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Old 6th November 2009, 12:57 PM   #891
udailey is offline udailey  United States
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Well, The B1 wont color the sound. Thats the nice thing about it. Mine didnt anyway and I had a standard LM3886 amp. I dont remember the volume changing any different before/after B1 but it may have been easier for my amp to do its job. I suppose its the difference between sources.
I wonder if changing the first resistor that goes to ground from signal in your amp would help since it would dump more of the signal to ground and probably give you further rotation on your pot, but that would basically be the same as putting that resistor parallel to your shunt resistors I was talking about.
You could find a pot of similar value to that resistor and replace that resistor with the pot. Then turn the amp on and dial the pot. Of course a pot with legs A W and B would only use legs A and W for this. So with the amp on you would dial it down very slowly and you might find a situation you like.
I know I would do it on my amp, but would do it more out of curiosity. Not saying that I am sure this would solve your problem, but I know that putting a resistor parallel on my pot did help me with a limited pot rotation problem I had.
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Old 6th November 2009, 01:21 PM   #892
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But surely, taking a 10k pot and adding a 15k series resistor, results in something VERY similar to a 25k pot, negating any SQ advantage to using a lower value pot you were suggesting in your first post. The fact is, for me, with a standard Audiosector LM3875 chip-amp, a 10k pot is not practical, and needs additional resistance for a line level signal. I will be testing a range of cheap carbon track pots in the near future to determine the most practical choice, and then replacing it with a stepped attenuator of the same value range.
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Old 6th November 2009, 02:04 PM   #893
udailey is offline udailey  United States
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No, its a resistor that goes parallel to the shunt side of your pot. So it goes from wiper to ground on your pot. This will make the shunt side of your pot move up in resistance more slowly. So the only drawback is that impedance will not be as linear, but if you are dividing voltage by a pot you have the A, W and B legs. A to W is signal in to signal out. Its series. W to B is Signal out to Ground. At W the signal decides to go to amp or to ground. The lower the resistance then the more goes to ground. Lower relative to A to W. So if we were to divide AW by WB the result would be more signal going to ground earlier in the rotation of the pot with things evening out a little and then shooting up towards the end of rotation. Thats my experience with it but the information for how it works is here
Tack a log taper onto a digital potentiometer - 1/20/2000 - EDN
and towards the bottom of this page
Potentiometers (Beginners' Guide to Pots)
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Old 14th November 2009, 02:58 PM   #894
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Hi Peter and the other here.

I dont know f here is the best place to ask for your suggestions in my problem.

I have bought 2 LM3875 premium kits a while a go. I havent got the time to finish them. No i spare some time and i put them together in 2 monoblocks.

As i asembled the first one and put it to work the output was loud enought but with a weird phenomenon , besides the hum (that i can avoid i think) i got after some second a cliping audible loud sound like mors signal besides the music.
even if i disconnet the input the noise insists.
I tried the other amp without the chasis and without cooling in the chip and there was no noise. But when i installed one the noise appeared. !!
Its loud and repetitive.
Any suggestions?

Nick
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Old 14th November 2009, 03:33 PM   #895
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Quote:
Originally Posted by tiglitosa View Post
Hi Peter and the other here.

I don't know f here is the best place to ask for your suggestions in my problem.

I have bought 2 LM3875 premium kits a while a go. I haven't got the time to finish them. No i spare some time and i put them together in 2 monoblocks.

As i assembled the first one and put it to work the output was loud enough but with a weird phenomenon , besides the hum (that i can avoid i think) i got after some second a clipping audible loud sound like mors signal besides the music.
even if i disconnect the input the noise insists.
I tried the other amp without the chassis and without cooling in the chip and there was no noise. But when i installed one the noise appeared. !!
Its loud and repetitive.
Any suggestions?

Nick
I have a suggestion Nick. It could be power supply noise of a kind that is almost impossible to remove with a regulated supply, emanating from these horrendous Ethernet communication cables that use the mains to send router information. Here in the UK, British Telecom have a wireless internet/TV/phone modem called the HomeHub, and it communicates to the TV box via the mains using Ethernet cables to electrical sockets. When I was using it, I got an awful noise like the one you describe in my hi-fi, and it was impossible to remove until I threw the electrical-socket Ethernet adapters away and simply used a single long Ethernet cable for the HomeHub to communicate with the digital TV box. BT is actually breaking the law in issuing these devices, by the way. There's a campaign to ban them. If you share your power supply, it could be somebody else using on of these things that puts junk data into the power supply. Just a thought.

These are the dastardly evil things:
http://www.amazon.co.uk/ComTrend-Pow.../dp/B001M06Y0M

Lucas
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Last edited by LucasAdamson; 14th November 2009 at 03:54 PM.
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Old 14th November 2009, 05:57 PM   #896
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Quote:
Originally Posted by LucasAdamson View Post
I have a suggestion Nick. It could be power supply noise of a kind that is almost impossible to remove with a regulated supply, emanating from these horrendous Ethernet communication cables that use the mains to send router information. Here in the UK, British Telecom have a wireless internet/TV/phone modem called the HomeHub, and it communicates to the TV box via the mains using Ethernet cables to electrical sockets. When I was using it, I got an awful noise like the one you describe in my hi-fi, and it was impossible to remove until I threw the electrical-socket Ethernet adapters away and simply used a single long Ethernet cable for the HomeHub to communicate with the digital TV box. BT is actually breaking the law in issuing these devices, by the way. There's a campaign to ban them. If you share your power supply, it could be somebody else using on of these things that puts junk data into the power supply. Just a thought.

These are the dastardly evil things:
ComTrend Power Line Ethernet Adapter: Amazon.co.uk: Electronics & Photo

Lucas
Thanx Lukas.
Its a little strange i havent thought of it and it might not be the case since in my building only 2 have internet including me and the other one is not so relevant to get those powerline adaptors. Any way i will ask hom to be on the safe side.
The strange thing is that this happened when i attached the heatsink !! Withiut heatsink no noise !! With heatsing a signal machine gun in slow rate.
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Old 14th November 2009, 06:00 PM   #897
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Is the heatsink grounded?
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Old 14th November 2009, 06:13 PM   #898
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Hi Peter

yes, the heatsink is grounded to the chg (chassis ground) of the amp board.
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Old 14th November 2009, 06:24 PM   #899
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Try installing small cap as described here: Commercial Gainclone kit- building instructions
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Old 14th November 2009, 06:37 PM   #900
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Thanx Peter I will try it.

But if is the case of electrical interference why this was not happend when i havent installed the heatsink?

And if i unplug all the other devices , fridge, routers, etc should i still get the noise? I ask this to eliminate all the other possibilities.

Thx

Nick
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