hi , i have a nos dac with 2,5V DC offset and use it with an output cap . what is the best preamp/buffer witch i can null this offset simply adjusting +/- power rail . can i make it with the BA3 ?
I would sum -2.5VDC (or subtract +2.5VDC) with the DAC output in the buffer/preamp instead. Trying to adjust the rails won't work.
film coupling cap have a sound, pio cap also , MKT another
so if i can do without i prefer .
so the best method is to put a -2.5V at the gate of the input chips to ground to null the + 2.5 from the dac , sounds easy for me to try .
thanks for the suggestion
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Hi,
OPAmps principle way of function is that they strive to keep the potentials at their two inputs identical.
A OPAmp does nothing else but to amplify the tiny error voltage, i.e. difference between its inputs.
The obvious seeming reasoning of the OPAmp amplifying a voltage that is fed to one of its inputs is basically wrong.
This voltage just off-balances its two inputs.
The little difference or error voltage between the inputs forces the output to ´move´ in such a way, that due to the connected feedback network the second input is forced to the same potential as the first input.
In other words, the output tries everyting it can to make the difference between the two inputs to 0.
This feature may be used here simply in that way that the second input may not be referenced to gnd-potential but to the 2.5V of the DAC-output.
See DS of ADIs´s AD1955, fig.7 ff. See to what potential the +inputs are referenced to.
See the differences to the gnd-referenced circuit in Texas´s PCM1794A datasheet, fig.24 and 25.
If the simple referencing doesn´t work out for a standard OPAmp look for the DS of Texas´s fully differential OPA1632, fig. 11.
The fully differential amp featues a Vocm input.
The voltage level applied to this pin appears at its outputs, thereby opening the possibility to generate an output offset or cancel an input offset-related output offset.
Similar possibilites with a single ended output are opened up by the AD1829/1830 from ADI.
See figs: 138-140.
jauu
Calvin
OPAmps principle way of function is that they strive to keep the potentials at their two inputs identical.
A OPAmp does nothing else but to amplify the tiny error voltage, i.e. difference between its inputs.
The obvious seeming reasoning of the OPAmp amplifying a voltage that is fed to one of its inputs is basically wrong.
This voltage just off-balances its two inputs.
The little difference or error voltage between the inputs forces the output to ´move´ in such a way, that due to the connected feedback network the second input is forced to the same potential as the first input.
In other words, the output tries everyting it can to make the difference between the two inputs to 0.
This feature may be used here simply in that way that the second input may not be referenced to gnd-potential but to the 2.5V of the DAC-output.
See DS of ADIs´s AD1955, fig.7 ff. See to what potential the +inputs are referenced to.
See the differences to the gnd-referenced circuit in Texas´s PCM1794A datasheet, fig.24 and 25.
If the simple referencing doesn´t work out for a standard OPAmp look for the DS of Texas´s fully differential OPA1632, fig. 11.
The fully differential amp featues a Vocm input.
The voltage level applied to this pin appears at its outputs, thereby opening the possibility to generate an output offset or cancel an input offset-related output offset.
Similar possibilites with a single ended output are opened up by the AD1829/1830 from ADI.
See figs: 138-140.
jauu
Calvin
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a little schema to help ?
Here's a simple circuit that nulls the offset from the DAC.
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Joined 2009
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Yes, this assumes that he has an op amp there anyway to adapt for the level shifting.
no in fact let me more clear , i had an opamp in my dac i"t s a discrete R2R dad with resistors ( like lavry gold dac) but i didm"t like the sound . so i take it out and would like to have a discrete output stage with or without gain for lowering the 10K output impedance.
the easy way is to put a 1uf coupling cap with a 250K load behind . but i would like to go without and need another solution to null the 2,5V offset .
it seems more easy with tubes , just dc couple the grid let it floating and increase the bias on the cathode. but i can"t find such solution with solid state preamp/buffer
anyway thanks for your help!
the easy way is to put a 1uf coupling cap with a 250K load behind . but i would like to go without and need another solution to null the 2,5V offset .
it seems more easy with tubes , just dc couple the grid let it floating and increase the bias on the cathode. but i can"t find such solution with solid state preamp/buffer
anyway thanks for your help!
Last edited:
Member
Joined 2009
Paid Member
looking at ZVN3310A data sheet it seems that 2.5V bias is on the low range for Ia. another candidate more appropriate for this gate voltage?
it seems were are closer now that of the beginning of the thread
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