The ML2520 is Whistle Rock Audio’s version of the classic API discrete opamp (DOA). According to the creator, the DOA has improved with each revision and is currently on rev 3.

Images courtesy of Whistle Rock Audio

Notable Features:

• Based on the 2520 discrete opamp
• Kit includes matched BC550C transistors
• Pre-assembled DOAs available
• Full assembly manual

Price: $17 for full kit

More info:

• Order from Whistle Rock Audio here.
• Read the assembly manual here. (PDF download)