Caution. Take all safety precautions for ear and electrical safety.
It arrived.
The Belotone Lyric beautiful chrome hearing aid.
It could not be believed how small it is. 6.5cm x 4cm x 2cm or 2.5in x 1.5in x 6/8in.
It arrived in somehow almost new condition. Thankfully it might have been replaced by a transistor hearing aid.
It's amazing, state of art electronics, electrons flowing all the time (class A operation) in vacuum. A microphone amplifier bought for so little. It contains 3 subminiature electron tubes. Thank you transistor.
The mercury very thick coin battery cell could be replaced if done very carefuly for safety with a A/3 rechargeable battery. It should last for long as each electron tube filament must consume about 10mA. The 15V battery might be replaced by a 12V garage remote control type battery practically lasting for ever. The operating manual clearly says that the amplifier can operate with just less power output even if the Vb goes down to 10V.
The Citizen Regent hearing aid must have enough space for fitting an ordinary AA rechargeable battery for the filaments.
(At your risk. All safety precautions must be taken. Batteries must be connected in series with a suitable fuse for safety).
The Beltone Lyric was cleaned in a similar way one would clean tape recording heads. Contacts too.
The impedance at 1024Hz of the headphone was measured 60Ω. The inductance 5mH.
The inductance of the output transformer secondary at 1024Hz was measured to be 30mH.
How would this preamp sound with an SM58, SM57? How with an MD441 U3? How with a Grampian DP4/L?
A suitable input transformer must be used to get inoedance to hi.
Or a mic that already has a transformer inside such as the vintage SM57 ie Unidyne III. Or an MD21 HN or HL or an M55 N.
How about MD211?
Or a crystal mic, say a nice Geloso or Sonotone.
How would it sound with its own (crystal?) mic?
Buying something ready may not have the joy the joy of building it yourself. But how would this little front end amp sound like?
Analog to digital converters needing hearing aids?
Further reading, the patent document of Beltone Lyric hearing aid:
https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=2495546A&KC=A&FT=D&ND=3&date=19500124&DB=EPODOC&locale=en_EP
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