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Old 2nd December 2005, 05:04 PM   #1
sfogg is offline sfogg  United States
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Default LM3875 measurements...

In case any were curious about these sorts of things I posted some measurements of a chipamp.com LM3875 premium kit here:

http://forums.klipsch.com/forums/642191/ShowPost.aspx

This is from the premium kit of about a year ago (v2 PCB) with the single rectifier board for both channels. Transformer is a Avel Toroidal 250va 18v+18v unit. Other then the addition of an inrush current limiter ahead of the fuse the circuit is stock without the zobel.

DC offset is around 100mv in one channel and 120mv in the other.

Shawn
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Old 2nd December 2005, 05:19 PM   #2
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Interesting! Can put a number on that, like % of dB? I would love to know what THD you measure at 1KHz,1 Watt, 8 Ohm.
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Old 2nd December 2005, 05:33 PM   #3
sfogg is offline sfogg  United States
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My analyzer only has about 80dB of S/N range.

But on 1kHz, 1w, 8 ohm.... (2.83v)

Second harmonic was voltage level of 9.01 E -4, about 70dB down relative to the fundamental.

Third harmonic was voltage level of 5.73 E -4, about 74dB down relative to the fundamental.

Anything else was below the noise floor of the instrument. The two numbers above were the average of 10 seconds worth of readings. The distortion itself was low enough that just with one reading it could be missed in the noise of the instrument itself.

Shawn
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Old 2nd December 2005, 05:45 PM   #4
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Thanks! Here's what I measured once:

http://www.diyaudio.com/forums/showt...772#post449772
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Old 2nd December 2005, 10:17 PM   #5
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Quote:
Originally posted by sfogg
My analyzer only has about 80dB of S/N range.

But on 1kHz, 1w, 8 ohm.... (2.83v)

Second harmonic was voltage level of 9.01 E -4, about 70dB down relative to the fundamental.

Third harmonic was voltage level of 5.73 E -4, about 74dB down relative to the fundamental.

Anything else was below the noise floor of the instrument. The two numbers above were the average of 10 seconds worth of readings. The distortion itself was low enough that just with one reading it could be missed in the noise of the instrument itself.

Shawn
if you null the fundamental with a notch filter you can examine the harmonics in greater detail -- of course the easy way to do this is with the output of a THD analyzer !!!
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Old 3rd December 2005, 01:13 AM   #6
sfogg is offline sfogg  United States
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Thanks, that is something I keep meaning to try sometime.. I just haven't gotten around to setting it up.

In reality though I tend to think in the vast majority of systems what happens below 80dB down is more or less irrelevant. For example if a person is listening with an average SPL somewhere in the 80s and their room has a 40dB noise floor then anything the amp is doing more then 80dB down is already 40dB buried beneath the noise in the room itself. Even taking out the noise floor in the room if you are listening at 80dB SPL anything the amp is doing more then -80dB down is going to be below 0dB SPL..... inaudible.

Shawn
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