AD811 as I/V stage for current DACs (and test some other op amps including Burson Audio op amps as I/V)

[grunf]

Did you already try the new version?

Not yet, next week I will order parts and if I'm lucky (i.e. enough free time) maybe next weekend

 

[grunf]

It is indeed a simple yet promising lil circuit to be tried.
Btw it seems clear @grunf you are using it as single ended output, what would actually be best way to use it in case of balanced output chips? Maybe using a small resistor or grounding the chip negative output?

In that case(balanced output), you need four AD811s, as I will use in my new DAC with two PCM1794s.

 

[grunf]

Hi grunf
Can I ask the dimensions of the board?
I got everything ordered and want to get the board made.

Klaus

The dimensions are 70*30mm, if I used SMD electrolytes the PCB would be even smaller, but I deliberately want TH UKZ at least while there are still some to buy.

Igor

 

[grunf]

@Radian, considering the lead spacing on the 8 pin dips is 2.54mm I'd estimte the board to be 55 ~ 60 ish mm long.
But the AD811 has low PS rejection so you need to use a very low noise high quality power supply. The power supply is where all the space in this design will be,

I agree, in my case the regulator and power supply are much more complicated and expensive than the IV stage itself.

 

[Radian]

Can I use two LT3045 each separately supplied?

10-0-10

 

[Radian]

I can get a 160mm X 100mm board made for
€26. 105um traces instead of 35um.
We can get 6 boards onto this area.
I need two for a differential setup.
grunf you can get two just for the cost of shipping if you like.
Shipping takes 7 days after payment.
Gerbers would be nice but pic works to.
We can up the area and put more boards on to it if interest exists.

Greets Klaus

 

[geoturbo]

In that case(balanced output), you need four AD811s, as I will use in my new DAC with two PCM1794s.

Thank you for your answer.
Not been clear sorry, I am using a balanced output chip (a pcm1794 per channel) into a single ended integrated amp (currently with passive iv stage). Since a single ended output is needed, what would be the best way to menage the negative ouput of the dac chip using the ad811 single ended output from the positive?

 

[grunf]

I understood correctly, it should look something like this, four AD811 for two PCM1794 .

 

[geoturbo]

..you know two much
will consider this.. I need -lowest- gain, I guess I was hoping to get away with a symplicistic reply of something like put a lowish resistor/ground the negative output thing but let’s see!

 

[Joseph K]

I understood correctly, it should look something like this, four AD811 for two PCM1794 .

Grunf, I think you have mis-interpreted the application of that 1kohm resistor..
The original Jung scheme is applying it !behind the feedback node. (below)
Your schematic means that you have eliminated the 0 ohm virtual ground that the Dac output wishes to see and now there is 1kohm I/V resistor in place, with some Volt of voltage compliance to tolerate for the Dac..

 

[grunf]

Yes, I think I made a mistake in the drawing, it is important that @geoturbo understood the point and the exact scheme is at the beginning of the topic, only he has to put four AD811.

 

[Itsmee]

@grunf, just found this thread, watching with intrest.

Had you thought of using some JFETS in the front end? - the same as the CEN IV https://www.diyaudio.com/community/threads/folded-cascode-cen-iv-with-fixed-rails.389776/

 

[Itsmee]

I've been thinking along these lines, input JFET's should have a minimum IDSS of 8mA.
I haven't built anything yet, too many projects on the go.

 

[Radian]

Is the AD811 only good for I/V duty or can it be used on voltage out dacs as well?

 

[batteryman]

You could use the +ve input but there are better audio opamps.

I've never been convinced that the 811 is a good choice, whereas the AD844 is, for current output dacs.

 

[Extreme_Boky]

Yeah... the post-DAC filter caps would (most likely) cause oscillations with AD811. Also, the Vout DACs are mostly differential-voltage-out DACs of some 2V pp... no need for an AD811, I'd say.

 

[grunf]

Is the AD811 only good for I/V duty or can it be used on voltage out dacs as well?

AD811 is not for voltage out dacs, a classic VFA op amp is sufficient there

 

[grunf]

Today I finished the prototype PCB for IV with AD811 but this time everything with SMD components except four 100uF/50v ukz and two AD811 with heatsinks.
For the first time I used DALE PNM non-magnetic series instead of RN60, and instead of polystyrene and polypropylene TH capacitors I used non-magnetic SMD COG capacitors. AD811 is soldered direct on the PCB.
So far I can say that in this application the PNM is much better than the RN60.

 

[grunf]

I've never been convinced that the 811 is a good choice, whereas the AD844 is, for current output dacs.

Have you tried AD811? It's not without reason that Walt Jung made an IV for TDA1541 with AD811.

 

[batteryman]

@grunf. Not tried them and happy with Sowter's IV transformers.

I was just sayingg it's not a good choice for a voltage output dac.