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 Forum index » DIY Hardware and Software » Thomas Henry designs
555 VCO output volume varying with frequency
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alanwilder81



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PostPosted: Tue Aug 22, 2017 6:32 am    Post subject: 555 VCO output volume varying with frequency Reply with quote  Mark this post and the followings unread

hello everyone.
self -explanatory title.
MY newly built 555 VCO exhibits a remarkable volume amplitude fluctuation depending on the frequency i set it to.
It gets louder as the frequency increases.
the effect is most pronounced with the triangle wave, as ,at low frequencies its is almost inaudible.
What might be wrong? i Suspect this is not a normal behavior !

thanksfor any suggestions
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JovianPyx



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PostPosted: Tue Aug 22, 2017 9:44 am    Post subject: Reply with quote  Mark this post and the followings unread

Can you show us a full schematic?

What it reads like is something similar to a VCO passed through a low pass filter, but you don't mention a filter. Have you measured the voltage of the output with an oscope?

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wackelpeter



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PostPosted: Tue Aug 22, 2017 10:46 am    Post subject: Reply with quote  Mark this post and the followings unread

Well at some extent it is normal behaviour or human ear response, what you experience.
The lower frequency of waveforms like triangle or sine are perceived at a "feeled" lower volume at the human ear... I guess it has something to do with the dynamics within the waves... for example in square or saw you have at least one flank of the wave a rapid change in Amplitude, which the human ear can detect much better. pls someone correct me if this is nonsense. Smile

It's somewhat similar to a 5Hz Square vs. 5 Hz sine at the same amplitude... The first you hear the clicks the latter you can't hear as long as your not amplifieng it to some extend were perhaps the membranes of your Speakers begin to flutter...

Best is to check at it with an scope... if they have the same amplitude and the amplitude stays the same everything is normal...

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wackelpeter



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PostPosted: Tue Aug 22, 2017 11:01 am    Post subject: Reply with quote  Mark this post and the followings unread

Interesting read covering the sine wave percepetion of the human ear:

http://www.physics.mcgill.ca/~guymoore/ph224/notes/lecture13.pdf

And well, uhhhm, a bit more detailed read about audio perception... only if you have enough time and patience...

http://whyyouhearwhatyouhear.com/TextandPDFFiles/pitch23.pdf

very complex from a first look... left me a bit... Confused scratch

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Blue Hell
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PostPosted: Tue Aug 22, 2017 2:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

Interesting read that 2nd link :-)
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alanwilder81



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PostPosted: Tue Aug 22, 2017 2:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

thanks guys..
i just wondered what the correct signals output levels were supposed to be like in the Thomas Henry 555 VCO.
I measured the output voltage with a digital multi meter, and observed the waveforms on the oscilloscope.
Can't remember whether or not they significantly changed in amplitude, when sweeping the frequency. Tomorrow i'll take a closer look !
I'll attach the schematics.
So, do you reckon that a steady amplitude on the scope, regardless of the frequency, means that everything is ok?
Or, do voltage outputs have to be measured with a DMM for a better response?
thanks


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alanwilder81



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PostPosted: Tue Aug 22, 2017 2:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

and yes, there's also part 2. However, i left out the sine wave part.
I only implemented the pulse wave !
i forgot to tell you guys that when sweeping the frequency knob not only did the perceived volume varied , but also the output voltage levels fluctuated !

Roughly , on low-mid frequencies triangle and sine the DMM reads 2 V output. When increasing to high frequencies the DMM reads about 4 volts.
Double as much.
that's what made me wondered what the hell is going on here ! Smile


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Grumble



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PostPosted: Tue Aug 22, 2017 11:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

human ears are more sensitive to higher frequencies plus you need to measure the output with a scope or use: https://www.zeitnitz.eu/Scope_en
https://youtu.be/qNf9nzvnd1k

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wackelpeter



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PostPosted: Tue Aug 22, 2017 11:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

I don't know exactly but i guess measuring the voltage with a DVM on a changing frequency output is not the best option, on the scope you can see level changes better.

What came into my mind is the HF-trimmer, might have something to do with it, as far as i understand it has something to do with compensating for lower Levels at high a constant output, might have to do with how it's generated: with an comparator as Long as the triangle output is over the treshold the comparator output Switches from rail to rail and giving a stable voltage out via the resistro divider at it's output...

But at least that would be the opposite from what your telling...

Also interesting, regarding the Square output, would be to know if the square frequency output also changes or at least the pulsewidth...
As with variable treshold via the triangle with changing Amplitude this could occour too, without changing the PWM manually or by CV...


...And at least, know my memories came back, there was an issue with the sine output being a bit low on some builds, which Fonik solved. There is a thread regarding this at the Forums here... Looking at my printed schematic the 100k R34 in the sine shaper is reduced to 56k, i guess that's how it ended up in my build, but i'm always a bit lazy with documenting my changes.

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wackelpeter



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PostPosted: Tue Aug 22, 2017 11:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

Grumble wrote:
human ears are more sensitive to higher frequencies



Correct, but only to a certain degree... when it reachs the upper Hearing Limits of the human ear then the perceiving curve goes down again.

With my tinitus and slightly deafness, i'm having trouble Hearing frequencies above 12Khz if i can trust my ears and DVM...
A Hearing test i had to absolve now twice, showed that i lost a lot of sensitivtiy in the frequency range of human speech... Not the best circumstances trying to be "musically" productive.

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alanwilder81



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PostPosted: Wed Aug 23, 2017 12:42 am    Post subject: Reply with quote  Mark this post and the followings unread

thanks all , Smile

Bastian, i believe the HF-trimmer is for tuning the VCO in case the higher portion of the spectrum sounds flat.
Can't exactly remember where i got that info though.
gonna give the amplitudes another look today and i'll tell you what
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