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4S preamp project
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=6392
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Author:  Suncalc [ 18 Dec 2018, 23:42 ]
Post subject:  Re: 4S preamp project

Gab wrote:
Am I right to think that this test shows that the problem does not come from the LED part of the circuit ?
Yes.

Can you describe again the noise on the left channel? Would you classify it as hum? hiss? crackle? popping? I am trying to get some clue as to why the problem would only be on one channel. I am also curious as to whether the noise is there before the cathode follower as well as after.

Author:  Gab [ 19 Dec 2018, 07:50 ]
Post subject:  Re: 4S preamp project

I think that a video is better than me trying to describe a sound. I’m not sure if i’m right but I would call this a hum ?

https://youtu.be/K6AkNfhyYQY

It might not be clear in the video, but the audio signal gets to both speakers undistorted. The hum is kind of adding itseft somewhere into the signal path, but doesn’t seem to affect the actual signal. Also important to mention is that the level on the power amp is only a hair over the minimum. The higher the level of the power amp is, the louder the hum gets.

Author:  Suncalc [ 19 Dec 2018, 12:21 ]
Post subject:  Re: 4S preamp project

That's 120Hz switching noise. Very characteristic of a power ripple coupling into the audio circuitry.

Can you draw out a schematic of EXACTLY what the PS looks like and how the preamp stages and buffers are tied in? There are two issues which need to be solved. First, why there is switching noise (i.e. higher harmonics of 120Hz) when you have an inductor in the power supply and two, why is it coupling into the left channel?

Author:  Gab [ 19 Dec 2018, 17:53 ]
Post subject:  Re: 4S preamp project

Here is the power supply drawing. I'll give a try to the preamp and buffer, but it is a lot more complicated to draw...

Edit : When you say "how the preamp and buffer are tied in", do you mean something like how the B+ gets from the power supply to the preamp and buffer sections ?

Author:  Gab [ 19 Dec 2018, 18:43 ]
Post subject:  Re: 4S preamp project

Maybe this will clarify the supply part of the circuit without being too complicated

Author:  Suncalc [ 19 Dec 2018, 22:47 ]
Post subject:  Re: 4S preamp project

The power supply looks electrically correct. One thing jumps out at me; the left and right preamp channels are drawing B+ from different places. Is the "left" channel of the preamp wired as Section 1 (i.e. pins 6, 7 & 8) or Section 2 (i.e. pins 1, 2, & 3) of the 12A*7?

Author:  Gab [ 20 Dec 2018, 08:41 ]
Post subject:  Re: 4S preamp project

The left channel is connected to the pins 1,2 & 3 of both valves

Author:  Gab [ 13 Jan 2019, 10:29 ]
Post subject:  Re: 4S preamp project

I looked at my preamp with a new (less old) scope and signal generator. I could find only one difference between the two channels. Before the buffer stage, both channels look exactly the same but the signal curve of the left channel gets assymetrical after the valve. The bottom of the curve gets flattened and the amplitude is lower of about 3 volts.

Also, I wasn’t able to actually see the 120hz switching noise. Could anyone explain how to narrow down the possibilities with the scope ? Thanks !

Author:  Suncalc [ 13 Jan 2019, 11:14 ]
Post subject:  Re: 4S preamp project

Gab wrote:
Before the buffer stage, both channels look exactly the same but the signal curve of the left channel gets assymetrical after the valve. The bottom of the curve gets flattened and the amplitude is lower of about 3 volts.
The buffer is designed to take over 100V without overloading. In the one channel you have a wiring or component error. Recheck all the components and connections in that buffer channel.

Oh, and I assume you've tried a different 12AU7 in the buffer socket? To rule out a bad channel in the tube?

Author:  Gab [ 13 Jan 2019, 22:17 ]
Post subject:  Re: 4S preamp project

Suncalc wrote:
Gab wrote:
Before the buffer stage, both channels look exactly the same but the signal curve of the left channel gets assymetrical after the valve. The bottom of the curve gets flattened and the amplitude is lower of about 3 volts.
The buffer is designed to take over 100V without overloading. In the one channel you have a wiring or component error. Recheck all the components and connections in that buffer channel.

Oh, and I assume you've tried a different 12AU7 in the buffer socket? To rule out a bad channel in the tube?


I actually use two 12au7 so I switched them. I couldn’t see any difference on the scope.

When the signal gets to the grid of the buffer, it looks exactly the same as the right channel. After the valve it’s different...

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