Wiring preamp and NIGC

miguel2 · 30th December 2003, 10:16 AM

Hi,

With Xtmas holiday and new year's coming I am having some time to finish some (unfinish

) things.
I am using a preamp based on OPA627/BUF634 and a NIGC after it. Before the pre I have an Alps blue 100k. My problem now is that as I turn the volume up a very large hiss comes from the speakers.
I have the input connected to the volume pot with microphone shielded wire (~25 cm) , being the pot on the front of the standard 17" case. Then it goes to the preamp some (~7cms) with cat5 cable and to the NIGC (5 cms) again with cat5. If I ground the output pin of the pot the hiss is gone so the problem is between input and the pot. I don't want to move the pot to the back of the case, as it is a bit full (It is a 2.1 setup, with LM3875 for the main speakers and a OPA549 for the sub).
This hapens both with a source connected or not. Any suggestions on how to deal with it?

Miguel

Nuuk · 30th December 2003, 11:24 AM

I think that we would need to see an actual wiring diagram to be able to help you Miguel.

miguel2 · 30th December 2003, 12:04 PM

Here it is. In red are the signal cables, black for power. Hope this is understandable.
The star ground is connected to chassis via 100ohm/0,1uF.

Miguel

Nuuk · 30th December 2003, 12:36 PM

The first thing that I can see is that there is no connection between the pot and the signal ground!

miguel2 · 30th December 2003, 02:11 PM

Sorry. The drawing is all from my head... The pot's ground goes to preamp's ground, then to NIGC ground and then to star ground. If I connect the output of the pot to pot's ground everything becomes quiet at full volume.

Miguel

Nuuk · 30th December 2003, 05:53 PM

Quote:

If I connect the output of the pot to pot's ground everything becomes quiet at full volume.

But that is only because you are shorting the signal to ground so none if it get's through to the amp!

About time we had some more help here!

hint - hint.

30th December 2003, 10:16 AM	#1
miguel2 diyAudio Member Join Date: Jun 2002 Location: Portugal	Wiring preamp and NIGC Hi, With Xtmas holiday and new year's coming I am having some time to finish some (unfinish ) things. I am using a preamp based on OPA627/BUF634 and a NIGC after it. Before the pre I have an Alps blue 100k. My problem now is that as I turn the volume up a very large hiss comes from the speakers. I have the input connected to the volume pot with microphone shielded wire (~25 cm) , being the pot on the front of the standard 17" case. Then it goes to the preamp some (~7cms) with cat5 cable and to the NIGC (5 cms) again with cat5. If I ground the output pin of the pot the hiss is gone so the problem is between input and the pot. I don't want to move the pot to the back of the case, as it is a bit full (It is a 2.1 setup, with LM3875 for the main speakers and a OPA549 for the sub). This hapens both with a source connected or not. Any suggestions on how to deal with it? Miguel

30th December 2003, 11:24 AM	#2
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	I think that we would need to see an actual wiring diagram to be able to help you Miguel. __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

30th December 2003, 12:36 PM	#4
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	The first thing that I can see is that there is no connection between the pot and the signal ground! __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

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30th December 2003, 02:11 PM	#5
miguel2 diyAudio Member Join Date: Jun 2002 Location: Portugal	Sorry. The drawing is all from my head... The pot's ground goes to preamp's ground, then to NIGC ground and then to star ground. If I connect the output of the pot to pot's ground everything becomes quiet at full volume. Miguel

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