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Old 3rd August 2004, 01:31 AM   #1
Nates is offline Nates  United States
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Default Question from a Newbie (no pointing and laughing please)

Well I've always wanted to build my own amplifiers (spent years in the car audio install business), and I've slowly come to terms with the fact that it's something I need to do. I've tried to read up as much as possible (internet research, Sloan's book, etc), and I've put together 2 small amplifiers from kits that I bought (velleman 7W, and a #88 10+10 kit who's name I don't know). Assembly was easy, as my soldering skills are quite good, but I'm REALLY undereducated in electrical theory, so I'm having trouble actually getting it to work. I know I'm missing something really basic here and I'm hoping that someone can fill me in. I've never really needed to worry about power supplies before, and I think that is were my problem lies (although it might be a preamp issue as well). Here's the deal:

1. built the amps (results were the same for both amps).
2. bought a dc adapter to pluc into the wall for 9V/1A
3. connected the adapter to the amp directly at V+ and Ground
4. connected speaker to amp directly
5. used an RCA interconnect off my cable box for input signal
6. Connected directly to amp
7. plugged amp in
8. speaker vibrates quietly
9. Chip and heatsink get hot
10... That's all (the chip and heatsink get hot without input as well).

BTW the speaker is 8ohm 1.5 W

Any help will be VERY appreciated, as I'm feeling REALLY stupid right now.

Any search headers would be appreciated as well.
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Old 3rd August 2004, 12:01 PM   #2
hjelm is offline hjelm  Sweden
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Any pictures or schematics to help us understand the problem?

You are sure that the powersupply is supposed to be single ended, hope it isn't a stupid question but i do not know the kits.

If the supply is single ended then you need a DC blocking capacitor on the output, check that it isn't broken.

Do you have a scope where you can look at the signal?

If not measure DC levels on in and outputs.

When you say that the speaker moves somewhat does it do that without input as well?

If you unplug and plug in the speaker does it pop, would indicate missing or broken DC blocking cap.
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Old 3rd August 2004, 02:14 PM   #3
Nates is offline Nates  United States
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Here are pics of one amp, schematics, and instructions:

http://www.electronics123.com/amazon/datasheet/k88.pdf

It is really just a learners kit, but I don't have anyone I know of to help me troubleshoot.

I'm sorry, but I don't have a scope.

Yes the speaker vibrates without input.

No it doesn't pop.

What should current levels be at the input and output?

Is my TV cable box a viable audio source to plug directly into this amplifier (just chopped off the end of an RCA interconnect and hooked it into the amp), or is there a better source?
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Old 3rd August 2004, 03:19 PM   #4
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I would use a cheap battery operated portable CD or mp3
player as the source. Just get a cable that plugs into the
headphone out and use that to feed the input of the amp,
it will allow you to adjust volume.

Triple check your soldering for any solder bridged shorts
and proper component placement.
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Old 3rd August 2004, 05:36 PM   #5
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Default Troubleshooting

Does the vibration of the speaker sound line 60 cycle hum of 120 cycle buzz. I know its hard to tell without a scope. If hte noise sounds like buzz, it may be that the power supply has too much ripple.

Also, have you really, really made sure all the capacitors are
installed in the correct polarity.

Otherwise, this is a really simple kit and if assembled perfectly, it shoult work.

Ad no, a cable box is an awful signal source. Use a discman or something like that. A least it will have a volume control.
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Old 3rd August 2004, 08:00 PM   #6
Nates is offline Nates  United States
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I've really gotta say, I really appreciate all of your feedback, I'll give it a shot and let you know what I find.

Would a source with too much ripple create a situation where NO sound other than the hum would come out?
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Old 3rd August 2004, 08:16 PM   #7
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Let's check the DC supply first.
Even without a scope we can get a brief idea if OK or not.

You can use a simple DMM and perform two measurements.
1. Turn DMM to DC voltage measurement: Which voltage do you get, between supply + to GND ?
2. Turn DMM to AC voltage measurement: Which AC voltage do you get, between supply + to GND ?
AC component should be small...
If AC is high, then it might help to increase C5 to 470uF or 1000uF.
If necessary or not is depending very much on the inside cap of your
supply adaptor....


Another traditional check: Polarity of all electrolytic caps correct ?

Good luck and stay tuned (diyaudio offers great help, I am
discovering the benefits of this right now in two other threads)

Markus
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Old 3rd August 2004, 08:20 PM   #8
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uhps, I was saying more or less the same as dmfraser...
..sorry for double pointing...
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Old 3rd August 2004, 08:49 PM   #9
Nates is offline Nates  United States
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GOT IT!!!!

Score one for the villiage idiot (that would be me).

Turns out that I had the power supply wiring backwards. Once I started checking for DC voltage (while hooked up) it was -3.6 volts. Flipped it and got +13.7 (which is a bit odd considering it's a 9V adapter, is that normal???). Now it plays great!!!

Thanks SOO much to everyone who replied to my post, I know you all have much much more interesting things to talk about than my problem, but you have no idea how frustrated I was getting (or how good this makes me feel). Now I can move on to some more advanced projects... Hoping to build a Leach before too long.
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Old 3rd August 2004, 08:50 PM   #10
sam9 is offline sam9  United States
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Was this the 7W the V-K4001?

Did you use the power supply, V-K1823? That's the one that's recommended.

Have you a DMM? If so what DC voltage do you get at the output with no input sigal? If you get the powersupply voltage at the output, that is typcal of the wrong component inserted somewhere or the correct one inserted incorrectly.
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