I'm building a motorcycle intercom and am using the LM368 IC for my amp. My circuit works well (surprisingly, I'm new to this).

I've got a button which I use as a push to talk switch. This works for the most part, however when I press the button it takes a good 5 seconds before I hear any output.

Below is my very rough schematic.

enter image description here

Any ideas what might be causing this lag?

Thanks all.

  • \$\begingroup\$ Your schematic needs some work. The way the LM386 is drawn, as the package outline, we have no idea of what connects to each of the pins. Check the datasheet to see how it's normally represented in schematics. Include pin names. \$\endgroup\$
    – stevenvh
    Apr 8, 2012 at 5:29
  • \$\begingroup\$ I concur, my schematic skills are not so clever. \$\endgroup\$ Apr 8, 2012 at 10:35

2 Answers 2


It is probably charge time of C5. C5 must reach full charge before the DC level of the amplifier is settled down. But C5 is charging through some large resistances, so it takes some time.

C5 is charging through about 70k of resistance. Resistors add in series, so R2 is in series with the input resistance of the LM386 (about 50k) and R3 is 10K series resistance on the other side.

The charge time is proportional to the RC product. Input RC is (70k * 10μ) or 0.7 seconds.

If C5 is too low a value, the amplifier sound will be tinny. If it is too large, you will get the long turn-on transient that you are seeing.

All to say, try reducing C5 to 0.1μF or 0.047μF.

  • \$\begingroup\$ I thought it might be the charge time of a cap. Many thanks Mark, I'll have a go at changing the values tomorrow. I will report back, thanks Mark. \$\endgroup\$ Apr 7, 2012 at 23:29
  • 1
    \$\begingroup\$ Cracking answer Mark, you were bang on. Changing C5 to a .05μF (probably a .047μF) worked like a charm. Many thanks :-) \$\endgroup\$ Apr 8, 2012 at 10:37

You have a monster 10microfarad Capacitor on microphone input and 220mic output Cap. It could be taking that long to charge up. No real need for such large caps on intercom type circuits. Try 1 mic or even 0.1 for input and 2mic or even 1 on output. Anything you have will tell you if this is the problem. The Caps just allow AC to pass but block DC. They also make high pass filters but the values you have might be for Very Hi Fidelity use of passing 10 cyc per second. Intercom is voice, maybe 100 cps needed.

  • 2
    \$\begingroup\$ Does 'mic' mean micro Farad? Please be explicit of the components you suggest to use. \$\endgroup\$ Nov 22, 2012 at 6:02

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