oscilloscope EQ question

[capnspoony]

Hi all

So I've just completed a two band parametric EQ .. and my question:

when i feed the input a 10K sin wave and then look at the ouput ... how does my frequency pot function when im feeding the input of the eq a specific value? .. the 10K is in the range of the eq band im putting the scope to.

I notice that it has a peak when the frequency pot "should"match the function generators sin wave frequency and lower amplitude to either side .. im assuming the peak is in fact when the Frequency pot is matching the value that im feeding it and the lows on either side are when it isn't matching the input frequency .. is this correct thinking?

because an eq boosts across a spectrum not a 10K sin wave .. how do I know my eq is working correctly without listening to it?

best
-richie

 

[gyraf]

Try looking at a lower-frequency squarewave in stead. Such one contains a lot of higher spectral components, thus easier to see on a 'scope.

Jakob E.

 

[capnspoony]

So i tried a few various low F square waves to see what I could come up with

and what I found was:

as I turned my frequency pot higher my wave acted as a sort of series expansion... add terms to approximate a function.. in this case a square wave .. as the pot was turned higher my output wave kept getting closer and closer to the input waveform ... I'm not sure I understand what I'm supposed to get out of that.

Are there any standard tests I should be running to understand what is happening inside the circuit?

thanks jakob

best
richie

 

[capnspoony]

bump?

 

[SSLtech]

Oh well I thought people would be al over this, but...

With a sine wave, as you sweep past the sine frequency (say 1kHz) with the EQ on full boost, narrow bandwidth, -you'll see the wave "grow" and then "srhink back to it's original level. The point at which it "grows" to it's highest peak means that your frequency sweep is set at exactly 1kHz.

Set the oscillator to different frequencies and repeat the test, you'll find the same result, but with the frequency "sweep" control at different positions.

Widening the bandwidth (or "Q") control will make the "jump" and subsequent "drop" back down in level less sudden, different boost settings make the "jump" and "drop" less high.

With a square wave, pick a low frequency, then look at the "flat" at the top of the square. Sweep the frequency upwards on a narrow bandwidth, and watch the "ripple" effect on the previously-flat waveform section. Note how the ripples 'tighten' up as boost frequency is swept upwards.

other than that, what do you want to look for... a 'scope is a diagnostic tool, but not so much of a "meter"...

Keith

 

[capnspoony]

thank you for the reply keith

I was wondering if there were any other ways to take advantage of the scope to see if my eq was working.

If those are the two key tests to do my eq passes both of them.

:razz:

I do have a question about it now though .. slightly different topic

Right now I have a simple wire as the in/out .. unbalanced.
I know my signal is fine .. via the tests with the scope.

When I hook the eq up to my 002rack I can't get any signal to come back in to my 002 rack.

When I hook up some crappy rat shack trafos I have to make it balanced .. it passes signal ..

Has anyone ever had this problem with thier 002?

I have my eq working on XLR in and out .. the outputs/inputs to my digi are 1/4inch line level. I have pin 1 to ground 2 Hot and I floated 3... when running the eq unbalanced.

-richie

 

[capnspoony]

edcors soon to replace the crap rat shack

 

[SSLtech]

If it's unbalanced, tie ring-to-sleeve or use MONO ¼" connectors for the connections, NOT the TRS with one terminal ignored. (unless you ignore the sleeve...)

Keith

 

[capnspoony]

thanks again keith

you must be a wizard!

or you know a little something about electronics :shock:

but thanks .. it works now balanced/unbalanced

-richie