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Old 29th August 2001, 11:31 PM   #11
ppl is offline ppl  United States
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Haldor: I agree with your Views that Protection is Needed and It is True that any General DIY amp as well as Mass market Consumer Amps Maby should have Every Protection Scheme Knowen to man. However I Build my amps along the Lines of the old Threshold amps I was convinced From Many Cases of Consumer and DIY Power Amps i have delt with, That MR. Pass Views at that time Regarding Current Limiting and the Like do Cause Premature Clipping and limit the True Power into Reactive (Real World Speakers)Loads.However i also use His Methods of Lots of output transistors and massive Heatsinks. Thermal cutouts are a good idea not only on the Transformer but the Output transistor Heatsinks as well. Yes the Mains fuse in concert with some Kind of Inrush Current Limiting via a Thermister or Soft Start relay work Rather well. The SOA of a transistor will Change with tempature as well as how mutch Voltage is across it and Foldback Current Limiting is a Constant type of Protection Method and is most often set for worst case conditions thus limiting Amplifire Performance at levels way below the Rated Power under normal operating Conditions. I have also removed the Current Limiting on lots of Consumer Amps and by adding Thermal Cutouts upon the Heatsinks and rail fuses Resulted in an Amp That sounded Better at all volume levels and would servive a shorted output with Hot output transistors. if Foldback type of limiting is wanted then My fav method is what is done on some Marantz solid state amps rather than Cut the Drive off fron the VAS stage thay connect the Transistors used to Current limit to a relay to shut down the amp or disconect the speakers. Another Problem with some Amps is that thay become Unstable when into Current Limit and I have seen quite a few Tweeters Blowen Because of this.

DISCLAIMER: The above are what i have found Works for me and your results may varry. The above concepts are for the Advanced DIY'er and should Not be attemted by those unable to Properly Calculate all the Elements Required to implement the System or Subsystems mentioned above.
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Old 30th August 2001, 11:33 PM   #12
haldor is offline haldor  United States
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Hi PPL,

I hope I'm not being too much of a boor. I'm new on this site and am probably shooting my mouth off more than I should. I'll confess, when it comes to DIY projects I have turned into a belt and suspenders kind of guy, probably because of all the smoke I've released over the years doing dumb things.

I'll never forget my first real electronics project, a 50W tube based Ham radio transmitter I built from a kit in high school. I was so proud of that baby, carefully following the directions, dressing the wires nicely, clean solder joints. Everything looked so perfect, right up until I plugged it in and the smoke started rolling out of the transformer. Turns out I'd wired the filter caps in backwards and when the transformer overheated the windings shorted and it just toasted everthing, I think I actually saw the windings glow for a second or two. It's amazing I wanted to have anything more to do with electronics after that.

Now, I always power up every new project with a variac and even then I don't stand too close until it's had a chance to warm up for a few minutes (1812 overture with exploding tantalums anyone). In my own case you're right, there can't be too many protective devices.

Peace,

Phil
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Old 31st August 2001, 08:22 PM   #13
ppl is offline ppl  United States
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Halder: No i do not find your Post Boring at all. Like I sed i tend to agree that for people just starting out should Use lots of Protection Methods untill thay get comfortable With Building and using the Equipment. I can relate to your Tranceformer Problem. I would have Thought that an AC mains Fuse would have saved that transformer.Like yourself i can remember the Tube era quite well. I had lots of problems with DIY Articals in the Likes of Popular Electronics, Radio Electronics ect. Most had lots of errors that later were Corrected in Print and quite a Few lacked Basic Protection Schems like the AC mains fuse.One Protection Method that i use Both in concert with a variac or as a stand alone Idea is to place a standard Household light Bulb in series with the Transformers Mains input. The Wattage of the Lamp being about half of what the Amp is Rated to output Total. This will light the lamp and save lots of expensive parts in the evbent of an error.Under normal operation the lamp lights Bright when power is first applied then gets dim as the Supply caps charge. If the Lamp stays bright then a problem is happening. This method is for Class Ab and class B amps since Class A amps Draw alot of Current the wattage of the Lamp will have to be raised. I can't say what woud be a Good Rating since I havent Built alot of Class A amps.Yes there are Times when just a Variac alone will allow to mutch Current into a circuit and thus i use this with the lamp for First time Circuits and Troubleshooting.
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Old 3rd September 2001, 08:13 AM   #14
ergo is offline ergo  Estonia
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I studied the possible ways of protection circuits a while back and actually the best method in my mind would be to use a specialised IC.

There seem to be three that are still available from some parts sources. These are

TA7317
HA12002
upc1237

One can find the datasheets from
http://www.freetradezone.com

These are a good solution as the total part count is quite low and at the same time you get
*power on delay
*dc voltage protection for output
*over current protection

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Old 3rd September 2001, 10:04 AM   #15
ppl is offline ppl  United States
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ergo: thanks for the Tip, However the link did not work for me. Yes IC solutions are great sisce thay do have low parts count. Most all mordern Consumer Amps and Receivers use these and thy work very well.
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Old 3rd September 2001, 10:10 AM   #16
ergo is offline ergo  Estonia
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I suppose the site is experiencing some technical problems. Try it again later as it is really great for searching datasheets. You have to register first time though, but they are polite and do not send unwanted spam....

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Old 6th September 2001, 12:17 AM   #17
haldor is offline haldor  United States
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Quote:
Originally posted by ppl
light Bulb in series with the Transformers Mains input. [/B]
A label printer we use to make used a lamp as a current limiting element. The lamp lets a quick pulse of current flow until the filament starts to warm up and increases the series resistance. Back when I worked in the technical support group I once got a call from a technician wanting to know why the lamp didn't light up when he opened the cover. I guess he thought it was like the light in your refrigerator. ;^) That really made my day.

Phil
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Old 6th September 2001, 04:35 AM   #18
ppl is offline ppl  United States
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Halder: in my 15 Plus years of using this technique I havent had a component fail because of a Defect of Improper Hookup. This method works real good for me and has saved manny an output transistor.
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Old 7th September 2001, 01:46 AM   #19
JBL is offline JBL  Canada
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Yeah all right but on the topic of short/overload/thermal protection for power amp waht is the best way to protect an amp.
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Old 7th September 2001, 07:21 AM   #20
hifiZen is offline hifiZen  Canada
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I think that simple current limiting circuits on the output are perhaps a better solution than foldback current limiting... with these, you shouldn't risk the possible instability of a foldback circuit, and the effect on sound quality should be nil if it's properly implemented... of course the trick is making sure that your load isn't going to draw enough current to reach the current limit. Now, of course with a simple current limit circuit, you're still going to deliver the max current into a short circuit, but if your output stage is robust, at least you'll be within the SOA, and your transistors will survive.

The GM75 amp by Neil McGann has a simple current limiting scheme which works well... you can find a carbon copy of Neil's extinct website complete with the schematics on my fossilized website (if it's still there...): http://members.nbci.com/_XMCM/hifiZen/kaa2.html

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