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LM3886 Progress

Started by DreamSeller, June 22, 2006, 03:15:27 AM

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DreamSeller

My LM3886 kit is in progress, I will post some pictures soon. Both PCBs a populated

I have a heatsink, Transformer, Fuse and PCB stands coming on monday.

Will update once I do some more.

One question though, For the offboard wiring should I use PC stakes? My offboard wiring is pretty terrible, I leave to much wire exposed on the upside of the PCB and it bends and snaps, Any pointers?

-Dream
A Broken Clock Is Right Twice A Day
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joecool85

What I do is push the wire through from the top, solder it on the bottom, then clip the end off to make it a nice small point.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

joecool85

Any more progress?  From anyone? 
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Fixr1984

Both of my board are populated but i havent gotten a power supply or heat sink as of yet.

joecool85

You mean no transformer and heatsink?  The transformer you can get a good deal on at partsexpress.com, as far as the heatsink, eBay is the way to go.

Here is an excellent one:
http://cgi.ebay.com/Wakefield-Engineering-Large-Brushed-Aluminum-Heat-Sink_W0QQitemZ7613908611QQihZ017QQcategoryZ66946QQssPageNameZWD1VQQrdZ1QQcmdZViewItem
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

DreamSeller

My transformer and heatsink arrived today.

Wiring progress starts after some reading.

-Dream
A Broken Clock Is Right Twice A Day
haggardfx.com - Electronics Resource Library

DreamSeller

#6
I know that I have said this before but this time but this time School is over not just holidays so I think this will actually be the time.

Well finally my final year of school is over, I think its about time I finished this pesky thing. This weekend I will look for an enclosure and wire everything up. My main concern is the whole AC power aspect as I have never wired anything from the wall before. I have already checked all my solders and components and I will do so again. I will wear gloves and plug it in to see how it goes.

To test it I will use the pre amp out from my other amp and see how it goes. I still havn't decided on a pre amp yet, one mor thing I can do now. Then I will move onto the LM1875 kit which I havn't even looked at yet...........

Anyhow the point of this was for warnings and pointers on testing this thing for the first time. Also do I need to test the output of the transformer before hooking it up to the power supply board?

Cheers

-Dream
A Broken Clock Is Right Twice A Day
haggardfx.com - Electronics Resource Library

teemuk

#7
Well, just some basic stuff...

The AC wiring in most circuits is pretty simple so there's not much that could go wrong. Still, checking everything few times before wiring is always beneficial and recommended. You seem to be well prepared, which is good ...BUT I recommend that you do nothing until you have built yourself a light bulb current limiter (a light bulb in series with the mains line and the amp) – unless you have one already, of course. That device will reduce the damage during faults drastically.

Do you need to test the output transformer? Well... of course you don't have to test anything but I dearly recommend you do so. Again, there's not much that could go wrong - but if something is wrong then you substantially reduce the damage since you will not be blowing up the rectifier/filter section. Transformers usually take a lot of punishment before they die – rectifiers and filter caps do not.

Similarly: Once you checked that the transformer wiring is correct (and measured that you have right secondary voltages), hook it up to the rest of the power supply and test the power supply circuit independently. If voltages look correct and everything seems to be fine you can connect the power amp section and test it (remember to drain the filter caps before that). Again, if everything is fine, you can connect the preamp. DO NOT put everything together at once: When you build modularly you reduce the damage area and also make troubleshooting easier since you know what module introduced problems. Makes sense, right?

Even more: During initial testing, leave out the speaker, dummy loads etc. Most SS amps work nicely without a load and you can do initial testing without the risk of ruining your speakers or heating up the amp too much (no load – no load current draw). Measure the power amp's DC offset, supply and input voltages. If they are correct and if there are no signs of malfunctions, oscillation etc. you can proceed and connect the speaker.

Do all this while keeping the light bulb limiter in series with the main line. Once you are 100% sure that everything is fine remove the limiter and test the circuit without it.

DreamSeller

Thanks for the info, as expected I have a couple questions

*Im still not exactly sure about where the Light bulb current limiter goes, could someone please claridy
*What is the safeest way to drain caps as I assume these things will have enough power in them to make me very hurting and angry
*What are the correct values I should be looking for when testing out the power amp?

