dac I/V convertion with very low distortion

[Joachim Gerhard]

I like that version even better. Now you return the base currents of the input stage too and it is very low noise. Noise may come in from the CCSs. the reason is that R3 and R4 are rather low. In my experience they should be at least 100 Ohm. That will provide that the noise of the CCS transistor is not amplified. Output impedance of the CCS will also be higher in proportion of that resistor. One way is to insert a green Led in the emitters of Q11 and Q12. That will make the voltage over R3, R4 aound 2.7V, the optimum value after Tietze-Schenk. For 50mA we end up with a bit over 50 Ohm for R3, R4. Not 100 Ohm but an improvement.

[Joachim Gerhard]

My comment was for version post 49. So where will you put that 1 Ohm resistor of version post 50 ? From the DAC output to the negative input of the opamp ?

[smms73]

Originally Posted by Joachim Gerhard

My comment was for version post 49. So where will you put that 1 Ohm resistor of version post 50 ? From the DAC output to the negative input of the opamp ?

Yes.

[smms73]

Originally Posted by Joachim Gerhard

I like that version even better. Now you return the base currents of the input stage too and it is very low noise. Noise may come in from the CCSs. the reason is that R3 and R4 are rather low. In my experience they should be at least 100 Ohm. That will provide that the noise of the CCS transistor is not amplified. Output impedance of the CCS will also be higher in proportion of that resistor. One way is to insert a green Led in the emitters of Q11 and Q12. That will make the voltage over R3, R4 aound 2.7V, the optimum value after Tietze-Schenk. For 50mA we end up with a bit over 50 Ohm for R3, R4. Not 100 Ohm but an improvement.

i agree with you, a low noise voltage reference can also be used instead of the led (lower serie resistance) .

the input offset trimming will be made in the base of q2, q5.

[Joachim Gerhard]

Very good that we agree on that too. I certainly will try an MC phono input with that topology.

[smms73]

Originally Posted by dirkwright

The DACs that you like have DC current on their output pins which will upset the balance of your circuit. The PCM1794a for example puts out -6.2mA of DC current. Something has to be done about that as well.

do you know why?
Yesterday i was think about this, and i have a theory . They do this for biasing the ne5534 in class a, in this way they decrease the thd of the circuit, so changig this amp op with another type can increase distortion.

what you guys think?

[Joachim Gerhard]

That may well be but are they this clever ? There are of cause other ways to bias an Opamp into Class A.
I use deliberate offset sometimes too. At some point in the chain the offset has to be removed though, be it a good quality cap, trimming or servo.

[dirkwright]

Originally Posted by smms73

do you know why?
Yesterday i was think about this, and i have a theory . They do this for biasing the ne5534 in class a, in this way they decrease the thd of the circuit, so changig this amp op with another type can increase distortion.

what you guys think?

No, I don't have a clue why they do that, and I doubt it is for the reason you suggest. All I know is that it will seriously upset this kind of circuit, since I have sim'd it myself.

[Joachim Gerhard]

In post 54 smms73 already suggested a method to trim the offset.

[Joachim Gerhard]

I know another way to trim the offset by wrapping feedback around the input stage.
That problem pops up in transimpedance MC phono stages too but the other way around by inserting a DC offset into the cartridge. The circuit i developed can solve both problems.