Re: Square wave testing and interpretation

"Chris Hornbeck" <chrishornbeckremovethis@xxxxxxx> wrote in
message news:8otbs317d7fnmqfkj53mncrng99hdib8jq@xxxxxxx

On Wed, 27 Feb 2008 23:46:25 GMT, "Dave"
<dspear99ca@xxxxxxxxx> wrote:

If you want a good all-in-one software test package,
have a look at the Right Mark Audio Analyser. It's
free, and works very well.

If one were to want to test an amplifier, would not the
amp's output signal require attenuation prior to
connection to the soundcard's input?

Yes. My favorite device for this purpose is a pair of 5K ohm 2 watt
potentiometers, soldered up to appropriate cables and connectors for
interfacing to the UUT and the input of the sound card.

Yes, this means that you need to solder and maybe drill some holes in a
little metal or plastic box to hold the parts.

I'm in the midst of trying to figure out some frequency
response problems in a tube amp (EL84 PP, ~15wpc) which
I've recently refurbished/rebuilt. Can I just keep the
amp's volume way down low? I think I should have no
more than 2V input to the sound card or risk frying it.

Any idea what size resistor (10R, 1k, 100k, etc.) I
might need to put in series with the output?

If you search google for "5K ohm 2 watt potentiometer" you will find any
number of sources. About $11.00 each.

You'll need several pieces to make an amplifier
measurement. First
is a load for the amplifier that can safely dissipate the
power. For an EL84-sized amplifier, resistors of 50 watt
rating should be fine; rig a way to keep them up in free
air.

The resistors will ideally be of a kind called "non inductive wirewound".
You should be careful to get resistors that do not dramatically change their
resistance when they heat up. The ones that Radio Shack sells are IME
problematical. I am informed that the following are good:

http://www.partsexpress.com/dayton-non-inductive-resistors.cfm

I have used the following and they are very good as well:

http://www.mouser.com/Search/ProductDetail.aspx?qs=osVJq4lsVJcRNnbAFdVahA%3d%3d


Second is a way to measure output voltage.

The tricky issue here is the fact that a lot of digital voltmeters have
lousy frequency response by design. Obviously, you want one that has good
accuracy over the audio band. Check the spec sheets carefully, and/or buy
some likely candidates with return priveleges.

You can count on the output of a modern sound card in a computer being
flatter from 100-10KHz than most DVMs.

Third is an attenuator for the drive signal to the
amplifier. This *may* be available in usable-enough form
in software; depends on the D/A (soundcard or other)
software.

The 5 K resistors mentioned above are good for this, as well. What you are
trying to avoid is the possibility that the output of your sound card picks
up additional noise and/or distortion as you turn its output attenuator
down. This is a pretty real possibility.

Fourth is an attenuator for the output signal - to -
soundcard.

See above.

Seems like a lot, but it's still just the easy to build
bits of distortion measurement. Also keep in mind the
A/D's bandwidth limitations.

If you want to spend a little money, you can find audio interfaces that
support sample rates up to 192 KHz, which gives you 96 KHz audio bandwidth.
Check out eMu and M-Audio for good values. You can find Rightmark tests for
most popular audio interfaces on line. They basically tell you the lowest
distortion measurements you can make with any degree of accuracy.

I say start out with the audio interface in your PC, and after you get some
experience with it, figure out if you want to go further.


.

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