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Old 26th August 2007, 07:33 PM   #1
engels is offline engels  Israel
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Default Help with a very simple 2002/383 audio amp

I'm stuck far away from home making an uber-simple audio amp for some presentation. I'm talking about this 8W circuit -

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the IC is 2002 which is an updated 8W version of 383, here's the datasheet:

TDA2002

I've made this circuit at least three times before without any problem (with 2002) but now something's wrong.

My PS consists of a 9-0-9V 0.3A transformer which goes into a bridge rectifier and then three filtering stages with 470uF/68V caps conected with 50 Ohm and then 100 Ohm 5W resistors. From there the power goes to pin 5 of the IC. That's the only difference from the schem above (no 2200 filtering cap). The load is two 4.5 ohm speakers connected in series for 9 ohm.

At first, when everything was (badly) connected with clips I wired the 18V secondary to the bridge and received 29V rectified and then 27V after the filtering. The amp was not working probably to a bad connection somewhere. The 2002/383 requires 18-20V so I wired the 9V to the bridge and had around 11V supply. After soldering everything point to point I was measuring 1.5V on pin 5. The power resistors were heating badly to drop 12V to 1.5V. Out of interest I connected the 18V again and the result was the same - 1.5V on pin 5 and lots of heat dissipated in the power supply to drop 27+ volts.

Needless to say the amplifier is not amplifying the input signal, although there are pops in the speakers when I touch the IC's legs.

So what's the problem here?
I've got two ideas:

1 - I've cooked the 2002 when it had 27V on pin 5 at the beginning. It should be able to handle 40V peaks though so I'm not sure.

2 - I don't have ground here (!!!) - all the sockets have only two holes. I think the problem is that I'm in Netherlands and Dutch people are extremely savvy.

HELP!!! I need to finish this damn presentation!!!!
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Old 26th August 2007, 11:06 PM   #2
BWRX is offline BWRX  United States
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Try putting C3 in series with R2 instead of in series with the inverting input. You can also use a lower value for C3 if you use higher value resistors for R2 and R3.

The amp should still work properly without an Earth ground at the outlet.
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Old 30th August 2007, 08:04 AM   #3
djk is online now djk
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"50 Ohm and then 100 Ohm 5W resistors"

Get rid of the resistors and your circuit will work.

"Try putting C3 in series with R2 instead of in series with the inverting input. You can also use a lower value for C3 if you use higher value resistors for R2 and R3."

Real bad ideas. Read the data sheet. This has an internal low value of feedback resistor, do not raise the external resistor values unless you understand the circuit. Don't even think about moving the location of C3.
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Old 30th August 2007, 04:55 PM   #4
BWRX is offline BWRX  United States
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Quote:
Originally posted by djk
Real bad ideas. Read the data sheet.
Good catch djk. I hadn't read the datasheet and just assumed it had a regular differential input. Sorry if my suggestion caused you more grief engels!
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Old 30th August 2007, 06:59 PM   #5
engels is offline engels  Israel
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Default tanx

Quote:
Originally posted by djk
"50 Ohm and then 100 Ohm 5W resistors"
Get rid of the resistors and your circuit will work.
100%!!!
you're right.
I've done it yesterday and it worked. I was probably spending too much time with tubes and for some reason made the PS look like it's from some old tube device.

tanx!!!!!!!!!!!!!!!!!
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