dac I/V convertion with very low distortion

[Shinja]

the opamp in the opamp cen? circuit is working in floating supply ,but the noninverting input is tied to GND.
so it is almost equivalent to working in grounded supply ,and inputted IV converted signal voltage.
therefore, the opamp is requied same property of ordinary IV opamp , high slewrate and short settling time.
additionally, small base current at input stage.(base bias current and current through Cob)

it may be fun to replace input stage of smms73's circuit with opamp.

 

[EUVL]

> mall base current at input stage.(base bias current and current through Cob)

Hence JFET input opamp, such as OPA1641 (post #272).

> the opamp requires same property of ordinary IV opamp, high slew rate and short settling time.

I think this affects more the IV input (opamp output) voltage variations than the current conveyed.
As long as +Vin is not leaking current to Gnd, Kirchoff's law takes care of the rest at the output (R_iv).


Patrick

 

[Shinja]

Hence JFET input opamp, such as OPA1641

yes I think it is good choice too.
OPA627 may be good why its input stage is bootstrapped.

I think this affects more the IV input (opamp output) voltage variations than the current conveyed.
As long as +Vin is not leaking current to Gnd, Kirchoff's law takes care of the rest at the output (R_iv).

but the output node of DAC should be stably regulated at 0V.
the voltage will be V/I converted with the Zout of DAC and conveyed to output.
this may affect especially pcm1704(Rout is 1kΩ) and so on.
this is why I planned to include the base common amp in the feedback loop, at first.

 

[smms73]

TAGUS-FET

This is special for Patrick. all JFET input.

with jfet we can simplify the circuit. But i dont have sk170/sj74 spice models. Can someone post them ?

This may perform better that the cen circuit as there is no voltage variation at the JFET´s, and don´t need a battery.

I dont have much experience with jfet and i have one question, if the gate voltage get's positive the gate current increase or in this type of jfet this is not a problem?

 

[smms73]

Or this one all jfet
( the current sources can also be made of jfet if one wants to )

 

[Shinja]

another plan of output stage

 

[smms73]

And this was the original circuit (or close )... very long time ago.
I was using tubes and no feedback at that time.
This was the input and voltage amplifier of a hibrid power amplifier the output was a triple with Transistors, this was not constructed.

Is a very good driver for a triode single end with that low gain valves like 2A3, 300b, 6c33c etc...

edit: the triode was the ecc88 (6dj8)

 

[smms73]

another plan of output stage

Very clever Shinja, nice way to get rid of the CEN battery.

 

[smms73]

But your circuit dont give a constant voltage for the input stage, my output topology do a better job in lowering distortion.
I think your idea was to get rid of the battery in CEN, was not it?

 

[smms73]

I have been looking for 2sa1015/2sc1815, and toshiba goes to stop production, but there is a SMD version 2sa1162/2sc2712. it seem a good choice, thanks Joachim and Shinja.

 

[EUVL]

A (self-biasing, a la Borbely) cascoded SEN (2SK170 cascoded with J111) or a cascoded CEN (2SK170/2SJ74 cascoded with J111 / J174) will protect the signal FET from Vds variations and has very very low distortion in itself. Simulated at -130dB, not measureable with an AP1.

The circuit is post=286 is cleverly "cheating" , as you basically create a floating supply with current sourced shunt reg. You might equally well use 2x TL431s. The current sources in the model are of course perfect, which will not be the case in rea life. And I do not like the idea that you create any extra path for the signal to flow back to ground (and not through the R_iv). How much that would be depends on the dynamic impedance of your current sources compared to the R_iv. And I bet it is not much more than 1000x (if e.g. you are using 2.4k R_iv, as I do).

We also had a long discussions at the other thread about fixed rails.
For the few that we tried, we always went back to floating supply with batteries.

It is so simple, and sounds so nice.
Why complicate things with 8 or more active devices when you can use 2 JFETs (or one opamp) and a battery ?

My prejudice. Don't let me discourage you to build and compare.


Patrick

 

[EUVL]

> I have been looking for 2sa1015/2sc1815, and toshiba goes to stop production,
> but there is a SMD version 2sa1162/2sc2712. it seem a good choice,

Take a look at 2SC3324 7 2SA1312.
I think you will like them more than 1162/2712.
Costs peanuts at Mouser.


