Big Problem!

[Matthewong]

I've tried to make a power amplifier using below schematic. But I'm facing a big problem. The problem is when I use more than 12VDC for the power amp, my scope will show some weird sine wave, the upper peak of the sinew wave become a short straight line while the lower peak is still in good shape. I think this will come out a distorted sound. Anyone out there can help me to solve this problem?

[darkfenriz]

Try R5=100k and R4=10k instead.
And check some bits and pieces like 10uF cap polarity and diodes polarity.

[Leolabs]

If possible,try to convert it to a dual polarity power supply

[Matthewong]

Ok, today I've tested your given advice and the problem is still same. Actually yesterday I have given wrong detail, actually the short striaght line not appear in the upper peak of the sine wave, but appear in the lower peak of the sine wave. Today I've tried to lower the input signal to become a perfect sine wave, and I tried to load a 4ohm resistor to the speaker output, and the lower peak of the sine wave become a short straight line and the upper peak of the sine wave become a very thick. I really have no idea what is going wrong here. Hope you guys can help me.

[Matthewong]

And 1 more thing I've forgotten to tell you, I've changed the BD907 and BD908 to MJE 2955 and MJE3055. Because I can't find BD907 and BD908, so I replaced them with MJE. I think the problem not from there because my friend told me that MJE is better than BD. Am I right?

[ilimzn]

Originally posted by Matthewong
And 1 more thing I've forgotten to tell you, I've changed the BD907 and BD908 to MJE 2955 and MJE3055. Because I can't find BD907 and BD908, so I replaced them with MJE. I think the problem not from there because my friend told me that MJE is better than BD. Am I right?

Did you ask him what are they better for?
The problem certainly IS from replacing BD907/908 with MJE2955/3055 because BD907/908 are darlingtons and MJEs are not. In the future it would be wise if you compared the datasheets of the transistors to see if they are proper replacements, using things like Ic, Vce, hfe. Pd which are far more concrete and useful than 'better-ness'

[moamps]

Originally posted by Matthewong
The problem is when I use more than 12VDC for the power amp, my scope will show some weird sine wave, the upper peak of the sinew wave become a short straight line while the lower peak is still in good shape.

Hi,

I think your amp may be exhibiting some stability problems. Could you measure the frequency of the sine wave? Does the signal appear at the amp's output even when the input is grounded?

You may wish to pull out the output transistors and check if the amp still oscillates. If yes, try connecting a small capacitor (100pF or so) in parallel with the RF resistor (30k).

Nota bene: BD and MJE transistors do not have the same pinning.

Originally posted by ilimzn
..... because BD907/908 are darlingtons ....

Think twice. I don't think so.

Regards,
Milan

[Tony]

datasheets are found here:
http://www.datasheetarchive.com/datasheet/pdf/4473.html

[jaycee]

According to the data sheets i found for the BD907/908 pair, they are not darlingtons, and the pinout is identical to the MJE2955/3055.

The MJE pair is not "better". They are slower by 1MHz, and have a lower hFE. None of this should be causing an issue however. I don't like the look of the ground layout on that PCB though, even though it is directly from the manufacturers data sheet.

When you get the sine wave on the output, do the transistors or the chip itself get very hot?

[ilimzn]

Originally posted by moamps
[BD907/908 darlingtons]
Think twice. I don't think so.

Just looked it up in a couple of books and you are right - apparently my datasheet is a mislabeled one for the BD901, which is a darlington.
Sorry, my mistake.