API Preamp (212L) Design Process

Located different schematics and data sheets for the amplifier
Recreated the circuits in LT Spice
Created our own simulation of the AP2520 Op Amp
Comparing frequency response measurement of the physical unit to the model

Original Schematic

The first step was to find the original schematic of the amplifier. Surprisingly this was relatively easy to find online, but we soon discovered that there was a little bit more to original circuit than the non-inverting OpAmp. The main sound of the API amplifier comes from the AP2520 OpAmp. This proved much more difficult to model.

Image from www.audiocircuit.dk

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Op Amp Design

API's signal processors are well-known for their unique Op-Amp design, which are still used in their modern products as well. We were able to find a schematic online to base our model on.

Image from https://gearspace.com/board/geekzone/66242-api-2520-schematic-original-vintage-version.html

Circuits on LT Spice

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212L

AP 2520 Op-Amp

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Frequency Response of the 212L From API

Image from https://soniccircus.com/product/api-212l-discrete-mic-pre/

Final Result

Unable to get the LT Spice model to match the expected response

Measured Response

This is our measured response of the API 212L preamp in Audio Studio B. We ran white noise through the amplifier, and put the output through the same transfer function calculations as the EQ. It is much closer to the expected response of the preamp, but it is still not exactly like the API predicted response.

Calculated Response

The final model was never able to get very close to the frequency response of the physical unit. We think that this could be because the model of the OpAmp was not as accurate as it needed to be. It also could be because the physical hardware will always behave differently than the components in LT Spice.

Future Development

In the future it may be better to train an AI model on how the API distorts the sound, and apply that to the signal. Our aim in this project was to detect how a signal was affected by the components and affect other signals in the same way. What we saw was that the API preamp not only boosted the highs but also had some internal distortion. The artifacts the amplifier adds could be a reason why the sound is so sought after.