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Old 5th September 2012, 08:45 PM   #1
HiZ is offline HiZ
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Default Beginners advice - signal input levels

Hello! This is my first time posting so please be gentle!

My current project is converting an old radio in to a 'connected' one. My main problem is matching my Airport Express to a low cost TDA2003 pre-built mono amplifier module.

I have opted for the Cebek device as it was cheap and looked to be just what I needed. 5W MONO POWER STAGE CEBEK - Fadisel

The source is an Airport Express, using the jack connector. I assume this is line level, rather than a headphone drive.

If I use the suggested 47K pot across the input, the gain is so huge and sensitivity so great that the audio is severely distorted when iTunes is set to full volume. I'd like the itunes to always be at full volume, with level adjustment set via the pot. What I also need to do is mono the output too.

So.. what I have done is connected the L and R via two 39k resistors to a 1k log pot, down to ground. The wiper and ground then feed the input to the amp.

I cant quite believe I need to pad down the audio voltage down by 39 times in order to achieve good results. This is an audio amp chip aimed at car audio, why so sensitive!?

All seems OK apart from the audio sometime sounding a bit crispy and raspy. When running a test tone though the system, its seems at around 50% volume I get very sudden appearance distortion, through until 70%, beyond that it sounds clean. Is this due to the resistor values Im using and do i need to refine the input design?

Any help appreciated.

Thanks
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Old 5th September 2012, 09:13 PM   #2
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I think that amp has input sensitivity of 100mV, this is very sensitive by modern standards. What you have done with the pad is quite fine. You could change the gain of the amp, but this may bring about instability.
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Old 5th September 2012, 10:01 PM   #3
HiZ is offline HiZ
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Quote:
Originally Posted by richie00boy View Post
I think that amp has input sensitivity of 100mV, this is very sensitive by modern standards. What you have done with the pad is quite fine. You could change the gain of the amp, but this may bring about instability.
Ok great, thats good to know!

I've tried some different -ve feedback arrangements. Leaving R1 as 220R, I've swapped R2 from 2R2 to 47R and its waaaay better now. I've reverted back to the 47K pot and left two 10k's on L and R.

Can you explain what problems I might get with 'instability'?

What I will say is that the heatsink is getting fairly hot just at moderate volume.

http://www.datasheetcatalog.org/data...cs/mXutuqt.pdf
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Old 6th September 2012, 09:30 AM   #4
AndrewT is online now AndrewT  Scotland
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Audio signals range from a few tenths of a mVac (at maximum output) to a few tens of Vac.

That 1000:1 ratio of maximum output signals requires amplification and/or attenuation to be built into "universal" audio systems.

For lots of detail go read:
What is Gain Structure?

While you are there, look at:
Audio Component Grounding and Interconnection - diyAudio
These two articles will be invaluable when you start building your amplifier.
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Old 6th September 2012, 05:54 PM   #5
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With no sound playing, if it is now getting hotter than before you did before the modification to the gain, that would suggest the amp is oscillating, i.e. instability.
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