The Sterling ST77 is a copy of the ADK A-51 / MXL 2001 in a larger mic body. The ST77 mics that we've seen use a typical low-cost 32mm Chinese K67 capsule, with plastic clamping rings. The circuit in this mic has numerous problems, resulting in a high noise floor and high distortion under moderate SPL. The stock ST77 exhibits a sharp, narrow-Q high-frequency peak of +4.5dB @ 11kHz -- the center of a net 9dB delta from 0dB @ 6500 Hz to +4.5dB @ 11kHz, back to 0dB @ 14.5kHz. This sort of sharp peak cannot be fixed via EQ later in the signal chain, because it exaggerates mouth sounds and breath noise. In some cases it can trigger distortion in the ADC as well. Our mod service replaces the entire signal path of the mic. Capsule choices You can choose the RK-12, RK-47, or our custom K67 from the T-67 microphone. All three designs work well with the linear circuit we will install. The service price on this page includes your choice of capsule. New Features The circuit upgrade provides two features not found in the original mic: A switchable 10dB attenuation pad A Cardioid/Omni pattern switch These two features are controlled by switches inside the mic. To access them, unscrew the bottom piece of the mic body (no tools required). Slide down the body cylinder. The switches are at the top of the circuit board. Platinum Circuit New in 2026, we have developed an updated single-stage JFET/transformer topology specifically for the ST77. The core circuit design is similar to our popular T-series microphones. But in this version, we've taken advantage of the ST77's larger housing to upgrade all of the components. All electrolytics are our own bespoke audio grade parts. Filter caps are oversized and bypassed. Noise filtering is excessive. The JFET is a modern, high-grade design that is individually biased, and configured to run at higher current. Transformer Options We have two transformer options. Both offer excellent performance. The Platinum transformer has slightly better resolution. Its higher cost is due to higher-purity copper windings, and import fees from Europe. Compatibility This mod is for the Sterling ST77 only. How it works After you buy this service, see our Service Policies page for instructions on packing and shipping your mic. Be sure to print the Service Request form linked from that page. On the Service Request form, specify which capsule you prefer: RK-47, RK-12, or K67D. The typical turnaround time is 5 business days. We can provide more detailed turnaround time estimates after you purchase the service. Costs The mod service includes the parts, installation labor, and testing. Return shipping charges (domestic US only) are included for the microphone. Availability This mod service is open to US residents only. (Residents of other countries might find that it costs less to purchase a finished microphone.)