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It is currently 17 Jan 2019, 01:36

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 Post subject: Re: 4S preamp project
PostPosted: 14 Jan 2019, 02:32 
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Joined: 23 Feb 2017, 02:02
Posts: 270
I just saw your video, thats deffnitley coming from the amp picking up magnetic or eltromatic radiation from transfomer.

I saw that ground switch makes a diffrence, thats a big hint most of the time this means you have done a poor ground wiering scheme and its causing the hum.
If you have ground switch off and theres no humm, that means you have poor ground wiering. And its not picking up noise from transfomer.

If your case is grounded to mains and then you connect ground to b+ negative using case thats a majour source of humm , I always found this to be massive issue.

A possible quick and dirty solution is to directly put parallel ground wires to mains ground from diffrent ciritcal ground points in system. Because the wires are in parallel directly to mains ground this will not become a ground loop and cause noise.


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 Post subject: Re: 4S preamp project
PostPosted: 14 Jan 2019, 16:36 
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Joined: 23 Feb 2017, 02:02
Posts: 270
You really want to have ground from mains connected somewhere over here, if it was connected at power supply section then theres long wire seperating ground to mains and resistance.

This means the amp ground has noise voltage introduced from currents in ground.

Looking at your photo a bit more you have implied that you grounded somehere near filter caps thats very bad having long wires before amp ground can meet mains ground, lots of noise.


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 Post subject: Re: 4S preamp project
PostPosted: 15 Jan 2019, 19:19 
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Joined: 06 Apr 2009, 10:08
Posts: 1520
Location: US Pacific Northwest
Gab;

Have you found your component/wiring error yet?

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Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog


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 Post subject: Re: 4S preamp project
PostPosted: 16 Jan 2019, 09:38 
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Joined: 19 Oct 2018, 15:30
Posts: 51
Location: Montréal, Québec
ILoveHiFi wrote:
You really want to have ground from mains connected somewhere over here, if it was connected at power supply section then theres long wire seperating ground to mains and resistance.

This means the amp ground has noise voltage introduced from currents in ground.

Looking at your photo a bit more you have implied that you grounded somehere near filter caps thats very bad having long wires before amp ground can meet mains ground, lots of noise.


The ground switch completely separate the chassis/mains ground from the signal ground. When the ground is "lifted", the preamp takes its signal ground through the output cables to the power amp. The preamp mains ground and signal ground are then completely separated. The noise is present with the switch in that position, and there’s JUST noise when the switch connects both grounds together.


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 Post subject: Re: 4S preamp project
PostPosted: 16 Jan 2019, 09:52 
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Joined: 19 Oct 2018, 15:30
Posts: 51
Location: Montréal, Québec
Suncalc wrote:
Gab;

Have you found your component/wiring error yet?


I haven’t actually found anything yet, but I have done some modifications to the left side of the buffer circuit. I rechecked all components values for the left channel of the buffer, and everything was correct. I just rearranged the components in a different manner in hope of getting rid of the noise.

I didn’t test the preamp after the modifications, but I will do it tonight. I don’t have a lot of time these days...

I am not shure if it was actually the case, but pins 4 and 5 might have not been connected together correctly. I made shure they are soldered as they should. Could have this caused the kind of issue I was experiencing ?


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 Post subject: Re: 4S preamp project
PostPosted: 16 Jan 2019, 12:10 
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Joined: 06 Apr 2009, 10:08
Posts: 1520
Location: US Pacific Northwest
Gab wrote:
Could have this caused the kind of issue I was experiencing ?
Yes.

If one half of the buffer tube was not heating, then that stage would simply act as a cold rectifier in the circuit. This would explain the clipping you were seeing on the buffer stage.

Again, the buffer (when operating correctly) can handle very large voltage swings (i.e. >100VAC) without clipping or distorting. When you said one channel was clipping I suspected a wiring or component problem, but lack of heater would also case the problem in one channel.

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Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog


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 Post subject: Re: 4S preamp project
PostPosted: 16 Jan 2019, 19:25 
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Joined: 19 Oct 2018, 15:30
Posts: 51
Location: Montréal, Québec
I have good news :

IT WORKS !!!

But, what I would describe as a low level hum is still audible with the volume down to 0. Totally listenable in my opinion, but still, there is room for improvement.

BUT, I think I messed up from the begining.

I checked the 4S with the scope tonight and the clipping I saw two days ago wasn't there anymore. I decided to try the 4s on my system, just to realize that the same f****** noise was still there. I was unplugging it to bring it back on the table to do further testing, but decided to plug it back and try the ground lift switch. We talked about it so why not. Bye bye 120hz switching noise. And I think I missed that from the start !!!

At the very least I should have try this before. Still, I might have more than one problem. The scope dont lie, the curve is now way cleaner than it was before.

It works better with both grounds connected together. Am I missing something or it's supposed to be the opposite ? Also, there's only noise on one channel when ground lifted. Does that means there's still something wrong, and it's ground related since the ground lift switch ''solves it'' ?


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