The Kuartlotron - keantoken's simple error-correction superbuffer

@Cney
there will be more issues to consider.
.
diskrete DAC has 1/2+V of the offset at the output? That is +1.65V if power supply is 3.3V most lightly. So the coupling C will be necessery?
.
Another thing is output resistance of the DAC that is tipicaly R value (from the R2-R ladder net). That is probably 2K to 5K? I dont know?
So there will be some attenuation because of 10K input at the buffer?
I have R value of parallel R2R net 10K-5K so 2.5K is equivalent generator resistance in my case. So 10K to ground at input was small value in my case and I had to move to higher values to avoid atenuation and LowPass analog filter needed higher R load to get proper Fo cut off... The output resistance og the dac is pretyy high i think.
.
And third thing - If You plan to use diferential mode 2 buffers and transformer after that to acheive SE, that transfprmer should have some sufficent inductance of primary. Because I think that output resistance of the buffer is not too low?
.
I tried JFET at the "input" branch, to increase Rin to several hudred Kilos. And Germanium AD161 at the output. With 340ohms series resistor from output tap to -V. To drive very low primary inductance transformer to get good LF bandwidthh...
I will post the shematic
.
Next - some small offset probably will be present at the output when power off buffers
and that offset probaly would not be tha same so the slight magnetization of the transfrmer will be present. That may not be very good?
 
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There's not really a need to cancel common mode noise before it goes to the RCA output. it's only a short lead inside the case so it is an option to just use one out per channel from the balanced section. Though I would only use half of the dac then. I would like to combine these so I would use both resistor ladders for the RCA as well. It's merely because I would like to have that. I could do it with an opamp after the Kuartlotrons but I would like to have the full output stage discrete.
 
@Cney
there will be more issues to consider.
.
diskrete DAC has 1/2+V of the offset at the output? That is +1.65V if power supply is 3.3V most lightly. So the coupling C will be necessery?
.
Another thing is output resistance of the DAC that is tipicaly R value (from the R2-R ladder net). That is probably 2K to 5K? I dont know?
So there will be some attenuation because of 10K input at the buffer?
I have R value of parallel R2R net 10K-5K so 2.5K is equivalent generator resistance in my case. So 10K to ground at input was small value in my case and I had to move to higher values to avoid atenuation and LowPass analog filter needed higher R load to get proper Fo cut off... The output resistance og the dac is pretyy high i think.
.
And third thing - If You plan to use diferential mode 2 buffers and transformer after that to acheive SE, that transfprmer should have some sufficent inductance of primary. Because I think that output resistance of the buffer is not too low?
.
I tried JFET at the "input" branch, to increase Rin to several hudred Kilos. And Germanium AD161 at the output. With 340ohms series resistor from output tap to -V. To drive very low primary inductance transformer to get good LF bandwidthh...
I will post the shematic
.
Next - some small offset probably will be present at the output when power off buffers
and that offset probaly would not be tha same so the slight magnetization of the transfrmer will be present. That may not be very good?

The output is SE 1.3V, Balanced is 2.6V RMS, Zout 625R. I would like to not use a transformer for Bal/SE if I can. I'm a mechanical engineer so I have some skill regarding electronics but I can't design or alter a stage in these kind of ways. The dac now has a simple buffer with 3 OPA1678, 2 directly at the output of the dac to the XLR and one performing the bal/SE conversion after that stage.
 
Sorry I didnt know that You already have some analog stage after DAC :(

It's just a simple opamp stage that you can buy with the Soekris DAM1941 as a test. But I'd like to replace it with a good transistor output for the XLR to drive longish cables in my studio and RCA to use it in my hifi set also.

I thought of Nelson Pass B1 but the Kuartlotron looked promising. I might try it for just the balanced section and I will see if I can sum the outputs for the RCA using an opamp or transformer and see how that works.
 
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I finally started building my boards but have a question. In this latest iteration, I have several Fairchild transistors on tape. Is it recommended to match Vbe for the buffer? I ran through the posts and wasn't quite sure if skipping matching would invite this to be unstable.
 
1941 has no offset on output. Output impedance is 560 ohm per half.

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IF You cut out all other nets from the output of R2R ladder network. Just FPGA and R2R net - it should have offset of 1/2 power supply at ladder net.
The output impedance for R2R based topology should be R value...
That is for DAC device only without any following analog stage, buffer or amplification stage
 
I tested one more version with JFET "input" and Ge "output" devices. Working very good. Same like full Ge, but with higher input R. For Ge-s I tried 2 types AC187 with lower Io, and AD161 with higher Io, but without owerhating without heatsink.
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I tested this because I have almost all elements on board. just exchanged 3 elements.
More common types for Ges anf JFET version is bellow.
 

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