Patrick

 

[smms73]

A (self-biasing, a la Borbely) cascoded SEN (2SK170 cascoded with J111) or a cascoded CEN (2SK170/2SJ74 cascoded with J111 / J174) will protect the signal FET from Vds variations and has very very low distortion in itself. Simulated at -130dB, not measureable with an AP1.

I have also think of that, but i don´t have jfet models to try. I have try with mosfet but was not very positive.
where do you buy sk170 sj74 ?
Is there an alternative? as the sk170 and sj74 are already discontinued.

I like the 2SC3324 7 2SA1312 specially the hight Hfe. Thanks.

 

[smms73]

another question Patrick if you don´t mind.
In yours CEN circuit you only use batteries or you use floating power supplies?

 

[dirkwright]

2SJ74:

.model J2sj74 PJF(Beta=92.12m Rs=7.748 Rd=7.748 Betatce=-.5 Lambda=4.464m
+ Vto=-.5428 Vtotc=-2.5m Cgd=85.67p M=.3246 Pb=.3905 Fc=.5
+ Cgs=78.27p Isr=129.8p Nr=2 Is=12.98p N=1 Xti=3 Alpha=10u Vk=100
+ Kf=26.64E-18 Af=1)
*$

2SK170:

.model 2sk170 NJF
(+ VTO=-5.211e-001 BETA=3.683e-002 LAMBDA=4.829e-003
+ IS=1.000e-009, RD=0.000e+000 RS=0.000e+000
+ CGS=5.647e-011 CGD=2.562e-011
+ PB=4.860e+000 FC=0.5)

 

[dirkwright]

I have also think of that, but i don´t have jfet models to try. I have try with mosfet but was not very positive.
where do you buy sk170 sj74 ?
Is there an alternative? as the sk170 and sj74 are already discontinued.

eBay has them, but they are old stock of course.

Linear Systems makes the LSK170 in 3 grades, but no complement. They have a spice model on their web site.

 

[EUVL]

If you search the forum, you will easily find the JFET models you need.
I have stocks of 2SK170/2SJ74 to last my lifetime. But you can still buy from Spencer Cheung, also here at the forum.
Just search 2SK170BL at the commercial sector. He still has some 2SJ74s.
And I would not buy from unknown ebay dealers. Too many fakes.

http://www.diyaudio.com/forums/swap-meet/131284-toshiba-2sk170bl-sales-12.html#post2689554

Alternative is to use SEN (N-JFETs only). Then you can use LSK170, 2SK369, 2SK372, and BF862.
The latter will be still around for a while, as far as I was told.
But you never know these days.

2SC3324 / 2SA1312 has much lower noise than the others mentioned.

And batteries only for me.
Only use transformers for power amp.
Everything else batteries. (Even one power amp is battery powered.)




Patrick

 

[dirkwright]

No spice models that I can find for 2SC3324/2SA1312, but I suppose they are functionally the same as 2SC2240/2SA970.

 

[EUVL]

Yes.

.model 2sa970 PNP(Is=465.4f Xti=3 Eg=1.11 Vaf=57 Bf=407.6 Ise=4.683p Ne=2.051
+ Ikf=.3998 Nk=1.192 Xtb=1.5 Var=100 Br=1 Isc=465.4f Nc=1.048
+ Ikr=6.032 Rc=2.343 Cjc=11.59p Mjc=.4014 Vjc=1.155 Fc=.5 Cje=5p
+ Mje=.3333 Vje=.75 Tr=10n Tf=1.252n Itf=1 Xtf=0 Vtf=10)

.model 2sc2240 NPN(Is=99.13f Xti=3 Eg=1.11 Vaf=422.2 Bf=352.8 Ise=1.179p
+ Ne=1.782 Ikf=.4704 Nk=.9631 Xtb=1.5 Var=100 Br=1.663 Isc=555.1p
+ Nc=1.796 Ikr=5.85 Rc=.2032 Cjc=7.561p Mjc=.2472 Vjc=.3905 Fc=.5
+ Cje=5p Mje=.3333 Vje=.75 Tr=10n Tf=1.295n Itf=1 Xtf=0 Vtf=10)


Patrick

 

[zinsula]

This post from syn08 regarding transistor noise, beta, rbb is very interesting.
Unfortunately, he didn't measure BC327/337.