Audio input from amp to laptop

Hi everyone,
I’ve recently installed an amp in my car and I’m wanting to use an oscilloscope program on my laptop to check for clipping,
I’ve already connected the output from one channel on the amp to a usb audio, video capture card and was able to see the wave form, unfortunately the usb card failed, so I’m presuming that the voltage was too much for it?
I’ve ordered another card as they are pretty cheap and I’m wanting to try again, if I use a transformer to step the voltage down would that result in the wave form dropping and not showing the clipping or as the original signal is clipped would that still show on the oscilloscope.
thanks in advance

 

Yes, the transformer will step down the voltage so will still show clipping. The issue is why your USB card failed. You should ALWAYS have an attenuator before the sound card so you don't clip (or damage) the soundcard with excessive voltage. A simple 10k pot will do the job nicely, rather than a transformer, which is much more expensive and not really the right tool for the job.

You don't really need a transformer, you just need an attenuator.

S.

 

Thank you for the reply.
I’ve had a chance this afternoon to try a transformer that I’ve taken out of an old faulty power supply, so free .
It has 3 wires on one side and 5 on the other, using a Bluetooth radio via the headphone socket and into the microphone socket on my laptop, I’ve been able to play a 1khz test tone from my phone and have been able to drop the voltage from 2 volts to several different voltages right down to 180 mv, so I should be able to keep the voltage down to an acceptable level, would you know the maximum input voltage that would be safe to use? I’m presuming less than 5 volts?

 

A power supply transformer is designed for use with 50/60 Hz so will have limited use at audio frequencies, however, as most of the energy is at low frequencies,that limitation won't be too significant. I would suggest that knowing little about the audio interface you're using, I would limit the voltage to 2v or less. That's why an attenuator is better than a transformer for this application as it isn't (seriously) limited in voltage or frequency response.

S

 

Thank again for your quick reply, I’ve just been reading up on pots and didn’t realise they were so cheap, I’m considering fitting a couple to the wiring in my car to balance 2 pairs of speakers running from my amp which are both on the same front channels from my head unit, 1 pair in the footwell and a pair in the front doors, from what I’ve just read it would appear that 100k logarithmic pots would be best, I really appreciate any help as I’m not really up on electronics.

 

You're welcome. Anything between 10k and 100k log pot will do fine to set levels BEFORE the amps. If you're thinking of using pots at the outputs of the amps, then the pots need to be a LOT lower in resistance, and of much higher power handling than normal pots.

S

 

Thank you again
I’d tried fitting a volume adjuster knob previously between the head unit and the amplifier, but that only provided very little adjustment, just before the maximum level and basically just killed the sound, I now believe that is because the BMW head unit uses 2.5 volts to the speakers (which I have now run to the amp in the boot) which is apparently quite high, normally only around 600 mv.