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.
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 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.
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.
Not sure though.
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.
Thank you! This is the first explanation that made sense to me.
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.
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=CQAgjCAMB...
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.
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
Hiss is reduced because of the attenuation.
It's quite possible a simple resistive pad would be equally effective as the whole thing.