messing with a 4049

[Greek Acrobat]

I'm just about to start messing around with a 4049 inverter. I've been checking out some schems using it and noticed most of them have a small cap and a resistor in parallel with each inverter.

I was wondering what they do and how changing their values would affect the sound? At a guess - would a higher value cap result in a bassier sound??

Sorry if this is a little basic but I haven't been able to find anything about this anywhere else.

[Peter Snowberg]

The resistor sets the amount of negative feedback. This is just like an opamp circuit with a resistor around it, so the ratio of that resistor and the input resistance is what sets the gain. You can use values from 10K to 20M in that spot, but I wouldn't go lower than 10K.

The capacitor provides a lower impedance path for super high frequencies. The effect is that it will "eat" high frequency energy. Because those inverter stages are so fast, you can get them of oscillate at several MHz without any problem. That sort of oscillation is not useful to us, so the idea is to get rid of it with those little caps. You may not hear a difference with or without them, but you'll sure see it if you put a scope across the output. They make a huge difference in shaping the tone of multi-stage circuits where the output will just get swamped with high frequency if you don't have them.

take care,
-Peter

[brett]

You can work out the effect of the capacitors by comparing their impedance with that of the fixed resistor.  For example, at the frequency where the impedance of the cap matches the impedance of the resistor, the gain of the stage will be halved compared with lower frequencies (because equal amounts of feedback are going through the cap and resistor).  At higher frequencies, the impedance of the cap is lower (more feedback, less gain), and at lower frequencies the impedance is higher (less feedback, more gain).  Neat, huh?  

For example, I use a 220pF cap and 100k resistor in each stage.  At 15kHz (the top of most people's hearing range), the cap impedance is about 96kohms (Z=1/(pi.freq.cap)), so the gain at 15kHz is reduced by about 1/2.

Don't forget that each stage multiplies this effect, so my two stages with 220pF caps actually reduce gain at 15kHz to 1/4 of the gain at low frequencies.  This removes some annoying high frequency "fizz" from the sound.

Some cap values that are worth trying are 100, 150, 220 and 330pF (generally you would use the smaller values if you use more stages, and vice versa).

[Ansil]

i was wondering about that formual .. cool now i know the answer to that. i was wondering about that.

[Greek Acrobat]

That's great guys, cheers. That equation is a wee gem - I have a feeling it'll be really useful. It makes a lot more sense now.

OT: I thought the limit of human hearing was 20kHz? I have a vague memory of trying it out at school and I think I managed to hear 22kHz. I know it decreases as you get older though (nothing implied  :wink: ) so I probably won't be able to pick that sort of frq up now. It'd be interesting to try that out.

[Ansil]

some people can hear up to 22k..  most musicians start to loose at 18k  ie cymbol crash

[brett]

I believe that the "whistle" noise from TV sets is at 14kHz to 16kHz.  So a good test of your hearing is how loud you hear the whistle.  I can hardly hear it at all, but I'm 43 and have listened to too much loud rock'n'roll.

(Also, I believe that "normal" hearing loss is about 2kHz per decade, so by the time you're 60, you're flat out hearing past 10kHz)

[Ansil]

are you talking about that annoying headache inducing noise that you keep turning off and on the tv just to make sure that is what you are hearing..   if so yeah i hear that.

i also hear flurescent lights whining sometimes when they flicker. and no one else around me hears it.. am i crazy..  oh yeah i already know the answer to that..  as long as there is no background onise. like big industrial fans.. i can hear a %^&*roach fart  even while i am asleep. let me tell you too those little bugger have gass