Why put a cap between DAC chip and IV opamp?

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If there is already a DC blocking CAP at the output of the Op-amp, why there is another cap between DAC and opamp?

Can we remove it?

Can the opamp work well the same with some DC at the inputs?
 

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Whether you can remove it depends on the supply voltages to your opamps. The DAC chip has only a single +5V supply so the offset at its output will be 2.5V. If you wish to handle the typical output level of 2VRMS from your opamp (say 3V peak) then your opamp positive supply will need to be 7.5V or more. The negative supply would need to be greater in magnitude than -2.5V.

Your DAC will most likely sound better if you get rid of the cap - not because of capacitor sound but because it will allow you to remove the following resistor which is needed to bias the cap.
 
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Looking at your schematic again I realize I've overlooked something so far. Your opamps are wired to have gain at DC because the two resistors in series going to the -ve input terminal are connected to 0V. Depending on the values of those resistors - i.e. how much gain you have - the output offset of the opamp might be excessive.

There are a couple of solutions - put a cap in series with those resistors, negative terminal to 0V. Or create a 2.5V reference (a TL431 with RC filter on its output will do fine) and return the resistors to that 2.5V reference.

If the gain of the first opamp is 2 (6dB) or less - the output offset will be <=5V and there won't be any problem because you have another 7V left to the voltage rails.
 
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The Cd Player has an output max of 2.2V RMS at 0dB.

That means 6.223 Vpp. The last stage is unity gain, and the first stage is gain 2, it means that maximun signal V+ is 6.2+2.5= 8.7V, thus can be easily managed with 12VDC i think.

I have removed the resitor too, and, as you said, improvement was audible (much more bass and clean open soundstage).
 

iflyingpig

Member
2016-02-03 5:51 am
These are DC block caps and from my point of view shall not removed:
1) PCM1710 have 2.5V DC on the outputs and 2.5V serves as AC center voltage.
2) If the OPAMP have gain, the DC will sure get amplified and this decreased the head room of OPAMP. Example: if 2 times amplified, the output will get 5V DC, and the head room will be VCC-5V. VCC is the positive supply of the OPAMP. This may lead to clipping.
3) If you want to improve the sound quility. You can change these caps into audio grade caps.
 
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