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Chip Amps Amplifiers based on integrated circuits

Yet another LM3886 Layout. Single-supply
Yet another LM3886 Layout. Single-supply
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Old 16th April 2019, 05:39 AM   #11
kipphebel3 is offline kipphebel3  Aland Islands
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Exactly
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Old 3rd May 2019, 02:07 PM   #12
kipphebel3 is offline kipphebel3  Aland Islands
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OK. This is what it looks like with a PCB from Aisler
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File Type: jpg PCB2.JPG (492.4 KB, 277 views)
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Old 3rd May 2019, 02:20 PM   #13
kipphebel3 is offline kipphebel3  Aland Islands
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Now, for the interesting part and here is where I expect to be educational... I measured it with my old HP8903A that has a baseline distortion of about 0.005%

The amp was fed from a TTI power supply at 35V and loaded by a 4 ohm resistive load. At low levels the amp adds very little to the baseline, but at higher levels I see a sharp increase in distoriton at lower frequencies. Based on what I see on the monitor output of the 8903A it is basically 2f, modulated with something. Any ideas what might cause this? @tomchr perhaps?

(oh yes, graphs are prettier in Office 2016 than in 2010)
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File Type: jpg HP8903A_Baseline.jpg (76.5 KB, 280 views)
File Type: jpg Distortion at 1V and 11V vs frequency.jpg (40.3 KB, 256 views)

Last edited by kipphebel3; 3rd May 2019 at 02:25 PM.
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Old 3rd May 2019, 03:01 PM   #14
kipphebel3 is offline kipphebel3  Aland Islands
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OK: Answering myself.. Putting a 100u low esr from pin 7 to ground halved the distorsion at 100Hz. Putting a 100u low esr over C3 (the Vcc/2 divider) lowered the distorsion by a factor 10.
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Old 3rd May 2019, 04:34 PM   #15
kipphebel3 is offline kipphebel3  Aland Islands
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I did some comparions to a cheap PCB I bought on eBay for prototyping. Schematic is the same and components are almost identical apart from the eBay one having electrolytics for C and Ci. As expected does the eBay PCB have pretty horrid THD at low frequencies, but adding a 100u across Ca fixes that. It does have somewhat higher THD at high frequencies possibly because of the layout.
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Old 2nd August 2019, 04:11 PM   #16
kipphebel3 is offline kipphebel3  Aland Islands
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Works well, but there is an annoying THUMP when I power up. Does not matter if I have the thing muted or not.
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Old 2nd August 2019, 05:57 PM   #17
blueskynis is offline blueskynis  Serbia
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Yet another LM3886 Layout. Single-supply
How did you connect the mute capacitor Cm. As shown in datasheet or as we discussed in this thread?
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Old 2nd August 2019, 07:25 PM   #18
kipphebel3 is offline kipphebel3  Aland Islands
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It is the same either way. The circuit thumps even if Rm is not connected to ground and if the cap is connected to ground or Vcc/2. With a single supply, the output of the amp will jump to Vcc/2 when the amp is enabled and I will get a thump no matter what. In fact, I can make it happen with just a speaker and an electrolytic. Apply 18V (Vcc/2) and I have a thump.

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Old 3rd August 2019, 07:05 AM   #19
palstanturhin is offline palstanturhin  Finland
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You hear the thump because the circuit loads the output capacitor to 1/2 Vcc.
A single supply amplifier is a "virtual ground".
Make the output capasitor also a virtual ground an see if it helps. (i.e. 2 output capacitors, one to Vcc and one to ground, speaker - to the mid connection point.)
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Old 3rd August 2019, 07:48 AM   #20
kipphebel3 is offline kipphebel3  Aland Islands
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Exactly, but a relay and timer will accomplish the same thing in less space
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