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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 3rd August 2019, 04:40 PM   #21
tomchr is offline tomchr  Canada
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Sorry. I didn't see the prod (@tomchr) until now. DIY Audio doesn't alert you when you do the @-thing.

You should be able to do better than 0.005 % THD+N with the 8903. Mine measured down to 0.0002% at 1 kHz with the 20 kHz LP filter enabled. Enabling the 20 kHz LP filter will mean that you're only counting three harmonics at 6.67 kHz, so I used the 80 kHz filter for frequency sweeps of the audio band.

It looks like you have some power supply modulation going on. If you were to measure the AC voltage on the power supply with the amp delivering 1 kHz into the load at a decent power level, I bet you'll find quite a bit of 1 kHz (and harmonics!) on the power supply. That'll creep through the VCC/2 divider. No surprises there. You should be able to do better with a voltage reference there.

You will always have a turn-on thump in an AC coupled circuit like that. The output capacitor needs to charge to VCC/2 somehow. The charging current flows through the speaker, hence, the thump. You'll need a muting relay and a resistor to ground on the 'inside' of the relay. My Guardian-86 would work well for you.

Tom
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Old 5th August 2019, 06:34 AM   #22
kipphebel3 is offline kipphebel3  Aland Islands
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Thanks! Too bad that the prod does not work I know that you are busy and did not want to bother you needlessly.

I must admit that I never ran the alignment procedures in the 8903 service manual. Perhaps I should just to make sure that it is set up correctly. Let me run the test with just a cable and with filters enabled to see what it says. As now, the measured THD is well below what should be audible.

Thanks also for verifying that an output cap will always create a thump, no matter what so if I want to get rid of that, then I need protection circuitry. Your Guardian is a novel idea. I always thought along the lines of relays.

Yes, even if the added cap will present a "capacitor" with value * Hfe of the transistor, the better solution is a proper reference, but I doubt I will redesign the PCB now. The specs are OK for a surround speaker and a single-supply will always be a tradeoff.

In this case, two of the amps will sit underneath the speaker on a pedestal and I wanted to minimise weight and small, dual-output SMPSes are few an far between.

It is just a hobby project after all
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Old 28th August 2019, 08:21 PM   #23
kipphebel3 is offline kipphebel3  Aland Islands
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Quote:
You should be able to do better than 0.005 % THD+N with the 8903. Mine measured down to 0.0002% at 1 kHz with the 20 kHz LP filter enabled
Mine only has a 30kHz LP filter and the service manual lists 0.01% as limit when doing performance checks. Mine is an 8903A, perhaps yours is more recent?
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Old 30th August 2019, 05:54 PM   #24
kipphebel3 is offline kipphebel3  Aland Islands
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This is my suggestion for a simple turn-on delay to isolate the speaker whilst power is applied. Just needs a suitable resistor to ground before the circuit.
Attached Files
File Type: pdf thump.pdf (16.1 KB, 24 views)
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Old 2nd September 2019, 07:17 PM   #25
kipphebel3 is offline kipphebel3  Aland Islands
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I published the layout on Aisler: Playground / Lm3886 by AndersG on AISLER
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