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I built this audio amplifier mainly with UA741, ICL7660 etc. The VCC is 5 volt from USB. enter image description here

But I found that the -VCC is quite unstable and this caused noise from speaker, which can hear even when music was played. And here is what my scope picked when nothing is played on computer from output (pin 6) of the Op Amp on the left.

enter image description here

And I picked the working frequency from pin 2 of ICL7660.

enter image description here

They lined up so nicely that's why I think it is the ICL7660 IC that caused the audible noise. Also, when music is played I found the -VCC become less negative significantly while VCC stays really stable.

So, can I improve this to get rid of that noise or must I make some other approach to fetch -VCC?

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    \$\begingroup\$ C2 (in the schematic) polarity is reversed. ICL7660 can only provide some 10s of mA (40 mA max without significant voltage drop) but you are driving a speaker with it? \$\endgroup\$
    – Long Pham
    Commented Feb 1, 2019 at 4:20
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    \$\begingroup\$ @LongPham Even 40mA is quite a lot for a switched-capacitor charge pump. \$\endgroup\$
    – Hearth
    Commented Feb 1, 2019 at 4:27
  • \$\begingroup\$ @LongPham Reversed cap fixed, thanks. My speaker is labeled as 0.5w 8ohm, if take it as a simple resistor, the required current is \$\sqrt{0.5/8}=250mA\$. Is there any voltage reverse chip that can provide that much current? \$\endgroup\$
    – Page David
    Commented Feb 1, 2019 at 5:43
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    \$\begingroup\$ To get yourself started you can use a USB wall adapter as your -5V supply. Since wall adapters have output their floating, you can connected the +5V output to ground in your circuit. \$\endgroup\$
    – sstobbe
    Commented Feb 1, 2019 at 16:09

3 Answers 3

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As stated in the comments, ICL7660 and ICL7660A series switched capacitor voltage inverteres shouldn't be used for current requirements more than 20mA because of high output impedance (around 100 Ohms). You can check this from the datasheet. The most common usage for this type of inverters is providing a supply for opamp-based circuits and RS-232 circuits. You might know that these circuits draw only a few mA of current.

If required voltage swing is not higher than ±2V then you can still use the single +5V supply with virtual grounding.

If you really need a negative supply with higher current then you can build a classical voltage inverter with MC34063 or LM2776 or equivalent:

enter image description here

enter image description here

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It’s not the Op Amp but rather the -Vcc supply that cannot drive <100 Ohms let alone 8 Ohms. You also lose ~4.5V swing due to 741 output swing = (Vcc+|Vee|)-3V and 2xVbe drops =1.5V . So neither 741 nor charge pump work well here.

The 741 was intended for >+/- 12V supplies and 20mA needs a current gain of >50 at 1A on emitter followers. So read data sheets carefully and supply must deliver 1A regulated voltage.

The 741 has 77 dB PSRR and 70 dB CMRR .

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  • \$\begingroup\$ You mean my negative supply is not robust enough or Op Amps and transistors circuit are designed badly and loss too much power? I am a little confused... \$\endgroup\$
    – Page David
    Commented Feb 1, 2019 at 5:51
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    \$\begingroup\$ Your charge pump supply has too high resistance ratio to your load. Read the specs for Io max. Use Ohms Law. The load R must be >> source and stable to achieve low V drop or ripple. It’s a divide ratio. Normally Zs is 1% of load not 10x bigger with 8 ohm speaker \$\endgroup\$
    – D.A.S.
    Commented Feb 1, 2019 at 14:05
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The UA741 does not have good -Rail Power Supply Rejection.

Use a different opamp.

enter image description here

Notice location of the 25pF capacitor. The right end is the large-voltage-gain node for the opamp. The left end is two diode drops (Q16,Q17) above the negative rail.

At high frequencies (fast trash on the -VDD rail), the trash on the -rail is copied onto the VOUT (base of Q18) and then heavily buffered to the output pin.

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    \$\begingroup\$ The UA741 doesn't have good anything, really. \$\endgroup\$
    – Hearth
    Commented Feb 1, 2019 at 4:28
  • \$\begingroup\$ Could you provide some example please? \$\endgroup\$
    – Page David
    Commented Feb 1, 2019 at 5:54
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    \$\begingroup\$ @PageDavid: Pretty much any opamp newer than 1967. The 741 is ancient. It was good for its day, but has been surpassed by just about everything in the 50 years since it first appeared on the market. \$\endgroup\$
    – JRE
    Commented Feb 1, 2019 at 6:57

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