Acceptable revisions to a noise generating circuit

Thread Starter

PeteHL

Joined Dec 17, 2014
340
The revisions involve switching from a single to bipolar power supply, and increasing the capacitance of two capacitors of the circuit. See the attached schematics of the original and revised designs.

To take the noise outputs of the circuit with respect to common of the bipolar supply, I reversed the polarity of electrolytic capacitors C2 and C7 connected to the collector of respectively transistors Q2 and Q3. This was done based on the fact that with the revised circuit I measured DC voltage of the collectors of Q2 and Q3 being several volts negative with respect to the power supply common.

The other revision involves increasing C1 and C3 from 25 uF to 100 uF, just because I don't have 25 uF caps on hand. C3 I believe blocks DC in the connection from the white noise output to low-pass filtering to produce pink noise; this means I think that increased capacitance of C3 is acceptable. The function of C1 in the circuit is unknown to me, so I'm uncertain if making C1 of greater capacitance than given can be done.

Thanks in advance for letting me know if my changes will or will not degrade noise generation of this circuit.

-Pete

AN-Gen.png AN-Gen-REV.png
 

Audioguru

Joined Dec 20, 2007
11,249
The polarity of the capacitors depends on the hFE of the transistors that is random. So measure the polarity before connecting the capacitors and measure the polarity again if a transistor is replaced. Non-polarized electrolytic capacitors are sold.
If you increase the value of the capacitors then they take longer time to charge.
 

Thread Starter

PeteHL

Joined Dec 17, 2014
340
The polarity of the capacitors depends on the hFE of the transistors that is random. So measure the polarity before connecting the capacitors and measure the polarity again if a transistor is replaced. Non-polarized electrolytic capacitors are sold.
If you increase the value of the capacitors then they take longer time to charge.
Yes, thanks.
 

Thread Starter

PeteHL

Joined Dec 17, 2014
340
Rod Elliott has designed a similar pink noise generator-

http://sound.whsites.net/project11.htm
http://sound.whsites.net/project11.htm
with a bipolar +/- 15V power supply, and substituting op-amps for transistors for amplification and filtering. What I find odd about his circuit is capacitor coupling between amplification and filtering stages (C4) and between the output of the generator and whatever that output is connected to (C8). I'm referring to the circuit diagram, Fig. 1, of his article. As I understand, op-amps are designed to have very little output offset voltage (relative to reference voltage).

So do you think that Rod's generator would be better or worse, or the same, as the all-transistor generator of my starting post?

Cheers,
Pete
 
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