EMI "IJ" line-equaliser for HB1 moving-coil microphone, 1930s

Alan Dower Blumlein was a pioneering electronics engineer who sought a more immersive listening experience, leading to the creation of “stereo” sound recordings. Alan Dower Blumlein and Herbert Holman designed this equaliser to be used with the new HB1 moving coil microphones at EMI.

Equalisers allow different frequencies in an audio signal to be changed independently. In sound recording they are used to adjust the signal from microphones to give a more accurate reproduction of the tone of the original sound source.