Osciloscope donation

[andrebarata]

Hello guys, i know this is a long shot, but i have to ask.

I'm an amator with electronics, but I have a need for learning and experimenting with it.
I wanted to make an HI-FI Amplifier, but I can't test it's performance and know how good it is without an osciloscope. which i can't afford.

So i'm wondering if anyone has one they can donate. I would love to share my achivements with the donator, and share everything i can.

I live in portugal, so i'm not sure how expensive the shipment could get, but i can ask the post office.

Thanks

[nctnico]

You are better off with a computer and a soundcard to measure distortion.

[andrebarata]

You are better off with a computer and a soundcard to measure distortion.

That sounds like a good idea... although I'm not sure if i can measure any frequencies outside 20-20khz with a sound card. But it may be effective enough. Do you know any software that can make that sort of comparison out of the box?

Thanks

[nctnico]

Most modern soundcards support up to 192kHz sampling rates (about 90kHz of bandwidth). You may need to increase the DC blocking capacitors to improve the low frequency response. Be sure to divide and limit the signal to the soundcard. A soundcard won't like a speaker signal at it's input.

[BravoV]

-> Visual Analyser (freeware)

[andrebarata]

Most modern soundcards support up to 192kHz sampling rates (about 90kHz of bandwidth). You may need to increase the DC blocking capacitors to improve the low frequency response. Be sure to divide and limit the signal to the soundcard. A soundcard won't like a speaker signal at it's input.

So with a 192 kHz sampling rate i should be able to take 9.6 samples of a 20kHz analog wave right? that doesn't sound bad. Yes, i guess in this case the output of the amplifier would need to be significantly reduced in amplitude for the line-in or mic of the card. Thanks for the tip.

-> Visual Analyser (freeware)

Thanks, the software looks pretty good, i'll guive it a try

[Mark Hennessy]

Start with a conventional analogue oscilloscope. You need wider bandwidth and calibrated high-impedance inputs than a sound card offers. A PC with a high-end sound card is useful for checking the detailed performance of a complete and debugged product, but useless for general circuit design and troubleshooting.

If you haven't already, read everything in this book: http://www.amazon.co.uk/Audio-Power-Amplifier-Design-Douglas/dp/0240526139

If you're an absolute beginner at this, start with a "gainclone". The LM3886 is an amazing chip for the money.

[andrebarata]

Start with a conventional analogue oscilloscope. You need wider bandwidth and calibrated high-impedance inputs than a sound card offers. A PC with a high-end sound card is useful for checking the detailed performance of a complete and debugged product, but useless for general circuit design and troubleshooting.

If you haven't already, read everything in this book: http://www.amazon.co.uk/Audio-Power-Amplifier-Design-Douglas/dp/0240526139

If you're an absolute beginner at this, start with a "gainclone". The LM3886 is an amazing chip for the money.

Ok, great tips.
I have checked some 2nd hand oscilloscopes in classified adds here, but there doesn't seem to be many of them and they's usually above 300$, but ill see if i can find a way to get one.

The book seems pretty detailed thanks, and i'm definitely gonna check the LM3886

[nctnico]

Perhaps you should try and talk the sellers into a better price. $200 is close to the price of a new low end digital oscilloscope. An old analogue scope should sell for much less. Some sellers will tell you to sod off while others will be willing to make a deal. It is a good idea to look at the completed listings on Ebay to see for how much a particular piece of equipment sells. That should give you a starting point for a reasonable offer.

If only a few pieces of equipment are for sale then there are not so many buyers as well so prices should be low.

[Simon123]

I have bought my scope for 20$ on flea market.
Its on vacuum tubes and goes only up to 200kHZ, but this should cover audio spectrum.

[madshaman]

If you can't buy an old distortion analyser where you are.  You can try to build one: http://www.cordellaudio.com/papers/build_a_thd_analyzer.shtml

I haven't built this circuit myself but the BOM doesn't seem that pricey.

[KedasProbe]

If you can't buy an old distortion analyser where you are.  You can try to build one: http://www.cordellaudio.com/papers/build_a_thd_analyzer.shtml

I haven't built this circuit myself but the BOM doesn't seem that pricey.

And he will test it with that oscilloscope he doesn't have. 

[madshaman]

If you can't buy an old distortion analyser where you are.  You can try to build one: http://www.cordellaudio.com/papers/build_a_thd_analyzer.shtml

I haven't built this circuit myself but the BOM doesn't seem that pricey.

And he will test it with that oscilloscope he doesn't have. 

The OP wants to build an amp, an oscilloscope is only so useful here (like checking for unwanted oscillation and the presence of noise).

You can't evaluate an amp's performance easily with a scope; it's not really the right tool.

As an aside, and it's only my opinion, the majority of computer soundcards, even the expensive ones don't have the THD and noise performance to make testing an amp with one worthwhile.

[madshaman]

Try hitting up local universities, colleges, TV repair shops, anyone who might use scopes.

Ask if these people might have a decommissioned or obsolete oscilloscope.  Be friendly and honest about your situation.

I'd bet money if you're persistent and keep knocking on doors, you'll b able to get yourself a decent free scope.