Post Output attenuation.

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Ethan

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I want to build a simple rotary switch attenuation box that I could patch inbetween preamps and my Digi 192IO. I would do my calculations based on the load impedance right? for instance, the digi 192's input Z is 12K, so if I wanted 5dB of attenuation my first step would be about 5K2, then 2K9, 1K6 etc... Is that right for a series attenuator?
 
No, you would want a voltage-divider at an impedance somewhat higher than the output impedance of the previous gear's output impedance - and somewhat lower than next stage's input impedance.

As most equipment happily drives 1K Ohm, that could be the impedance of your attenuator.

Jakob E.
 
Could you explain to me why I would use a voltage divider to attenuate rather than a series attenuator (yes I do realize this is also a kind of voltage divider...which is part of the confusion) that matches the input impedance of the gear it will be going into? For instance, if it is going to be used in front of my 192IO, which has an input Z of 12K, what would be the consequence of designing the series attenuator to match that impedance? I understand Jakob's answer but I don't know why that would be preferred over a series style attenuator... with the matched Z (to the 192IO) series attenuator, since the input Z of the 192IO is higher than any preamp output Z I've seen... Wouldn't that give me the best voltage transfer? what am I overlooking?
It's more important to me that I understand rather than just build something that works for better or worse.

Thank you very much!
 
> I want to build a simple rotary switch attenuation box ... Wouldn't that give me the best voltage transfer? what am I overlooking?

If you want to attenuate, you do NOT want best voltage transfer. You want "bad" voltage transfer. Duh!

It is very possible your input will not be happy looking out at 12K. Most modern inputs want a low-Z source to damp-out crap on the line and in the input wiring. It is rare to find a modern output that is as high as 600 ohms actual-impedance.

As Jakob says: you want an attenuator that has an input impedance no lower than the rated load of the source (possibly 600Ω), and the lowest possible output impedance, and known voltage-ratios. The best approximation to this is a Potentiometer. Given those exact values, use a 631.57Ω pot. If you use switching instead of a rubby-pot, that's your total resistance. But since most outputs are just as happy with 1K||12K= 923Ω loading, a 1K pot is convenient. Or try a 5K pot: the output impedance can be as high as 1.25K, wich is still "low" compared to 12K loads. And significantly lower than the 6K line impedance you would get with a series resistor pad and -6dB attenuation.
 
Please excuse this hijack. I've been pondering how to add output attenuation post-transformer without having unwanted side effects, but was reluctant to post before experimenting. I'd assumed that just adding a pot in between the two devices (transformer out 600R mic pre and 10k in A/D) would introduce unwanted loading effects, changing the tone of the pre relative to the pot setting. However, based on what you say, PRR, it sounds like this is the way to go. So in that case, a dual 1k or 5k log pot in series with XLR pins 2 &3 should do the trick? What about potentiometer tolerances causing a difference in level between the + and - signal?

Thanks!
 
I thought NYDave had posted nice plans for this. Not sure they're directly suited (600 Ohms comes to mind) but they'll be at least a nice starting point.
 
Jakob, are there M* Vari-Mu schems around??????
So in that case, a dual 1k or 5k log pot in series with XLR pins 2 &3 should do the trick? What about potentiometer tolerances causing a difference in level between the + and - signal?
This way you would add serial resistance. Use only one pot connected to 2 and 3 and the wiper being your new output-2. Simple voltage divider.
cheers
Jens
 
[quote author="jensenmann"]This way you would add serial resistance. Use only one pot connected to 2 and 3 and the wiper being your new output-2. Simple voltage divider.
cheers
Jens[/quote]
Okay this is what I was confused about. They never gave an application for voltage dividers in class. It was just this thing that existed. Starting to make the correlation now. Thanks! :sam: :thumb:
 
Wouldn't a single pot across a balanced output, usung the wiper as a new output-2 unbalance the signal?
 
yes, probably - but it will also alter output impedance depending on setting. Often, we want to avoid this - so a straight-T attenuator is the most common for this:

bridge-attenuator.gif


Here's an inexpensive useable potentiometer:
http://www.alltronics.com/resistors.htm (500 OHM LINEAR 4-GANG POT WITH SWITCH - $2.50 each)

Jakob E.
 
> using the wiper as a new output-2 unbalance the signal?

What is wrong with that?

Yes, it isn't "neat". But at these impedances, in studio situations, it does no harm. Even the theoretical "harm" may be less than the real-world harm of running signal through or past multiple switch/pot wiper contacts.
 
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