Note
HDP-21 (Electro Voice model 637)
HDP-21A (Electro Voice model 638)
The HDP-21 was introduced in 1962 as a companion for the HX-10 and was kept in service until about the time the SB-101 was introduced in 1967, when it was replaced by its close cousin, the HDP-21A. The change was purely cosmetic. Both of these hi-impedance dynamic mics have exactly the same specs, notably the 300 to 3000 Hz response.
The HDP-21 and 21A both use the EV model 428 “Touch to Talk” desk stand, with satin finish chrome-plated die-case zinc base and handles. By at least 1973, EV was shipping the 21A with a gray anodized base and handle. The use of anodized parts was an attempt to keep the units looking nicer longer, as sweaty hands would eventually tarnish the bases and handles of the plain aluminum versions.
Element: dynamic
Frequency response: 70-10000 Hz
Impedance: 150Ω or Hi-Z
Output level (150Ω or Hi-Z): –57db
Case: chrome-plated die-case zinc
Photos, general information and specifications from "Heathkit: A Guide to the Amateur Radio Products," by Chuck Penson, WA7ZZE. Used with permission.