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Old 28th January 2013, 04:33 PM   #91
johnr66 is offline johnr66  United States
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I'm worried about the "micas" too. Why are they even there? This appears to be a single rail job. The insulator will just add to the thermal resistance. Also having the grounded heat sink will aid in shielding.

+1 to AndrewT's advice but you are too far along now.
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Old 28th January 2013, 05:12 PM   #92
Philfr is offline Philfr  France
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Hi,
Yes it's single rail. So it's better without heatsink ? Why, (except shielding) ?
Pin 3 is connect with TO220 and ground, no.
For loop and big inductances, it's true, but it's my first diy amp.
When I saw the components arrive, I thought "problems"
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Old 28th January 2013, 07:52 PM   #93
JMFahey is offline JMFahey  Argentina
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Oh, I forgot about "single rail".
Yes, *in this case* you can avoid the micas ..... but not the grease.
Or leave the silicone pads and consider them "grease".
Genarally I always use insulators, even on single rail jobs, so I can ground wherever I like using a piece of wire, instead of where the chip is bolted to.
But in this *very very* particular case, there's not much chassis to speak of , so, be my guest.
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Old 28th January 2013, 07:53 PM   #94
johnr66 is offline johnr66  United States
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Quote:
Originally Posted by Philfr View Post
Hi,
Yes it's single rail. So it's better without heatsink ? Why, (except shielding) ?
Pin 3 is connect with TO220 and ground, no.
For loop and big inductances, it's true, but it's my first diy amp.
When I saw the components arrive, I thought "problems"
No, you NEED the heatsink! you do not need the insulators between the IC and heatsink for single rail use as the tab on the IC is circuit ground potential. Use heatsink paste between the IC and heatsink for good thermal conductivity.

The reason why I don't care for those insulators (when not necessary) is because they add quite a bit of thermal resistance, something like 1 c/w. If your dissipating several watts as heat, the IC will be several degrees hotter. Heat is the enemy of electronics.

Even on my dual supply amps, I let the heatsink have the negative rail on it and insulate it from the chassis.
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Old 29th January 2013, 06:34 AM   #95
Philfr is offline Philfr  France
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Hi,
Thank's, I see..
This amp is to drive a 95 db subwoofer and I need only 5/10 w, maybe in this case it's not a big problem.
I remembered about inductance, TDA's run up to only 200 hz.. What that might create at these frequencies ?
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Old 29th January 2013, 09:52 AM   #96
JMFahey is offline JMFahey  Argentina
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Quote:
I remembered about inductance, TDA's run up to only 200 hz.. What that might create at these frequencies ?
Well, there's a difference in concept here.
The TDA may be fed only <200Hz signals, because it's meant to drive a subwoofer.
Fine.
*BUT* the chip amp itself *still* has wide frequency response, way into the 10's of KHz and above, and still can be unstable, oscillate, etc.
That's why you must still be careful with grounding, wiring layout, etc.

On another thread, somebody commented that he did not use the Zobel network, because "when he connected it the resistor smoked".
Proof of *strong* HF oscillation !!!!
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Old 29th January 2013, 12:07 PM   #97
Philfr is offline Philfr  France
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Yes, I think you are right
1 : test and try like this
2 : how to prevent or reduce oscillations in my case ?
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Old 29th January 2013, 01:29 PM   #98
johnr66 is offline johnr66  United States
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Quote:
Originally Posted by Philfr View Post
Yes, I think you are right
1 : test and try like this
2 : how to prevent or reduce oscillations in my case ?
You won't like the answer to number 2. It involves tearing down what you already built and reassembly on a board with short lead lengths, star ground and RF input filter... However, since you are far along, just complete this amp and test it. If it sounds good to your ears, that all that matters.
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Old 29th January 2013, 03:51 PM   #99
Philfr is offline Philfr  France
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Hi John,
I agree that you said..
But I have two others ways :
3 thrown out by front window
4 frame it for my living room
No, it's a joke
I do not do things like everyone else, we'll see !
Thank's for comments..
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Old 29th January 2013, 04:55 PM   #100
JMFahey is offline JMFahey  Argentina
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Quote:
3 thrown out by front window
4 frame it for my living room
No, it's a joke
Who said it's a joke?
I have done both .... and worse.
Just picture a 40 Lb (20 Kg) construction type sledgehammer and imagine possibilities.
Click the image to open in full size.
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