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Old 12th August 2010, 11:45 PM   #521
Bill_P is offline Bill_P  United States
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Quote:
Originally Posted by ClaveFremen View Post
Hi Tom,

sure it is, a lot of people use with success those 2x22V transformers but some have better regulation than others and this could be a problem...

It's for such reason that I asked to measure that voltage so that we can compare a working one with a faulty one and understand if regulation is a concern.

I agree that mounting the uninsulated version, with the due care, is pretty easy but all things that could go wrong with it simply doesn't exists with the insulated one.

In fact all my builds were done with the insulated version, simply easier and safer.
Transformer regulation should not be an issue since the amplifier should be at idle with no input signal when the relay is supposed to close. There may be a significant difference in the AC mains voltage at various locations and that may have an effect.

The insulated LM3886 has twice the thermal resistance from chip to case compared with the uninsulated device. If mounted with care using a good insulator, the uninsulated device will have a lower thermal resistance to the heatsink. The chip will then run a bit cooler.
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Old 13th August 2010, 01:03 AM   #522
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Quote:
Originally Posted by ClaveFremen View Post
Hello Ben,

try to measure voltage between these two points:

Mine (2x24V sec.) measure 27V

It would be nice if someone with a working MyRef with 2x22V transformer measure also that voltage.

I suspect that your transformer, under load, regulate not so well to a voltage unsuitable for the protection circuit.

Original specs were 2x24-25V but later people found that 2x22V was better suited for a 4 Ohm load but such voltage is borderline for the protection circuit.
Hi Dario,

I measured the voltage across the two points. I read 2.5 vdc on both boards. I jumped another 470 ohm resistor across the one already there, the only effect being the led shining more brightly.( I did this to both boards as well) The un-rectified part of the board measured correct at 23vac.

Being that the voltage is not even close to where it should be on that part of the board, and the fact that both the boards are nearly identical in symptoms, I will start by going through my placement of the components carefully to make sure I did not mix anything up. I should have time to do this this weekend.

Thanks again, Ben
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Old 13th August 2010, 01:13 AM   #523
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Quote:
Originally Posted by Bill_P View Post
The insulated LM3886 has twice the thermal resistance from chip to case compared with the uninsulated device. If mounted with care using a good insulator, the uninsulated device will have a lower thermal resistance to the heatsink. The chip will then run a bit cooler.
Does this statement hold true when one factors in the thermal resistance of a mica chip and 2 layers of thermal compound? It seems National Semi. would have used some insulating material in the TF that has reasonable thermal transfer characteristics, maybe even better than mica. I am back on the fence again.

TIA,

rick
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Old 13th August 2010, 01:26 AM   #524
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Bill P, could you provide a range of safe values to try in parallel with R14, in case someone doesn't have another 470 at hand? There may be more serious problems than the relay that cause an amp to not work, but with all this talk about marginal operation of the protection circuit, I thought that was the easiest way to eliminate at least one possible problem. You're certainly correct that all those other elements could be contributing factors. That is one reason why I always advocate using a smaller tolerance part when it's available. When every element is closer in value to the design, it's got to make everything else operate more efficiently, likely to last longer, and possibly sound better.

Dario's tutorial is a good demonstration with nice pictures of common sense technique for building this or any other electronics kit. I have a few suggestions: build a pair of amps simultaneously. Solder a single connection on one amp, then set that aside to solder the identical connection on the other amp. Alternating like this allows each component to cool slightly between soldering heats. It also might help reduce mistakes. And I would never solder an entire chip without letting it cool down between heats. I'm also a little surprised that you recommend soldering anything from the top of the board. That just scares me, and any beginner should probably not attempt it. There are other ways to hold things perfectly in place, if that's really so important. Who cares if a part is slightly askew? I'd much rather have that than a burned cap or fried chip. I would also recommend that each passive component be measured before inserting in the circuit. This really cuts down on errors. Dario, you might also mention the "light bulb" tester for the first power up.

Peace,
Tom E

Peace,
Tom E
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Old 13th August 2010, 02:26 AM   #525
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Quote:
Originally Posted by Lazarus500 View Post
Does this statement hold true when one factors in the thermal resistance of a mica chip and 2 layers of thermal compound? It seems National Semi. would have used some insulating material in the TF that has reasonable thermal transfer characteristics, maybe even better than mica. I am back on the fence again.
I am amazed at how much I have missed reading and re-reading this thread. The life of a a DIY newbie I guess. It seems if you use an insulator, you don't also use thermal paste. That makes sense with the grey flexible piece I received but how about the mice one? The same here, mica without the paste? I am still leery of my skill as this will be the first heat sinks I have set up. As long a you use a proper shoulder washer, you simply use the insulator, washer, and a steel bolt/nut and tighten it all up? I really worry about the bolt contacting the inside of the mounting hole on the chip? Am I just paranoid? I guess people are very successful using the T and not the TF chip but would find comfort in the TF but am reluctant to give up any advantage in performance.

rick
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Old 13th August 2010, 02:43 AM   #526
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Use a nylon bolt and nut, available at most hardware stores for a few cents.

