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 17 Sep 2019, 00:41

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 94 posts ]  Go to page 1, 2, 3, 4, 5 ... 10  Next
Author Message
PostPosted: 29 Sep 2008, 18:07 
Offline
User avatar

Joined: 04 Jun 2008, 22:27
Posts: 146
Location: Brazil
Hello all!

So, Ive just finished building my new bridged-chips gainclone, but I'm running into some problems. The amp seems to be oscillating, since past a certain volume it starts making some crackling noises during transients. The chips also run too hot to touch the heatsinks.
Today I made a lot of changes to the amps, but the problem only got worse:

-Put a zobel network: 1uH inductor in parallel to 10R resistor
-Put a low pass filter at the input of the 3875s (680R with 470pF caps)
-Put a 100pF cap at each of the input jacks
-Put a 100nF with a 470uF cap at each of the V+ and V- rails right at each 3875 supply pins.

Right now my filter at the input of the 3875s is a 3.3n with a 680R (which gives me a cutoff frequency of around 70kHz), but the amps still crackles.

My power supply is just a 23-0-23 tranny with a 35A bridge rectifier. After that theres a 4.7K resistor in each rail (to discharge caps) followed by a 1uF cap, a 10uF cap, 2 10.000uF caps, 1 560uF cap, 2 more 10.000uF and at the end a 100nF cap (thats all I have on each rail). Like I said, today I also put a 470uF cap and 100nF on each rail soldered directly to each chip pins as recommended by the datasheet to avoid oscillation.
Attachment:
DSC02059small.JPG

The amplifying section is just like the ones suggested in page 6 of the Application Note 1192 Overture Series High Power Solutions from National Semiconductor. The schematic is shown below.
Attachment:
MGC.PNG

So, I need some help. What else could be causing this??? I believe I covered it all, but the thing wont stop making this scary crackling noises!

Thanks for any help!


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


_________________
http://www.regenceaudio.com
http://valvuladosvintage.blogspot.com.br/


Top
 Profile  
 
PostPosted: 29 Sep 2008, 18:32 
Offline
Editor
User avatar

Joined: 28 May 2008, 21:53
Posts: 4579
Location: Winnipeg, CANADA
Hi Bernardo

bsgd wrote:
So, Ive just finished building my new bridged-chips gainclone, but I'm running into some problems. The amp seems to be oscillating, since past a certain volume it starts making some crackling noises during transients. The chips also run too hot to touch the heatsinks.
Today I made a lot of changes to the amps, but the problem only got worse:

-Put a zobel network: 1uH inductor in parallel to 10R resistor

A zobel is typically a capacitor and resistor in paralell. In the LM3886 Gainclone Amplifier Kit I built, the Zobel consisted of a a 0.1uF capacitor and a 2.7R resistor.

_________________
[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]


Top
 Profile  
 
PostPosted: 29 Sep 2008, 19:29 
Offline
User avatar

Joined: 04 Jun 2008, 22:27
Posts: 146
Location: Brazil
gmilitano wrote:
A zobel is typically a capacitor and resistor in paralell. In the LM3886 Gainclone Amplifier Kit I built, the Zobel consisted of a a 0.1uF capacitor and a 2.7R resistor.

Yes, right, I did try that too, but my problem only got even worse (more crackling). I did it just like this:
http://www.tech-diy.com/LM4780_bridgedamp.htm

Is this the right way to do it when using bridged chips? Cause I really didnt expect it to make things even worse, but it certainly did!

_________________
http://www.regenceaudio.com
http://valvuladosvintage.blogspot.com.br/


Top
 Profile  
 
PostPosted: 29 Sep 2008, 22:32 
Offline
Editor
User avatar

Joined: 28 May 2008, 21:53
Posts: 4579
Location: Winnipeg, CANADA
I haven't tried a bridged amp, but I would expect that the application note would work. The tech-diy amp is essentially the same as the AN. I would implement the Zobel as shown. I can't see how the zobel would make it worse? Just to cover the basics, I imagine the chips are isolated from the heatsink.

_________________
[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]


Top
 Profile  
 
PostPosted: 30 Sep 2008, 21:40 
Offline
User avatar

Joined: 04 Jun 2008, 22:27
Posts: 146
Location: Brazil
Hi Gio! I too dont understand why the schematics on the application note dont work. I will try again the Zobel network as shown as I've now made some mods and that might make it better.
The chips are the insulated version (TF) and the case is wood, so even if the isolation from some of the LM3875s is in trouble, it should still be fine.

