The Ultimate Integrate Part 1.

I jumped on the DSP bandwagon as part of my 3-Way Retro build. I only planned to use it as an active XO and for some room correction. But working with the DSP and SigmaStudio left me with the feeling that it was somewhat under utilised in that role. I am very satisfied with my current setup of DIY analog pre- and power amps. Inserting a DSP in-between the two is far from optimal. An extra A/D to D/A conversion and hard to match gain structure to mention a few of the issues. I played with the idea of building a preamplifier built around the DSP. It would be easy to recreate the NOOLOM filters with a DSP and the digital volume control would track even better than my stepped attenuator. But I really didn't find much added value in recreating my analog PreFUNK with a DSP core.

That was my initial thought but it only took one idea to make a huge difference. I had used the DSP to correct my speakers for some defects in the response right? My ears are somewhat damaged so I have to correct them with hearing aids. My DSP got four output channels and this is where I started to see a totally new opportunity. Hearing aids are great but far from optimal for music listening. There is no way such smal devices can come close to the comparable huge membranes of my beloved AKG K550 headphones. But the hearing aids got DSP built in to compensate for my hearing deficits, something my AKG K550 does not. What if I would use two of the output channels to feed the speakers and the other two to feed a headphone amplifier? Would I be able to use the DSP to EQ my AKG K550 to a response curve countering my hearing deficits? I just had to try it out so I bough another DSP to experiment. I made my initial tests over Christmas and I was totally blown away. I used the audiogram I got from my audionome to recreate PEQ filters in SigmaStudio to counter my hearing deficits on my left and right ear. I then connected a switch through GPIO so I could toggle these PEQ filters on and off while listening. The difference was like night and day. I felt like I was 20 again listening with the filters on. I could fully enjoy music for the first time in years. This was truly a game changer for me so I decided to go forward and design an integrated amplifier around the DSP. Below is a logical block diagram of what I wanted to achieve.

I am going to reuse must of the components from previous builds. The RIAA amplifier and headphone amplifier will be Rod Elliotts P06 and P113. Both are solid designs and great performers. They will fit this build perfectly. I will use the DSP and TPA3255 from 3e Audio. The DSP is super versatile and the TPA3255 got such a clean sound. I will build a new source selector because I would like to replace the rotary switch as a selector with radio buttons, more about it later.

Next up, the power of a DSP. I can implement a lot of functionality in software instead of analog filters as add-on boards. This is what I plan to use the DSP for. A mono switch (great for the ones like me still listening to vinyls). Separate EQ sections for the speakers (filter and room correction) and the headphones (hearing deficit correction). A shared loudness function (I don't know how useful it will be with headphones) and volume control (perfect multichannel tracking with only one potentiometer) and cross-feed for the headphones (I personally like it).

The power section is quite straight forward. I have previously used a SMPS600RS to power my TriPath and Class-D amplifiers. It got clean power on the secondary side but it is injecting quite some noise back on the mains. I have always wanted to try a traditional unregulated PSU with a Class-D amplifier and now is the time. So I gutted an old Sony amplifier of some useful parts. The main transformer is a Sony Audio E-core +38-0-38V. It is center tapped so I will build a board to rectify it to 38V, perfect for the TPA3255.

I am planning to use a small toriodal transformer and Rod Elliotts P05-Mini as the +15-0-15V PSU feeding the RIAA and headphone amplifiers. I will build another 12V PSU to drive relays and CMOS logics.

The final question, what should it look like? I want to give this amp a proper enclosure. All metal with a brushed aluminium front panel. I want to keep it clean with sane defaults and a minimum of controls. It would leave the faceplate quite empty. Leave it like that or fill it with something? Big ass VU-meters? ;)

Concept has been bench tested and works better than I thought. Most of the needed boards are already built. VU-meters has been fabricated. Enclosure has been ordered but it seems to take time for them to deliver. CAD drawings for the face plate is done. I have managed to get a lot done since Christmas but it feels like I still have a long way to go...