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Need help with my new buffered gainclone!
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Author:  bsgd [ 29 Sep 2008, 18:07 ]
Post subject:  Need help with my new buffered gainclone!

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!

Author:  Gio [ 29 Sep 2008, 18:32 ]
Post subject:  Re: Need help with my new buffered gainclone!

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.

Author:  bsgd [ 29 Sep 2008, 19:29 ]
Post subject:  Re: Need help with my new buffered gainclone!

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!

Author:  Gio [ 29 Sep 2008, 22:32 ]
Post subject:  Re: Need help with my new buffered gainclone!

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.

Author:  bsgd [ 30 Sep 2008, 21:40 ]
Post subject:  Re: Need help with my new buffered gainclone!

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

Author:  RiGuy [ 01 Oct 2008, 07:40 ]
Post subject:  Re: Need help with my new buffered gainclone!

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

Author:  Gio [ 01 Oct 2008, 12:46 ]
Post subject:  Re: Need help with my new buffered gainclone!

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.

Author:  bsgd [ 01 Oct 2008, 18:03 ]
Post subject:  Re: Need help with my new buffered gainclone!

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.

Author:  bsgd [ 01 Oct 2008, 18:07 ]
Post subject:  Re: Need help with my new buffered gainclone!

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!

Author:  RiGuy [ 01 Oct 2008, 19:13 ]
Post subject:  Re: Need help with my new buffered gainclone!

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

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