards from ESP are a great start, but far from what is needed to make up a complete preamp. I was planning to etch the rest of the boards myself, which led to some constraints. Through hole components (which is great when soldering yourself both for construction and any repairs in the future) and single layer boards to keep it simple.

I use a free copy of DipTrace to design my PCBs in combination with a DIY UV exposure box made out of an old scanner and a bug zapper, it works great. I also use a free copy of TINA-TI to simulate my designs if needed (its like a virtual breadboard with an endless supply of components).

I already made the decision to utilize micro relays as switches whenever possible. The first thing I needed was a power supply with an aux relay for the 12V I was planning to use to power all other relays. I was seeking inspiration from the Mark Hennessy's preamp, especially the power supplies. I wanted to try out the low-dropout LM2940 12V regulator and came up with something like this (click images for higher resolution).

It would prove problematic since the LM2940 was oscillating like crazy. At first I could not understand why, but I later realized that I missed to read the fine print about Output Capacitor ESR Limits in the LM2940 datasheet! I finally replaced the LM2940 with a bog standard 7812 using the same PCB and it worked perfectly with a +/- 12V AC (safety) transformer.

Next up was a board for switching between inputs and a muting relay for the unbalanced preamp output.

The second input is for a CD player and got trim pots for level matching with the first input that will be used with a turntable. The other two inputs are for A/V and AUX and are assumed to have their own attenuators. S1 is for switching between inputs with a 4-way rotary switch and S2 to operate the preamp output muting relay (K5).

The next board I designed was an input buffered cross-feed front-end of the headphone amp. I used Dr. Meiers extended cross-feed as my filter and the board is supposed to be connected permanently (but can be bypassed or  left out if cross-feed is not wanted).

The board also holds the muting relay for the phones output (K1) operated by S1. I had one problem with this board when the output transistors on my P113 board initially was running scorching hot. I first could not understand why until I realized that I accidentally swapped J5/J7 and J6/J8. It bypassed the resistors R9/R10 to the ground and I am using the P113 with a minimum of output resistance (using output resistors inline on the output jack instead). Lesson learned, always check wiring before powering.

I also needed a board to hook up balance and volume controls. This board was made with my initial assumption that the NOOLOM filter would go after the first stage of P88, before this board and J4/J6 and J3/J5 was supposed to be connected to a second stage of P88 and the cross-feed board. This would not work, but more about that later. The board was also used for the mono switch (relay K1).

The NOOLOM board might need some explanations. First of all the name NOOLOM, it's an acronym for the title of a Swedish article by Ingvar Öhman named "Några Ord Om Ljust Och Mörkt". It would translate into something like "a couple of words about light and dark" in English. It outlines a couple of root causes for recording imperfections and presents a circuit with three different filters in order to compensate for these imperfections. The filters might be called something like "minor bass lift", "major bass lift" and "treble antiresonance" in English. The two different bass lifts can be used together for additional amplification allowing for nine different combinations (all about being one of them).

When to use the different filters are explained in the article, but it's quite obvious in real life. The major advantage over regular tone controls are a few good options over unlimited fiddling not knowing exactly when and where to stop. It is also easy to get perfect channel separation compared to tone controls based on regular potentiometers. It is built to counter common recording imperfections not for room correction. That might be seen as a drawback to some.

The circuit is quite old and based around two TL074 op-amps. I wanted to make a new version of the circuit based around, the more modern op-amps and with relays instead of off the board switches. This is what I ended up with.

J7 is for minor bass lift, J8 for major bass lift and J9 for antiresonance. I used the option of 12.1 kHz for antiresonance as suggested by Ingvar Öhman in the article. I had no previous experience with the NOOLOM circuit so I followed the original design without thinking too much. I wish I had designed it with more flexible and robust inputs and outputs, but I was planning to stick it between two other op-amps and I didn't think too much about it. That was a major slip-up. I stuck it after the P88 input buffer before the balance control as suggested in the P88 assembly instructions. I was a bit surprised when both channels where somehow shorted to ground with balance control in full left or right. It took a while to realize that  if I put the balance control R5L/R5R on the P88 board is supposed to work together with the 10k potentiometer used for balance control. I totally bypasses it when I inserted the NOOLOM circuit in-between. I could have designed it for it, but I hadn't. I was planning to feed the P88 output buffer and headphone cross-feed after the volume control but I really had to move the NOOLOM board somewhere else.

The real solution would have been to redesign the NOOLOM board, but I had already built it. The quick fix was to stick the balance control after the P88 input buffer, move the NOOLOM board after the volume control and split the signal to the P88 output buffer and the cross-feed board after the NOOLOM board. I had to change the input resistance from 10k to 100k on the NOOLOM board. I used 100k on the cross-feed board and the P88 output buffer because I was planning to replace the fake log potentiometer with a true log stepped attenuator. More about that later.

I made one final board. Nothing fancy, just a board to use as a junction for all power feeds and as a star ground of sorts. There are a lot of boards to power and all the relays with their own ground. Note that P05C and P05-Mini must share a common ground since they share the same transformer.

J1 to J6 is powered by P05C and J7 and J8 by P05-Mini. J12 to J15 is used to power the relays and J9 to J11 is used for the 20V unregulated aux supply. J17 is used as a general point for the signal ground.

There is a huge potential for improvement of the design for all of these boards, especially the ground plane. But I didn't even know if it would work out so this first build was something like a prototype. There are pros and cons with a modular design like this. A lot of boards and interconnects. It is far from perfect, but great when trying things out. Each board can be tested separately and it is also good if I ever want to redesign and reuse just a couple of boards in another configuration.

This marks the end of this first post about my ESP based DIY preamp. The next post will be about the construction of the preamp.