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Old 24th November 2004, 05:41 AM   #1
duffy is offline duffy  New Zealand
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Default Input transformers and IC input impedances

I want to use a Lundahl LL1550 transformer as a stepdown/splitter 4:1+1 as shown as *connection F* in the this datasheet:

I want two outputs so I can run power IC's with active XO inputs for my Jordan JX92s/Aurum Cantus G2Si speakers. Tenatively thinking of an inverted LM3785 on the ribbon and inverted, bridged LM3886's on the cones @ 4500Hz XO or so. I intend to try all the possiblities for comparison.

My simple question is this:

Assuming I load each secondary with a 5k resistor (or 2k5 + 2k5 for the balanced, bridged connection) I will have 160k ohm input resistance. But what does the IC *see*? In other words, does the IC *see* 5k or must I add a series resistor for the IC input in order to define the feedback ratio?

Secondly does the secondaries static resistance play any effect, or is it swamped out by the loading resistor?

Thanks, duffy
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Old 24th November 2004, 08:22 PM   #2
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At DC your chip input will see the DC resistance of the secondary.
At any other frequency it will see the seondary inductance in parallel with the 5k and in parallel with 1/16th of your source impedance. Roughly.
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Old 25th November 2004, 06:35 AM   #3
duffy is offline duffy  New Zealand
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Thanks for the reply Two Spoons,

Exactly what I was looking for.


Duffy, (Kiwi in crazy Tokyo)
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Old 25th November 2004, 07:59 PM   #4
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Ah! That would explain the flag!
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Old 2nd December 2004, 06:08 AM   #5
duffy is offline duffy  New Zealand
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Default IT / attenuators / impedance

I thought about your answer for a couple of days, Two Spoons, and decided that I was still confused (as always!). To make sure, Two Spoons, or anyone else, for that matter,

1, Is the feedback ratio defined as:

Rf/Rin or

Rf/Rin ll secondary inductance ll source impedance*step down ratio squared


(by inductance you are refering to idle inductance??)

If it's the latter, then assuming a high source impedance of 10K, then effective Rin is around 600ohm, and the ratio is very subsceptible to changes in source. In this case, I guess the smartest thing to do is to load the tx primary with a high 100K pot?

2, Separately, if it's the former,!=@does it make sense to vary Rin to use as a volume control? I assume this is what a series attenuator is. For example, if Rf is 220K, could a 12 step attenuantor be sometime like:

Rin 5K, 10K, 20K, 40K, 80K, 160K, ....etc

I realise these are not log values, but you get the idea.

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