ePreamp01 – design and build | evolutionarytheory

It’s been a while since my last post I admit. I was working for the last two years producing/co-writing/engineering/mixing/programming and mastering for the following album:

http://leesafar.bandcamp.com/album/laundry-after-midnight.

This album consumed almost every night over that two year period, but, just after I finished it, I got back into some long-awaited DIY audio!

I was contacted by a friend of a friend about the possibility of designing and building a two-channel microphone preamplifier.

Their specs included:

+48V phantom, switchable on both channels.
Phase reverse per channel.
Separate XLR inputs and outputs, fully balanced.
20dB pad input for line-level signals.
Low noise.
High gain (up to 70dB).

So I set about designing and testing various circuits.

The design I settled upon was originally going to be 2 stages of BigFetBloke gain stages (designed by Tamas of GroupDIY).

Here are some photos of the prototypes I made of these Discrete Operational Amplifiers:

Unfortunately, I had quite a few issues powering these devices from +-24V rails derived from a custom PSU run from a Jaycar 17VAC plug-pack.

After quite a few extra LTSpice simulations and some thinking, I radically redesigned the entire device to use an e2510 and e2520 Discrete Operational Amplifier per channel, instead of two stages of the BigFetBlokes.

Here are some images of the gain stages (first the e2510 then the e2520):

These are customized DOA’s I designed myself for low current drain and lower +-16V power rails.

These fixed the power issue and ran beautifully off of the Jaycar plugpack power supply.

Here is a photo of the custom power supply fitted into the final unit using laser-cut perspex mounting brackets I made (they were the first thing I ever made using a laser cutter!):