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Old 26th January 2009, 08:32 PM   #1
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Default Line out using LM4562

Hello everyone,

I'm new to electronic design so i need your help.

I want to amplify the output of a DAC (TDA1543) in order to feed it after to a power amp and control the volume.

I made some rough calculations and a schematic using an LM4562.

Can you please have a look and give me some comments.

Thank you very much
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Old 27th January 2009, 10:56 AM   #2
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hi bluetech

you can use the IVY linestage in stereo configuration, look at the manual

http://www.twistedpearaudio.com/linestages/ivy.aspx

it sounds fantastic with opus dac
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Old 27th January 2009, 11:24 AM   #3
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With a 5 V single supply there will be no 2 V output. The output pp voltage will be less than 2,5 V. That leads to a maximum Ueff of (2,5/2)/SQR(2) = 0,88 V.

You should revise your output filter. The capacitors are much too small and they will form a high-pass filter with the following equipment. That means, the filter frequency depends on the input impedance of the power amp. Change the amp and the filter frequency changes. Not good. Therefore R34 and R36 should be connected between the capacitors and the output connectors.

Then the filter frequency will be 1/(2*PI*R*C). With 100k and 80 pF you get nearly 20 kHz. Everything below that will be attenuated. You don't want that, do you?

Very much the same applies to the input filter. Place a resistor (~ 5*P3 or bigger) to ground after the capacitor. Then calculate the high-pass frequency. For a bandwidth from 50 Hz upwards choose a filter frequency below 10 Hz. Take into account that the filters add up, so the lower the better.
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Old 27th January 2009, 12:09 PM   #4
paulb is offline paulb  Canada
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DAC output of 80mV doesn't sound correct.
Besides that, you might want to do some reading:
http://headwize.com/projects/showfil...=opamp_prj.htm
http://headwize.com/projects/showfil...=cmoy2_prj.htm
The headphone amplifier is effectively the same as what you want.
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Old 27th January 2009, 12:45 PM   #5
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Using such large resistance values raises two issues -- noise and stability. See National's product folder
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Old 27th January 2009, 01:28 PM   #6
AndrewT is online now AndrewT  Scotland
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I wonder if the DAC output is 20mA?

20mA across 150r is about 3Vpk ~=2Vac

But 2Vac is ~ 3Vpk ~ 6Vpp.

Is the output from the DAC 20mApk or 20mApp? or something else?
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Old 27th January 2009, 04:06 PM   #7
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The output from the DAC is +/- 25mV, I think!
The DAC is the TDA1543
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Old 27th January 2009, 09:13 PM   #8
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Quote:
Originally posted by pacificblue
With a 5 V single supply there will be no 2 V output. The output pp voltage will be less than 2,5 V. That leads to a maximum Ueff of (2,5/2)/SQR(2) = 0,88 V.

You should revise your output filter. The capacitors are much too small and they will form a high-pass filter with the following equipment. That means, the filter frequency depends on the input impedance of the power amp. Change the amp and the filter frequency changes. Not good. Therefore R34 and R36 should be connected between the capacitors and the output connectors.

Then the filter frequency will be 1/(2*PI*R*C). With 100k and 80 pF you get nearly 20 kHz. Everything below that will be attenuated. You don't want that, do you?

Very much the same applies to the input filter. Place a resistor (~ 5*P3 or bigger) to ground after the capacitor. Then calculate the high-pass frequency. For a bandwidth from 50 Hz upwards choose a filter frequency below 10 Hz. Take into account that the filters add up, so the lower the better.

Regarding the voltage out, i have changed the gain to 400 to give an output of 1v (you said it will be 0.8). I may do a dual 5V suuplly at the end.

I have changed the high pass at the output to 10Hz.

I have added the 5*P3 resistors at the input even though i don't understand what they do.

How can i set the low pass frequency at 20KHz?

What is the SQR??

Thank you very much
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Old 27th January 2009, 09:31 PM   #9
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Excuse but i don't know much about opamps.

I had a look on a book and made some assumptions.
correct me if i'm wrong.

Both filters on input and output should be high pass and for a required frequency of 50Hz I should set them to 1 fifth (10Hz) in order the 3db point to be approximatelly at 50HZ.

The low pass frequency will be the highest frequency in the signal times the gain.

Am i near at all?
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Old 27th January 2009, 09:38 PM   #10
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Sorry, I have just decode the SQR thing, it's sruare root!
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