1
\$\begingroup\$

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?

|improve this question
\$\endgroup\$
  • 1
    \$\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 '14 at 19:46
  • 6
    \$\begingroup\$ Both of those parts suck almost equally for single supply use. \$\endgroup\$ – Matt Young Jul 17 '14 at 19:50
  • 1
    \$\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\$ – Chris Stratton Jul 17 '14 at 19:58
  • 1
    \$\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 '14 at 0:52
  • 1
    \$\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 '14 at 17:56
4
\$\begingroup\$

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.

|improve this answer
\$\endgroup\$
  • 1
    \$\begingroup\$ I agree, but @MJXS had it working! ("Riddle me that Batman") \$\endgroup\$ – George Herold Jul 17 '14 at 20:05
  • 1
    \$\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\$ – Spehro Pefhany Jul 17 '14 at 20:23
  • 2
    \$\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\$ – WhatRoughBeast Jul 17 '14 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 '14 at 17:54
  • \$\begingroup\$ B..bbbbut I thought any circuit produced by a Maker (TM) is ipso facto perfect!? \$\endgroup\$ – Cuadue Oct 13 '17 at 16:54
2
\$\begingroup\$

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. (?)

|improve this answer
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.