Amplifier Bass with problem

[aguantesoda]

hi! someone can helpme with this problem?

the amplifier have this caracteristics:

Power supply: 25 + 25 6 amper
RL: 8 or 4 ohms
Tip 142 and 147 in output(Darlington pair)

So, the problem is, when I put on the amp, it made a strange sound, such as a continuous sound, very deep;

I suppos I have a problem in the circuit, or a problem when I made the circuit, but where?

the inputs and outputs are right, and vcc too.

[edl]

Hi,

Your amp is probably oscillating.

-let's solder a Boucherout-cell on the output: a 10ohm resistor in series with a 100n foil cap, going from the speaker-out to the ground (so paralel with the speaker)
-let's solder a 47nF or 100nF foil cap in paralel with Q4 (the cap goes to collector and emmiter of Q4)
- let's change C5 to 100pF, or even to 220pF.

I'm sure these modifications will help you. (If not, let's check input cap, and C3, C4)

Best regards,

[edl]

On the margin, with +/-25V supply two pair of darlingtons in the output will be more then enough. If you don't go below 4ohm, then 1 pairs in enough.
TIP142/147 has good safe operating area...

[MikeB]

The 39k from bc548 collector to plus rail is WAY too big. That should be more like 3.9k (using 39k to LTP from neg rail). Also try to skip the 1000uf, if they are not wired correctly they do more harm than they help.
Also, bc640 has to dissipate ~450mw, too much for a to92, it will cook. Use bd140 instead.

Also, you have set gain to 1:100 !!! Increase the 390ohm to ~1k-

Mike

[aguantesoda]

ok, I have changed the values of resistances; but nothing happens.

when I put on(enciendo) the amp, the speaker(his cone) moves to the front. its is not normal, I suppos....

the circuit has changed the values only...

[aguantesoda]

where is the trouble?

[MikeB]

You should have said at the beginning that the speaker cone moves to the top. This indicates large DC-offset. I hope you only connected cheap test speaker, this amount of DC-offset can/will destroy a speaker.
It's likely that you have a construction fault or bad part somewhere.
Power up the amp without speaker connected, measure DC-offset at output and voltage across the 3.9k.

Mike

[edl]

R6 need to be about 2,7k. Or put a 4,7k trimmer there, and slowly trim for lowest offset.

And what about these?!:

let's solder a Boucherout-cell on the output: a 10ohm resistor in series with a 100n foil cap, going from the speaker-out to the ground (so paralel with the speaker)

[richie00boy]

Have you set the pot P1 470 ohm to the middle so that there is 235 ohms on each side of it?

[MikeB]

Originally posted by edl
R6 need to be about 2,7k. Or put a 4,7k trimmer there, and slowly trim for lowest offset.

How did you calc 2.7k ? I get 3.9k...

Yes, the trimpot would also be a possibility.

Mike