DIY Audio Projects Forum
 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

DIY Audio Projects Forum

Welcome to the DIY Audio Projects Message Forum. Use these forums to discuss Hi-Fi audio and to share your DIY Audio Projects. Registration is free and required to post messages and view the file attachments. Registration will only take a minute and registered users do not see any advertisements. After you have completed the online registration process, check your email (including spam/junk folder) for the verification email to activate your account. New members are under moderation - so your posts will not be visible until approved by a moderator. See the Read Me 1st, Forum RULES and Forum FAQ to get started on the forum.

It is currently 27 Jun 2019, 01:03

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 81 posts ]  Go to page Previous  1, 2, 3, 4, 5, 6, 7, 8, 9  Next
Author Message
 Post subject: Re: 4S preamp project
PostPosted: 18 Dec 2018, 23:42 
Offline
Project Author
User avatar

Joined: 06 Apr 2009, 10:08
Posts: 1586
Location: US Pacific Northwest
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.

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


Top
 Profile  
 
 Post subject: Re: 4S preamp project
PostPosted: 19 Dec 2018, 07:50 
Offline

Joined: 19 Oct 2018, 15:30
Posts: 117
Location: Montréal, Québec
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.


Top
 Profile  
 
 Post subject: Re: 4S preamp project
PostPosted: 19 Dec 2018, 12:21 
Offline
Project Author
User avatar

Joined: 06 Apr 2009, 10:08
Posts: 1586
Location: US Pacific Northwest
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?

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


Top
 Profile  
 
 Post subject: Re: 4S preamp project
PostPosted: 19 Dec 2018, 17:53 
Offline

Joined: 19 Oct 2018, 15:30
Posts: 117
Location: Montréal, Québec
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 ?


This post has a file attachment. Please login or register to access it. Only Registered Members may view attached files.



Top
 Profile  
 
 Post subject: Re: 4S preamp project
PostPosted: 19 Dec 2018, 18:43 
Offline

Joined: 19 Oct 2018, 15:30
Posts: 117
Location: Montréal, Québec
Maybe this will clarify the supply part of the circuit without being too complicated


This post has a file attachment. Please login or register to access it. Only Registered Members may view attached files.



Top
 Profile  
 
 Post subject: Re: 4S preamp project
PostPosted: 19 Dec 2018, 22:47 
Offline
Project Author
User avatar

Joined: 06 Apr 2009, 10:08
Posts: 1586
Location: US Pacific Northwest
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?

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


Top
 Profile  
 
 Post subject: Re: 4S preamp project
PostPosted: 20 Dec 2018, 08:41 
Offline

Joined: 19 Oct 2018, 15:30
Posts: 117
Location: Montréal, Québec
The left channel is connected to the pins 1,2 & 3 of both valves


Top
 Profile  
 
 Post subject: Re: 4S preamp project
PostPosted: 13 Jan 2019, 10:29 
Offline

Joined: 19 Oct 2018, 15:30
Posts: 117
Location: Montréal, Québec
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 !


Top
 Profile  
 
 Post subject: Re: 4S preamp project
PostPosted: 13 Jan 2019, 11:14 
Offline
Project Author
User avatar

Joined: 06 Apr 2009, 10:08
Posts: 1586
Location: US Pacific Northwest
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?

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


Top
 Profile  
 
 Post subject: Re: 4S preamp project
PostPosted: 13 Jan 2019, 22:17 
Offline

Joined: 19 Oct 2018, 15:30
Posts: 117
Location: Montréal, Québec
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...


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 81 posts ]  Go to page Previous  1, 2, 3, 4, 5, 6, 7, 8, 9  Next

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 19 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
DIY Tube Projects :: DIY Tube Amp Kits :: DIY Speaker Projects :: DIY Solid State Projects :: DIY IC / Op-amp Projects :: DIY Phono Projects :: DIY Cable Projects :: Hi-Fi Audio Schematics
© diyAudioProjects.com - Terms of Service - Privacy Policy