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Old 11th August 2004, 06:59 PM   #1
benmanf is offline benmanf  Israel
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Talking Yay!!!!!!my amp is complete and working!...but...--->

well i built my first amplifier - Progect 3a
it is working but here are some problems:

1. when i disconnect the input signal, its starting to make noises from the speaker (bzzz..)

2. the volume trimpot isn't working, the amp is always at full volume.

3. when the pot stands at max resistence the vol. is still maximum...
but when i switch to minimum resistanse the volume dont change
but it makes some noise form the speakers and the temperature of the output transistors starts to increase VERY fast

PLEASE HELP MEEE!

here is the pic: (ITS STILL MONO, NOT FINISHED MAKING STEREO YET, NOT FINISHED MAKING A BOX...)
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Old 11th August 2004, 07:19 PM   #2
hughmon is offline hughmon  United States
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Umm, if you're referring to the trimpot VR1 in the schematic, that's not a volume pot. That's the output bias adjustment. And yes, if you turn it too far in one direction, things will get hot VERY FAST!


Hum when you disconnect the input is normal for laying out on the floor like that (it's stray noise pick-up).
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Old 11th August 2004, 07:48 PM   #3
benmanf is offline benmanf  Israel
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OOO
CAN YOU EXPLAIN ME WHAT IS BIAS???(LOL IM NOT GOOD IN THIS...IM A BEGINNER AND IM ONLY 14 YEARS OLD HEHE...LEARNING )

LOL THANKS FOR CORRECTING ME ABOUT VR1 (IM A BIGINNER...)
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Old 11th August 2004, 07:57 PM   #4
hughmon is offline hughmon  United States
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Assuming you're not pulling my leg, the output bias is simply the idle current through the output transistors with no signal applied. If the current's too high, things get hot rather quickly (there may be a "bang!" at some point), too low, and the distortion goes up. There should be a recommendation somewhere in the literature for your amp design on setting this.
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Old 11th August 2004, 07:58 PM   #5
PMA is offline PMA  Europe
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You might prefer to study Rod Elliott's project carefully, he explains everything very well, and you will also find articles about amplifier basics on his pages.
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Old 11th August 2004, 08:09 PM   #6
benmanf is offline benmanf  Israel
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THANKS ALL SO NOW I KNOW THAT MY AMP WORKS GOOD!(MY FIRST)

I REALLY NEED TO READ SOME OF THE ARTICLES AT ROD ELLIOTT'S SITE

BTW - its sound quality is fantastic! VERY NICE AMPLIFIER!
exactly what i need in my room ...
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Old 11th August 2004, 08:18 PM   #7
Vivek is offline Vivek  India
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benmanf, I appreciate your enthusiasm but you should do some serious reading or you might harm yourself.
All the best.
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Old 11th August 2004, 09:33 PM   #8
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you need to add an input resistance from input to ground, in front of the input coupling capacitor, C1. Tis probably why the circuit makes noise when the input isn't connected; the output impeadence of your pre-amp is the input resistance when it is connected.

you might try to add a current source for the VAS stage instead of R9 & R10.

Try connecting a couple of power diodes, 1A in series from the base of Q5 to base of Q6. Power diodes have a larger forward voltage, but with control of the bias servo Q9, these diodes would form a "latch" to keep the outputs from having to large DC bias. Also, when one transistor conducts it will push the other into cuttoff so not as much heat would be produced.

Also, you should put the circuit in a metal enclosure to sheild from any stray RF noise that may bounce around in your circuit and cause undesirable effects.
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Old 11th August 2004, 09:50 PM   #9
joensd is offline joensd  Germany
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You should setup the bias as written in the article, then check for DC voltage on the output.
Ideally use some dummy speakers or resistors to test the amp for at least a little while before you connect your beloved speakers.

It may seem a stupid advice but it isnt; Ive seen a good friend being in the same situation as you, being as happy about his new amp as he was angry about his burnt voice coils.

Congratulation to your amp. Quite amazing for your age.

Cheers
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Old 11th August 2004, 10:45 PM   #10
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Benmanf,

Q9 is the bias servo. it should be mounted to the heatsink with the output transistors. The operation theory is: because a BJT tends to conduct more when heated, the bias for these devices is different at low temperature than at high temperature. It takes less Vbe to turn them on when hot. So your bias circuit has to have a dynamic responce to heat. As the outputs get hot, they heat the servo transistor and cause it to conduct more, turning the outputs off more. The idea is to get an overall temperature coefficient of around 1. If the servo isn't mounted with the output transistors, then it is kind of useless and the outputs will go into thermal runaway. (bang!!)

To set your output bias, remove the speaker or load resistance, and measure the voltage accross R13 & 14. Adjust the trimpot VR1, to saturate Q9, turning the outputs off. You should see 0V accross these resistors. slowly turn the pot on until you just begin to see voltage. This is where you want to bias the outputs.

If your DC bias is not 0V, I would suggest changing R2 from 22K to 20K and add a 5K pot in series with it. Even better, get a 15 or 20 turn pot for more accuracy. This will allow you to fine tune the DC bias on the output.

You seem to have a pretty good grasp on things, just some minor technicalities.

PS: If the hisssssssssss is to overbearing try putting a small capacitor say 10-50pF from collector of VAS transistor(Q4) to the base of Q2.
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