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Modifying stepped attenuator

post #1 of 15
6/28/11 at 9:35am
Thread Starter 

I have a few stepped attenuator from eBay.

 

I was thinking of removing the resistor and the wire on the lowest step.

 

The purpose is so that at the minimum volume setting, the source is actually totally disconnected from the amp.

 

Is there any downside to this?

 

I am wondering if there would be a pop sound when the switch is turned between the lowest step and the next? I also wonder if the same thing would happen if a rotary switch is used as a source selector for the same reason.

 

Thanks.

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post #2 of 15
6/29/11 at 6:13pm
Thread Starter 
Come on people, I am sure it isn't a tough question. :)
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post #3 of 15
6/29/11 at 8:35pm
What kind of attenuator is it? Ladder, series or shunt?

Why don't you just try it and find out?
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post #4 of 15
6/29/11 at 9:11pm
Thread Starter 

It's the ladder type. I can't try it out as the amp isn't working yet, and I need to know this for a parts purchase that will go to the amp, so that it would work. :p

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post #5 of 15
6/30/11 at 6:20pm
Depends upon how the input to the amp forms the signal.

Does the input to the amp support a coupling circuit of any kind?
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post #6 of 15
6/30/11 at 7:31pm
Thread Starter 

The amp in question are AMB's Beta 22 and Alpha 20.

The sources are the typical commercial stuffs, CDP, Bluray player, DAC, and things like that.

I have a pre amp that hooks them together, during source switching there are minimal pop, often inaudible. However it went dead and I have to build something to replace it. A better way of putting my question might be: would simple rotary switch cause popping when used as an input selector in these amps?

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post #7 of 15
6/30/11 at 9:01pm

A popping source selector is not generally a problem with your amp, it is generally a problem with your sources. It means your sources have varying levels of DC offset at the outputs. 

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post #8 of 15
6/30/11 at 10:17pm
Thread Starter 

So I guess, if the source is not going to pop with the input selector rotary switch, it's not going to pop with the removal of those resistors on the stepped attenuator?

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post #9 of 15
6/30/11 at 11:05pm

 

Quote:
Originally Posted by Navyblue View Post

So I guess, if the source is not going to pop with the input selector rotary switch, it's not going to pop with the removal of those resistors on the stepped attenuator?

 

Removing the ground reference at the input of an amp is a risky idea. There is always the resistor between the wiper and ground, but why tempt fate with an amp like the β22?

 

If you want to isolate the amp from the source do it with the source selector. 

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post #10 of 15
6/30/11 at 11:33pm
Thread Starter 
Please pardon me, how is it different when cutting the source by means of a stepped attenuator and a source selector? Since both are basically 3 pole rotary switch?
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post #11 of 15
7/1/11 at 6:53am
The source selector can be made using a make-then-break component. Wired properly, this can insure the ground reference is always maintained when switching between source inputs.
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post #12 of 15
7/1/11 at 8:19am
Thread Starter 
Quote:
Originally Posted by jdkJake View Post

The source selector can be made using a make-then-break component. Wired properly, this can insure the ground reference is always maintained when switching between source inputs.
 


I intended to reserve a position for no input. With a 3 pole switch where the ground is also switched, does it mean that I can't do that since the ground contact will be broken?

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post #13 of 15
7/1/11 at 8:54am
Is there a particular reason you want to switch the grounds?

In any case, nothing says that your reserved input position cannot be wired to properly terminate the input signal to the amp.
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post #14 of 15
7/1/11 at 10:21am
Thread Starter 
Quote:
Originally Posted by jdkJake View Post

Is there a particular reason you want to switch the grounds?


My (limited) understanding is that, it is a bad idea to connect the ground of various sources? Say I have 5 sources connected to an amp, it is better to use a 3 pole switch (left, right, ground) than a 2 pole switch (left and right, and leave all signal grounds permanent connected).

 

Quote:
Originally Posted by jdkJake View Post

In any case, nothing says that your reserved input position cannot be wired to properly terminate the input signal to the amp.

 

Sorry I don't get what you mean. Say with the input selector, I have a switch position that is not connected to any RCA jacks (as a source off), what do you suggest that I do?

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post #15 of 15
7/3/11 at 7:11am
It is my understanding that the preferred scheme is to switch the grounds such that a reference to the input device ground can be formed without unwanted contamination from other sources of ground. That being said, I have seen many builds, even high-end commercial builds that tied the ground returns together. I assume this is done for economy reasons rather than signal integrity reasons. I have built my switch box both ways. Without actually measuring the noise on the grounds, there was little to no audible difference. I preferred the switched ground as it is more technically correct (as I understand it). I am sure this can (and will) continue to be debated.

As for input termination, you can always leave that last input floating or you can consider establishing a ground reference to the amp and terminate the signal input with a high value resistor, say 10k ohm or so. You could probably get away with a much lower value (say ~600ohm) as well, but I would think the higher values would be preferable.
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