I know that audio signals are AC signals (negative/positive voltage).

Assume a typical Fuzz (guitar pedal) effect circuit below. It seems signals stay between 0 and 9 V (9V is the power/battery voltage).

Also, when I look at some through an oscilloscope however, the GND is considered 0 V, and the voltage goes from 0 V to some mV/V, only positive. I used for this a line level signal (not guitar audio output, since I don't have a guitar myself).

So I'm a bit confused about the 'negative/AC' voltage. Can someone clear that up?

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    \$\begingroup\$ It is always positive at the collector of Q2 but the 2.2mf capacitor only allows the AC to be coupled in and the 0.01mf cap only allows AC to be coupled out the output of your guitar should not have any DC offset and the output wont have any either. Try simulating the circuit in LTspice or similar. \$\endgroup\$ – Warren Hill May 7 '20 at 9:38
  • \$\begingroup\$ @WarrenHill I know know what you mean ... I already put some similar circuit in microCap and I see negative voltage before the first cap and after the last, and in between only positive. So it's the capacitors that remove the 'AC' \$\endgroup\$ – Michel Keijzers May 7 '20 at 9:47
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    \$\begingroup\$ A capacitor cant have a DC current so the output will be biased around 0V measured across the output resistor. There will be a DC bias on the bases of both transistors but the input capacitor means that the guitar output is also 0V biased. \$\endgroup\$ – Warren Hill May 7 '20 at 9:52
  • \$\begingroup\$ Ok, and the DC bias on the bases of both transistors will depend on the battery/voltage power (which is 9V, thus bias around 4.5V or somewhere between 0 and 9 at least). \$\endgroup\$ – Michel Keijzers May 7 '20 at 10:03

The audio in/out signals are AC, with no DC component, i.e. biased around 0V DC. The transistors need DC bias voltages to operate. There are DC removing caps on the input and output. So the signal at the transistors is same AC signal but biased around some DC voltage.

  • \$\begingroup\$ Yes I see it indeed... I simulated something similar in microCap and I see negative voltages before the first cap and after the last cap and in between only positive voltages. At the transistor I would expect than a signal biased around 4.5 V ? (half the power ?) As if it would be around 0 V, the signal would be clipped right? \$\endgroup\$ – Michel Keijzers May 7 '20 at 9:52
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    \$\begingroup\$ Yes, if that was an op-amp circuit powered with 9V battery, the op-amp input and output would be DC-biased to 4.5V. This is similar, exact bias could be simulated. \$\endgroup\$ – Justme May 7 '20 at 9:59
  • \$\begingroup\$ Thanks for the explanation. I will need to check microCap to see how this biasing works exactly. But I'm glad at least I have the AC/DC issue cleared up. \$\endgroup\$ – Michel Keijzers May 7 '20 at 10:02

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