ICIETDIYEUR
Senior Member
Hello 
Concerning OPAMPs, I am not going to address here SUBJECTIVE considerations that are often linked to a kind of 'myth', 'fashion', various opinions concerning listening or even commercial orders...
I will try to remain OBJECTIVE (my goal is not to convince but to inform) first by observing the circuit used and then by relying on the manufacturers' data concerning the OPAMPS.
The circuit used has capacitors at the inputs of the OPAMPS for two reasons:
- a bipolar OPAMP creates OFFSET if these input impedances are not identical then we use these 'ARTEFACTS' to remedy it, even if it does not correct the OFFSET totally...
- the manufacturer, thanks to this 'TECHNICAL PIROUETTE', will be able to offer a device at a much lower cost price to gain more margin (a NE5532 DIP8 costs about 0.50 euros at MOUSER)
That being said, another important parameter is the value of this input resistor (actually it's impedance).
Here is a comparative table from TEXAS INSTRUMENTS to observe the behavior of BIPOLAR OPAMP and FET according to the input resistance. It will be noted that after a certain value, it is obvious that a FET OPAMP will be preferable to a BIPOLAR OPAMP.
The additional advantage of an OPAMP FET is that it will not create OFFSET if the input resistors are not identical:
we can then do without capacitors, i.e. a 'simplified' circuit without having the various 'effects' of an additional component on the signal.
It can be added that a chemical type capacitor will not work properly if it does not have a minimum potential difference across its terminals, which is the case here while an 'MKP' type capacitor (e.g. WIMA MKS2) is perfectly designed for this use.
Here is another comparison table that shows the 'THD+N' according to the value of the input resistor ->
The reading being done, I leave you 'alone' to judge but I will simply indicate that the OPA1642 (double version of the OP1641) is modern and with a very reasonable cost while the OPA827 is of older technology (although excellent) and only exists in a single version, which implies using two of them on a 'special' adapter whose overall cost will be much higher.
So finally, here is the modified diagram as it should be after observing my remarks ->
I simply added 220pF capacitors 'close' to the OPAMPs (the slots are empty on the PCB) which allow for an improved FFT such as the one I proposed for the O-NOORUS D1 amp because the circuitry is comparable almost in every way.
Additional addition:
The OPA1656 is also a very good choice that remains just as affordable while in the following table the OPAMPs OPA2828 (double opa828) and OPA2156 are starting to enter a much higher 'price range'.
Have a nice day
Concerning OPAMPs, I am not going to address here SUBJECTIVE considerations that are often linked to a kind of 'myth', 'fashion', various opinions concerning listening or even commercial orders...
I will try to remain OBJECTIVE (my goal is not to convince but to inform) first by observing the circuit used and then by relying on the manufacturers' data concerning the OPAMPS.
The circuit used has capacitors at the inputs of the OPAMPS for two reasons:
- a bipolar OPAMP creates OFFSET if these input impedances are not identical then we use these 'ARTEFACTS' to remedy it, even if it does not correct the OFFSET totally...
- the manufacturer, thanks to this 'TECHNICAL PIROUETTE', will be able to offer a device at a much lower cost price to gain more margin (a NE5532 DIP8 costs about 0.50 euros at MOUSER)
That being said, another important parameter is the value of this input resistor (actually it's impedance).
Here is a comparative table from TEXAS INSTRUMENTS to observe the behavior of BIPOLAR OPAMP and FET according to the input resistance. It will be noted that after a certain value, it is obvious that a FET OPAMP will be preferable to a BIPOLAR OPAMP.
The additional advantage of an OPAMP FET is that it will not create OFFSET if the input resistors are not identical:
we can then do without capacitors, i.e. a 'simplified' circuit without having the various 'effects' of an additional component on the signal.
It can be added that a chemical type capacitor will not work properly if it does not have a minimum potential difference across its terminals, which is the case here while an 'MKP' type capacitor (e.g. WIMA MKS2) is perfectly designed for this use.
Here is another comparison table that shows the 'THD+N' according to the value of the input resistor ->
The reading being done, I leave you 'alone' to judge but I will simply indicate that the OPA1642 (double version of the OP1641) is modern and with a very reasonable cost while the OPA827 is of older technology (although excellent) and only exists in a single version, which implies using two of them on a 'special' adapter whose overall cost will be much higher.
So finally, here is the modified diagram as it should be after observing my remarks ->
I simply added 220pF capacitors 'close' to the OPAMPs (the slots are empty on the PCB) which allow for an improved FFT such as the one I proposed for the O-NOORUS D1 amp because the circuitry is comparable almost in every way.
Additional addition:
The OPA1656 is also a very good choice that remains just as affordable while in the following table the OPAMPs OPA2828 (double opa828) and OPA2156 are starting to enter a much higher 'price range'.
Have a nice day
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