4 ms·

Can someone explain how that circuit works? They mention it buffers with a gain factor of 0.1, but I don’t understand how the four op-amps play together to eli

Can someone explain how that circuit works? They mention it buffers with a gain factor of 0.1, but I don’t understand how the four op-amps play together to eliminate the hiss.

mikewarot 3y ago
The 1k resistors on the left don't need to be there if you have 2 batteries. They waste battery. The same could be said for the ground "amplifier". Hiss is reduced because of the attenuation. It's quite possible a simple resistive pad would be equally effective as the whole thing.
pie314isi 3y ago
it doesn't. it only works incidentally. The author isn't an EE and has designed a circuit as if an artist (or an AI) drew a picture of what a circuit looks like. The weird topology voltage divider attenuates the signal. the op-amps buffer the output. The 470R resistors do nothing. The 1k/1k virtual ground does nothing. The rest of the authors writings smell of /r/iamverysmart
JKCalhoun 3y ago
Yeah, and two op-amps for the ground filter has me stymied as well. Slightly off-topic, someone pointed out that MAX232 chips make a great power supply for op-amp circuits like this (using charge-pump capacitors) one because they can hand you +/- 9V from a 5V source (which could be a USB power brick). I'm not sure but I would guess that the current needs of his audio filter would be within what the MAX232 could handle. I made a small PCB based on that idea for experimenting with op-amp circuits on a bread board.
- burnerthrow008 3y ago
  It’s not a ground filter, but a virtual ground. The point is to divert current from whichever power rail is further from ground, into the ground from the input device. That makes it so that the two “main” opamps see a common mode voltage of 0V. Opamps generally perform better near that point. I’m not sure that it’s really necessary here with modern op amps, and the author may have just copy-pasted the circuit from somewhere.
- rzzzt 3y ago
  I think one of the op-amp inputs should be flipped around so it works in inverting mode. Then their amplified output is summed.
- bArray 3y ago
  > They mention it buffers with a gain factor of 0.1, but I don’t understand how the four op-amps play together to eliminate the hiss. I'm not entirely sure, but it appears that this is setup to amplify the ground noise. I'm also not entirely sure why you would want a gain of 0.1, you would probably want to preserve as much amplitude information as possible. Also the power circuit on the left shows a resistor ladder with 9V over a 1k resistor on one side, and the same for the other. 9 / 1000 = 9mA, x2 is 18mA. Added with the amplification of noise, I can see where 25mA may come from. > Yeah, and two op-amps for the ground filter has me stymied as well. I think this is because it's the ground for both the left and right channels, which is potentially two op-amps worth of power and you don't want this dumped through the headphone jack. I haven't tested this, but would probably go for a simpler design like this: