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When looking at schematics of amplifiers, filters etc, I often find that the output to the audio jack is going through a resistor in the 470-680 ohm range and also shunted to ground using a high value resistor, like in this example:

schematic

simulate this circuit – Schematic created using CircuitLab

What's the meaning of R2, I just know it's something like a "load" resistor. And how about R1, is it needed for when nothing is connected to signal? What are appropriate values for those resistors? Do I need them when I connect between two op-amps or just to the output jack? I hope someone can clarify those questions or point me to a source where I could read more about these basic topics.

Extra question: what happens when I add a coupling capacitor between R1 and R2? Version with added capacitor:

schematic

simulate this circuit

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    \$\begingroup\$ My guessing the R2 is the output impedance matching, R1 is probably for eliminating some noise from cable. \$\endgroup\$ – Marko Buršič Oct 20 '15 at 14:05
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    \$\begingroup\$ When driving a pair of headphones, R2 combined with the headphones functions as an attenuator. This cheap solution comes at the price of poor damping which affects sound quality. For a pair of headphones (or speakers in general), you want the output impedance as low as possible to make the cone follow the drive voltage as close as possible. Leaving R2 out is not always a viable (and cheap) option, about which is written in one of the answers below. \$\endgroup\$ – jippie Oct 20 '15 at 15:09
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The TL081 is a low power amplifier, not meant to drive headphones. The roughly 600ohm R2 does, or can do, a number of things, depending on what this circuit was intended for.

1) Protects the output from excessive currents if low impedance headphones are plugged in. The amplifier itself is already protected against short term overload, but not continuous overload.

2) Increases the output impedance into a more standard 600 ohms, if it's intended to be used with other audio gear.

3) Decouples the amplifier output from the capacitance of long co-axial lines. This is important for stability.

4) If the amplifier were a different higher power one, then R2 would attenuate the signal down into headphones.

R1 appears to be a 'belt and braces' sort of component. It doesn't do any harm. It does provide a DC path across the output, whether the amplifier is powered or not. If a decoupling capacitor were provided between amplifier and output, then it would indeed be useful to maintain 0v at the output.

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  • \$\begingroup\$ thanks a lot, that answers almost everything. about 3), how does this decoupling work? i realize long cables function as capacitors, but in what way does the resistor decouple? \$\endgroup\$ – jilski Oct 20 '15 at 15:49
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    \$\begingroup\$ @jilski opamps don't like a capacitor slapped right on their output. With R2 there, they are driving a resistor in series with the capacitor, which they can handle. Trust me, you probably don't want to know why they don't like driving capacitors. If you don't trust me and still want to know, then try analog.com/library/analogDialogue/archives/31-2/… \$\endgroup\$ – Neil_UK Oct 20 '15 at 16:32
  • \$\begingroup\$ alright, will trust you and also read that doc. Thanks a bunch for helping, appreciate it a lot. \$\endgroup\$ – jilski Oct 20 '15 at 16:42
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  • Opamps generally don't like lots of capacitance on their outputs. Why is a different question.
  • Long cables look like capacitors. (2 conductors with insulator between)
  • R2 disconnects the amp from the cable to keep it from oscillating.

The tradeoff is that the device's output is weaker (higher impedance) than if the resistor wasn't there, which will make it interact more with external stuff including:

  • A long cable's capacitance to make a lowpass filter.
  • Another device with similar output to make a passive mixer, provided that they don't go into overcurrent mode because the output resistors aren't that big.

As for R1, I don't see that it does much of anything. If this were an input, then it would keep it somewhat stable when nothing is plugged in, but for my designs I would consider it to be too weak to do much else. The voltage divider effect is negligible because of the extreme ratio.

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  • \$\begingroup\$ Alright, thanks a lot. What happened if I put a 10µF coupling cap between R2 and R1? Would it make R1 more useful/logical? \$\endgroup\$ – jilski Oct 20 '15 at 15:23
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    \$\begingroup\$ @jilski - Yes, it would make sure that the output end of the cap was close to 0V if nothing was plugged in (i.e. the bias voltage if there is one is already established across the cap.). This would help prevent a horrible click as you connect something to the jack. \$\endgroup\$ – user1844 Oct 20 '15 at 15:49

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