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LM3886 / LM4780 5KHz-20KHz frequency response issue
LM3886 / LM4780 5KHz-20KHz frequency response issue
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Old 1st December 2015, 12:00 PM   #1
SunShade is offline SunShade  Italy
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Default LM3886 / LM4780 5KHz-20KHz frequency response issue

I have built a simple non inverting LM4780 amp. Input resistor is a 40k towards ground, plus 1k going to the non inverting input (I also have polipropylene caps as low / high pass filters but I disconnected them for the test). Feedback is a 21k resistor. The inverting input is then connected to ground through a 1k resistor plus a 47uF electrolythic cap
Power supply (30V) filters are 10K uF, plus there is a 1k uF cap on voltage supplies close to the LM4780, along with a 0.1uF bypass
Attached the Rightmark analysis I get using an UCA202 soundcard (both loopback and through the amp)
Load to amp is a 10ohm 5W low quality resistor, with a 470k pot in parallel. Connection to soundcard is through tap at 11kohm

A couple of questions if anybody has experience with LM3886 or LM4780 or can suggest a cause:
- why the decreasing frequency response on the amp starting around 5 kHz. at 20KHz the frequency response seems to be down about 2 dB

- why the 50Hz noise /dynamic range / THD+noise peaks on the amp (it isn't the power supply, should be at 100Hz) and not on the loopback

- why the 50Hz peaks on both the amp and the soundcard on the Intermodulation distortion
Attached Files
File Type: pdf Amp Rightmark test2.pdf (76.6 KB, 64 views)
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Old 2nd December 2015, 09:00 AM   #2
AndrewT is offline AndrewT  Scotland
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post your schematic and a pic of the pcb with wiring.
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Old 3rd December 2015, 08:01 PM   #3
twest820 is offline twest820
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50Hz looks like poor ground management. Roll off above 5kHz may be an RC lowpass against the 459k source impedance to the sound card.
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Old 4th December 2015, 01:09 AM   #4
tomchr is offline tomchr  Canada
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The bandwidth limitation is most likely the 470 kΩ pot into the input capacitance of your test equipment/ADC. I bet if you use a 10 kΩ pot, you'll see a much wider bandwidth.

With the values you mention for the feedback network, you should have a bandwidth in excess of 200 kHz.

Some potentiometers cause distortion. I haven't systematically gone through every type, but I did see the THD shoot up dramatically when I used a 250 Ω wire wound pot as a voltage divider, similar to what you're doing. A better solution would be to use a pair of metal film resistors as a fixed voltage divider. Just be careful not to burn too much power in the voltage divider. I'd stay well below (>5x) the power rating of the resistors to avoid the resistors themselves contributing to the THD.

+1 on the mains hum issues. You'll probably benefit from reading through my Taming the LM3886 pages here: Taming the LM3886 Chip Amplifier

Tom
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Old 4th December 2015, 05:29 AM   #5
danielwritesbac is offline danielwritesbac  United States
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Check for this error:
source -> series resistance -> input cap -> amplifier
This order may reduce the midrange and treble response.
That's actually quite popular.
It also causes the popular, but never desirable "volume knob sounds good at only one spot" problem, which is vexing.
A buffer can let you get away with that trick (without the volume knob problems), but otherwise, there can be excessive caveats.

Fix it by rearranging the order to this:
source -> input cap -> series resistance -> amplifier
Although the fix could make the midrange and treble subjectively less pleasant, indeed it should measure more cleanly that way. Although not so hot for music at utterly full blast, it would be perfectly fine for intelligibility at the TV. . . and measuring.
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Last edited by danielwritesbac; 4th December 2015 at 05:32 AM.
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