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Old 14th December 2014, 10:29 AM   #1
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Default Chipamp.com DC output voltage

Hello All,

I have finished the assmbly of a Non-Inverted LM3886 Kit from from chimp with two separate power supply boards and transformers.

I did all the measurements from the transformer to the amp board and all seems to be within the specs. DC out put from V+, PGND+ from both PS boards is around 31.2V which seems normal as the transformer output around 23V.

The manual says to measure the DC on the output terminals. Quote from the manual"Power the amplifier up without any load connected. Measure the output DC voltage between the + and – output terminals to verify that a low DC offset is obtained (usually < 100mV)"

I did that and measured around 65mV on the left (edit) channel and about 119mV on the right (edit) one.

I'm not sure but I don't think it is normal that there should be such a difference between the channels. What could be causing it ? I tested the amp on a pair of car speakers which went fine and then hooked it up to my main speakers and there was no hum or crackles or such. In fact it sounded quite good. I'm just a bit worried that it might cause some sort of failure (fry my speakers) at some point

Thank's !!

Greetings.

Attached a pic of the assembly in a temporary MDF case.
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Last edited by r100; 14th December 2014 at 03:38 PM.
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Old 14th December 2014, 10:33 AM   #2
AndrewT is offline AndrewT  Scotland
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You need to remove the speakers AND remove the input interconnects AND add a shorting plug to the inputs.
Then measure the output offset/s.
Come back with new measurements.
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Old 14th December 2014, 10:34 AM   #3
AndrewT is offline AndrewT  Scotland
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The heatsinks won't cool well, placed like that.
If you put a lid on the cooling will be even worse.
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Old 14th December 2014, 11:25 AM   #4
Mooly is offline Mooly  United Kingdom
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A 119mv offset won't do any harm (its less than 2mw dissipation into 8 ohms).

In absolute terms and by modern standards anything over around 20mv might be considered as "high". Without knowing the circuit details and how well balanced the LM3886 input currents are, its impossible to make a judgement call.
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Old 14th December 2014, 11:33 AM   #5
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Having resistors that are at the opposite ends of there tolerance at the + and - inputs can easily cause a fair amount of offset on their own.
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Old 14th December 2014, 12:09 PM   #6
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Thank's for all your feedback !!

Quote:
Originally Posted by AndrewT View Post
You need to remove the speakers AND remove the input interconnects AND add a shorting plug to the inputs.
Then measure the output offset/s.
Come back with new measurements.
Measurements with shorted RCA plugs are about:

L +25.3mV
R -23.1mV

Quote:
Originally Posted by AndrewT View Post
The heatsinks won't cool well, placed like that.
If you put a lid on the cooling will be even worse.
Thank's for pointing that out. I will most definitely rethink the cooling. Bigger heatsinks are on order
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File Type: jpg L.JPG (91.8 KB, 140 views)
File Type: jpg R.JPG (112.6 KB, 135 views)
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Old 14th December 2014, 12:59 PM   #7
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Quote:
Originally Posted by jerluwoo View Post
Having resistors that are at the opposite ends of there tolerance at the + and - inputs can easily cause a fair amount of offset on their own.
Hi jerluwoo, I measured the all the resistors before installing them and they all seemed to be very close to their respective values.
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Old 14th December 2014, 01:25 PM   #8
Mooly is offline Mooly  United Kingdom
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If the offset changes when you couple to a source (the inputs) then there are few things to check.

1/ Is the input to chip amp AC coupled. If so, then that means DC offset present in the source can not affect the overall offset.

2/ Is there a problem with hum/oscillation/noise when the inputs are shorted and this is in fact giving the altered and misleading offset measurements.

3/ The real check should be to put a scope on the output and confirm there is no low level oscillation present when connected to a source component.
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Old 14th December 2014, 01:33 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by r100 View Post
...........Measurements with shorted RCA plugs are about:

L +25.3mV
R -23.1mV
............
That's much better.
Now leave both outputs and inputs open and measure again. Measurements 3 & 4.

Then connect your source with volume set to zero and measure again. Measurements 5 & 6.
Then turn volume up as you monitor the voltage on one channel. See if the output changes, or reaches a maximum and falls again. Measurement 7.
Note the maximum reading and where in the rotation it occurs.
Repeat for the other channel. Measurement 8.

Post all 8 measurements.
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Last edited by AndrewT; 14th December 2014 at 02:01 PM.
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Old 14th December 2014, 01:53 PM   #10
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Quote:
Originally Posted by Mooly View Post
If the offset changes when you couple to a source (the inputs) then there are few things to check.

1/ Is the input to chip amp AC coupled. If so, then that means DC offset present in the source can not affect the overall offset.

I did the test with shorted RCA plugs so I'm not sure I understand your question.

2/ Is there a problem with hum/oscillation/noise when the inputs are shorted and this is in fact giving the altered and misleading offset measurements.

With shorted RCA plugs + speakers attached there is no hum, buzz or any other funny noises. In fact, it is very quiet.

3/ The real check should be to put a scope on the output and confirm there is no low level oscillation present when connected to a source component.

I'd love to do that but I don't have one
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