amp output testing

[vitalstates]

I'm wanting to connect the output of a power amp to the the line in on a cheap soundcard. The intended maximum output or the amp will be around 2.8v into 8 ohms. I've read that to safeguard the soundcard a 2k2 resistor can be put in series with the line in to prevent any damage, but havn't been able to find any further info......I will be using a dummy 8R load on the power amp.

has anybody done this or can anybody suggest how to safely connect power amp output to soundcard line in?.

Ed

[firechief]

Just using a resistor in sires is not a good enough answer, not a complete solution. We need to know a little more about the amp (like model) and that 2.8 volts into 8 ohm's implies a very low power amp. More information please.
Also, by using the search feature you could find where this has been discussed already.+

[Gordy]

+1

You need to consider the maximum ouput of the amp under accidental conditions, i.e. if you accidentally hit the volume knob!

However, most important is the maximum input of the sound card. Some are limited to 1 or 2 Volts.

Either way you will need to make an attenuator for between the 8 Ohm load and the sound card input.

[Arnulf]

I'd add clamping diodes to attenuator output just in case - from output to ground in antiparallel connection.

Besides, you're probably going to test your amplifier and to test it you're going to have to try running it at significantly higher levels (where more distortions occur) than just 1W so you're going to need a suitable attenuator anyway.

[vitalstates]

thanks guys

I was a bit naive in thinking it was a straightforward question, apologies....

all the amps are mine so there are no model numbers...they've all been tested for sensitivity using sig gen and scope...I now want to connect them to the pc as I have a number of VST plugs that will do FFT and plot distortion.....but need to get the signal into the pc without destroying the soundcard.....the sig gen is very precise so I had not anticipated putting more than 1 watt out of the amps, i.e 2.8 volts......just thought there may be an easy way to safeguard the soundcard....

Ed

[Arnulf]

Why protect the sound card if you don't intend to overload its input ?

Here, some more utterings that go well along with above

[vitalstates]

Originally Posted by Arnulf

Why protect the sound card if you don't intend to overload its input ?

Here, some more utterings that go well along with above

are you taking the ****?
did I offend you in some way?
did I ask something silly?

please explain

[tomchr]

Originally Posted by Arnulf

Why protect the sound card if you don't intend to overload its input ?

Because sometimes sh....tuff happens! Like when I accidentally rigged my spectrum analyzer to the 300-ish volt G2 supply rather than G1. I didn't INTEND to do that. And I was only off by one pin... Thankfully, the spec an was intelligent enough to short the input to ground so aside from fusing a relay, the analyzer did not suffer any damage.

Now, back to the original question - which in my opinion and experience is quite valid. The idea behind the input resistor is to limit the current that will run through the input ESD protection of the A/D converter or preamp on the sound card during an over-voltage event. Typically, these ESD structures will handle up to 1 mA or so for short periods of time. So the 2.2 kOhm suggested will probably be OK for 5 volt circuits (assuming the sound card runs on 3.3 V). A better option would be to use a tranzorb (zener diode optimized for handling transients) and limit the available current with a resistor or fuse. If the current is sufficiently limited, I suppose you could use a pair of zener diodes wired cathode to cathode. Put the series combination of the two zeners from "hot" to "ground" on the input of the sound card. For a more robust solution, look at what Pete Millett is doing on his soundcard interface.

~Tom

[aardvarkash10]

how about doing the obvious - put a buffer between the amp output and the soundcard?

[firechief]

Well put tomchr!. As well as most of the suggestions in the thread so far. I was just going to reference Pete Milletts work. It dosen't have to be too painfull to protect your sound card. Search this vast collection for things relating to using an audio card for measurment, as it has been discussed throughorly. (sp? yea, yea, I know. Some form of protection is advisable.