Bruel & Kjaer - 2804 , documentazione :
https://www.bksv.com/media/doc/bp0218.pdf
http://www.technicalaudio.com/pdf/Bruel&Kjaer/Bruel&Kjaer_Microphone_Power_Supply_Battery_Driven_2804.pdf
http://www.livingston-products.com/products/pdf/105231_1_en.pdf
