Oscillating amp.

[DaViruz]

I just had my first go at drawing a class A headphone amplifier, this is what i came up with:
http://hem.passagen.se/jocke999/classas.gif
(open it in a new window, if you get a funny error message in a silly language, just place the cursor at the address bar and press enter)

Don't laugh :-), it is the first audio amp i've ever drawn, my main goal was to make it actually amplify the sound, quality of the sound being extremely low priority.
I just built the darn thing, when i finally managed to cool it enough to get a stable DC offset near zero, i hooked up a pair of old crappy $5 headphones and powered it up, and got scared to death. The output was oscillating at about 500Hz, fairly loud.
Anyone got any pointers on how to solve this?

[Elso Kwak]

Originally posted by DaViruz
I just had my first go at drawing a class A headphone amplifier, this is what i came up with:


Don't laugh :-), it is the first audio amp i've ever drawn, my main goal was to make it actually amplify the sound, quality of the sound being extremely low priority.
I just built the darn thing, when i finally managed to cool it enough to get a stable DC offset near zero, i hooked up a pair of old crappy $5 headphones and powered it up, and got scared to death. The output was oscillating at about 500Hz, fairly loud.
Anyone got any pointers on how to solve this?

Hi DaviRuz,
I would like to give some free advice but your schematic did not show even not with a right mouse click.
http://hem.passagen.se/jocke999/classas.gif
Apart from the fact that you call Swedish a funny language, which I find funny , something goes wrong with your link to the picture. What works best on this forum is uploading the picture from your PC. If necessary resample/resize with the program : www.irfanview.com (free of charge)

[DaViruz]

Originally posted by Elso Kwak

I would like to give some free advice but your schematic did not show even not with a right mouse click.
http://hem.passagen.se/jocke999/classas.gif

Try entering the url in the address bar and press enter, it's the damn host that refuses all requests with the wrong http-referer.


Apart from

Originally posted by Elso Kwak
the fact that you call Swedish a funny language, which I find funny , something goes wrong with your link to the picture. What works best on this forum is uploading the picture from your PC.

I did call it a silly language ;-)

Will upload it instead, i usually put all images and stuff on a friends box, but it's down at the moment.

[LennyK]

why you use OE on the output?
maybe try amplyfing signal on the input with BC546 and thn use BD177 as OC follower

if you don't want to do so then try adding some local feedback - put smal resistor from BD177 emiter to -12V

PSU regulated?

good luck

Lenny

[DaViruz]

Here is the pic, sorry for the hassle.

[DaViruz]

Originally posted by LennyK
why you use OE on the output?

No idea, i just picked one.

maybe try amplyfing signal on the input with BC546 and thn use BD177 as OC follower

if you don't want to do so then try adding some local feedback - put smal resistor from BD177 emiter to -12V

PSU regulated?

I'm using a ATX supply, since that's the only thing i've got that i can get +/-12V from at .7A

good luck
Lenny

Thanks. :-)

[Elso Kwak]

Originally posted by DaViruz
Here is the pic, sorry for the hassle.

Hi Daviruz,
No problem, I now see a very clear schematic.
First you will need a coupling cap between your headphone and the amps output as otherwise it has a large offset voltage upon it.
Furthermore with Darlington connected transistors it is very unusual to take the ouput at the collector. It is usually taken from the emittor and the gain provided by some earlier, i.e. upstream stage.

[DaViruz]

Originally posted by Elso Kwak

Hi Daviruz,
No problem, I now see a very clear schematic.
First you will need a coupling cap between your headphone and the amps output as otherwise it has a large offset voltage upon it.
Furthermore with Darlington connected transistors it is very unusual to take the ouput at the collector. It is usually takan from the emittor and the gain provided by some earlier, i.e. upstream stage.

Do i really need the dc blocking cap at the output when i can adjust the dc offset with VR1?
Will redraw it to take the output from the emitter too, since that seems to be a more usual approach. By the way, why is that? Is the more powerfull transistors more unsuitable for voltage amplifing then the small signal trannies?

Thanks for the help so far, much apriciated.

[jam]

Try a small value resistor in series with the base of Q2 and Q1(500 ohms). Also add decoupling caps 10uf (tantalums if possible) to the supply lines (as close to the circuit as possible). You may also try some degeneration on Q1.

Jam

P.S. I suspect the output needs to be taken from the emitter of Q1.

[halojoy]

Good Start, DaViruz
This is how we do when we first try to figure out
how amplification with this thing called transistors works.

This time next year,
you might shows us all some new ideas.
And teach some new beginners how to do the trick
of designing an amplifier!
(While you are experimenting! Please use an 100-470uF Capacitor in series with your headphone output.
One of mine headphones took the shortcut to the heaven of headphones)

The Times They Are A'Changin' /Dylan 1964
Things Have Changed /Dylan 2000

"People are crazy and times are strange
I'm locked in tight, I'm out of range
I used to care, but things have changed"