Schematics!

So after a long time, the design was set and the schematics completed. All the work was done in Cadsoft's Eagle, and uses a lot of custom library components just to make sure the PCBs don't have any mistakes.

The unit follows the above diagram, with all inputs being THAT 1200 series ICs and most of the outputs being THAT 1646 ICs. As the majority of the unit is designed to be clean and quiet, these seemed the best option without taking up too much PCB space or cost. The outputs that aren't using the THAT chips are from the main gain stage, which have the option of either the above chips or lovely API 2503 output transformers. The mix amps use DOAs to further increase the flexibility. For this unit I will be using a John Hardy 990c and THAT 1646 combo for the 'modern' bus, and a GAR 2520 paired to a Classic API 2503 transformer for the 'vintage' bus.

Full schematics are available here:
HYDRA Full Schematics

Design Changes

Just after I finished the first PCB I moved to London to start a industrial year working at a pro audio repair shop. It was a great year, but it meant the project slowed down quite a lot as I had less free time. I also started the rethink some of the design choices.

I decided to change to virtual earth summing at this point as I wanted every individual channel to be switchable, as opposed to having just 2 groups of 4 channels hard wired to the 2 different buses. Although this could still have been done, it would have meant doubling the number of switch poles (due to needing to back-ground the resistors on a passive mix amp) and therefore increased the cost considerably.

I also reconsidered using a tube gain stage, partly due to my total lack of tube experience and the added cost of having a separate HT PSU.

I made this block diagram and started to plow on with the schematics.

Input PCBs

I came up with a basic idea of all the features and started working out how to go about it, which took a while as it was the first time I had designed anything any more complicated than a DI box. About this time I was still in uni and one of my lectures was helping me bounce the ideas around and the project was moving along pretty fast.

At this point the unit was going to use 'passive' summing with transformer inputs, with 1 gain stage using a DOA and the other tube based. After playing around with the ideas for a while I went ahead and produced a prototype PCB of the input section to test the concept.

A little daunting as it was the first PCB I had ever had made and I was a little unsure ordering it from PCB Cart in China, but it came a lot faster than I expected and looked top notch.

The PCBs all worked out except that the Cinemag input transformers had to be mounted on the bottom as I messed up the footprint. Not too bad for my first attempt.

How It Started

This project was first conceived just after I finished building my first piece of DIY audio equipment, an SSL 4000 bus compressor clone. I was thinking about what I wanted to build next, and although there is a massive range of DIY projects available, I found myself wanting some features that I couldn't currently find anywhere.

At the time I had been playing around with the amazing Slate Digital VCC and listening to some blind comparisons of different DAW and analog mixes, and I guess I was greedy and wanted to build something perfectly suited to my situation (but who wouldn't?). From the blind comparisons it was clear that all the analog mixes sounded better than the digital versions, so the obvious thing to do was build a summing box that could give the big analog sound to a small project studio.

But then you are stuck with the choice. Solid state or tube? Fast and clean or dark and grungy? But why stick to one sound, after all you don't use the same preamp on every instrument. So there it was: I wanted to build a summing box in which you could submix to a choice different flavour of mix amps. Each bus would have inserts so that you can put a compressor over just the drum bus or whatever you wanted to try.

I wanted to include a decent monitor controller as well, with separate mix/monitor outputs so that you didn't have to go through the AD/DA before listening to the mix.

My SSL bus comp with controversial perspex front panel. After building this some people said that perspex isn't such a good material for a front panel, as it won't block EMI/RF, which is true. Now that I have more knowledge on the matter I probably wouldn't do it again, but I haven't experienced any interference problems yet. You're always learning how to improve these things.

Elliott

Welcome!

So this is my new website, mainly to document the build of a mixer that is being built for my dissertation. It will also include other pro audio related posts, as well as my random interests such as music, photos etc. I see this blog mainly as an online repository for me to keep every in one place, but I guess there's no harm in it being a public blog either.

Although the build of the mixer is already completed and I am now on the test phase, I will be documenting what I did throughout the design and build stages for posterity.

Cheers
Elliott