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Old 12th August 2010, 11:29 AM   #511
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Quote:
Originally Posted by ben-jam-in View Post
The Relay is not switching on both boards. The led is very weak. I am using transformers with 22 Vac secondarys.
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.
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Old 12th August 2010, 01:28 PM   #512
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Default Lm3886tf

For various reasons, I have decided to use the fully insulated LM3886TF. Parts Express and Digikey carry these for about $7.30 each but both are out of stock until the end of the year. So I googled for other sources and came up with this Futurelec link.

NATIONAL SEMICONDUCTOR | LM3886TF/NOPB | LM3886 Series TO220-11 High Performance 68 W 8 MHz Audio Power Amplifier - Future Electronics

Their price is $4.40 and they have 1600 available with zero customer rating. Does this raise a red flag with anyone? Could these be counterfeit chips? How could you know? Anyone have thoughts on this?

TIA,

rick
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Old 12th August 2010, 01:39 PM   #513
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I've bought a pair on ebay, they seem genuine (thery're identical to the others I have buyed from RS Components) but I've yet to power on those monoblocks...
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Old 12th August 2010, 08:47 PM   #514
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Default be carefully of bc546

Hi guys, if you have problem with relay (protection circuit), it may worth to check which bc546 you use, as far as I know the pins of Philips bc546 is different from other maker's, exact opposite
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Old 12th August 2010, 10:01 PM   #515
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My amps from the last group buy use 22v transformers and they run just fine. The protection circuit operates AS DESIGNED. Andrew, if you don't believe that and you want to modify yours, that's fine and we'll be happy to learn the results. In the meantime, however, perhaps you shouldn't go on so much about how the circuit is improperly designed. You might be misleading other builders into thinking that's the problem when it might not be.

If your relay doesn't click, there is probably something wrong with your amp. It happened once to me (out of four total builds) and I never could find the cause. I tried replacing active parts until I wrecked the board from desoldering/soldering. I finally just bought a new board and started over with all new active parts. Worked fine on the first try. It cost a few bucks for the new board, new chips, and new transistors, but it was well worth it. I suspect the 318 and transistors are sensitive to soldering heat, static electricity, and lots of other things, and sometimes they are defective from the start. I can't imagine a passive part failing unless it's overpowered or otherwise abused, or far out of tolerance to begin with. I wouldn't expect that of parts of this level of quality.

Or perhaps it IS a balky relay. I just thought of a simple test for those with non-clicking relays. Since R14 determines the power going to the relay and it's near the edge of the board, it would be very easy to turn on the amp and hold another resistor in a pair of insulated pliers and touch it across the leads of R14. You can do this from the top of the board, without having to solder anything. With a second resistor (what size?) in parallel, it will allow more current to flow to the relay. If that's the cause, the relay should click, the amp should play, and you will know what to fix. I don't think this will endanger any other parts of the amp. Don't endanger yourself, either!

Why would anyone use the insulated chip? There is no difficulty in completing the build with the uninsulated chip if it is isolated from ground, which is really quite simple to do. In fact, that should be the easiest part of the entire build.

Peace,
Tom E
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Old 12th August 2010, 10:09 PM   #516
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Quote:
Originally Posted by Lazarus500 View Post
For various reasons, I have decided to use the fully insulated LM3886TF. Parts Express and Digikey carry these for about $7.30 each but both are out of stock until the end of the year. So I googled for other sources and came up with this Futurelec link.

NATIONAL SEMICONDUCTOR | LM3886TF/NOPB | LM3886 Series TO220-11 High Performance 68 W 8 MHz Audio Power Amplifier - Future Electronics

Their price is $4.40 and they have 1600 available with zero customer rating. Does this raise a red flag with anyone? Could these be counterfeit chips? How could you know? Anyone have thoughts on this?

TIA,

rick
Unfortunately, National Semiconductor stopped of giving samples some months ago. For sampling ends. For example, sampling a MyRef_C
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Last edited by regiregi22; 12th August 2010 at 10:18 PM.
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Old 12th August 2010, 10:10 PM   #517
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The protection circuit operates as designed most of the time. The capacitors and transistors do have significant parametric variations and the wrong combination of values could cause malfunction in a few cases. This kind of issue might only show up in a few cases of a fairly large quantity build. It's all a matter of statistics.

Parallelling a resistor across R14 is a good idea for test purposes. Try another 470 Ohm resistor across the existing 470 Ohm resistor on the board and see if the relay closes. I don't think the relay itself is much involved in the problems a few builders have reported.

Regarding Philips BC546 transistors, they were not supplied in the group buy. I checked the datasheet anyway and did not see any pin configuration difference between Philips and the other manufacturers.
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Old 12th August 2010, 10:15 PM   #518
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By the way, nobody has said anything about the excellent job that Dario has made with his tutorial. It is quite straightforward and has useful tips about assembly problems that may raise, or tips for getting components well placed.
Thanks

Regards,
Regi
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Old 12th August 2010, 11:05 PM   #519
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Quote:
Originally Posted by regiregi22 View Post
By the way, nobody has said anything about the excellent job that Dario has made with his tutorial. It is quite straightforward and has useful tips about assembly problems that may raise, or tips for getting components well placed.


Any suggestion to improve it (or my english..) it's welcome.
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Old 12th August 2010, 11:15 PM   #520
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Originally Posted by madisonears View Post
My amps from the last group buy use 22v transformers and they run just fine.
...
Why would anyone use the insulated chip? There is no difficulty in completing the build with the uninsulated chip if it is isolated from ground, which is really quite simple to do. In fact, that should be the easiest part of the entire build.
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.
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