Need help understanding distortion meter !!!

[Peter Simonsen]

Man this is a first for me.. ..sorry for the newbie question ;-)

Now Jakob E told me face to face..(thanks Jakob) the basics on how to operation such a unit.(see picture).

And I actuelly got the unit to work based on Jakobs fine words of whisdom, but here we go with my question..when I have set the correct level on the unit/range ..then switch to the distortion mode..find the correct freq notch with the freq wheel and see the meter needle starts to fall off..or down the scale..till it stops..how do I read the scale ???

Is it correct if I say the the top scale is the reading in volts...???
the middle scale would be the distortion in %
and the last scale would be in db´s..???

If we take a look at the picture again...would it be correct to say that I`m having a approx distortion reading of 0.6% at 100Hz..???

Or am I totally missing the point here..???

Thanks for any help..

Kind regards

Peter

 

[gyraf]

Hi Peter,

If you think of it voltage-wise, you have on the meter (if I read the sensitivity setting correctly - set to 1mV full-scale) some 0.2mV residual signal.

As you started out with calibrating the unit to a reading of 1.000V with full signal, your remaining (distortion) figure is 0.02% of that, or some -75dB.

:razz: nice meter, that one...!

Jakob E.

 

[Peter Simonsen]

[quote author="gyraf"]Hi Peter,
As you started out with calibrating the unit to a reading of 1.000V with full signal, your remaining (distortion) figure is 0.02% of that, or some -75dB.

:razz: nice meter, that one...!

Jakob E.[/quote]

Hi Jakob,

Thanks..so much..man I´m a bit slow here..let me think about it a little longer and see if it somehow stars to make sence to me..*GGG*..And yes the meter is not bad at all..just my brain ;-)

Thanks again

Peter

 

[gyraf]

Think of it this way:

you throw in a signal at a some level, and set the level reading to 1.000 in "set level" mode.

then you notch out completely the fundamental frequency by setting the unit to "distortion", and finetuning the frequency control untill a minimum reading is reached.

what is now left is the distortion plus noise.

the ratio of leftover-to-original signal is your distortion ratio - be it in percent or db.

you can set the input to accept different signal levels for a reading of 1.000 because you often need to measure dist at diverse levels - so an "absolute" mV-reading wouldn't make sense.

Jakob E.

 

[PRR]

> Is it correct if I say the top scale is the reading in volts...???
the middle scale would be the distortion in %
and the last scale would be in db´s..???

No.

The meter can read from 1mV to 300V, a range of 300,000 to 1. Except a meter can't be read accurately over even 10:1 of range (imagine reading 0.001V on the 1V range). So it has to have a switch. It could be switched 1, 10, 100..., but then it is hard to read something like 0.15V. So they switch it in 1, 3, 10, 30, 100, 300... steps. When you use a step that starts with "1..", you use the top scale that reads 0-1. And when you use a step that starts with "3..", you use the middle scale that reads 0-3.

And at any step, you can also read the bottom "dB" scale, and add the number on the switch. You are showing "-14dB" on the "0dB" range, so your signal is -14dB. (Apparently referenced to 0.775V.)

> when I have set the correct level on the unit/range ..then switch to the distortion mode.. find the correct freq notch with the freq wheel and see the meter needle starts to fall off.. or down the scale.. till it stops.. how do I read the scale???

Ignore the Set Level function for a moment. Set up your signal so you have 1 Volt of 1KHz coming out. The meter reads 1V total output. Now switch in the filter and null the 1KHz signal. What is left is the distortion and noise. It will normally be MUCH less than the signal. If THD is about 10%, and you start with the 1KHz tone at full-scale, then with the 1KHz filtered out you will get about 1/10th
reading. The distortion is 1/10th of the desired signal, and 1/10th is also 10%.

If your amp is any good, THD will be much less than 10%. You might just barely be able to see 1% distortion (1/100th of full scale) without changing the meter range. But it would not be accurate: depends where you put your eye and how well the meter is zeroed. But if you switch down 2 steps on the meter, the indication is 10 times higher and easy to read.

So THD is 100%*(filtered signal)/(unfiltered signal).

If the unfiltered signal is 3.45V (3450mV), and the filtered signal is "0.25" on the "10mV" range, then the filtered signal is 0.25*10mV= 2.5mV. 2.5mV/3450mV= 0.00072 ratio of distortion+noise to signal. We like THD in percent so multiply by 100%: 0.07% THD.

> I'm having a approx distortion reading of 0.6% at 100Hz..???

Well, the gain switch is on the "1V" range so you read the 0-1 scale and see 0.18V.

If the original signal was 1V, and you did not change the gain range when you cut-in the filter, then this looks like 100*0.18V/1V= 18% THD.

 

[Peter Simonsen]

PRR,

My god..that post of yours made it all clear to me..you have a way with words and how you put them together that just keeps on making so much sence..thanks so much..;-)

So both PRR & Jakob..I´m on my way now and have been testing for the last couple of hours..worse case of THD+noise I could get ( using varius input/output voltages) was 0.871%..(this is the tube preamp I was talking about Jacob)..that not soo bad for a tube class A unit is it..???..Not that I care too much..it sounds good to me, and thats the number one important thing to me..*S*

Thanks again..it has been great help from both of you..another small step on the way down the road of electronics.. :thumb:

Kind regards

Peter