-Dream
A Broken Clock Is Right Twice A Day
haggardfx.com - Electronics Resource Library

teemuk

QuoteIm still not exactly sure about where the Light bulb current limiter goes, could someone please claridy

Info about the light bulb current limiter:

http://www.diyguitarist.com/PDF_Files/AmpCurrentLimiter.pdf
http://www.repairfaq.org/samnew/tvfaq/tvtslbt.htm
http://www.geofex.com/Article_Folders/SPO_Test.htm

QuoteWhat is the safeest way to drain caps

Safest way to drain the capacitors is through permanently attached high-value "bleeder" resistor(s). This way you don't have to touch the caps and you can avoid bleeder probe slips etc. For example, in the amps I have built I have used the power indicator LED and it's series resistor as the bleeder circuit. This automatically drains the caps when power is disconnected. However, if the resistor is high in value (what it realistically must be) the draining takes quite a lot of time and thus can be a bit annoying.

So... For testing purposes you can build a wire where the other end has an alligator clip that you use to form a solid ground connection. This wire terminates to a suitable value resistor (read more about RC time constants from http://www.kpsec.freeuk.com/capacit.htm), which further connects a "probe" end that you stick into the capacitor terminal you wish to drain. Thus, the capacitor discharges its current to ground through the resistor. Monitor the process with a voltage meter to ensure that the cap has drained. When working with high-voltage capacitors the bleeder must be left permanently attached to prevent dielectric absorption from recharging the capacitor again. (The bleeder is of course removed whenever the circuit is powered on again).

Anyway, don't stress yourself too much with this. Draining capacitors is a concern only when:
- Capacitors have a high voltage charge
- Capacitors have a high energy charge (large capacitors > 1000 μF)
Your filter caps likely fill the second clause.

You likely know it already but you can also drain a cap extremely fast by shorting it (this is the typical scenario when accidents take place). I do not recommend this approach because
- The high short circuit current may ruin the capacitor
- The high short circuit current can ruin the tool you use in shorting the capacitor (or the capacitor may weld itself to the metal surface)
- Most importantly: The current peak caused by sudden discharge may destroy delicate semiconductors you have in the circuit. Expect logic circuits, opamps, FETs and alike to suffer the most.

QuoteWhat are the correct values I should be looking for when testing out the power amp?

Frankly put, you are the one who should really know the answers to this question but - for reference - typical values are:

DC offset: few millivolts max.
AC in output without load and without input signal (oscillation test): extremely low or non-existential
Voltage of inverting input = voltage of non-inverting input
Vcc+ and Vee-: Depend on design. I assume you know what these should be; anything within few volts is likely ok. Expect a smaller reading when the amp is loaded and higher reading without any loading.
DC resistance output-common without load = very high
DC resistance output-common with load = about nominal speaker load
I can't say anything about current draw without schematics but the light bulb limiter or excessive heating up of parts should tell you if there's something wrong with this.

joecool85

The LM3886 kit from chipamp.com has a capactor drain resistor on each large power cap, I did the math and also talked to Brian, they drain in 20-30 seconds on their own.  I'd wait 1 minute before messing with them just to be sure, and you can double check by hooking a voltmeter up to them to see what they are at for voltage.  Ideally you want them below 1v, not so you don't hurt yourself, more so you don't hurt the circuit if you short something while messing with it.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

DreamSeller

Cheers Joe. All is soldered and enclosure is ready for after testing.

As for testing a family member of mine has been working with electronics for a while so he is comin along to make sure I dont mess it up.

Finally seems to be coming together. Will post purctures soon.
A Broken Clock Is Right Twice A Day
haggardfx.com - Electronics Resource Library

joecool85

Quote from: DreamSeller on November 18, 2007, 04:07:32 AM
Cheers Joe. All is soldered and enclosure is ready for after testing.

As for testing a family member of mine has been working with electronics for a while so he is comin along to make sure I dont mess it up.

Finally seems to be coming together. Will post purctures soon.

Awesome!  I'll be waiting.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

DreamSeller

A Broken Clock Is Right Twice A Day
haggardfx.com - Electronics Resource Library

DreamSeller

A Broken Clock Is Right Twice A Day
haggardfx.com - Electronics Resource Library