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Old 25th December 2008, 11:21 PM   #1
jamesk is offline jamesk  United Kingdom
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Default Please help me begin my knowlege quest!

Happy Christmas everyone!


Background
info about the project I am about to embark on

I failed to persuade anyone to buy me a new mastering console for my christmas present this year, and seeing as I can't afford one either, I had a Christmas revelation that I'd make the thing myself!!

I have limited electronic knowledge - this sounds foolish - but hold on.... I've been researching my needs for an analogue mastering console and have concluded that everything I need it for can be achieved using passive components - basically it's just signal path, resistors and switches for attenuation, stereo operations and routing.


Basically I want to make this:

Click the image to open in full size.

Forget the meters for now as I will need to use some active components and am not there yet.

The most important part of this console is a high quality rotary switch based passive attenuator with 23 steps. I can get a high quality all in one kit from hifi collective (after much hunting around). I can get a balanced stereo kit which is great.

My problem
Looking at the very simple schematic of a passive shunt attenuator, there are switch selected resistors going to ground as expected, but there is a 10K MASTER resistor before the switch. What in the name of spongebob squarepants is this for? Why must I chuck away some of my audio signal's strength for seemingly no reason?

This for me is very much bewilderment at the very first hurdle and it sucks. I feel as though something "simple" like this could be found in page 2 of text book X. But for all my persistent searching, now and in the past, I just get more confused about electronics in general.

So essentially could someone point me in the direction of a no-messing-around source of information to get me started in electronics, or at least explain about this resistor?


Thank you in advance x100
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Old 27th December 2008, 09:35 AM   #2
Spiny is offline Spiny  United Kingdom
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The input resistor is the upper part of the divider network.
I attach a simplistic circuit of the attunator with the formula. The actual attenuation is also affected by both the output resistance of the driving circuit and the input resistance of the following circuit. These two are normally termed impedances as they vary with frequency to a degree.

All the stepped attenuaters do is switch different bottom resistors. the output then gets a different portion of the signal. One problem of the simple form is the output resistance varies vastly and can cause problems in the next stage.

You may find this book a good starting point. Covers the basics and more.

Electronic Circuits, Fundamentals and Applications. By Mike Tooley


Those forum members with more knowledge will I'm sure provide other books and links to information and correct any errors in the simple explanation above
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File Type: gif attenuator.gif (1.5 KB, 44 views)
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Old 27th December 2008, 02:00 PM   #3
jamesk is offline jamesk  United Kingdom
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thanks for the reply, i will check that book out ASAP.

But as for the resistor, although I am grateful for the formula you gave, I still don't know the actual reason it needs to be there.... I just don't get why it is there... I'm just taking peoples words for it and don't understand the purpose. Lets imagine all the schematics depicted a chicken drumstick instead of this resistor... and there was ohms law explaining how the chicken drumstick's impedance adds up to 7 banana bucks.... I still don't know why I need a chicken drumstick in my circuit!

But anyway, when you say the problems in changing impedance can mess up the following circuit (£2000 amplifier) I am worried... how would I make a minimal signal path passive balanced attenuator without resistors?



lol
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Old 27th December 2008, 02:50 PM   #4
Spiny is offline Spiny  United Kingdom
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Passive attenuators imply resistors to loose the unwanted part. You can terminate a network so the input and output resistances are constant - this would I think require more wafers on the switch to add in extra resistors, not my field - I'll have to look up.

The top resistance is used as part of the attenuator (its R1 in the diagram) the input signal is shared accross the two resistors and you tap the portion you want at the mid point. Without the top resistor you are just connecting direct to the output any attenuation is then provided by lower resistor against the output resistance of the first stage, this is probably ill defined and may be subject to frequency variation. the result will be unpredictable and poor. (and your first stage may object to having its output taken to ground via a .1 or 0.2 ohm resistor at the maximum attenuation setting.

With the 10K in place the minimum resistance the first stage sees is 10K when the max attenuation is applied - the resistance increases from here.

Input stages (the next stage ) do not generally mind low resistance drive so the output side is OK.

Easier (and cheaper) to use a couple of OP-Amps to buffer the signal with the attenuation between them.
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