Seventh Circle Audio

I found this USB Oscilloscopeonline. Would this, or something like it, be adequate in callibrating the N72 preamps kits?

If not, are there any other PC interfaces that would be? I'm trying to find a cheaper route than buying a stand alone unit

The device you're referring to has a stated maximum input of 20Vpp, not even half of what you'd need to measure the output of the N72 at clipping. For $150, you can easily get a real scope like this, as just one of many, many examples.

Given that the scope is called for only to eyeball the waveform on the N72 for symmetric clipping, it doesn't call for any great precision, so it appears that this device should suffice.

On the other hand, there are a raft of used scopes on eBay that you can get for even less, and these are Real scopes and are likely to be a lot better.

Or for even less money you could take them to a tech and have the tech set 'em up.

Or for even less money you can skip the scope altogether (the scope tweak is optional.)

IMHO, if you really want to have a scope around in your arsenal of test tools, buy a real scope, and if you don't want to own a scope, use a tech or skip the step. This device looks like it makes for a pretty lousy tool (very limited frequency response, for starters.)

I missed the amplitude limitation. So it's lousy in both axes....

hmmm, the 'take it to tech' sounds like a good idea. where would i go to find a service like that. just look in the yellow pages for an electrition?

edit:
I guess i could always 'buy' one from fry's and then take it back when i'm done

Electricians aren't the right bunch for this. Find a TV/radio repair place or the like (if they still exist). Anybody with a scope that even halfway knows how to use it can follow the instructions and get the N72s set up properly. Or a pro audio or home theatre place with a repair shop. The adjustment takes about a minute, once you've taken five to read the instructions. Seems like a few calls ought to be able to track somebody down.

Couldn't you just use a DAW with a Sine Wave Generator and a realtime Waveform Display (like Waves "PAZ Meters", or some of the tools in Wavelab)? Then set visually for symmetric clipping? I was going to try this, but I have not gotten around to it. Seems like your standard DAW has all of the tools you would need - but I am still a DIY Noob .

Or is there something about ultra-sonic frequencies or exact voltage measurements that need to be analyzed with a scope?

You'd at least have to put some padding in front of it to get the levels low enough so that you don't get digital clipping--the output levels in the clip test are quite extreme.

You could use a DAW for this, though it doesn't have the kind of instant feedback that a scope does.

Like dkatz says, you're likely to clip the input of your converters long before the N72 clips. Even something like an Apogee AD16X clips at a maximum +24dBu, still a few dB short of where you need to be.

I have a few 30dB pads - I think that should pad down the input enough . My RME ADI-8 takes +19dBu for 0dBfs - so I'd assume padding that a further 30dB (or 60dB with 2 pads in series) should easily allow clipping the output of the pre w/o clipping the AD input.

I'll try it when I am off next weekend (Yes, I work the holidays ).

So, all I would do is look for symmentrical clipping of a sine wave in the DAW's waveform display? And adjust the bias until symmetrical clipping is achieved? Is there a desired test signal - or use a standard 1KHz sine? Maybe a 100Hz tone would be better? The Wavelab meters and Waves PAZ plug-ins are pretty much instentaneous (within a few mS), so this should not be too hard IMO. They "capture" the waveform (maybe "Track" the waveform is what I mean?) in the screen just like a Scope can - so it is not like you have to look at a regular scrolling waveform display in your wave editor. It is "realtime" minus a few mS of latency, and of course your scope's bandwidth is dictated by your fs.

Well, did you ever try it???

Nineteen Billion \$m[1]:Well, did you ever try it???

I've used TrueRTA software/soundcard/PC based O'scope with a 30 dB pad for this purpose (www.trueaudio.com). Their free demo software includes a signal generator and o'scope capability that uses the soundcard output for the signal source and input as the o'scope input. That, a PCf, and the pad is all that you need.

The software also gives the RMS voltage, so you can calibrate the output using a DMM, and then use that to calibrate the input. Although that's not needed to adjust an N72. Works like a charm.

HOT!

did you build the 30db pad? If so, got a schematic?

It would be awesome if you had detailed instructions for your procedure. Perhaps some pics (pushing it)? Maybe it could be a sticky

Yeah! Moreover, are pads really as straightforward as a couple resistors, or can they vary in quality?

I guess I mean to ask: is attenuation easy to keep linear?