white noise gen. replacing a ua741 with lm741cn

[oram]

Hello i expect i'm doing something very dumb here... but i replaced a ua741 with a lm741cn in this circuit, http://electro-music.com/forum/phpbb-files/whitenoise_969.jpg.

It doesn't work, have i chosen an incorrect replacement here?

thank you.

[Seljer]

What the voltage on the base and emitter of the transistor? and whats the voltage on all of the opamp pins?

[oram]

hello seljer and thank you

base = 11.3     Emitter = 0.90

pins:
1= 0.04   2=0.15   3=1.22   4=0.40   5=11.49   6=9.1   7=0.01   8=0

[PRR]

> replaced a ua741 with a lm741cn

That's fine.

However the circuit shown is for *bi-polar* supplies (note no output cap). And is also incomplete (note no connection at pin 3). Below I show a single-supply implementation.

And the Emitter should be zero (isn't it connected to V-?), the Base should be 7V higher. The 11V number you show means the emitter-base junction is NOT broken-down, as it needs to be (in this circuit).

[PRR]

Further.... this is probably not a WHITE noise source. The 47nFd against 2K2 is a 1500Hz bass-cut. It has no "rumble".
The corner will be a little lower due to transistor impedance. But I think not-much.

I think the "47n" cap should be 100 times higher. Which is 4.7uFd. May as well use a third 10uFd. Since this will be an Electrolytic, polarity sensitive, put the "+" side toward the 2K2 and transistor.

[oram]

that's a lot of work you've put in here for helping me. Thank you.

I'm just gonna spend some time trying to understand fully what you have written and i'll let you know how i get on.

thank you

[oram]

"And the Emitter should be zero (isn't it connected to V-?)"

apologies...i don't understand this, the schematic how i see it, the emitter connects to the 1mega and 2k2 +12v? isn't it the base that connects to -v?

I was also under the (incorrect) assumption i could run this circuit off 9v? i see you have put in 12v

thank you

[PRR]

> "And the Emitter should be zero (isn't it connected to V-?)"

SORRY! I was wrong. Yes the BASE connects to V-, which is probably Ground and zero Volts.

> 9v? i see you have put in 12v

Your reported voltages include 11.3 and 11.49. So I ass-umed nominal 12V. What is your actual supply?

> run this circuit off 9v?

It needs more than 7V, the base-emitter breakdown voltage.

When we used to do these things with 9V batteries, they worked for a while, but as the "9V" battery ran-down to 8V they got weak, at 7V the hiss would fall way off. 9V is possible (especially wall-power instead of battery) but 12V is better.

There's other, maybe better, ways to make white noise. The old digital chip is long gone, but you can do pseudo-random with a couple CMOS chips. For pure analog, both halves of a TL072 working at gain about 300 each will give about a tenth-volt of hiss over a 25KHz band and reasonably flat.... hmmmm, you need to null the 200Hz 1/f noise corner. As a "spice sound" that's do-able, but if you need *flat* white hiss for measurements you need calibration or a better method.

[PRR]

> "And the Emitter should be zero (isn't it connected to V-?)"

SORRY! I was wrong. Yes the BASE connects to V-, which is probably Ground and zero Volts.

> 9v? i see you have put in 12v

Your reported voltages include 11.3 and 11.49. So I ass-umed nominal 12V. What is your actual supply?

> run this circuit off 9v?

It needs more than 7V, the base-emitter breakdown voltage.

When we used to do these things with 9V batteries, they worked for a while, but as the "9V" battery ran-down to 8V they got weak, at 7V the hiss would fall way off. 9V is possible (especially wall-power instead of battery) but 12V is better.

There's other, maybe better, ways to make white noise. The old digital chip is long gone, but you can do pseudo-random with a couple CMOS chips. For pure analog, both halves of a TL072 working at gain about 300 each will give about a tenth-volt of hiss over a 15KHz band and reasonably flat.... hmmmm, you need to null the 200Hz 1/f noise corner. As a "spice sound" that's do-able,

but if you need *flat* white hiss for measurements you need calibration or a better method.

[oram]

thanks PRR.

apologies, i don't get on here at minute enough to respond swiftly.

The circuit was running off 9volt wall supply. Buti ha been messing about with a couple of double transistor based circuits that ran off 12so maybe i hadn't taken the voltage back down.

I kinda figured when i started messing around with these circuits that battery supply wouldn't be an option.

"HISS" is grand, i realise i've been slightly miss leading since you mention this. I'm just trying to learn to filter in hiss (white noise as i say) to existing circuits. Like the noise function on my analogue synth.

I will build the circuit you've drawn out this week and get back to you. What is the value of the output capacitor please, i read it as 2m but on researching this i can't understand what this means. I 'm probably just miss-reading.

I appreciate your help and time

Oram

[psychedelicfish]

What is the value of the output capacitor please, i read it as 2m but on researching this i can't understand what this means. I 'm probably just miss-reading.

What you read as "m" is in fact the greek letter mu (µ), which stands for micro. 2uF (we quite often replace "µ" with "u") is the correct value, though 2.2uF is probably more common and will work just as well. 2mF would mean 2 millifarads, which is equal to 2000uF and is a very big (and somewhat expensive) capacitor for an application such is this.

[oram]

thank you psychedelicfish

so obvious now...doh!!

[oram]

When we used to do these things with 9V batteries, they worked for a while, but as the "9V" battery ran-down to 8V they got weak, at 7V the hiss would fall way off. 9V is possible (especially wall-power instead of battery) but 12V is better.

the initial 12v circuit broke  down at 10v and silenced. In case you were interested. You made it work though, great. Still awaiting my cmos chips to test the 9 volt version.

[oram]

Maybe this needs to go under a different topic. But the circuit i'm trying to make involves this "noise" circuit blending into a line fed through the pedal. for example a guitar signal. Just i'm unsure which circuit/blend /mixer to use. I initially thought the back end of the buff an blend, but then read so much oither stuff i got confused.Then i looked at the ggg mixer for ages and thought surely there's a simpler solution for a single "noise" circuit to be fed to an existing signal.

apologies for my lack of terminology here, probably why i haven't found what i need. Maybe someone can just pin point me with the correct circuit please?

regards

oram