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Author Topic: An tube pre impedance question. . .  (Read 1267 times)

Beezoboy

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An tube pre impedance question. . .
« on: March 19, 2005, 10:54:28 PM »

So a friend and I are building an Altec 1566a preamp from the schematic and with some help on other sites, and we were wondering if the input transformer was completely neccesary. (150/15k)

I have heard that a mic pre's input impedance should be 10x higher than its source. Well in this preamp it goes from the transformer stage straight to the first tube stage. So what if I skipped the transformer. Since tubes operate at much higher impedances couldn't I just hit the tub or would there be anoise issure in theory?

Sorry if I sound dumb, but lots of transformerless tube pres say the input impedance is 15000, and not 150.

Beez
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Consul

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Re: An tube pre impedance question. . .
« Reply #1 on: March 20, 2005, 07:44:57 AM »

The input transformer isn't only for matching impedences. It is also there to unbalance the signal from the microphone, and usually with a better CMRR (Common Mode Rejection Ratio) than most electronic unbalancing methods.

Using a step-up also provides you with some "free" voltage gain, which means less noise at the first tube input stage (less voltage gain needed). The final output stage usually provides plenty of current gain, enough to drive the transformer out anyway.

I would definitely keep the input transformer there.

EDIT: I should point out that I'm somewhat of a beginner at this myself, even though I've been at it a couple of years, on and off. What I've stated above should be more or less true, though, at least in general.
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Darren Landrum

"Never be afraid to try something new. Remember that a lone amateur built the Ark. A large group of professionals built the Titanic." - Dave Barry
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