If you have Xduoo MT-602 and your channels pop when plugging in headphones. DC offset tutorial.

[SeriousApple]

MT-602 is class a push and pull design amp. So there is some DC offset that can cause popping sounds when plugging in sensitive low impedance headphones. Sometimes even 50-60 Ohm headphones can have a popping sound. Which means your driver is not sitting at its resting position its either in or out. Which may also affect the sound.
You want to have DC offset as close to 0mv as possible on both channels L+R.

Step1

Remove your tubes. Your board wont come out with tubes in.

Step 2

Remove these 4 screws using a small Philips head screwdriver.

Step 3

Remove the knob and remove the hex nut with a plier and then the washer will come off also.

Step 4

Pull the pcb board out. Make sure you don't touch any chips and discharge yourself by touching a metal object. Also wear a anti static wrist band if your wearing a sweater or you have a carpeted floor. If you cant then just be very carful and avoid touching the pcb directly as much as you can and place board on a piece of wood or something. Placing it on a mouse pad is not good idea either buy yes I have done it here so don't @ me in the comments for this.

Step 5

Plug in an aux cable in the front headphone socket. Also plug in your power and turn on the amp. (MAKE SURE YOUR PCB IS ON A PIECE OF WOOD OR SOMETHING NON CONDUCTIVE WOOD TABLE TOP SHOULD BE FINE NOTHING METAL) And set your volt meter to millivolts denoted by mV. Now use your probes to test the voltage on the other end as shown here in the photos below. Use black probe for ground and red for channels. Now you want to adjust your DC offset with a small Philips head screwdriver by adjusting those two small potentiometer very very very minutely until you get a value close to 0mV 10mV to 5mv should be fine to avoid major popping and offset. Obviously you can check the voltage and adjust at the same time so go back and forth between adjusting and checking. Finally you want both channels either positive or negative offset not one negative and one positive.

Step 6

Put your MT-602 like before and enjoy your pristine Head Amp.

All the best.

Disclaimer: I am not responsible for any damages to your equipment if you follow my directions. Open your MT-602 at your own risk.

Final Thoughts: This procedure might work on MT-601, Mt-603 and MT-604 as well. PS. I would not want to touch MT-604 its a fully balanced head amp leave that to the factory experts.

 

[parsecn]

I really appreciate the post. I received the MT-602 about a week ago from supplier Aoshida HiFI-AU.

Aoshida shot me a post-purchase email explaining that the product is relatively new and asked if I was happy with the sound, product in general, etc. I'm a fan of the sound and provided only positive feedback with the exception of the slight pop - wasn't a deal-breaker, and I let them know I will most definitely keep the unit, regardless. I also linked them to your post asking if your method was recommended to address the issue, etc.

The Aoshida rep reached out the engineer who designed the xDuoo MT range, who provided the following:

"Dear friend,

The xDuoo engineer has finished reading the steps. He said that this step is correct, and they have adjusted through this method before. Therefore, this way is feasible.

But one thing to remind is that you must be very careful in the process of disassembly and assembly, and use appropriate and professional tools to do the step, so as to avoid accidental damage to the machine.

Hope our suggestion can help you.

Thanks again!"

I have yet to adjust my unit but will tackle in the coming week as I have time. It's great knowing we have a simple way to pull those values into line.
Cheers

 

[SebaMK82]

Hello, I have the 602 and I am very satisfied with its performance. My headphones are relatively sensitive (AD500X) but luckily I don't have any popping or popping issues.
My only drawback is the channel imbalance at low volumes, which means that I always have to listen at medium/high volume to avoid channel imbalance.
Does the procedure in this publication help with that or is the imbalance issue something else?
What tubes are those lengths shown in the image? I have changed the stock ones for some JAN GE5654W and I think there is a slight improvement but nothing crazy, do you recommend others?
Greetings and thanks for your excellent contribution.

 

[infinityfine]

Will this be able to power a 600Ohm DT880?

