Cleaning Up the Noise + Selector Switch Anomalie - diyAudio
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Old 14th September 2013, 11:27 PM   #1
BRSHiFi is offline BRSHiFi  United States
Previously known as kingden
 
Join Date: Aug 2008
Location: Evanston, IL
Default Cleaning Up the Noise + Selector Switch Anomalie

I am building a line stage in a "glass op-amp" type fashion. It has two grounded cathode stages DC coupled to a long tail. The long tail is DC coupled to a current sourced cathode follower. A feedback loop sets the gain. The power supply uses an active regulator. The schematics are attached.

I am having some issues with noise and a selector switch that does not completely isolate the signals. When the switch is in another position, you can hear the music pretty clearly with the volume up. What switches do not do this? On some El-cheapo solid state units I repaired for a client I had the same problem.

I will discuss the noise issues as soon as I am advised on the switch problem. Also, what do you think of the design? It sounds very good. The distortion is low and is very punchy.
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Old 15th September 2013, 12:03 AM   #2
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by kingneb View Post
I am having some issues with noise and a selector switch that does not completely isolate the signals. When the switch is in another position, you can hear the music pretty clearly with the volume up. What switches do not do this?
The problem may not be with the switch. It could be with the wires that go to and from the switch.

The best way to switch signals is to have balanced signals that go to a double pole switch and have the signals in cables that are twisted pair and shielded, grounded on ONE end. Lacking balanced signals you can still use shield cable right up to the switch terminal.

Also never run the cables parallel to each other. They should come to the switch from different angles.

This same applies to EVERY wire in the ampler, do not rn any in parallel, cross them only at right angles and make full use of the 3 dimensional space.

You can test the switch. Get a signal ganerator and place a test tone on one side and check the other with a good, high impedance AC micro voltmeter.

What I'm suggesting is that th signals are jumping over because via the electric fields. So use both eletrocstatic shields on the signal wires and the "inverse square law" (distance).
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Old 15th September 2013, 12:15 AM   #3
Art M is offline Art M  United States
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Quote:
Originally Posted by kingneb View Post
I am having some issues with noise and a selector switch that does not completely isolate the signals.
The Capacitive coupling between switch contacts/ lines is the
Major problem resulting in CrossTalk.

Suggested solutions: Decrease signal line impedances. Cascade switch
contacts and terminate Off position lines to Ground.

Switch Quality has a relatively minor effect on this coupling.

As suggested by Chris, electric fields are the Major problem.
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Old 15th September 2013, 12:29 AM   #4
BRSHiFi is offline BRSHiFi  United States
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The cables are shielded. The way I have it wired is with shielded two-conductor cable. The black cable carries the left channel signal, the red, the right. The shield is grounded on one end (RCA jack bank). I take it the wires may be too close (see photo).
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Last edited by BRSHiFi; 15th September 2013 at 12:32 AM.
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Old 15th September 2013, 12:41 AM   #5
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by kingneb View Post
The cables are shielded. The way I have it wired is with shielded two-conductor cable. The black cable carries the left channel signal, the red, the right. The shield is grounded on each end. I take it the wires may be too close (see photo).
Odd that you'd place left and right channles inside the same cable. You will have a worse cross talk problem that way, although your ears will not notice so much.

Also never ground BOTH ends of a shield. That allows current to flow. Only ever ground ONE end only. With one end grounded there is no way current can flow. Avoid ground loops.

One thing you might do is duplicate that resister you have across the input. I think you might be useing one resister after the switch. Better to place the resister directly on each input jack. Now your un-selected leads are terminated .

It does make a difference where physically the resister is. Current flows through that resister and of course in th wires it connects to. Placing it on the input jack keeps the current out of the amp and out of the switch,

Also, the over all grounding plan matters. The input should be ground near the jack, even using non-isolated jacks. Then the signal grounds are star ground and finally the star ground connects to the input jack ground stud. There is also a safety ground near where the AC comes in. All the shields conect to the signal ground one one end only. This way zero current flows in the chassis or the shields.
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Old 15th September 2013, 12:55 AM   #6
BRSHiFi is offline BRSHiFi  United States
Previously known as kingden
 
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It seems you posted before I edited. The shields are connected at only one end (the ground of the RCA's.

Quote:
One thing you might do is duplicate that resister you have across the input. I think you might be useing one resister after the switch. Better to place the resister directly on each input jack. Now your un-selected leads are terminated .
I do not have resistors across the inputs, just the volume control. Should I place, maybe a 100k on each input jack from signal to ground?
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Old 15th September 2013, 01:28 AM   #7
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by kingneb View Post
I do not have resistors across the inputs, just the volume control. Should I place, maybe a 100k on each input jack from signal to ground?
Yes try it. But remove the resistor you have now or else you'd have two in parallel for the selected input.

Placing the resister after the switch allows you to only use one resister but placing it before the switch keep some of the signal out of the switch and the wires. Also your sources will continue to "see" a load when they are unselected.

So you end up with multiple copies of R5. The schematic remains the same. Just that now you mound R5 on the input jack and because you have several jacks you use several resisters but only one is switched in at a time. The unselected ones are just loads for the sources.

Last edited by ChrisA; 15th September 2013 at 01:30 AM.
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Old 15th September 2013, 01:33 AM   #8
BRSHiFi is offline BRSHiFi  United States
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That resistor in the schematic is representative of the volume control.

First there is the RCA bank, with cables running to the switch. The switch runs to the volume, which runs to the first stage.

How should I handle this?
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Old 15th September 2013, 02:46 AM   #9
ChrisA is offline ChrisA  United States
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Oh, R5 is a volume control. You can't have a 100K resister in parallel with a 100K volume control. It would mess it up. Make the grounding resistors 10X bigger (at least) then the volume control. So use 1M resisters. But maybe even make the pot smaller. The pot in effect determines the input impedance of the preamp. 25K would work. You'd get slightly less thermal noise too.
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Old 15th September 2013, 03:39 AM   #10
BRSHiFi is offline BRSHiFi  United States
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Now to address some of the hum and hiss issues. I tore apart the grounds and signal cables for better wiring.

I labeled a photo for where my various ground points are. The main star point is a solder lug that makes contact with the chassis. Where do you advise I run the ground wires based upon that layout?

The previous layout had a low level hum that changed with the volume. Changing around tubes varied hiss levels. There was some low level buzz that changed with the volume as well. There was a ground loop somewhere because I, while diking around, occasionally had AM radio interference.

I uploaded a revised schematic denoting which stage is which and some other info as well. Note the right channel is not entirely built.
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