New built LM3886 buzz

[cent88]

I just builded another LM3886 to replace LM4780.
the buzz is gain together by increasing volume pot, until to the max and it gone. Is it volume pot problem? using blue alps.

[AndrewT]

No.
when the volume pot is at maximum the input from the RCA/phono is shorted to the amplifier input. This is often a very low resistance circuit and the low impedance attenuates the Hum.
More likely you have a wiring error.
You may have a DC coupled amplifier or possibly a mixed AC & DC coupled amplifier.

You need to help us to help you.
post schematic and pics

[cent88]

The schematic same as in the LM4780 datasheet.
The hum is louder when I touch the volume knob.

Tested with
no source, loud hum.
ipod 3.5mm jack and there is a hum.
PC sound card RCA, hum is lower a lot barely audible(almost non but hiss at volume above 50%)
All no hum at max volume pot except for no source.

http://i290.photobucket.com/albums/l...0120111434.jpg
http://i290.photobucket.com/albums/l...0120111439.jpg
http://i290.photobucket.com/albums/l...0120111442.jpg

[AndrewT]

good quality pics, but why are they 3Mega pixel?
Why are they 500kB?
Why are they on photobucket?

I see lots of mains cables twisted to minimise loop area.
I cannot see any signal wiring twisted to minimise loop area.
It looks like you have converted a dual secondary to centre tapped at the amplifier.
Two twisted pairs to feed these 3 insulated terminals is not correct.
A twisted quad is OK, or convert to centre tapped at the transformer and bring a twisted triplet to the amplifier.

Which fig/page in the datasheet? There are 7 to choose from !

[cent88]

no. it is dual bridge there. four of the dual secondary's terminals blocking them.
I use coaxial for signal, the red cable. but the solid core speaker out pick up a lot handphone frequency.
There are 4 twisted pair feeding the 2 x 3 insulated terminals.
2 twisted(V+/V-) to feed each channel.
and another 2 twisted pair are for each channel ground.

http://www.national.com/ds/LM/LM4780.pdf
I use the Reference PCB schematic, at the most bottom page of PDF.

[AndrewT]

fig7 page21 ?
Fig7 has RF attenuation and Output Zobel and is AC coupled.

It must be a wiring fault.
What is the output offset of both channels?
Does shorting the RCA to signal ground change the output offset?
Does adding a dummy 10r resistor across the speaker terminals change the output offset?

Does disconnecting the signal line of one channel change the hum?

[cent88]

yup. fig7.
I followed the schematic exactly, without the 15pF input and zobel.

additional
100k pot before input coupling cap
2.7ohm resistor between circuit ground and speaker ground + psu ground (this solved my previous DC offset transformer buzz in LM4780 to speaker).

I'm wondering which causing the problem because all the tracks are well spacing at least 0.5mm from others.
Wrong connection of coaxial can cause buzz? the coaxial I'm using is a dual type(for microphone). internal signal wiring red for signal and black connected to shield to ground.

[AndrewT]

Originally Posted by cent88

fig7.
without the 15pF input and zobel.

additional
100k pot before input coupling cap
2.7ohm resistor between circuit ground and speaker ground + psu ground (this solved my previous DC offset transformer buzz in LM4780 to speaker).


Wrong connection of coaxial can cause buzz? the coaxial I'm using is a dual type(for microphone). internal signal wiring red for signal and black connected to shield to ground.

fit RF attenuation. Try the 15pF(or upto 47pF) direct across the RCA. But also install 330pF to 1nF from the +IN side of RB1 to signal ground at the bottom of Rin1, repeat for ch2

Fit the Zobel to the PCB, not at the speaker terminals.
Where is the 2r7 fitted?

Screened 2core is not coaxial.
The flow and return are the 2cores. The screen is connected at the source end only. By connecting the multiple screens all at one end avoids the screens becoming the ground loop.
But you have multiple returns. These can form ground loops if wired incorrectly.
Draw a sketch of how you have wired the signal returns from RCAs to Pot and from Pot to PCB and from PCB to PSU and from speaker to PCB and from Main Audio Star Ground to ????

[cent88]

How to measure output offset?
I don't have DMM and low ohm resistor with me now.

on soundcard
disconnecting either input channel does not create hum.

ipod/no source
disconnecting either input channel create hum on both speakers.

[AndrewT]

you can buy very basic test instruments for very little money.
A well performing DMM reading down to 199.9mVdc and 199.9mVac full scale with resolution of 0.1mVdc/ac can be as cheap as $4 or £2.50 for 1% tolerance on full scale reading.

Do not buy an all singing and dancing DMM with lots of extra facilities. What you need now is a good cheap voltmeter. After you become experienced in the basics, then decide if you want/need to invest in more test/build equipment.