Another I/V stage…

[Justcallmedad]

I recently bought an Audio Aero DAC, and I would like to change the OPA 2134 I/V stages, the DAC chip is an AD 1853 (balanced outputs).
For more information on this converter:

http://www.audioaero.com/ Products then Prima D/A Converter 24/192 MKII
http://www.anagramtech.com/
http://www.6moons.com/audioreviews/audioaero/prima.html

The sound is very musical, near vinyl, but it lacks a bit of transparency, 4 op-amps and a tube in the signal path!
I wonder if this I/V stage adapted from an old input circuit of a power amplifier design that I tested a few months ago (the project is in standby waiting for a really good source, CD or turn-table, - now I have both ), could be a good challenger…


I/V Schematic

It’s a first simulation design, probably I will go for a folded cascode, for the moment I just focus on the I/V conversion.

Help and comments are welcome.

[triode_al]

Please use a variable resistor 20 turns for the load resistor. Methinks that you'll need 1.500 ohms for a good output.
At least in my design, with a 3,5 mA current source to the bottom rail ans TDA1541 this was the optimum value to get 0 dB and an undistorted 0dB 1 kHz sine output.
I used a 2SK1078 miniMOSFET and no source resistance. The resistance will give quite some AC effect on the DAC output and the ideal is 0 mV AC IMHO.

[Justcallmedad]

Thanks triode_al,

What do you mean by "the load resistor"? The 2 * 22R resistors between the
fets, for dac load ?

I have in mind to use a trim-pot there to adjust the input for the dac at
0V, and try to lower the resistors values to 10R, but as the fets present
a "load" around 30R, I think that it will not change a lot, in my case the
reference is 2.7V for the AD1853, but for simulation it’s easier to
reference the circuit to ground.

Have you a schematic of your design? As well as the SK1078 datasheet, it
seems not to be an easy part to get, does the complementary part exist ?

[Justcallmedad]

This is the folded cascode design :

I/V_Conv_Folded Cascode.

Probably better with cascoding Q7/Q8.
DAC output is 3mA AC peak to peak, for those who are skill in the art of I/V conversion, which current values do you recommend, and what would be their transistors choice?
I didn’t built it yet, I will do the first tests with MPSA06/56 for Q5/Q6-Q7/Q8 biased at around 15 mA, maybe it’s too much and 6/8mA with SC2240/SA970 will be better?

[Justcallmedad]

Not yet build it, probably this w.e. here just a thd simulation:

I/V thd.

At full scale 6V pk to pk output thd is mostly 2nd at around –80 dB.
At 2V pk to pk 2nd thd is at – 90 dB.
And at 1V pk to pk, -100 dB.

Probably real results will be a little worse.

Input impedance is 11R (2*22R in //) Is it lower enough for the AD 1853 ? I think I could go for half the value with this fets 2*10R and increase their bias current to 4 mA.

If someone is experienced with the AD 1853 advises are welcome.

[triode_al]

The folding cascode changes things of course.
I have no experience with AD 1853. Only philips 154x. And they have a current source. So the input impedance is crucial, and will make a different current in both halves. I recollect Rbroer has designed I?V cascodes of various types.

BTW have you ever seen the input stage of Le Monstre? There the FETS are in the signal path (not just current sources) while the nice harmonic degradation comes from a real cascode. This seems to me a good desgin direction. I add the file of the original article. The little monster from Jean Hiraga is reknowned.

2SK1078 is a MOSFET in SMD format, very low drain resistance and not loading the DAC-output resistance at all. I loaded the bootom with a current source (2SK30) and this wirks fine. But of course a different design direction! Literally also: it is single ended.

[Justcallmedad]

Hi, triode_al,

Yes I know the “Le Monstre” I build it a few years ago, it’s a very very good sounding amplifier.

Actually after reading the rbroer threads, I give up (for the moment the CFP, as it seems to be less musical than the simple common base stage).
I am currently working on his design called “Less simple I/V stage”, measures are quite good, I am tuning it, current sources values, probably replacing the current source that feeds the common base and the folded cascode, by a simple resistor.

I will listen it, in a few days after some more tests and tunings, and post here or on his thread the measuring results, then listening feelings.

Could you post a schematic of your design?

[Jocko Homo]

The input Z will be higher than 22R/2, as you will not have zero ohms looking into the JFET/BJT compound. That figure usually goes down as you up the current. Porbably will have second order dominant, and it will be much more than -80 dB. Could be some cancellation to bring the number down.

Jocko

[Justcallmedad]

Hi Jocko,

I give up for the moment the CFP complementary design, and start to study your “simple” design, well more precisely the folded cascode design, as rbroer did on his “less simple I/V for TDA1541 thread, as described in my last post.

I report here, the circuit:



Replacing T11 the upper current source by a resistor, give the same measuring results and I think that it will improve sound quality, I will check this in a few days.
Cascoding T5 by fet T9 don’t improve measures, what is your opinion on it?
All bjt’s are 2SA970/2SC2240 biased at 6mA is it ok?
Measuring results (on real circuit) are nice, around -80 db thd 2, -90 db thd 3, at 0 db output, for 3mA pk to pk input, noise is a little less than -130 dB.

I will post my actual schematic tomorrow.

Did you carry some test and/or listenings on “hybrid” CFP fet/bjt, it seems that classic bjt CFP don’t sound “good” or musical, even if measures are much better.

Have you experiment/design with the AD1853 DAC?

Thanks to help us and share your knowledge for us, to learn a little more about musical circuits design.

[Dave]

Audio Aero have this "Digital to Analog converter High quality IC analog attenuator" in the list of features for the Prima DAC. Out of interest what chip are they using??