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Old 5th February 2011, 09:39 PM   #11
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Originally Posted by mattjuk81 View Post
Excellent thanks will I need to lift the ground with a resistor when connecting those pads to the chassis?
Looks like Peter has sorted your problem Just on the resistor, the only time that one would contemplate doing this is when there is a problem with earthloops, However simply adding a resistor is not a safe way to deal with it. Use of an earthloop breaker as shown in this article by Rod Elliot Earthing (Grounding) Your Hi-Fi - Tricks and Techniques is a much safer way to deal with the problem (if indeed there is one).

If the other equipment you are connecting to doesn't have a safety earth (ie double insulated equipment of a commercial nature) then you shouldn't run into earth loop problems.

Tony.
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Old 6th February 2011, 12:00 AM   #12
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Thanks for the help I just want to make sure I do everything correctly
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Old 6th February 2011, 03:54 PM   #13
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Originally Posted by Peter Daniel View Post
BTW, using rail fuses with LMXXXX chip amps is a bad idea; in case only one fuse blows, an excessive DC offset may occur and damage your speakers.
Using no fuses is an even worse idea. Everywhere in the world the use of fuses is obligatory by safety regulations.
If the output stage blows, there will also be an excessive DC offset. That is why decent amplifiers have decent DC protection.
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Old 6th February 2011, 04:48 PM   #14
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Originally Posted by pacificblue View Post
Using no fuses is an even worse idea. Everywhere in the world the use of fuses is obligatory by safety regulations.
If the output stage blows, there will also be an excessive DC offset. That is why decent amplifiers have decent DC protection.
And why the assumption that there is no fuses?

A properly selected fuse on transformer's primary always worked for me. This is also what I see implemented in other manufacturers decent amps.

Can you present a case example where rail fuse is superior to transformer primary fuse and situation where blown rail fuse creates less damage than blown primary fuse in case of LMxxxx chip amp with speakers connected?
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Old 6th February 2011, 06:53 PM   #15
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Quote:
Originally Posted by Peter Daniel View Post
And why the assumption that there is no fuses?
There are obviously none behind the secondaries.

Quote:
Originally Posted by Peter Daniel View Post
A properly selected fuse on transformer's primary always worked for me. This is also what I see implemented in other manufacturers decent amps.
An amplifier that does not comply with elementary safety regulations can never be decent.

Quote:
Originally Posted by Peter Daniel View Post
Can you present a case example where rail fuse is superior to transformer primary fuse
Fuses behind the secondaries are chosen according to the transformer’s nominal output current. They protect the transformer from overload (current that is only slightly higher than the nominal current for an extended time) and short-circuit (current that is much higher than the nominal current even if only for a short time).

Primary fuses must be oversized to 1,5-3 times the nominal current to withstand the inrush current. They can only safely protect the transformer against shorts in the primaries. They may protect the transformer against some shorts with low enough impedance behind the secondaries. Other shorts with not low enough impedance and certain overload situations will however not trip the primary fuse(s).

The workaround to use an inrush current limiter can only partly improve the function of the primary fuses. It can be shown that there are still situations possible where even a right-sized primary fuse won’t blow in spite of a fault on the secondaries, due to the damping effect of the windings and transformer losses. In fact I had to do such calculations during my formation time, so I can assure you that skipping the secondary fuses is a bad idea, just as bad as putting them anywhere else than right behind the secondaries. They don’t belong behind the rectifiers, smoothing caps or wherever else you sometimes see them, but directly behind the transformer secondaries.

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Originally Posted by Peter Daniel View Post
and situation where blown rail fuse creates less damage than blown primary fuse in case of LMxxxx chip amp with speakers connected?
A blown fuse does not create any damage. The damage will rather be created if the primary fuse fails to blow when a secondary fuse would have blown (see above). And with a decent DC protection in place even an inadequate or non-working undervoltage protection in the chipamp will lead to no harm when only one rail fuse blows.
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Old 6th February 2011, 08:36 PM   #16
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Those are certainly all valid points and thanks for the input. I don't mind if people fuse the amps in any possible way, including speaker's output, if that makes them feel better.

