Any sonic reason to not build a series stepped attenuator?

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bobschwenkler

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Dec 8, 2005
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Olympia, WA
I'll be building one for a monitor controller, balanced stereo between modern DA and active speakers. If I use good resistors is there going to be a real problem with running my signal through a chain of them?

I could build a ladder, but of course, I'd need twice the decks. If I can still get very good performance out of a different configuration, I would choose the cheaper route.

Any advice on what type to choose baseed on experience (which I don't have much of)?
 
Why not just build a stepped potentiometer? Attenuating a signal just by adding series resistance is not a good way to go; the source impedance rises as you attenuate, which will skew the frequency response.

You can do it with two decks. Unbalanced will work fine. It's line-level and I presume you're not running it hundreds of yards, between different buildings, etc.
 
What exactly is a stepped potentiometer? How is that different than a stepped attenuator (a switch)?

The goldpoint website states that a series attenuator maintains a constant input impedance. http://www.goldpt.com/how.html

Are you suggesting I only utilize one side of a balanced signal for this project?
 
Sometimes it's easier just to draw a picture--then I can use this again next time the question pops up, which is about every six months or so.

PotsNLadders.jpg
 
[quote author="NewYorkDave"]Sometimes it's easier just to draw a picture--then I can use this again next time the question pops up, which is about every six months or so.
[/quote]

thanks for the pictoral answer NYD! :thumb:
i made a file of the image under "attenuators" so i wouldn't have to ask the same question in may '07! :green:
cheers,
grant
 
Funny, when you said "series" attenuator, I thought you were talking about switching in ever-larger series resistors to achieve attenuation. Now, looking at that Goldpoint page, it seems we were both thinking of potentiometers. What they call a "series" attenuator is actually a potentiometer. Some audiophile sites call that a ladder (which it is not--see my diagram), whereas Goldpoint uses the term "ladder" for what is in fact an L-Pad.

Audiophile semantic variations aside, the correct definitions are:
Potentiometer: A resistor with a third terminal variable between either of the two ends, either continuously or in steps.
Ladder: cascaded Pi attenuators with nominally constant input and output impedances.
L-Pad: A two-element attenuator with ganged series and shunt legs, with nominally constant input impedance and a variable output impedance.
 
So your drawing and what I had in mind are very similar. The main similarity, and the issue I was originally concerned with was with passing my audio signal through so many resistors. If I get good resistors do I have much to be concerned about?

That is good that you looked at the page and discovered that (for me); I had been looking through my "Handbook for Sound Engineers" and was seeing some discrepancies between the webpage and my book also, but was unsure about what was really what.
 
I've seen continuous poentiometers that have detents along the whole way of travel. This may be what you are refering to as a stepped potentiometer... I think your original question has to do with stepped attenuators, and weather the resistence should be series or not.

For several reasons, I like setting these up so that the signal is only going through one resistor at a time. Firstly, your signal is is going through less circuitry when you use one resistor at a time.

Secondly, in series, all of the resistances add up, and if you don't pay close attention to picking out the proper values along the way, you may end up short (or excessive) at the end of the chain.

If you set it up with only one resistor at a time, each step doesn't affect the other step, so if you find that all of your steps are perfect, except for one right in the middle of your sweep, you only need to change that one resistor to get it right, and that change will not affect the other values downstream.
 
I've been thinking my head off about this and still can't figure out what to do. I'm referencing the "Handbook for Sound Engineers" by Glen Ballou and various websites for information. Here are the questions that I can't figure out.

1.How important is it to build an attenuator with both constant input and output impedance (with the setup mentioned above in the thread, specifically, Rosetta 800 -> Dynaudio BM6A)?

2.What are average output and input impedances for devices like these? I was guessing 500 and 50,000.

3.If I did build a stepped potentiometer or ladder attenuator, how significant will signal degradation be if I'm working at an attenuation level that inserts 15 resistors into the signal path?

4.Dave, if you're reading this, my book has a diagram similar to yours of the ladder attenuator, but additionally shows a resistor between the input and switch wiper. It also states that a ladder attenuator only reflects a stable impedance to its source. Is there an explanation for this difference?

Other comments: I have a four deck ELMA switch to work with.

I calculated out values for an attenuator using a separate balanced Pi for each step, and even when I have a 1:5 ratio of Zi:Zo (as opposed to a 1:100 like if it were 500:50.000, which has a minimum loss of more like 25dB) there is a minumum loss of ~13dB, so it seems like a balanced Pi won't really work.

For various reasons, I think I'd like to build an attenuator that has a separate attenuator for each step.

This will not just be a snap to construct, so I'd like to do it right the first time and not have to buy new resistors and start all over. What kind of attenuator would be the best to use in this design? There is a lot of (too much) information out there, and I don't have enough experience to make my own informed decisions here...
 

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