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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: Re: Transformer-based passive pre, biamp with active XO, need impedance help please posted by Saurav on April 1, 2002 at 13:05:52:
> With the 100k input impedance of your Waves, I don't see any problems driving them in parallel with your XO.Combining this with what you said later, that a transformer doesn't have an impedance of its own per se (ignoring the DCR) - this means that I need to consider the parallelled impedance of the Waves and the XO, on my CD player and phono stage - is that correct? Basically, there is no impedance buffering, so the final effective input impedance reflects directly onto the source device(s), with the appropriate multiplication factors thrown in.
Correct. With the 100k input impedance of your Waves, you really shouldn't have to worry about the input impedance of your XO. Let's say your XO's input impedance is 10k. In parallel with 100k, you'd get a combined parallel impedance of about 9k, which is shouldn't present any problems even for tube output stages.
> If you haven't wired up your passive yet, may as well wire to a parallel pair of RCAs. No need to throw additional connections into the mix (i.e. the Y-adaptor) if you don't have to.
OK, that's what I was thinking. Any resistors/caps to be put in series/parallel anywhere to isolate the outputs, or something like that? Also, from what I read here, it seems it's advisable to have a 2-gang input selector which switches both hot and neutral of the RCAs. Some people mentioned R or [R + (R || C)] circuits across the RCA for noise or isolation, I don't remember. Would anything like that be needed for multiple outputs? I would prefer not adding resistors, because that would appear to negate the advantage of using a transformer-based attenuator in the first place.
Don't see any reason why you can't drive the outputs directly.
One more question - does anything change if this device uses autoformers? At this point, I'm not exactly sure what's in there - I have it on loan from someone else.
I don't believe so, but I'm not as intimately familiar with autoformers as transformers and I could be overlooking something so take that for what it's worth. :)
> This works for unity (1:1) and step-down (2:1). In a step-up situation, such as 1:2, then you have to turn the math around.
Which would mean that I would be strongly advised to make sure my sources have enough output voltage that I don't need to wire anything as step-up, right? In fact, it looks like the impedance "situation" is better as one steps down further.
Right. And yes, the impedance situation improves as you attenuate more. Input impedance increases and output impedance decreases.
Good luck. Let us know how it turns out.
se
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Follow Ups
- Re: Transformer-based passive pre, biamp with active XO, need impedance help please - Steve Eddy 13:43:55 04/01/02 (1)
- Re: Transformer-based passive pre, biamp with active XO, need impedance help please - Saurav 13:53:52 04/01/02 (0)