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PostPosted: 01 Oct 2008, 19:18 
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It could be those transformers right near the chips. Other than that, I don't know where else to check.

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PostPosted: 01 Oct 2008, 20:08 
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RiGuy wrote:
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

Hi Ri!

I'm not using a pot actually. I'm just using a separate tube preamp, that I'm 100% positive works fine.
I also tried connecting the amp directly to my CDP, but didn't help... lots of crackling noises from my speakers!

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PostPosted: 01 Oct 2008, 20:10 
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gmilitano wrote:
It could be those transformers right near the chips. Other than that, I don't know where else to check.

Well that I didn't think of! But what to do? Maybe I'll disconnect that one try another trannie, far from the chips.

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PostPosted: 01 Oct 2008, 21:17 
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The 120Hz hum makes me suspicious. Before you move those transformers, you can try and slide some grounded shielding to see if that makes a difference. You don't want those transformers close to the inputs or the chips.

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PostPosted: 01 Oct 2008, 21:30 
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Just doing a little research.
So in BTL an 8 ohm load looks like a 4 ohm load to each IC. The LM3886 datasheet states that for 4 ohm load the supply is +/-28V. This works out to about 40W of power dissipation. For parallel with 2 ICs then a 4 ohm load looks like 8 ohms. 3 chips makes this 4 ohm load look like a 12 ohm load to each chip. All of the Overture chips do not work well at 2 ohms, too hot and Spike protection will kick on fast. You will almost ALWAYS get less power at 2 ohms than 4+ ohms. So this means the design of any multichip amp needs to be designed so each IC sees 4 ohms or higher. Then based on what each IC sees you must use the correct supply voltage range to keep power dissipation around 40W.

That might explain the heat.

AN1192 mentions this about noise and noise on ground that gets amplified.
Quote:
The non-inverting input resistance, Rb, is used to create a voltage drop at the non-inverting terminal to offset the voltage at the inverting input terminal due to the input bias current flowing from the output to the inverting input. Generally, the value of this resistor equals the value of the feedback resistor so that the output offset voltage will be minimized close to zero. However, if this value is too large, noise can easily be picked up which will be amplified and seriously affect the THD+N performance. If the resistor is eliminated and the terminal is grounded, the THD+N performance will be much better, but it will not necessarily be optimized. By connecting the non-inverting input directly to a ground reference, any noise on that ground will be directly injected into the amplifier, amplified and thus will also affect the THD+N performance. The best solution is to use a value of resistance that is not too large that it picks up stray noise and not too small as to be affected by ground noise fluctuations. The value used in the previous plots was a 3.32kΩ resistor. It should be noted that this is not necessarily the optimized value and can change with varying circuit layouts.

Also, Alex88 from shine7.com mentions in his stereo LM3875 that:
UPDATE 10 Sept 2004 : This GC circuit works fine as long as you don't get high frequency interference. I found out after I placed a DAC on top of this GC, some HF noise is present on the speaker output when the DAC is on. My guess is that the noise comes from the DAC's digital circut, and got coupled into the amp. To solve this problem, I added various HF filters :

1. Add a RC filter at input RCA socket, valuse used are 680 ohm and 470pF

2. Add a 220pF capacitor between the + and - signal input pin of LM3875.

3. Add 0.1uF bypass capacitors in parallel to the 100uF power decoupling capacitors.

After adding all these the HF noise is completely gone.

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PostPosted: 02 Oct 2008, 05:14 
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gmilitano wrote:
The 120Hz hum makes me suspicious. Before you move those transformers, you can try and slide some grounded shielding to see if that makes a difference. You don't want those transformers close to the inputs or the chips.

Slide some gounded shielding where? Around the tranny?
Oh,forgot to mention, the tranny itself is connected to the bus ground.

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PostPosted: 02 Oct 2008, 05:28 
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Great reserach you did there!!!

