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Old 8th May 2014, 02:47 PM   #11
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Quote:
Originally Posted by sangram View Post
24-position, 4 pole MBB for the ladder type attenuator. Needs two resistors per channel, per position.

24-position, 2-pole MBB for series type. Needs one resistor per channel, per position and one common resistor.

4-position, 2-pole MBB for input selection. I use 4-pole switches for this and use the third and fourth pole to switch grounds and short the unused inputs, this helps with crosstalk between inputs. Because the switch is MBB, this shorts out the input and completely suppresses the pop associated with switching inputs when there is active signal on more than one.
Oh that's a good idea. I didn't like the idea of combining my grounds. Now I understand what the resistors do for volume control but I'm having trouble understanding what function the resistors in the rotary switch play.
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Old 8th May 2014, 02:54 PM   #12
sangram is offline sangram  India
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All pots and attenuators apply a resistive divider to the input signal to achieve attenuation. As the pot/switch is turned, the series resistance reduces and the parallel resistance increases, the ratio of these resistors controls the attenuation.

The ladder attenuator basically has a resistive divider for each specific switch position. The series attenuator uses a single resistor in parallel, and a series of resistors in the series path.

The usage of a 4-pole switch does not obviate the combining of input grounds, I use a small resistor in series with the input grounds to connect to chassis (0.1 ohm or so). If you didn't, the shorting idea wouldn't work.
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Old 8th May 2014, 03:01 PM   #13
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Thanks for the explanation.

Currently the plan is to go with a series type stepped attenuator for my volume control. For the source and output selector switches I don't see the advantage for going with a ladder type stepped attenuator over shunt type stepped attenuator. Won't the ladder type just add more complication to the build?
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Old 8th May 2014, 03:04 PM   #14
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Poles and Positions: Sorry i was having a brain fade moment there

Poles is how many separate channels you have. For example to switch and left and right audio signal you would have four poles, + and - for left channel, + and - for right channel, so you need a switch with four separate poles.

Positions is how many clicks the switch has, for example if you wanted to switch an audio input between four outputs you would need a four position switch.

To put it all together you would need a four pole, four position switch to do all of the above.

A very cheap option is the vintage Russian hardware on eBay, there is a huge variation of pole and position numbers available. A few examples..

http://www.ebay.com/itm/281321318149
http://www.ebay.com/itm/231001582511
http://www.ebay.com/itm/271474674493

Last edited by mcandmar; 8th May 2014 at 03:07 PM.
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Old 8th May 2014, 03:07 PM   #15
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I'm a bit confused. I thought sangram was helping me with finding a rotary switch that would best work but I'm guessing he was talking about attenuators for volume control and rotary switch for source and output selection. That makes more sense. Right? you wouldn't want to use a stepped attenuator for source and output select.
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Old 8th May 2014, 03:10 PM   #16
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Correct, rotary switchs and attenuators are two entirely different things. Confusion arises in the case of the Elmas as they use the same mechanical switchs, but with different circuit boards attached to them, hence they look very similar at first glance.
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Old 8th May 2014, 03:16 PM   #17
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Quote:
Originally Posted by mcandmar View Post
Correct, rotary switchs and attenuators are two entirely different things. Confusion arises in the case of the Elmas as they use the same mechanical switchs, but with different circuit boards attached to them, hence they look very similar at first glance.
Ahh I see. Thanks for those links. Those prices are lot easier to bare.

Quote:
Originally Posted by sangram View Post
The usage of a 4-pole switch does not obviate the combining of input grounds, I use a small resistor in series with the input grounds to connect to chassis (0.1 ohm or so). If you didn't, the shorting idea wouldn't work.
So even if I used a rotary switch I would have connect the input ground to a common ground?
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Old 8th May 2014, 05:27 PM   #18
sangram is offline sangram  India
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No, not neccesarily.

You could choose to switch the ground, but then the inactive inputs cannot be shorted to ground.

You could alternatively choose to short the inputs, in which case all will need to see similar ground potential, hence the grounds need to be combined. In order to get some amount of decoupling, you could use a small value resistor below about 1 ohm to send each input ground to ground common which is used as the input reference, and then send the selected ground to the output for the output reference. I shall try to draw up a schematic tomorrow.

Re series vs ladder attenuators, in the series version the input impedance changes with every position of the switch. The ladder type is constant impedance re the input. Usually the choice of series versus ladder attenuator depends on the components before and after the attenuator. Yes the ladder type is more complex and a pain to put together, but it has its benefits.
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Old 8th May 2014, 06:00 PM   #19
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Quote:
Originally Posted by sangram View Post
Re series vs ladder attenuators, in the series version the input impedance changes with every position of the switch. The ladder type is constant impedance re the input. Usually the choice of series versus ladder attenuator depends on the components before and after the attenuator. Yes the ladder type is more complex and a pain to put together, but it has its benefits.
Add to that in a ladder attenuator you only have two resistors in the circuit at any given time vs a series attenuator which adds all the resistors together in series.
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Old 9th May 2014, 03:25 AM   #20
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Wow thanks for the quick response. I'll need 2 right? One for each channel?
The one I linked was stereo, so unless you need balanced you only need one

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