Whitenoise

Free-for-all discussion about PAiA products and the kinds of audio you produce with them.
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mackemint
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Whitenoise

Post by mackemint »

Hello!
I figured the Fatman could do with some whitenoise so i found myself a simple schematic for this here:
http://forum.allaboutcircuits.com/cache ... 59_7lo.jpg

I figured it'd be nice to use since the voltage is the same and would be small and easy to build into the Fatman.
I figured i'd just use the voltage from IC20:3 and set the output to IC17:3 like with the sub-oscillator modification.
Any tips for an unexperienced circuit bender on this subject?

Regards
/M
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PAiA-Scott
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Re: Whitenoise

Post by PAiA-Scott »

The IC20 pin 3 voltage is only 8v instead of the 12v shown in the linked schematic, but this might still work. If not, substitute PNP transistors and shift things down so it's connected with the ground in place of +12 and the -12v supply from IC14 pin 2 in place of the ground/minus_dc_pole.

Also, instead of going direct to pin 3 of IC17, go through a resistor (as the VCO1/VCO2 mix does via R57. The VCO waves here are about 4v and the output from the noise circuit is probably only about a volt or less so a resistor about a 1/4 the value of R57 would likely be suitable.

The schematic in this link uses a zener diode for a noise source (followed by two series-connected gain stages). If you shift the three parts to the left of the first gain stage down to operate from ground to negative twelve, and use a much smaller resistor as the input to pin 3 of IC17, it might result in a significantly simpler means of adding a noise source. A switch could be inserted before the resistor into IC17 for Noise On/Off. To add a potentiometer for Noise Level control, use one that has a great value (1M) so it doesn't load the noise source and connect it between the coupling capacitor and the new resistor into IC17. Wire it as shown in FatMan schematics for the Output control R113.

http://www.maxim-ic.com/app-notes/index.mvp/id/3469
Scott Lee
scott@paia.com
mackemint
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Re: Whitenoise

Post by mackemint »

Hey!
Thanks for the advice. I quickly realized that the voltage out from IC20 was too low, so I figured I'd just add another regulator (a 7812) to provide the DC.
This seems to work, although I get a loud noise on the output (50 Hz, that goes nicely with the bassy tones, gives quite a PWM twang to it ;)
I've measured the AC on the output from my 7812 to approx. 0.47 V, compared to 0.02 or something out from IC14.
I tried putting a 1uF cap from pin3 to pin2 on my 7812, but the hum only gets slightly weaker..
Measuring the capacitor it settles nicely at .97uF.

So I should add a resistor to increase the voltage before going to IC17?
I figured you put a resistor to decrease voltage in a circuit.. Is this in any way similar to a pull-up resistor?

I think I saw that schematic when I at first researched this project but i decided to go with the other one since I had all those components at home atm. ^_^
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PAiA-Scott
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Re: Whitenoise

Post by PAiA-Scott »

The positive dc supply is more heavily loaded than the negative, so there is more ripple on it. Adding some resistor-capacitor filtering before your added 7812 might do the trick, ie about 10 to 100 ohms resistance and about 1000 to 3300 uF capacitance (look at the R117-C30 filter as an example).

The resistor linking the noise to the input, works along with resistor R59 to scale down the voltage to the input of IC17. Since you'll be using one that is about a fourth of the value of the one for the VCOs (100K R57), the weaker noise signal will be comparable to the stronger VCO1/VCO2 Mix signal.
Scott Lee
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mackemint
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Re: Whitenoise

Post by mackemint »

Hey, your pole reversing trick did it!
However, the output of the noise is quite quiet. Without the R56*1/4 (i put a 27k one) the noise sounds fine, but it makes the oscillators quieter by quite alot.
With it, the oscs stay loud but the noise is inaudible! :S
I figured maybe making the 100k from the output to gnd bigger or maybe even gettin rid of it might do something.
Stay tuned! ;)
mackemint
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Re: Whitenoise

Post by mackemint »

Hello!
After some experimenting with values on the 1/4 of r57 - resistor I've found that 3,3k has the best volume to not conflicting with the other oscilators-ratio.
However, the volume of the noise is still very quiet.
Will the decoupling resistor of 100 k also set the output volume?
There is also some hum still on the output..
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PAiA-Scott
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Re: Whitenoise

Post by PAiA-Scott »

Maybe some DC is getting through the output coupling capacitor -- was it reversed with the changes to be on the negative supply?

The 100k load resistor shouldn't be the cause of any trouble.
Scott Lee
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mackemint
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Re: Whitenoise

Post by mackemint »

No I didn't reverse it. Is it just a polarity reversal you're talking about?
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PAiA-Scott
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Re: Whitenoise

Post by PAiA-Scott »

Yes. If the capacitor polarized and backwards with respect to the voltages on its two ends, then it could be failing at its job of passing audio fluctuations but blocking dc level.

The noise circuit side of the capacitor will have a dc voltage reading and the filter input side will be practically zero.

If the circuit is now connected to the negative supply, the minus pole of the capacitor will need to be connected towards the noise circuit and the plus pole towards the filter input.
Scott Lee
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