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Old 16th August 2012, 08:05 PM   #21
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by neutron7 View Post
I don't see how the input from your chipamps could fry anything through the 1k resistor. Even a 30v spike on the chip input could only produce 30ma through the resistor.

What would be able to do something would be a ground differential. That could happen if the mp3 players ground is different from the amplifier. Can you check for a voltage between your amps ground and the ground of your mp3s power supply?
If the players battery powered then the ground assumes whatever potential it happens to connect to. 0 volts or a 1000000 volts it doesn't matter. No current flows. If the player is AC powered then its a whole different ball game.

30 volts at 30 ma doesn't seem much but consider a CMOS chip running at 3.3 volts that draws a micro amp or two. It would certainly zap that.

We'll probably never know what really happened with this one. All we can do is make best guesses as to the most probable scenarios and design to try and avoid that.
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Old 16th August 2012, 09:15 PM   #22
vputz is offline vputz  United States
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Thanks, Mooly. I just spent another fair amount of time in the lab checking continuity of ground et al, doing O-scope traces of the input and output lines, etc. It looks great still. I may just make a "line in condom" with the caps and resistors and some rca jacks until I have time to fix the circuit proper.

The player is (was) indeed bat powered. Ah well. I was getting tired of it and its proprietary connector anyway, so a replacement was coming; this just gets it here a bit sooner.
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Old 16th August 2012, 11:45 PM   #23
vputz is offline vputz  United States
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Taking my new MP3 player's life in my hands, the amp works just as well as it ever has... but when I turned it on, the UPS connected to the same circuit cycled (with speakers but no input attached). I've got a wicked powerup/down thump issue. Guess I need to learn a bit more before recommending this to friends
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Old 16th August 2012, 11:58 PM   #24
vputz is offline vputz  United States
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Toroidal transformer, blah blah inrush current (I'm guessing that's my issue with the PSU cycling). Sounds like the "cheap solution" is to put an NTC thermistor on the primary windings. How do I determine which part to use?
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Old 17th August 2012, 12:36 AM   #25
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I recall reading datasheet from a Wolfson IC intended for mp3 players suggesting a certain output implementation to avoid the headphone out (some class d amp) from going unstable when connected to a high impedance load like the imput of your gainclone.

Maybe the manufacturer cheaped out and that's what happened here?
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Old 17th August 2012, 07:14 AM   #26
Mooly is offline Mooly  United Kingdom
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Switch on thumps are part and parcel of many designs and I don't know how "silent" the LM3875 is in real use. Some chips have circuitry designed to minimise this effect built in.

A speaker relay and small delay is the best solution very often. I don't think inrush current is the cause.
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Old 17th August 2012, 02:09 PM   #27
vputz is offline vputz  United States
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I don't think inrush current is the cause of the mp3 zap, but I do think it was what cycled the UPS. Honestly with the transformer only being 250-350VA, inrush isn't even a big worry so much as an annoyance and I can live without soft start on this if I need.

Still need to make my input decoupler, though.
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Old 18th August 2012, 01:46 AM   #28
vputz is offline vputz  United States
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Irritating addendum:

It got better. I'd made the filter dingus (Mooly, I made a hacked version with too-small caps, and I think it was proof of concept; passed signal well and periodically fuzzed out; I'll try with proper parts at some point), and was listening to music through a new MP3 player, when as I went to toss out the old one I tried power again. Works like a champ.

So, I'm healthily in WTF mode, and still puzzled, but I suppose I should count it as a blessing.
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Old 18th August 2012, 07:25 AM   #29
Mooly is offline Mooly  United Kingdom
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I guess stranger things have happened
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Old 18th August 2012, 10:39 AM   #30
AndrewT is offline AndrewT  Scotland
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is post28 telling us the amp did not eat the mp3?
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