EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Tenderfoot on October 22, 2024, 05:30:18 pm
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Impedance in series is simply added, or so I thought. So why do two 150 ohm primaries add up to 600 ohm according to this data sheet. Am I missing something here, or reading it wrong? Just for information I'm looking to convert a single-ended audio sig-gen output to balanced, for a general purpose test lead.
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Twice the windings gives 4 times the inductance. Lookup the formula to find inductance per number of windings.
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Hi Tenderfoot,
since the two windings are on the same core
and inductance goes by the square you get
2x2x = 4-times the impedance.
Have fun
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I should have known that, I guess at one time I did. But who puts audio through transformers nowadays. Many thanks for your fast replies.
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But who puts audio through transformers nowadays
plenty of people
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Yes, I'm always amazed to be reminded of all the high-end studio microphones that are routed through transformers, right at the source. You'd think that they would degrade the signal somehow, but apparently they don't.
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4x the inductance ... but that's not the impedance.
The impedance is with a given recommended load. If you aren't using it in ancient circuits, that probably won't be the impedance.
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A good transformer (probably expensive) run well within its ratings, should not degrade the audio.
Check out the data sheet for this transformer: note that the distortion rises at low frequencies and higher power.
(0 dBu = 0.775 V RMS, the voltage that gives 1 mW across a 600 ohm load.)
https://www.tubesandmore.com/sites/default/files/associated_files/p-t-lehle-mz-l.pdf (https://www.tubesandmore.com/sites/default/files/associated_files/p-t-lehle-mz-l.pdf)
(That vendor has two transformers in that case size, with different impedance ratios, from that manufacturer.)
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Such a transformer 'matches' or 'reflects' the impedance on one side, to an impedance on the other side. Impedance transfer is related to the turns ratio and is proportional to N squared. If you don't have 150 on one side you won't present 600 on the other side, or vice-versa.
Your application of single-ended to push-pull (balanced) is very common. There is still a substantial commercial market for valve related amps (that use output transformers to match impedances) and signal line related transformers. Many vintage oscillators and distortion meters and signal measuring instruments use a high quality input transformer that allows a variety of matching configurations.