Peace,
Tom E
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Old 13th August 2010, 07:57 AM   #527
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Quote:
Originally Posted by madisonears View Post
Bill P, could you provide a range of safe values to try in parallel with R14, in case someone doesn't have another 470 at hand?
Based on an earlier post by Bill_P I built mine with 100R at R14. I have 25v secondaries and this puts just under 24v across the relay. Putting a 120R across the 470R will result in around 100R parallel, so anything bigger than this should be OK. For secondaries less than 25v, values slightly lower should be also be fine.

Cheers
Geoff
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Old 13th August 2010, 11:26 AM   #528
Bill_P is offline Bill_P  United States
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Quote:
Originally Posted by geoffcooper View Post
Based on an earlier post by Bill_P I built mine with 100R at R14. I have 25v secondaries and this puts just under 24v across the relay. Putting a 120R across the 470R will result in around 100R parallel, so anything bigger than this should be OK. For secondaries less than 25v, values slightly lower should be also be fine.

Cheers
Geoff
Yes, I agree with your recommendations. With a 22V transformer secondary I used 68 Ohms.
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Old 13th August 2010, 11:33 AM   #529
Bill_P is offline Bill_P  United States
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Quote:
Originally Posted by Lazarus500 View Post
Does this statement hold true when one factors in the thermal resistance of a mica chip and 2 layers of thermal compound? It seems National Semi. would have used some insulating material in the TF that has reasonable thermal transfer characteristics, maybe even better than mica. I am back on the fence again.

TIA,

rick
The LM3886 package is made of plastic, not a particularly good thermal conductor. The thermal resistance from chip to the metal tab on the 'T' package is 1 degree Centigrade per Watt. For the 'TF' plastic package it is 2 degrees Centigrade per Watt. If the interface materials (insulator, thermal paste, etc) between the 'T' package and the heatsink have a thermal resistance less than 1 degree Centigrade per Watt, the cooling advantage is with the 'T' package.

Nylon screws typically do not have very high shear strength and I very much prefer an insulating shoulder washer with metal screw. A proper washer will not allow contact between the tab on the LM3886T and the mounting screw.
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Old 13th August 2010, 11:55 AM   #530
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Hi Tom

Quote:
Originally Posted by madisonears View Post
Dario's tutorial is a good demonstration with nice pictures of common sense technique for building this or any other electronics kit.


Quote:
Originally Posted by madisonears View Post
I have a few suggestions: build a pair of amps simultaneously. Solder a single connection on one amp, then set that aside to solder the identical connection on the other amp. Alternating like this allows each component to cool slightly between soldering heats. It also might help reduce mistakes.
In fact is what I did, I'll write it in the tutorial.

Quote:
Originally Posted by madisonears View Post
And I would never solder an entire chip without letting it cool down between heats.
Usually I do it alternating pins side to side so that the chip don't get too hot on a single point but remember one thing... in automated factories everything is soldered together with a solder bath and in non automated ones the worker solder one entire chip at a time...

I'll better specify also that.

Quote:
Originally Posted by madisonears View Post
I'm also a little surprised that you recommend soldering anything from the top of the board. That just scares me, and any beginner should probably not attempt it. There are other ways to hold things perfectly in place, if that's really so important.
Regarding faston tabs this is necessary...

If you place a little lifted the tab and solder it when you apply force to it you stress the bottom copper layer, if soldered by both sides starting from the top this is avoided.

The only other componens where I suggest to fix their posistion with a little of solder on top side are TO-220 diodes, Zeners and R1,R4.

I'll better specify also that.

Quote:
Originally Posted by madisonears View Post
I would also recommend that each passive component be measured before inserting in the circuit. This really cuts down on errors.
You're right, I'll better specify also that.

Quote:
Originally Posted by madisonears View Post
Dario, you might also mention the "light bulb" tester for the first power up.
I never used it... and I don't think it necessary at all if the due care is taken.

BTW I can mention it.

Thanks for the suggestions.
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