DIY Audio Projects Forum

Hi guy's,
my latest project is a amp for a smaller subwoofer so i got myself a pcb from china and as always it's no documentation
whatsoever but at least was the values for the components printet on pcb.
My question is regarding 3 pcs of trimmer pots (50K) one for each chip, what are these pot's doing ? what value am i
looking for ? where is the test points ?
It's been a couple of amp projects last 3 years but i never seen this kind of adjustment before on my amps and i have
a idea is some kind of DC offset adjustment.

Unfortunately, without a schematic, any suggestions would only be guesswork w/o experience with this board

Do you have the skills to reverse-engineer the board?
(though I have noticed some Chinese PCB's using multilayers just to keep people from finding out where they stole the design from.... errrr... reverse engineering their wonderful intuitiveness )

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-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
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Those skills are something i dont have but i just got a e-mail with a schematic maybe it help you and me both.

If the trim pots are to go where the screen print says "50k" then I would put full size pots mounted off the brd there. These points are for connecting to you volume controls not trim pots.

But then I'm just taking a stab. All if the schematic is accurately drawn trim pots are not drawn with arrow heads but flat heads. It's an odd way to control volume but there are many ways to fry chips.

_________________
Projects: "Precision" - 300W Class AB Holton power amp | "retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp |

For me it's sounds pretty weird to have 1 volume pot for each chip on a mono block

Those trim pots are to adjust the DC offset of each chip. I actually built the same boards and they sound really good. you have to adjust one at a time to get them close to zero Volts. I got mine to 9mV on one channel and 5mv on the other.

Here is a picture. The DC offset voltage can be measured by using voltmeter, one probe to the output point, and another probe to the ground. The signal ground and the power ground are the same. If there is a voltage, then adjust the trimmer to reduce the voltage. If the output DC is so low ( say below 5mv) then there is no need to adjust. For 'one by one' it really means connect the 0.22 ohm resistors one by one to ensure each LM3886 has the minimum offset before hook all three together.pin #3 is the output but when the 0.22 resistor is in place (again- one at a time) that should provide continuity all the way to the main output pad - which should be easier and safer to measure.
Remember, those adjustments are interactive between chips and may require readjusting as you work.

To yakusa,
million thanks for a great explanation and superb picture which cleared all mystery regarding this amp project.

You're welcome,

Just make sure to keep the rails below ±33V (I'm assuming you will use a 2ohm subwoofer). you want to stay below the 8 amp current limit the the PA150 has. I did some test with mines and with ±35 V and 100HZ sine wave my out put was 22 Vac. I used a 6ohm and a 4 ohm dummy load since my speaker are 6ohm but can dip down to 4.5 ohm. I ended up with 80 watts and 120 respectably. With a 2ohm load i may get 240 watts but that would be over stressing the amp.

dear sir,
I just got the same pcb as yours, I am very beginner, I've tried as you picture, but I still do not understand how to adjust the dc offset, you say to disconnect the resistor 0.22ohm and then adjust the trimmer to reduce the voltage. And you say one probe to the output point, it's mean the probe touch to the output speakers ( + ), and another probe to the ground?? I've tried as you describe, but the lm3886 chip were getting hot.
Please help me, step by step, because i'm very blind for this, and sorry for my bad english, hope you can help my problem.
Thanks.