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
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
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/ll265/VincentChang88/310120111434.jpg
http://i290.photobucket.com/albums/ll265/VincentChang88/310120111439.jpg
http://i290.photobucket.com/albums/ll265/VincentChang88/310120111442.jpg
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/ll265/VincentChang88/310120111434.jpg
http://i290.photobucket.com/albums/ll265/VincentChang88/310120111439.jpg
http://i290.photobucket.com/albums/ll265/VincentChang88/310120111442.jpg
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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 !
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 !
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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.
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.
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?
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?
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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.
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.
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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 ????
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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.
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.
Here is my lm3886 layout. poor drawing.
red line is signal ground.
High Res
http://www.diyaudio.com/forums/gallery/showfull.php?photo=6175
red line is signal ground.
High Res
http://www.diyaudio.com/forums/gallery/showfull.php?photo=6175
Hi Cent,
You have a Signal Ground at the left end of the 2r7.
You have Power Ground at right hand end of 2r7.
You have a second channel repeating both these grounds.
You have linked, but not shown the PSU Zero Volts to the Pgrounds.
You have linked the input RCAs, but not shown, to the pot.
You have linked the pot but not shown to the amp inputs.
You have created a group of (not one, but many) grounding loops.
Please draw out ALL the grounding connections, cables, screens, PSU Zero Volts, chassis connections etc.
Label every connection so that we can discuss those that need to be removed and those that need to be moved.
Or go and download and read
http://www.diyaudio.com/forums/diya...udio-component-grounding-interconnection.html
You have a Signal Ground at the left end of the 2r7.
You have Power Ground at right hand end of 2r7.
You have a second channel repeating both these grounds.
You have linked, but not shown the PSU Zero Volts to the Pgrounds.
You have linked the input RCAs, but not shown, to the pot.
You have linked the pot but not shown to the amp inputs.
You have created a group of (not one, but many) grounding loops.
Please draw out ALL the grounding connections, cables, screens, PSU Zero Volts, chassis connections etc.
Label every connection so that we can discuss those that need to be removed and those that need to be moved.
Or go and download and read
http://www.diyaudio.com/forums/diya...udio-component-grounding-interconnection.html
I had read the http://www.diyaudio.com/forums/diya...udio-component-grounding-interconnection.html a lot of times before building both of my chipamps. The only things I did not follow are, not connecting the RCA ground to chassis(earth) and components ground to chassis(earth).
My input RCA ground terminal-> pot -> PCB
the buzz just goes away when I earth the RCA terminal's ground.
Actually how is a ground loops? it flows in through the ground point of chip, input(by shunt resistor) or through air around the circuit to amplifier section?
What is a must to avoid it?
My input RCA ground terminal-> pot -> PCB
the buzz just goes away when I earth the RCA terminal's ground.
Actually how is a ground loops? it flows in through the ground point of chip, input(by shunt resistor) or through air around the circuit to amplifier section?
What is a must to avoid it?
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Normally amp impedance is higher or lower than source?
plug to ipod or no source,
the hum is gain by increasing volume pot.
plug to soundcard or no source(earth either ANY ground point).
totally no hum.
my ground cable from psu is across the trans and twisted AC lines, does the hum injected into ground cable from trans/AC lines?
plug to ipod or no source,
the hum is gain by increasing volume pot.
plug to soundcard or no source(earth either ANY ground point).
totally no hum.
my ground cable from psu is across the trans and twisted AC lines, does the hum injected into ground cable from trans/AC lines?
Discovered the major causes of low hum due to the IEC input socket close to grounding point of PCB and speaker.
This a lesson for me, never ever place ground path or signal path near to the spaced Live and Neutral line.
EMI of ~230Vac much stronger than <30Vac
EMI of main lines much stronger than toroidal transformer
correct me if I'm wrong.
This a lesson for me, never ever place ground path or signal path near to the spaced Live and Neutral line.
EMI of ~230Vac much stronger than <30Vac
EMI of main lines much stronger than toroidal transformer
correct me if I'm wrong.
help please....
Builded another set of LM3886, also buzz.
In this new circuit, I had removed 2.7ohm resistor(isolator between low level signal ground.chip and higher level signal.speaker/power supply) and re-route speaker ground closer to power supply ground. No changes to the buzz and now I can hear 'pops' from the speaker by switching on/off the light, fan and motor. Added in 220pF across the inputs, eliminated the 'pops' but not total of it.
And the buzz.
Whats wrong with the circuit?
Rin's value?
POT removed, input floating - buzz
with POT, LOWEST volume(shorted to ground?), input floating - no buzz
with POT, higher volume, input floating - buzz
with POT, input from sound card(output impedance 99ohm?) - no buzz.
with POT, input from handphone, ipod and..small devices etc - buzz.
Earth any point of the ground with conditions above - no buzz.
Builded another set of LM3886, also buzz.
In this new circuit, I had removed 2.7ohm resistor(isolator between low level signal ground.chip and higher level signal.speaker/power supply) and re-route speaker ground closer to power supply ground. No changes to the buzz and now I can hear 'pops' from the speaker by switching on/off the light, fan and motor. Added in 220pF across the inputs, eliminated the 'pops' but not total of it.
And the buzz.
Whats wrong with the circuit?
Rin's value?
POT removed, input floating - buzz
with POT, LOWEST volume(shorted to ground?), input floating - no buzz
with POT, higher volume, input floating - buzz
with POT, input from sound card(output impedance 99ohm?) - no buzz.
with POT, input from handphone, ipod and..small devices etc - buzz.
Earth any point of the ground with conditions above - no buzz.
I had replaced the red shielded wire with cat5 cable for input signal and 18awg for input ground.
RCA terminals are isolated from chassis.
I don't think there is a ground loop, high DC offset?
Earth wherever the ground path no buzz.
Connect to sound card(Buffer) no buzz.
Connect to ipod/mp3 player/handphone or input float, buzz(without connecting ground to Earth).
I have 2.7ohm/0.1uF zobel at the output but still amplifying the incoming/outgoing call signal of handphone.
RCA terminals are isolated from chassis.
I don't think there is a ground loop, high DC offset?
Earth wherever the ground path no buzz.
Connect to sound card(Buffer) no buzz.
Connect to ipod/mp3 player/handphone or input float, buzz(without connecting ground to Earth).
I have 2.7ohm/0.1uF zobel at the output but still amplifying the incoming/outgoing call signal of handphone.
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