RiGuy wrote:
Just doing a little research.
So in BTL an 8 ohm load looks like a 4 ohm load to each IC. The LM3886 datasheet states that for 4 ohm load the supply is +/-28V. This works out to about 40W of power dissipation. For parallel with 2 ICs then a 4 ohm load looks like 8 ohms. 3 chips makes this 4 ohm load look like a 12 ohm load to each chip. All of the Overture chips do not work well at 2 ohms, too hot and Spike protection will kick on fast. You will almost ALWAYS get less power at 2 ohms than 4+ ohms. So this means the design of any multichip amp needs to be designed so each IC sees 4 ohms or higher. Then based on what each IC sees you must use the correct supply voltage range to keep power dissipation around 40W.

That might explain the heat.

Well, mine is bridged connected to a 8Ohm load. So, since each IC is actually seeing 4Ohms, its more understandable that they get quite hot. Yesterday I did some measurements and they were at 136F.
I even tried running them separate, no bridged (to test each chip and make sure they were fine) and they did run cooler. Now I know why!

RiGuy wrote:
AN1192 mentions this about noise and noise on ground that gets amplified.
Quote:
The non-inverting input resistance, Rb, is used to create a voltage drop at the non-inverting terminal to offset the voltage at the inverting input terminal due to the input bias current flowing from the output to the inverting input. Generally, the value of this resistor equals the value of the feedback resistor so that the output offset voltage will be minimized close to zero. However, if this value is too large, noise can easily be picked up which will be amplified and seriously affect the THD+N performance. If the resistor is eliminated and the terminal is grounded, the THD+N performance will be much better, but it will not necessarily be optimized. By connecting the non-inverting input directly to a ground reference, any noise on that ground will be directly injected into the amplifier, amplified and thus will also affect the THD+N performance. The best solution is to use a value of resistance that is not too large that it picks up stray noise and not too small as to be affected by ground noise fluctuations. The value used in the previous plots was a 3.32kΩ resistor. It should be noted that this is not necessarily the optimized value and can change with varying circuit layouts.

Well, I may chnage that resistor and see what happens. Mine is exactly 3.32K as suggested.

RiGuy wrote:
Also, Alex88 from shine7.com mentions in his stereo LM3875 that:
UPDATE 10 Sept 2004 : This GC circuit works fine as long as you don't get high frequency interference. I found out after I placed a DAC on top of this GC, some HF noise is present on the speaker output when the DAC is on. My guess is that the noise comes from the DAC's digital circut, and got coupled into the amp. To solve this problem, I added various HF filters :

1. Add a RC filter at input RCA socket, valuse used are 680 ohm and 470pF

2. Add a 220pF capacitor between the + and - signal input pin of LM3875.

3. Add 0.1uF bypass capacitors in parallel to the 100uF power decoupling capacitors.

After adding all these the HF noise is completely gone.

Hey I found that too! Hehehe All those mods are already there in my amp. The only difference are:

1- I added a 3.3n and 680ohm (just to be 100% sure no HF would get in, I know I exagerated a little).
2- I used 100pF instead. I'll change that to 220 and see what happens today.
3- Each chip has a pair of 100n soldered directly to its pins going from V+ to GND and V- to GND.

I've spent 3 full days trying to get this amp to work and still couldnt :x hehehe. This is driving me crazy. Lets see if today I can finally hear some good music from it. :xfingers:

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PostPosted: 02 Oct 2008, 05:33 
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Stop and plull the plug.

If a chip is getting hot you have a major issues.

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PostPosted: 02 Oct 2008, 05:43 
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mwhouston wrote:
Stop and plull the plug.

If a chip is getting hot you have a major issues.

Maybe I am missing something but which particulsarly chip is being bridge.

Some e.g. like the LM3875 are not meant to be bridged easily. Yes there are other ways to bridge them.

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PostPosted: 02 Oct 2008, 06:05 
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mwhouston wrote:
mwhouston wrote:
Stop and plull the plug.

If a chip is getting hot you have a major issues.

Maybe I am missing something but which particulsarly chip is being bridge.

Some e.g. like the LM3875 are not meant to be bridged easily. Yes there are other ways to bridge them.

Hi Mark. Yes, Im using exactly 3875 (insulated version) chips and I've followed the National application schematics to do it! Its the one posted here on my intial post.
These are not meant to be bridged easily?? Well, lets say that right now Im really aware of that!!!! Hehehehehe

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