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I constructed this circuit as shown for use as a simple headphone amplifier. I was so happy with myself that I made a second one for stereo, as well as using a 3915 to make a very simple volume visualizer.

The circuit uses a uA/LM741 as a "preamp" for the 386 audio amplifier. Through my reading, I learned there are much better op-amps to use for audio applications than a 741, such as the TL071. So, imagine my gaiety when I discovered a set of TL071s in my parts box. Well, I'll just plug them in (swapping the TL071 in where the 741 is without changing the circuit otherwise) and...

No dice. I have to change the bias on pin 3 (the noninverting input) so that it's biased at Vcc/2, which ends up limiting the amount of "volume" I can get out as well.

My question is, why can the 741 have its NI input hooked to ground and still function fine, but the TL071 needs a "typical" mid-supply bias?

I know that the 741 is a BJT-based design and the TL071 is a JFET op-amp, and you'll have to excuse me but I'm still just learning about FETs at the moment. Is this because of the larger offset voltage in the TL071 because it's harder to match device parameters in a FET?

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    \$\begingroup\$ Hi there and thanks for your prompt response! Pardon my confusion - there is no difference between the circuits besides that I merely swapped one chip with another, and V- is at ground (I am operating single supply). Or do you mean that the TL071 is intended for single supply use? \$\endgroup\$
    – MJXS
    Jul 17, 2014 at 19:46
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    \$\begingroup\$ Both of those parts suck almost equally for single supply use. \$\endgroup\$
    – Matt Young
    Jul 17, 2014 at 19:50
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    \$\begingroup\$ With either op-amp, you need to bias the input to be between the supplies, so if you are running single-supply, negative inputs are unusuable, and A/C coupled inputs need to be biased to a virtual ground. \$\endgroup\$ Jul 17, 2014 at 19:58
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    \$\begingroup\$ If you cannot swap op-amps in a simple audio circuit, it must contain a spectacular design flaw. Without a schematic, and one which accurately reflects what was actually built, it is difficult to comment on. \$\endgroup\$
    – Kaz
    Jul 18, 2014 at 0:52
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    \$\begingroup\$ Thank you to everyone who responded - it's sad when even a newbie like me can understand the flaws in a circuit designed and published by someone with "15 years of design experience" \$\endgroup\$
    – MJXS
    Jul 18, 2014 at 17:56

2 Answers 2

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Well, if that circuit does in some way function it's well outside of the normal operation of a 741. The common mode range of a 741 only goes to within a couple volts of the negative rail, below that the current sinks that bias the differential front end will no longer function.

enter image description here

Even with a single-supply op-amp the circuit will not function properly because the op-amp output cannot swing below ground.

enter image description here

A horrible, horrible circuit and he did not demonstrate it working, just producing a horrid buzzing noise.

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    \$\begingroup\$ I agree, but @MJXS had it working! ("Riddle me that Batman") \$\endgroup\$ Jul 17, 2014 at 20:05
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    \$\begingroup\$ @GeorgeHerold Ah, inquisitive, aren't we? The output can't swing to the negative rail either- so it ends up being a kind of class A amplifier biased by the output not-quite-railing (at around a volt). That's my theory. The bias current may give it a bit of offset that allows it to produce both halves of the waveform to some degree, if the input voltage is small and the pot is turned towards the high gain end. \$\endgroup\$ Jul 17, 2014 at 20:23
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    \$\begingroup\$ I agree that there is no way on God's green earth that this thing ought to work. The video shows an electret microphone producing some sort of response with no excitation, too. Well, Johnathan Swift had it right when he referred to "the innate perversity of inanimate objects." \$\endgroup\$ Jul 17, 2014 at 21:00
  • \$\begingroup\$ I can confirm it does indeed work (to some limited degree - the gain settings are very specific on the 386 possibly as a result). Now, in light of this conversation, I understand why it shouldn't. \$\endgroup\$
    – MJXS
    Jul 18, 2014 at 17:54
  • \$\begingroup\$ B..bbbbut I thought any circuit produced by a Maker (TM) is ipso facto perfect!? \$\endgroup\$
    – Cuadue
    Oct 13, 2017 at 16:54
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Weird, I don't know how that first circuit worked with a 741.
Maybe there was enough current leaking out of the inverting input to bias the the input coupling cap to some value that let it run. Biasing the non-inverting input to mid supply was the right thing to do. To get rid of the clipping (low volume) maybe you could reduce the gain of the first stage and let the LM368 do more. (?)

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