Hi everyone, please help me with this Douk Audio U3 headphone amp or one of these days I'll hammer it. I've tried every way I know, but when I turn it on or when I insert the jack an annoying bump comes to the headphones and, since I have a wonderful (at least for me) Philips Fidelio H2HR I don't want to ruin it, so I'm doing my tests with a cheap Sony (9.90 € !!!). For the rest the amp works well but I don't know how to avoid this noise. I contacted the company and they gave me some advice that I already knew. What can I do? Thanks.
I assume you are using a standard 1/4" (6.35mm) TRS jack/plug for the headphones? These will produce a brief short to ground on insertion/removal and is just a detriment of the jack type. One thing I would do is measure the DC offset on the amp output (L-Gnd, R-Gnd). Any offset will exacerbate this issue.
Thank you for your quick response!
Yes, I always use 6.35mm jack, I really hate 2.5mm jack.
So there can be an offset at the output? Then I will never use it again! Tomorrow I will do some measurements.
Yes, I always use 6.35mm jack, I really hate 2.5mm jack.
So there can be an offset at the output? Then I will never use it again! Tomorrow I will do some measurements.
The U3 seems to be a single-supply design. That makes a turn-on thump relatively hard to avoid - the output has to be brought up to ~1/2Vcc when powering up, yet at the same time you want good low-frequency response from it.
About the only thing you can do is looking for the Vcc/2 reference voltage divider, which should normally be an R/RC kind of affair with resistors around 10 kOhms, and make the capacitor as big as possible. Even that would only minimize turn-on thump, power-off is still relatively hopeless (and would also require a diode from midpoint to +Vcc to discharge the capacitor safely). This is a hard problem to crack without using some form of output muting e.g. via a relay, and the U3 is just a cheap little amp without the parts budget or space for such frills. It's still a good bit more fancy than my old BTech BT928 amp that used to retail in a similar price range 15 years ago.
About the only thing you can do is looking for the Vcc/2 reference voltage divider, which should normally be an R/RC kind of affair with resistors around 10 kOhms, and make the capacitor as big as possible. Even that would only minimize turn-on thump, power-off is still relatively hopeless (and would also require a diode from midpoint to +Vcc to discharge the capacitor safely). This is a hard problem to crack without using some form of output muting e.g. via a relay, and the U3 is just a cheap little amp without the parts budget or space for such frills. It's still a good bit more fancy than my old BTech BT928 amp that used to retail in a similar price range 15 years ago.
Put your safety glasses on and give a good old beating.
Now you'll be forced to buy something better, which shouldn't be all that difficult. 😉
jeff
You're discharging the output sound-coupling cap, which sits at 1/2 of Vcc, every time you plug AND unplug the headphones.
Output cap should have discharging resistor after it. I dealt with it with hamptone pre, it was needed for radford, and especially for last heaphone amp I built, designed by Hugh. I had click at first, then I experimented with value of discharging cap till pop was acceptable.
Its extremely easy just to put resistor on output jack inside.
Its extremely easy just to put resistor on output jack inside.