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Old 6th February 2010, 12:30 AM   #1
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Default strange problem with DiY amp

Hi, 5 or 6 years ago I haver built this power amp
Motorola Hi-Fi power amplifier - circuit diagrams, schematics, electronic projects

it is built with TL071cn and 2*28V
this was my first diy amp, and hasn't been used at all since then, till today

I found it in some parts box, and decided to put it together,
amp "works" and sounds great, however, it has one strange problem, when input stage is opened, DC voltage appears on output and power darlingtons get really hot in matter of seconds (would get smoked if left more then 3-4 sec. this way)

when input is connected, there is no anny issue at all and amp works great

I wouls appreciate if someone could help me solve this

thanks,

Slobodan
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Old 6th February 2010, 01:58 AM   #2
sregor is offline sregor  United States
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Normally, I would guess an oscillation problem, but after looking at the circuit, I would wonder if your are removing the ground to the non inverting (pin 3) input and letting the input float? If you want to continue troubleshooting the problems, try putting resistors in the out put collectors so they don't burn up or overheat.
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Old 6th February 2010, 02:32 AM   #3
a.wayne is offline a.wayne  United States
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Maybe here :


Before the first “turning on” you must short circuit the inputs of the amp, and put a mA-meter on the output, than turn the amplifier on, and tune the R13 pot, to decrease the DC current on the output, to some uA-s, or in a lucky situation to zero. I was able to decrease it to 10 uA.


Just a thought .......

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Old 6th February 2010, 03:04 AM   #4
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Could be the floating input is picking up RF and oscilating at the output.
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Old 6th February 2010, 03:07 AM   #5
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This looks more like rf oscillations. Try adding a resistor from the input to ground , as low as possible. Like say 10 K ohms. This will prevent the input from being 'open' when you remove the source . Alternatives would be to change the capacitor C1 on the nfb resistor . C2 provides feedback at HF. Wonder how it's value affects the circuit. The opamp on the circuit shows a 741 ! That's very slow compared to what you have put in. Maybe the c1, c2 cap values need to be changed.

Did you try any other type of opamp ? But it's a good idea to keep a 10 or 20 K resistor at the input to avoid an 'open' circuit situation.
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Last edited by ashok; 6th February 2010 at 03:23 AM.
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Old 6th February 2010, 03:31 AM   #6
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whoa, I added a 10k resistor between input and gnd, no problem any more, however, it seems to me that sound has changed, hmmm, just my imagination or?

btw thats nice suggestion with caps value, I think these are already adapted to 071, can't remember really, it's been a long time since I built this

tomorrow will check this, its 4,30AM so going to catch some sleeping time


thanks guys btw

Last edited by bobekucor; 6th February 2010 at 03:34 AM.
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Old 6th February 2010, 03:33 AM   #7
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Quote:
Originally Posted by bobekucor View Post
whoa, I added a 10k resistor between input and gnd, no problem any more, however, it seems to me that sound has changed, hmmm, just my imagination or?

thanks guys btw
10K might be loading your preamp a bit.
Try 22k and 47k and check for oscilation and difference in sound.
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