Right now, after bypassing the tube, I noticed Im getting too much 120Hz and too much hiss from the speakers. At first I thought this was the tube (Im using AC heaters) but now I know its the LM3875s. Atill not sure why.

I have already changed the feedback loop (tried 100pF in series with 33K across each feeback resistor). I also tried just the 100pF alone (made the things much, much worse). I also tried to use a bigger inductor (6.8uH) and some 1nF caps accross the speaker terminals. Nothing helped at all. Oh, and I tried some clamp onf chokes too on the speaker wire. Nothing.

Im really running out of ideas now. Something is definitely wrong here but I just cant find where the problem really is. I've read the 3875 datasheet and made all the suggested mods to keep the 3875 from oscillating (soldered feedback resistor right at 3875 pins, but them short so they wont capacitively couple to anything, placed 100nF and 470uF caps at each supply rail soldered directly to each 3875 pins, placed 100pF caps at the input to avoid HF from entering the amp) . Still, nothing.

If anyone has any, any suggestion at all, I'd really appreciate! I dont wanna throw this project on my junk pile!

I knew I should have stuck to tube amps :? ... hehehehe

_________________
http://www.regenceaudio.com
http://valvuladosvintage.blogspot.com.br/


Top
 Profile  
 
PostPosted: 01 Oct 2008, 07:40 
Offline
Project Author
User avatar

Joined: 06 Jun 2008, 20:12
Posts: 972
Location: Houston, TX
Figured I would throw this out there. If you have any of the energy saving flourescent bulbs.... turn them off then test radio. I get really bad hiss and I think its a 60hz buzz in the radio upstairs with those lights on. Turn them off and it instantly disappears.
Ri

_________________
DIY Lightspeed Passive Attenuator


Top
 Profile  
 
PostPosted: 01 Oct 2008, 12:46 
Offline
Editor
User avatar

Joined: 28 May 2008, 21:53
Posts: 4579
Location: Winnipeg, CANADA
Hi Bernardo, a couple things to consider. Try an input resistor. 47k to 100k from input to ground. Also, look for ground loops as a ground loop could cause the oscillation too.

_________________
[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]


Top
 Profile  
 
PostPosted: 01 Oct 2008, 18:03 
Offline
User avatar

Joined: 04 Jun 2008, 22:27
Posts: 146
Location: Brazil
RiGuy wrote:
Figured I would throw this out there. If you have any of the energy saving flourescent bulbs.... turn them off then test radio. I get really bad hiss and I think its a 60hz buzz in the radio upstairs with those lights on. Turn them off and it instantly disappears.
Ri


Hi Ri!

Well, I do have some fluorescent bulbs in my kitchen, but they were off during testing.

_________________
http://www.regenceaudio.com
http://valvuladosvintage.blogspot.com.br/


Top
 Profile  
 
PostPosted: 01 Oct 2008, 18:07 
Offline
User avatar

Joined: 04 Jun 2008, 22:27
Posts: 146
Location: Brazil
gmilitano wrote:
Hi Bernardo, a couple things to consider. Try an input resistor. 47k to 100k from input to ground. Also, look for ground loops as a ground loop could cause the oscillation too.


Hi Gio!

I already place a 100K resistor at the input (together with a 100pF cap to avoid HF) but didnt help. Also, there is no ground loops, I carefully checked everything. Today I tested the 3875s to make sure they are OK and they are.

Right now I have my right channel exactly like in the AN1192 paper, tube bypassed, zobel network attached (2R7 and 100nF) and nothing. I believe I have covered it all, read all threads I could find around the web, but still cant get this amp to work. I dont even know if its really oscillating as I did all possible to fix oscillation problems but its still crackling. If its not oscillation, I dont know what it is!

_________________
http://www.regenceaudio.com
http://valvuladosvintage.blogspot.com.br/


Top
 Profile  
 
PostPosted: 01 Oct 2008, 19:13 
Offline
Project Author
User avatar

Joined: 06 Jun 2008, 20:12
Posts: 972
Location: Houston, TX
I think you should redo the solder joints on your volume pot. That was a cause of a lot of crackling for me. And if you are using wires to connect to the pot make sure to check or just resolder each end. The pot end and the amp end and the RCA end as well as the connection to ground.
Ri

_________________
DIY Lightspeed Passive Attenuator


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 94 posts ]  Go to page 1, 2, 3, 4, 5 ... 10  Next

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 5 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:  
cron
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