 

[solderdude]

The DC level may well need re-adjustment after a while when the tubes age.
Also this amp does not have a DC protection (just like most other cheap amps) and would not recommend to connect it to expensive low impedance headphones.

 

[luddi]

Sorry to resurrect this post but maybe my problem can be fixed with this as well. I use the MT-602 with Hifiman Sundaras and whenever I turn off the amp there comes a loud "pop/crack" sound from the headphones. This is my first tube or hybrid amp. Is this normal or should I be worried of damaging my headphones? Could this be caused by the DC offset?

 

[Obelix]

Another resurrection. I have a potential issue with my MT-602 in that my DC Offset is 100mV in the right channel, and 60mV in the left. After trying to adjust it using this method, which is almost impossible, as it is constantly fluctuating by around 15mV or more in both channels, making it impossible to set. After getting it to around 6mV in both channels (all the while fluctuating up and down). After using it for an evening, and now checking it now, the DC offset is back up to 100mV and 60mV. Should it fluctuate this badly, and almost reset back to what it was only hours ago? I'm using this to drive an LCD-X which is scaring me a little. I'm probably going to ship it back under it being faulty.

Any technical knowledge on this would be appreciated.

 

[solderdude]

Yes, this is to be expected with this kind of design.

These cheap amps are basically toys of questionable quality as indicated by the price. A novelty device.

 

[DVDdoug]

I see this is an old thread, but 1950's tube technology AND a DC offset?

 

[Obelix]

I believe I may have fixed the issue now. It seems somewhere along the line the screws might have been turned more than needed, as after a full 180 turn, the offset now sets correctly at 00.1. Would you recommend a topping L50 in replacement of this amp? Does that have DC circuit protection?

 

[Obelix]

Sorry to resurrect this post but maybe my problem can be fixed with this as well. I use the MT-602 with Hifiman Sundaras and whenever I turn off the amp there comes a loud "pop/crack" sound from the headphones. This is my first tube or hybrid amp. Is this normal or should I be worried of damaging my headphones? Could this be caused by the DC offset?

I honestly would unplug the headphones prior to switching off the device as I do now. I measured a full 1V that is sent to the drivers at shutdown. I don't think it's a deadly voltage, but possibly better to be safe. When powering up the device there is less of an issue, due to the mute circuit, but after the mute circuits switches off, only 100mV is then reaching the drivers, which falls to 000.1

 

[anthony 5]

I honestly would unplug the headphones prior to switching off the device as I do now. I measured a full 1V that is sent to the drivers at shutdown. I don't think it's a deadly voltage, but possibly better to be safe. When powering up the device there is less of an issue, due to the mute circuit, but after the mute circuits switches off, only 100mV is then reaching the drivers, which falls to 000.1

I do unplug the headphones prior to shutting off eleminating the pop sound in the earphones coming from capacitors emptying no delay switch on this unit

 

[anthony 5]

Another resurrection. I have a potential issue with my MT-602 in that my DC Offset is 100mV in the right channel, and 60mV in the left. After trying to adjust it using this method, which is almost impossible, as it is constantly fluctuating by around 15mV or more in both channels, making it impossible to set. After getting it to around 6mV in both channels (all the while fluctuating up and down). After using it for an evening, and now checking it now, the DC offset is back up to 100mV and 60mV. Should it fluctuate this badly, and almost reset back to what it was only hours ago? I'm using this to drive an LCD-X which is scaring me a little. I'm probably going to ship it back under it being faulty.

Any technical knowledge on this would be appreciated.

Sorry to resurrect this post but maybe my problem can be fixed with this as well. I use the MT-602 with Hifiman Sundaras and whenever I turn off the amp there comes a loud "pop/crack" sound from the headphones. This is my first tube or hybrid amp. Is this normal or should I be worried of damaging my headphones? Could this be caused by the DC offset?

I do have the same problem i unplug headphones before switching off no delay switch on this unit pop sound coming from capacitors