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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 12th May 2024, 10:43 pm   #1
RadioHist
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Default Matching Brown microphonic amplifier to 2000 ohm

I have collected three variants of the S. G. Brown Carbon capsule Microphonic Amplifiers. Two of them are designed to work with 120 Ohm loudspeakers. I do not have the 120 Ohm variety (only 2000 Ohm) I presume I need a matching transformer 120 to 2000 Ohm for maximum power transfer. However, I have yet to find such a transformer. I also presume I am not the first to encounter this problem. What is the source of such a transformer? I'm thinking the primary need handle a few tens of milliamperes.
Comments welcome.
Robert
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Old 14th May 2024, 11:59 am   #2
Herald1360
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Default Re: Matching Brown microphonic amplifier to 2000 ohm

Sounds like you'll need a transformer with about 1:4 ratio, primary to secondary. Not ideal, perhaps, but a small 120V to 30V mains transformer, using the 30V secondary as primary would probably work well enough.

Looking at a circuit found online suggests that the amplifier already has a transformer output so dc shouldn't be an issue (unless the circuit is for a 2000ohm output amplifier and the 120ohm version has none) but even so a relatively small (25VA perhaps) TX should be OK. They work well enough to use as a bodge fix output transformer for a small single ended valve (tube) amplifier or radio.
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