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Old 5th October 2005, 07:23 PM   #1
Progg70 is offline Progg70  Sweden
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Cool My new amp have a strange behave help!

Hi there!
Now Iím testing my project. It plays nice (the first impression ;-)) and the signal, both music
and the sinus wave looks find when I have the speaker hooked up.
As soon I replace the speaker with a resistor for 4 ohm I got a very hi frequency who is all over
the screen and the sinus is not there anymore.
The music signal is not there either but there is still music and it sounds still the same.
The DC offset seems to be effected and starts to rice.

Please can someone tell me whatís happen?
What can I do to solve the problem?


Regard

Progg70
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Old 6th October 2005, 08:54 AM   #2
EC8010 is offline EC8010  United Kingdom
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You have RF oscillation. I'm afraid attempting to offer a cure for that over the Internet is very hard. There are so many things that could be causing it:

Input leads near output leads.
Poor power supply decoupling.
Missing/too small gate-stopper resistors on power FETs.
Poor earth layout.

I'm amazed no smoke has come out. Normally, RF oscillation is very brief and terminal. Assuming it's a transistor power amplifier, borrow a copy of the "Power Amplifier Handbook" 3rd Ed. by Douglas Self for construction tips.
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Old 6th October 2005, 09:25 AM   #3
dhaen is offline dhaen  Europe
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Some things to try:
Add local decoupling to the power supply pins of the opamp to 0v.
Wire the output stage supplys back to the power supply separately using thick wire epecially for the 0v line.
Try removing C3. In many cases this can cause instability.
Add a small cap (22pF ish) between each opamp's output and inverting input.

Observations:
C2 may cause a large inrush of current at startup, and a DC bounce.
There is no control (servo mechanism) to correct for DC on the output.
The volume control will affect the DC on the output due to opamp offsets.

Conclusion:
I wouldn't connect this to a good speaker for fear of damage untill you have the instability and DC control sorted out - sorry...
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Old 14th October 2005, 02:36 AM   #4
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Default osc

why don't you have any feedback?
put 47p caps at R4 and R12. Then worry about isolating the power supply garbage.

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Old 16th October 2005, 07:31 PM   #5
Progg70 is offline Progg70  Sweden
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Cool Itís aliveÖ

I donít like signal feedbacks, OK you need one for the VAS stage but then they are more of the negative nature for a signal.
A DC feedback for the DC offset could be an option.

What do you mean with the ďThen worry about isolating the power supply garbage.Ē?
How should I put the caps? Parallel with the resistors or to groundÖ?

I have now put the amp in a better chassis and better wiring so everything works now.
I have about 20 Ė 100 mV DC offset without any servo, so Iím pleased with that.

The sound I very nice and the thing get hot, nice. I have it in my room for 15 2M and the effect is enough.
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Old 17th October 2005, 07:54 PM   #6
Progg70 is offline Progg70  Sweden
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Cool How calculate the output power

If I have +/- 12 V power supply and 4 ohm speaker, what is the output power then?
What is the formula? See the circuit in the beginning of the thread.

Regard

Progg70
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