Hello Synthius, Doesn't look too bad for a first time build. I don't know a lot about the distortion output of the National chipamps, so I can't comment in details. I sure that some here will have ideas how you can make it even cleaner. It's nice you have access to such a nice piece of measurement equipment. So, do you get to graduate?!
Peace,
Dave
Peace,
Dave
Why does the right channel have distortion that is almost an order of magnitude higher than the left channel?
Also, how long do you have access to that AP? 😉
Also, how long do you have access to that AP? 😉
Wow man. For a Bachelor's degree? Oh, that beats the heck out of my statistical business analysis (exams without a calculator), computer programming, quantum physics, and other timewasters, etc. . . I'm so very jealous. 😀
BWRX said:
I was also suprised by the higher distortion of the right channel. I'm going to redo this test again sometime this week.
The AP is a sharp piece of equipment, gonna take advantage of it before I graduate.
From your first post:
Is the channel that has the higher distortion the same one that made the strange sound when powering down?
Perhaps you could try replacing the chip and measuring it again if you've tried just about everything else to fix the problem.
synthius said:
synthius said:
Is the channel that has the higher distortion the same one that made the strange sound when powering down?
Perhaps you could try replacing the chip and measuring it again if you've tried just about everything else to fix the problem.
BWRX said:
Both channels make the sound when power down, I originally thought it was only one channel but its both.
Although, There is some very low audible hum when amp is idling on the right channel. Its only noticible when I put my ear up to the speaker. I'm thinking maybe this could be the reason for the increased distortion??
I used a point to point board to create the stereo LM4780 so this could be attributed to my layout and soldering.
I'm going to run a few more test once i get back to the AP. also going to test the THD of my paralleled LM4780, will post that later in week.
This is about "ideas" of things to check out.
Location of the hum is probably the power circuit; however, this being a stereo chip with only one channel humming. . . means the reason for amplifying the hum isn't actually in the power circuit.
Noise:
On this page is one possible approach for reducing the hum from the power circuit: http://diyaudioprojects.com/Chip/Synergy-LM3875-Gainclone/index.htm So, reducing the noise could possibly reduce the problem. Also, it could be useful to review "ground loop breaker" circuit.
Cause:
Hey, anyway, that's (above) a fix to get rid of the hum, but it doesn't address the main issue of why its getting amplified. Perhaps some gel flux (huge can of that in any Radio Shack) on top of some of the connections, and a re-heat, could stop hums from being amplified.
Other location:
Its possible that the hum is on one channel of your preamplifier, so you might want to diagnose to see if its amplifier or preamplifier.
If preamplifier or ground loop at input circuit, you might want to try "lift the signal ground" by 4R to 10R. That technique is illustrated http://www.diyaudio.com/forums/showthread.php?postid=1477270#post1477270 in the photo in that post, as the 10R at the far left side.
Due to the small timeframe, there's some ideas above about reducing the source of the noise rather than its actual cause. I wouldn't normally recommend a "band aid" fix, except for the short timeframe.
Location of the hum is probably the power circuit; however, this being a stereo chip with only one channel humming. . . means the reason for amplifying the hum isn't actually in the power circuit.
Noise:
On this page is one possible approach for reducing the hum from the power circuit: http://diyaudioprojects.com/Chip/Synergy-LM3875-Gainclone/index.htm So, reducing the noise could possibly reduce the problem. Also, it could be useful to review "ground loop breaker" circuit.
Cause:
Hey, anyway, that's (above) a fix to get rid of the hum, but it doesn't address the main issue of why its getting amplified. Perhaps some gel flux (huge can of that in any Radio Shack) on top of some of the connections, and a re-heat, could stop hums from being amplified.
Other location:
Its possible that the hum is on one channel of your preamplifier, so you might want to diagnose to see if its amplifier or preamplifier.
If preamplifier or ground loop at input circuit, you might want to try "lift the signal ground" by 4R to 10R. That technique is illustrated http://www.diyaudio.com/forums/showthread.php?postid=1477270#post1477270 in the photo in that post, as the 10R at the far left side.
Due to the small timeframe, there's some ideas above about reducing the source of the noise rather than its actual cause. I wouldn't normally recommend a "band aid" fix, except for the short timeframe.
Synthius -- the THD+N% should (actually) be lower at 45W than 1W with the LM4780 -- and the numbers you have look pretty high compared to what I have been able to derive from the chip in both bridged and parallel mode.
jack
jack
jackinnj said:
I was looking at the lm4780 dtatsheet and noticed that it should be lower also. Since I was in a hurry today and I just learned how to operate the AP I might have miss labeled the chart. I'm going to sit down and repeat the measurements again this week.
danielwritesbac said:
Thanks for the suggestions daniel, you've been very helpful with all my questions. I will make sure I reference you in my Final Paper...and the rest of you guys who have been helpful
jackinnj said:
Were the bridged and parallel circuits you are referring to made on a point to point board or PCB?
synthius said:
Set the bandwidth to 20Hz --80kHz and run it, this is the standard that National uses, then run it 20Hz to 30kHz -- run it with the 400 Hz High Pass filter (this will tell you what kind of power supply artifacts are coming through.)
You'll be surprised too when you cover the chassis/box as radiation from a fluorescent bulb can work its way into the results.
Lastly, make sure that you are using reasonable quality inter-connects. Cheapo's are no match with the AP, or other high quality instruments like the Tek SG505/AA501 or Boonton 1120. If the lab has Pomona's grab them.
Here are the Final Test results
Stereo Channel THD+N
Subwoofer THD+N
Not sure why the distortion is so high below 50 Hz.
When I observed a 20Hz sine wave output on a scope it had little concave and convex humps on the waveform. THis was evident with frequencies below 50Hz. Anybody have an idea of what the problem could be.
FFT of subwoofer , 50Hz input signal
Stereo Channel THD+N
An externally hosted image should be here but it was not working when we last tested it.
Subwoofer THD+N
An externally hosted image should be here but it was not working when we last tested it.
Not sure why the distortion is so high below 50 Hz.
When I observed a 20Hz sine wave output on a scope it had little concave and convex humps on the waveform. THis was evident with frequencies below 50Hz. Anybody have an idea of what the problem could be.
FFT of subwoofer , 50Hz input signal
An externally hosted image should be here but it was not working when we last tested it.
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