How can I measure distortion on my mic pre?

I have a cheap tube mic preamp and want to measure the amount of distortion I add when cranking up the tube overdrive.  I don't own an oscilliscope.  Is there a VST tool that does this kind of thing?  I have Googled this and have seen some VST tools that will measure another VST's distortion but have yet to find a tool to measure the incoming signal into the DAW.  

Thanks.

Rimshot

You'd need to use a single sine wave as an input to the mic pre, then use a tool like SPAN to watch for harmonic content.  I'm not aware of a plug that will take the spectrum and calculate harmonic distortion, but just looking at the spectrum will give you an idea.

Sound forge could make you a good sine wave sample, then feed that out your sound card at a really low level back into the mic pre and monitor the resulting input with SPAN, you should see the harmonic content come up as you crank the overdrive.

I'm not sure why you'd want to do this though, it's a given that the tube drive is adding distortion on purpose so the only measurement that matters is how it sounds.

Thanks for the info Mike.  I am not sure I can hear the tube distortion I am adding by cranking up the tube preamp overdrive.  If it was easy, I would like to see the harmonic distortion while I a listening to get a better idea of what I am doing to the sound.  Sometimes it sounds warm and other times not. 
 
Rimshot

Jim Roseberry


If you have a test-tone generator
You could run a sine wave (at very reduced level) into the mic preamp and record several passes using different gain structures on the preamp.  Zooming-in, you can see the results of the preamp.


Had the message open (waiting for a moment to respond).
Looks like several others had the same basic idea.   

Hi Jim,
 
Thanks for the reply.  I have asked if anyone knows of a sine wave generator for the iPhone.  I am going to check on that.  That might be my only direct way of generating a tone without purchasing a sine wave generator. 
 
Rimshot

In the hi fi world we were measuring very low amounts of harmonic and transient intermodulation distortion using distortion analysers and they are interesting and helpful. In your case you are looking at much larger amounts of distortion being introduced on purpose. It is easier to see and hear as a result.

I would be interested in when the preamp was being very clean and at what point any distortion was being introduced. There are some signal level things to consider. Our signal is at various levels at each point in the signal chain. The first level is the level just outside the preamp being fed in. A continuous sine wave at that point of various frequencies and levels would be desirable.  (hardware oscillator and true rms voltmeter very handy at this point) 

(It is important to know how any balanced and line inputs are configured. The block diagram of the pre will tell you this. Some lnputs actually bypass certain input stages in some pres, others do not. They just attenuate the signal and run it in through the Mic pre anyway.)

Inside the pre amp the first input gain stage may be overloaded by the input signal itself. The first stage can often feed a second gain stage and then various amounts of distortion can be added in by pushing the second stage from the first stage. (or an output transformer being driven hard by the input gain pre, Neve likes coupling one stage to the next with transformers, he uses them a lot) The second level is from any first stages inside the pre into its output stage. The third point in the signal chain is the signal and it's level leaving the preamp itself. (Hardware oscilloscope true rms voltmeter and hardware spectrum analyser good bits of kit to have hanging on the output)
 
I would start with the input gain set down low and having any output controls set high. And then looking at the waveform to see how clean it is in real time. You can record of course and playback. You can also use oscilloscope software and there are free ones that will show you quickly. A spectrum analyser would be good to have on hand to see if any other harmonics start rising up. 

I would be increasing the level of the input signal slowly while monitoring its level as well (true rms voltmeter) while observing what is happening to the waveform at the output of the pre amp. Time domain waveforms (such as normal DAW wafveforms) will start to distort their shape too as distortion sets in. The spectrum analyser (frequency domain waveforms) will start popping up other harmonics and you will be amazed by how much!

Square waves are excellent test signals too. They show up all sorts of ringing and oscillation issues that may be inside the Mic pre. But be aware of what the harmonic structure is like for a square wave when looking at it on the spectrum analyser. Sine waves are excellent because there is only one fundamental and it is very easy to see harmonics popping up. The square wave on the output will tell you a lot from its shape change while in time domain mode even. 

I would keep the output high but set the input gain now much higher eg for a microphone perhaps recording a softish instrument. I would do the level shift input thing again to see at what point the output wave form changes its shape too much and when the harmonics start to appear. Then you will have a range of input voltages that you know the pre is clean and from what point it starts distorting. You also need to see what happens when you drive that input gain stage hard and into the output stage that is turned down lower. Waveforms are going to change shape sooner and harmonics will start to rise up as well. You need to note where the controls have to be set in this mode and for ultra clean modes on your pre too.

It is well worth finding all this out. You may be introducing distortion in small amounts without knowing it simply because you are not fully sure how your pre is behaving and what it is doing to your input signal on various control settings. (And the range of input levels hitting the preamp) All of this stuff applies to expensive mic pres too. 

I have got one of those Behringer valve mic pres and I did the same with that. A series of tests which now allow me to set those input and output controls for either very clean operation or quite dirty if need be on top adding in some interesting colour. 

Here is a link to a virtual oscilloscope that was recommended by Sound on Sound. It is only very small and free. It has a very stable trigger mode which allows you to come in very close while still maintaining a perfectly still waveform for more detailed observation.

http://www.jaggedplanet.com/VST/jscope.html

Hey Rimshot,

If you don't have a test-tone generator,
Some DAW applications can generate test-tones.

• Samplitude
• Audition
• Studio One Pro

All can generate a sine wave.

The sine wave could then be burned to an audio CD.

Or, if you have a synthesizer, you could use that to generate a sine wave.

While you are in all this testing mode you can test any audio device to see what range of input voltages it works over and if and when it introduces distortion.

I have gone on about it before but doing level calibrations in and out of your interfaces is also an interesting thing to do. In a previous system I had three sound cards running at once and I discovered that one of them was inverting all of its 8 outputs for some reason. 

Another good thing to do is to see what things like your Pro Channel modules are doing to the signal and how they are distorting things (on purpose of course) The Console emulator is another process which will show up some interesting things in terms of adding harmonics and things to input signals and distorting waveforms.

You certainly can use your DAW as an audio testing station. Many DAW's have test tone signal generators as Jim suggests and there are free and paid ones as well that can generate a whole lot of stuff. There are level meters as well. But this is where you need to calibrate your audio interface so you know what the actual voltage is that appears on any output for any given level. You could generate test signals and route them to an output on your interface. From there over to your unit under test and back through an input again. You could run the spectrum analyser and a nice CRO plugin on a track and monitor the input signal that way. You would be able to run everything in real time. And make recordings too of course of any testing procedure.

You can also use your DAW test setup to check polarity of every speaker and microphone you have. You create an signal that is not symmetrical. ie the top half of the signal is different to the bottom half. You can setup a microphone into a CRO now and run the test signal out to every speaker in your studio (one at a time of course) The mic is placed in front of the speaker and you will see if the speaker is moving the correct way or not. The CRO will display the test signal either the right or wrong way up. I have a bunch of mono speakers placed around my studio to assist me with monitoring in other parts of my room. I found some of those were wired the wrong way around.

Once a speaker is calibrated you can test every mic you own to make sure every diaphragm is moving the right way as well. I found a few mics out of several that were wired incorrectly displaying the waveform the wrong way around. I was using one of them for high hats at one stage. (that sound would have been wrong and hard to mix)