https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWEB2WZIICxgEwGZMA2QgDmRMJAUiqpoQFMBaMMAKAEMQdISQSaeHvxoBOEOKbj08OCCZ5oosHjyiSohKOSjMmPEgVKV+gaLzlMkVeFmRO3XtwSUhNHC4leptu-JzQhMi4OB4I5JDmfEwBQbiYojjIcASafDLwbABOjul6IG7g+TSYydm5BcKFhSVlAO6VglXCHpT2DYVg+YUCUGwNPHyilINFmH0DTkk4FTVsAF6zpI0FyzMAJgwAZhwArgA2AC5M8wwAdgxZbABK4MiUYIlj4E80JW9QnzAI5WD3IIRxn8Hq9fHBfv9HjNgdwrJ8Mu07iM4aMcHDEaNAUiAeN7H8aGAHjhooTwMTuI5HPQoNB0D98eARp4WEyRpSeNSYOwGaSofJeU9ofQ3rSaewcjCsawZliCZkJf80QTcLCPhk2AAPfmQZDgAjyQjSCwgcZokD7baHTUFOEmArkPW603jC1bK1aljobjLJhYbjBE2wkBZACWAHMABbu7XpbQGvBUwNmsNndb9RkvGbDTMTCQgrOUKxvdPZosVMuImF8gzQ17pms5ht4ZaIpvLaot9PS1XgFWtXPd-u9fuI3q4dIq8cDyfk7uytgAeRWvSI4zHfSXDb5q5zEHsm5anh3-b3bAAzhmp2PyR8dvszwxzxmh+5PLeOPfH4t+2XRr+QJsOwHMcpwXFcOSFL+R5yHK4KTHwzZ8H+MH1lU7bCBWbAAPY2jinxWOoNKyPcODmLoCB4LgSBcvC2EFAmsompEfDUaKEDsDhBh4e8hGsRA0JsEAA https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB...
  - cwillu 3y ago
    The author repeatedly mentions that his new headphones are a bit too sensitive for his liking, hence the hardwired gain adjustment.
    - sbuk 3y ago
      They're studio headphones. Clearly he doesn't know how headphones work...
zh3 3y ago
It's just a couple of buffers - the 10k/1k sets the gain at 0.1, shorting the 10k's would make it 1x. The two op-amps with outputs tied together with 1 ohm are just creating a virtual ground (effectively paralled to double the current handling). I can't see how this could reduce 'hiss' if it there's something wrong in the first place,all seems very unlikely though. Possibly it eliminates hum (which mobile devices are generally hideous for, when charging) but the details are likely to be setup-dependent. Or maybe RF is leaking out of the phone causing the hiss in those particular headphones, and all this is doing is stopping that.
- vitiral 3y ago
  My guess was that the signal is good but the audio output can't drive the right amount of current for the high end headphones. Not sure though.
loph 3y ago
I'm not an EE, but I play one in my home lab. I don't understand why this circuit needs an artificial ground at all, since the power source is two 9-volt batteries in series. AFAIK that could make a dead-simple bipolar power source +/- 9 volts and get rid of about half the parts. The 10:1 voltage divider is also an interesting concept. It seems to me that what this thing does is attenuate the input signal and then provide a low impedance output to drive the 'phones. I wonder if the writer ever considered something as simple as shunt resistors. Perhaps a "real EE" can enlighten me. FWIW, I use a AudioEngine D1 USB DAC to drive headphones from a computer. It was not cheap, but neither is my time. For portable use I am rocking (with RockBox) a old iPod "video" with aftermarket SD card mod.
burnerthrow008 3y ago
As other posters mentioned, two of the opamps create a virtual ground. (Op amps generally have optimal performance when the input is halfway between the power rails, and the two batteries in series don’t guarantee that will happen). The other two each drive one channel. The 0.1 gain is to reduce noise created by the DAC, which is independent (in this case) of the volume control. So the author is turning up the volume on his device, to compensate for the 0.1x gain. Accordingly, DAC noise at the speakers is 1/10th what is was before, but the signal is the same.
- ZoomerCretin 3y ago
  > The 0.1 gain is to reduce noise created by the DAC, which is independent (in this case) of the volume control. Thank you! This is the first explanation that made sense to me.
jrmg 3y ago
I’d also like to understand where the ‘explosion of noise’ on changing tracks on the phone comes from.

