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Topic: audio distortion reading from SD card wav (Read 610 times) previous topic - next topic

Boffin1

On a recent project, I used the wave shield  circuit to read wav files from an SD card, and it worked, but the audio quality was bad ( very distorted )

I have since removed the TL072 voltage follower, and I am feeding the audio direct from the 11Khz filter into an external amplifier, and the quality is outstanding ( I have been using the ISD chips for years, with mediocre results )

I havn't got time to investigate what was causing the distortion ( on both modules that I made ) but I would be happier having the input and output AC coupled and biasing the amp at VCC/2 - even if the specs say it is rail to rail .

I did a search on the forum and seem to be the only one who had this problem...........
With my mobile phone I can call people and talk to them -  how smart can you get ?

davekw7x

#1
Sep 29, 2010, 04:49 pm Last Edit: Sep 29, 2010, 07:38 pm by davekw7x Reason: 1
Hmmm...

I see the ladyada wave shield schematic shows a TL072 output amplifier.  That's the schematic symbol, but I can't believe that a TL072 will work in that application.  Not unless there is some device designated "TL072" that is not the same as the original (30-year old) TI part http://focus.ti.com/lit/ds/symlink/tl071.pdf   (See Footnote.)

If your board actually contained a TL072, then I offer the following observations:

1. The ladyada description does not mention the Op Amp device type, but it says that it is rail-to-rail.  The TL072 is definitely not rail-to-rail.  No way.

2. The ladyada description says that the Op Amp can supply 100 mA per channel.  The TL072 definitely can not supply 100 mA output current.  No way.

3. I have not actually tested a TL072 using a single 5V supply, but I have used that family of devices (many, many, many years ago) using +- 12 Volt balanced supplies.  I wouldn't expect the TL072 to be very linear with a single +5 volt supply.  Not at any output current, let alone at with an output current driving a low impedance speaker or headphone device.  No way.

4. I do not have a wave shield to test or to inspect but I have to ask whether you obtained the shield from ladyada (adafruit) or from somewhere else?

5. I would be interested to hear from anyone else who has used this product.  What is the marking on the 8-pin audio output device?  I mean, there are dual Op Amps, in that same package with the same pinout, that will operate on a single 5-Volt supply.  I'm wondering if the ladyada schematic guy had simply used the '072 Eagle library component and didn't change the value on the schematic to indicate the actual chip used (maybe a TLC2272 or some such thing).  I mean, the layout could be OK but the schematic is misleading.  I don't see an actual Bill Of Materials listed anywhere. (See edited Footnote.)


Regards,

Dave

[edit]
Footnote:
Now I see a parts list http://www.ladyada.net/make/waveshield/parts.html, and the part is a TS922, not a TL072. The TS922 looks a little more like what I would expect. (But I find the practice of connecting Op Amps in parallel to be somewhat "creative"---is this really kosher?)

The question is: did your board really have a TL072?[/edit]

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