As pointed out, calculating proper fuse value may be tricky, and that's one area where secondary fuses in a chip amp can be risky, if not only for a reason that when improperly chosen it can blow without actual fault condition and that can damage the speakers.

And talking about decent (sounding) amps without rail fuses, here's a link to one such example: 6moons audio reviews: FirstWatt F5
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Old 7th February 2011, 06:40 PM   #17
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Originally Posted by Peter Daniel View Post
As pointed out, calculating proper fuse value may be tricky,
Not at all. The transformer's nominal secondary current determines the fuse rating. A slow-blow characteristic is good enough for that task. The next higher current rating is sufficient, if the nominal current does not coincide with an available fuse rating. A smaller fuse is proof of having wasted money on a too big transformer.

The important thing is that those fuses are there to protect the transformer, nothing else. It is not possible to protect the amp or its components with fuses. That can only be done by electronic protection systems and thermal limiters, if at all.
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Old 27th April 2011, 02:58 PM   #18
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More discussion on secondary fuses here: Chip amp power supply- a beginners guide
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Old 16th March 2012, 04:48 AM   #19
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Default Need a little help.

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Originally Posted by Peter Daniel View Post
I assume you are using a single chassis.

There is no need for any additional star ground in case of dual mono setup, as each amp pcb features on board star ground.

The earth ground from power entry module connects directly to chassis and each amp board connects to the same ground point as well (run wires from CHG pads).

BTW, using rail fuses with LMXXXX chip amps is a bad idea; in case only one fuse blows, an excessive DC offset may occur and damage your speakers.
It took me forever to find this quote. The quote pretains to LM4780, but I assume it applies to LM3875 as well.

What if I will have 4 chassis? 1 chassis left amp, 1 chassis left power supply, 1 chassis right amp, 1 chassis right powersupply. Right now on 2 peices of wood, but will be moved to nice seperate 4 x chassis after I make it all work right.

I think this photo of my first amp build must be how "not to do ground for dual-mono". I ended up with a little buzz in the speaker. I guess I was over thinking it, and got confused between dual-mono and stereo grounding layouts.

So I should unsolder this, then move my earth connection to CHG, connect PG+ / PG- to board directly, and no need to worry about any other star ground, is that right?

Any other things I need to do differently, because I have seperated the rectifier and amp boards. I'm specifically looking for how to build an embellical cord between the two chassis.

Thanks for all the help and support. I feel like I'm almost there.

Sincerely,
AlexQS
Attached Images
File Type: jpg starground error.JPG (495.8 KB, 240 views)
File Type: jpg Amp Complete.JPG (253.1 KB, 225 views)
File Type: jpg Earth.JPG (371.1 KB, 222 views)

Last edited by AlexQS; 16th March 2012 at 04:55 AM. Reason: fix typo
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Old 20th March 2012, 03:24 AM   #20
AlexQS is offline AlexQS  United States
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Default Buzz is gone

Hi, not sure if anyone is here...

If it helps any new guy out there to know, when I heard the buzz I instantly assumed it was a ground problem.

It turned out to be that anytime I connected the Coax CATV line from my cable company to my TV, in the vicinity of the amps that the TV would leak RF and make my speakers buzz.

It was a lot of deductive reasoning, and some nice guys on the diyAudio forum were very helpful to me. My first build, and now that the buzz is gone, it's the best sounding amp I have ever owned! Very open, detailed, good imaging, and depth. The bass is very solid, and I only have 10uF on the ps. They are driving what I think of as a heavy load, a 3-way Transmission Line speaker. The little LM3875 with a big powersupply is doing a great job.

I'm sure I'll build another one. Not sure what to try next though.

Later!

AlexQS
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