Can someone explain how that circuit works? They mention it buffers with a gain factor of 0.1, but I don’t understand how the four op-amps play together to eliminate the hiss.

mikewarot 3y ago

The 1k resistors on the left don't need to be there if you have 2 batteries. They waste battery. The same could be said for the ground "amplifier". Hiss is reduced because of the attenuation. It's quite possible a simple resistive pad would be equally effective as the whole thing.

pie314isi 3y ago

it doesn't. it only works incidentally. The author isn't an EE and has designed a circuit as if an artist (or an AI) drew a picture of what a circuit looks like. The weird topology voltage divider attenuates the signal. the op-amps buffer the output. The 470R resistors do nothing. The 1k/1k virtual ground does nothing. The rest of the authors writings smell of /r/iamverysmart

JKCalhoun 3y ago

Yeah, and two op-amps for the ground filter has me stymied as well. Slightly off-topic, someone pointed out that MAX232 chips make a great power supply for op-amp circuits like this (using charge-pump capacitors) one because they can hand you +/- 9V from a 5V source (which could be a USB power brick). I'm not sure but I would guess that the current needs of his audio filter would be within what the MAX232 could handle. I made a small PCB based on that idea for experimenting with op-amp circuits on a bread board.

burnerthrow008 3y ago
It’s not a ground filter, but a virtual ground. The point is to divert current from whichever power rail is further from ground, into the ground from the input device. That makes it so that the two “main” opamps see a common mode voltage of 0V. Opamps generally perform better near that point. I’m not sure that it’s really necessary here with modern op amps, and the author may have just copy-pasted the circuit from somewhere.
rzzzt 3y ago
I think one of the op-amp inputs should be flipped around so it works in inverting mode. Then their amplified output is summed.
bArray 3y ago
> They mention it buffers with a gain factor of 0.1, but I don’t understand how the four op-amps play together to eliminate the hiss. I'm not entirely sure, but it appears that this is setup to amplify the ground noise. I'm also not entirely sure why you would want a gain of 0.1, you would probably want to preserve as much amplitude information as possible. Also the power circuit on the left shows a resistor ladder with 9V over a 1k resistor on one side, and the same for the other. 9 / 1000 = 9mA, x2 is 18mA. Added with the amplification of noise, I can see where 25mA may come from. > Yeah, and two op-amps for the ground filter has me stymied as well. I think this is because it's the ground for both the left and right channels, which is potentially two op-amps worth of power and you don't want this dumped through the headphone jack. I haven't tested this, but would probably go for a simpler design like this: https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWEB2WZIICxgEwGZMA2QgDmRMJAUiqpoQFMBaMMAKAEMQdISQSaeHvxoBOEOKbj08OCCZ5oosHjyiSohKOSjMmPEgVKV+gaLzlMkVeFmRO3XtwSUhNHC4leptu-JzQhMi4OB4I5JDmfEwBQbiYojjIcASafDLwbABOjul6IG7g+TSYydm5BcKFhSVlAO6VglXCHpT2DYVg+YUCUGwNPHyilINFmH0DTkk4FTVsAF6zpI0FyzMAJgwAZhwArgA2AC5M8wwAdgxZbABK4MiUYIlj4E80JW9QnzAI5WD3IIRxn8Hq9fHBfv9HjNgdwrJ8Mu07iM4aMcHDEaNAUiAeN7H8aGAHjhooTwMTuI5HPQoNB0D98eARp4WEyRpSeNSYOwGaSofJeU9ofQ3rSaewcjCsawZliCZkJf80QTcLCPhk2AAPfmQZDgAjyQjSCwgcZokD7baHTUFOEmArkPW603jC1bK1aljobjLJhYbjBE2wkBZACWAHMABbu7XpbQGvBUwNmsNndb9RkvGbDTMTCQgrOUKxvdPZosVMuImF8gzQ17pms5ht4ZaIpvLaot9PS1XgFWtXPd-u9fuI3q4dIq8cDyfk7uytgAeRWvSI4zHfSXDb5q5zEHsm5anh3-b3bAAzhmp2PyR8dvszwxzxmh+5PLeOPfH4t+2XRr+QJsOwHMcpwXFcOSFL+R5yHK4KTHwzZ8H+MH1lU7bCBWbAAPY2jinxWOoNKyPcODmLoCB4LgSBcvC2EFAmsompEfDUaKEDsDhBh4e8hGsRA0JsEAA https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB...
- cwillu 3y ago
  The author repeatedly mentions that his new headphones are a bit too sensitive for his liking, hence the hardwired gain adjustment.
  - sbuk 3y ago
    They're studio headphones. Clearly he doesn't know how headphones work...

zh3 3y ago

It's just a couple of buffers - the 10k/1k sets the gain at 0.1, shorting the 10k's would make it 1x. The two op-amps with outputs tied together with 1 ohm are just creating a virtual ground (effectively paralled to double the current handling). I can't see how this could reduce 'hiss' if it there's something wrong in the first place,all seems very unlikely though. Possibly it eliminates hum (which mobile devices are generally hideous for, when charging) but the details are likely to be setup-dependent. Or maybe RF is leaking out of the phone causing the hiss in those particular headphones, and all this is doing is stopping that.

vitiral 3y ago
My guess was that the signal is good but the audio output can't drive the right amount of current for the high end headphones. Not sure though.

loph 3y ago

I'm not an EE, but I play one in my home lab. I don't understand why this circuit needs an artificial ground at all, since the power source is two 9-volt batteries in series. AFAIK that could make a dead-simple bipolar power source +/- 9 volts and get rid of about half the parts. The 10:1 voltage divider is also an interesting concept. It seems to me that what this thing does is attenuate the input signal and then provide a low impedance output to drive the 'phones. I wonder if the writer ever considered something as simple as shunt resistors. Perhaps a "real EE" can enlighten me. FWIW, I use a AudioEngine D1 USB DAC to drive headphones from a computer. It was not cheap, but neither is my time. For portable use I am rocking (with RockBox) a old iPod "video" with aftermarket SD card mod.

burnerthrow008 3y ago

As other posters mentioned, two of the opamps create a virtual ground. (Op amps generally have optimal performance when the input is halfway between the power rails, and the two batteries in series don’t guarantee that will happen). The other two each drive one channel. The 0.1 gain is to reduce noise created by the DAC, which is independent (in this case) of the volume control. So the author is turning up the volume on his device, to compensate for the 0.1x gain. Accordingly, DAC noise at the speakers is 1/10th what is was before, but the signal is the same.

ZoomerCretin 3y ago
> The 0.1 gain is to reduce noise created by the DAC, which is independent (in this case) of the volume control. Thank you! This is the first explanation that made sense to me.

jrmg 3y ago

I’d also like to understand where the ‘explosion of noise’ on changing tracks on the phone comes from.