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Old 23rd February 2008, 10:33 AM   #21
Dr_EM is offline Dr_EM  United Kingdom
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Here is a layout on stripboard I did for the 2030AV chip. It worked perfectly well but overall I had poor grounding (no real central point and small leads used) and managed to destroy this amp in a test using an earthed oscilloscope . The layout is fine though if grounded properly and has been used at max volume for hours as an amplifier for en electric drum set. Note the ceramics right next to the chip and the 100uf caps on the side, these are essential!
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Old 23rd February 2008, 10:44 AM   #22
AndrewT is offline AndrewT  Scotland
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Hi,
these chipamps all operate in ClassAB.
When there is zero input signal the output is just noise (mVac).
The quiescent current taken by the chip is given in the datasheet.
The transformer runs with just a tiny load and the smoothing caps have nearly peak voltage on them.

When a music signal is present the output might be around 1W. The PSU must supply that 1W of output + quiescent. The transformer still has a tiny load.

When the very rare occasions come along that require maximum power for a few fractions of a second the transformer does nothing extra. All the transient power comes out of the smoothing caps. The transformer then tries to charge the smoothing caps back up during each peak of the mains supply waveform. This recharging could coincide with with the max power peak or more likely the recharge will come soon after the peak power transient has passed.

The average load that the transformer sees is mostly the 1W+Quiescent. You will find that a PSU works well when the total maximum output power <=transformerVA/1.5, but, there is an acceptable economic range of VA.
I use VA between 1*total output power to 2* total output power. Most other builders work within this range.
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Old 23rd February 2008, 12:31 PM   #23
Paswa is offline Paswa  India
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Quote:
I use VA between 1*total output power to 2* total output power
thanks for the explanation.
total output power means RMS, ok so I use 50VA (just!)?
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Old 23rd February 2008, 01:06 PM   #24
AndrewT is offline AndrewT  Scotland
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except that very small transformers have a very high regulation.
I would suggest no smaller than 100VA and preferably >=150VA for a small power amp.
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Old 23rd February 2008, 01:22 PM   #25
Nordic is offline Nordic  South Africa
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And for the few cents' diffirence you may as well go 180VA if buying new,,,
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Old 23rd February 2008, 02:41 PM   #26
Paswa is offline Paswa  India
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So I am making it my final decision 180VA

now the PCB #1, there may be some schematic mistakes. I'll correct 'em later. But friends, I'll request you to post your comments and suggestion regarding this PCB.

It was made using ExpressPCB. I think this is the best PCB designing software. In my xprience I found it more comfortable and easy to use software even than the well known Eagle
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Old 23rd February 2008, 06:26 PM   #27
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Suggestion: Move the LM1875 to the edge of the board so that a heatsink may be used.

Power supply scenerios:
With 144VA, it will handle Three LM1875 chips, IF there's -per each LM1875- 4700uF smoothing caps, per rail, at the power supply board. (and additinal 220uF or more per rail onboard the amp)

With 144VA, it will handle Two LM1875 chips, IF there's -per each LM1875- 2200uF (or more) smoothing caps, per rail, at the power supply board. (and additinal 220uF or more per rail onboard the amp)

With 72VA, it will handle Two LM1875 chips, IF there's -per each LM1875- 4700uF smoothing caps, per rail, at the power supply board. (and additinal 220uF or more per rail onboard the amp)

With 72VA, it will handle One LM1875 chips, IF there's -per each LM1875- 2200uF smoothing caps, per rail, at the power supply board. (and additinal 220uF or more per rail onboard the amp)

With 36VA, it will handle ONE LM1875 chip, IF there's -per each LM1875- 4700uF smoothing caps, per rail, at the power supply board. (and additinal 220uF or more per rail onboard the amp)

Production models quoted are at 28+28vdc, operating 4 ohm loads with audio at maximum. . . and generous heatsinks.

Efficiency for use of above figures depends on amplifier PCB--EDIT: So listen to Andrew on amplifier efficiency.
Accuracy for use of above figures depends on transformer quality (not a peak-rated or low-temp transformer).


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Old 23rd February 2008, 07:00 PM   #28
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Quote:
Originally posted by Paswa
So I am making it my final decision 180VA

now the PCB #1, there may be some schematic mistakes. I'll correct 'em later. But friends, I'll request you to post your comments and suggestion regarding this PCB.

It was made using ExpressPCB. I think this is the best PCB designing software. In my xprience I found it more comfortable and easy to use software even than the well known Eagle
hi Paswa.

The footprint of the LM1875 seems wrong to me. Are you intending to bend the leads to fit?

Increase the width of the traces, especially the ones that carry higher currents.

regards
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Old 23rd February 2008, 07:14 PM   #29
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Quote:
Originally posted by danielwritesbac
Production models quoted are at 28+28vdc, operating 4 ohm loads with audio at maximum. . . and generous heatsinks.
Hi Daniel,

Following up what Andrew said, read about regulation of toriods. Note how significant it is for small toriods (~20%). It will mean the output voltage of the PSU using the transformers you susgested will vary. I would also guess those small toriods will get quite warm.

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Old 23rd February 2008, 07:36 PM   #30
gootee is offline gootee  United States
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Where to begin? :-)

Move and rotate C1 and C2 so that they connect as close as possible to the LM1875's power pins. Then bring a huge, fat ground trace (or a copper pour area) up to their ground ends. Also then rotate and move C3 and C4 up as far as you can, close to C1 and C2. It might help to move the power connector so it's aligned with the power pins.

Fatten all of your traces, by a LOT.

Add more connectors, so that the power supply decoupling ground, the output/zobel ground, and the signal ground, can each have their own completely-separate ground-return conductors, to the power supply. (Do some searches for 'star ground'.)

If you can squeeze R3 under the chip, that would probably be good. You can mount it on the bottom of the board.

Someone else will probably chime in, here, too. And there are already some threads with good discussions about PCB layout.
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