ePreamp01 – design and build

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!

leesafar_laundry_after_midnight

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:

_MG_9050_thumb

 

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):

_MG_8902_MG_8899

 

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!):

 

Here are some images of the final build, which tested beautifully, and sounds completely amazing:

And here are some photos of the final device with custom front-panel fitted.

The custom is extremely happy with this design and the performance of the final unit.

He is using it to DI guitar, record microphones/keyboards and various other duties.

The final unit uses transformer-balanced inputs and outputs and 2 equal-stages of gain for a maximum gain of 70dB!

_MG_0024 _MG_0022 _MG_0021 _MG_0020 _MG_0019

 

If you have any questions about this build just let me know!

More projects coming soon….

 

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