UNIO a minimalist I/V converter

[smms73]

Joachim

when you said 0.5 volts input you mean 0.5 volts RMS. and may I ask why you need such a high output voltage.

 

[Joachim Gerhard]

Well, my Class A power amps have rather low gain and a 10kOhm input impedance.
I found also that a preamp with a bit too much gain sounds better. I do not know why though. Also because i listen to vinyl most of the time ( i hope your forthcoming DA converter persuades my more into digital ) there can be very short, high energy pulses because of dirt and scratches. 6V RMS maximum output with low distortion would be good.

 

[smms73]

but 470 ohms is strange, the 6.2ma dc offset will develop 2.9 volts dc at the output of the dac, i think the dac has protection diodes, but i may be missing something here.

 

[smms73]

UNIO HV

As promised, the preamplifier for Joachim Gerhard.

It uses a circuit (Q7,Q8) that keep the voltage drain source of the input jfet at a constant level to minimize distortion due to early effect.

without this circuit the distortion for input voltages of 0.2v pp, 1v pp, and 2v pp are:

0.0013% 0.0074% 0.019%.

with Q7,Q8 the distortions are:

0.000017% 0.00038% 0.0017%.

This circuit was never test in a real circuit and i never see a circuit like this before, so it has to be build a prototype to see if it works in real life. In simulation it seems stable

 

[Joachim Gerhard]

Plus-minus 40V !
That is some serious sledge hammer.

 

[smms73]

V2 can be a 12v supply, if one use a 40 volt supply the ccs transistor has to be another one ( increase power dissipation).

 

[Joachim Gerhard]

At the moment i have only a plus-minus 24V supply. What do you recommend as modification to use that voltage ?
I have also not seen that circuit before, although elements remind me of something.

 

[smms73]

I intend to use 2 current source to lower the voltage drop at the output resistances. This can be done with 24 volts no problem. Tomorrow I will post the modifications.

 

[Joachim Gerhard]

Of cause i can make the minus supply lower. I just do not have a high quality supply with plus 40V although i can make one.

 

[Joachim Gerhard]

OK; fine, i am busy anyway but i can build it at the end of next week.

 

[smms73]

The input jfet can be k170 or other. Or use a BC337.
I am also a bit busy , but I intend to make a prototype of this circuit.

 

[smms73]

I am also thinking in a circuit with mosfet at the input.
But the ltspice give to much gate current in mosfet and jfet. And the distortion measurement of mosfet seems worse than in real life.
Do you know any low noise mosfet?

 

[Joachim Gerhard]

Usually P-channel laterals are low noise.

 

[Joachim Gerhard]

This could work :http://documentation.renesas.com/doc/products/transistor/rej03g0122_2sj76ds.pdf

 

[smms73]

I know this lateral mosfet very well. And I have some pairs, I will try with them as they have low drain gate capacitance. But I was thinking in a vertical mosfet. Nelson pass uses irf610 a lot.
Normally I use mosfet in power switching, and in last years the vertical mosfet are getting very good. But I don't know if they are any good in low level linear circuits. I have to try

 

[Joachim Gerhard]

Zetex makes some small ones but generally the N-cahnnels are very noisy.

 

[Joachim Gerhard]

Gm is good in this ones but i do not know the 1/f noise.
http://diodes.com/datasheets/ZVP3306A.pdf

 

[smms73]

I will see if I can make the circuit I have been thinking with p Chanel mosfet, as you recommend.

 

[smms73]

Ok thanks. This have to wait. I have already start build the dac boards. One with pcm1794a and another with tda1541a . So I will be busy . And I already have enough IV converters .

 

[Joachim Gerhard]

Maybe in 10 years you are Mr. IV Stage