# DIY Analog stage for Zhalou and other V-out DAC's



## regal

Really what the Ori and Zapfilter analog stages are doing is simple: differential to single ended buffer.

 I am baffled by the lack of info on audio sites on making this basic upgrade to a voltage out DAC

 This page describes I think a good method to do this:

http://ultranalog.com/cdenhancer/sac...ancer_evo3.pdf


----------



## TzeYang

are you sure Zapfilters use the same thing?

 To me it looks like a very very very simple discrete opamp with single ended output stage 
	

	
	
		
		

		
			





 The IV conversion is also done with this circuit too? in anyway, even cetoole's IV converter is much complext than this lol.


----------



## Pars

A voltage-out DAC doesn't need I/V (current-to-voltage) conversion... I haven't studied these at all, but I would assume that it is much simpler than I/V for a current-out DAC. It might basically be just a buffer. The complexity of the zapfilter comes from it being configurable for both v-out and i-out DACs, and both single-ended and balanced (IIRC). As far as its use as a single-ended I/V converter, I would doubt that it would outperform such DIY stage as cetoole's, Jocko's etc.


----------



## joe_cool

Perhaps you don't realize this analog DAC circuit is normally a multi-pole low pass filter. Some commercial DAC chips are single-ended with no need for external differential conversion.


----------



## 00940

If it's a differential output, you could easily use a dynalo with only one pair of output transistors to perform the balanced-unbalanced conversion and analog filtering.


----------



## majkel

I guess there are two reasons why DIYers don't care about V-out DACs.
 1) Such a DAC is nothing but a linear source, you just need a filter/amplifier stage. There are lots of schematics for active low-pass filters.
 2) V-out DACs are inferior to I-out DACs because they have already built-in I/V converter which is of arguable sonic quality for die-hard audiophiles. Some say V-out DAC is a I-out DAC with a mediocre op-amp I/V converter inside.


----------



## TzeYang

Quote:


  Originally Posted by *Pars* /img/forum/go_quote.gif 
_A voltage-out DAC doesn't need I/V (current-to-voltage) conversion... I haven't studied these at all, but I would assume that it is much simpler than I/V for a current-out DAC. It might basically be just a buffer. The complexity of the zapfilter comes from it being configurable for both v-out and i-out DACs, and both single-ended and balanced (IIRC). As far as its use as a single-ended I/V converter, I would doubt that it would outperform such DIY stage as cetoole's, Jocko's etc._

 

hmmm, but it seems to me that the Zapfilter also mentioned IV conversion in their circuit. I thought we were talking about zhaolu and it's siblings? Those arent voltage DACs are they?


----------



## joe_cool

Quote:


  Originally Posted by *TzeYang* /img/forum/go_quote.gif 
_hmmm, but it seems to me that the Zapfilter also mentioned IV conversion in their circuit. I thought we were talking about zhaolu and it's siblings? Those arent voltage DACs are they?_

 

The Zhaolu DACs use the AD1852 and/or CS4398 DAC chips. These are differential voltage output converters. I believe the Benchmark DAC1 uses an AD1853 which is a current output DAC.


----------



## 00940

Quote:


  Originally Posted by *majkel* /img/forum/go_quote.gif 
_I guess there are two reasons why DIYers don't care about V-out DACs.
 1) Such a DAC is nothing but a linear source, you just need a filter/amplifier stage. There are lots of schematics for active low-pass filters.
 2) V-out DACs are inferior to I-out DACs because they have already built-in I/V converter which is of arguable sonic quality for die-hard audiophiles. Some say V-out DAC is a I-out DAC with a mediocre op-amp I/V converter inside._

 

The new generation of voltage out DACs (like the wolfson wm8740 or cs4396) isnt' working that way. They use the switched capacitor topology, not an internal opamp like the TDA1547 did.


----------



## TzeYang

Quote:


  Originally Posted by *joe_cool* /img/forum/go_quote.gif 
_The Zhaolu DACs use the AD1852 and/or CS4398 DAC chips. These are differential voltage output converters. I believe the Benchmark DAC1 uses an AD1853 which is a current output DAC._

 

thanks. So which means if i want to modify the zhaolu's output stage, a buffer that takes differential input will work?


----------



## TzeYang

because the output does not have sufficient current to "turn on" the input transistor bases of the opamps.


----------



## Cauhtemoc

Quote:


  Originally Posted by *majkel* 
_Some say V-out DAC is a I-out DAC with a mediocre op-amp I/V converter inside._

 

And some say that mankind never really went to the moon, but it doesn't make that true either.


----------



## joe_cool

Quote:


  Originally Posted by *Shopper* /img/forum/go_quote.gif 
_With a DAC like the AD1852 that has differential voltage outputs, do you strictly need low Ib opamps in the analog stage in order not to have DC offset on the line output?_

 

No, the impedances are kept low to control the noise figure. See page 14 in the datasheet for a similar circuit.

AD1852 datasheet


----------



## regal

Quote:


  Originally Posted by *majkel* /img/forum/go_quote.gif 
_I guess there are two reasons why DIYers don't care about V-out DACs.
 1) Such a DAC is nothing but a linear source, you just need a filter/amplifier stage. There are lots of schematics for active low-pass filters.
 2) V-out DACs are inferior to I-out DACs because they have already built-in I/V converter which is of arguable sonic quality for die-hard audiophiles. Some say V-out DAC is a I-out DAC with a mediocre op-amp I/V converter inside._

 


 Maybe we should care with people making a lot of money installing a few transistors in Zhalou's. 

 A modern V out DAC is not a I-out DAC with a built in I/V. Study up on Sigma Delta.

 To me sharing ideas so people can save a dime is what DIY is all about. People love the OMZ's.

 The real trick is the differential to single ended transfer, to be honest I have trouble grasping it and how to adjust the gain to match the different V out p-ps of different chips.

 Why not make a project out of it.

 Or even better how about a discrete analog stage for the EZDAC. Again the differential to single end and the high p-p current out of the PCM1794 make plugging in a Jocko, Ctool, or D1 difficult for me. 

 I like this idea better since it is DIY start to finish. I'll start a new thread if people would like to make it a project.


----------



## 00940

Quote:


  Originally Posted by *regal* /img/forum/go_quote.gif 
_The real trick is the differential to single ended transfer, to be honest I have trouble grasping it and how to adjust the gain to match the different V out p-ps of different chips._

 

That's quite easy to understand with opamps (and your discrete stage is just a discrete opamp): http://www.analog.com/library/analog..._Ch2_final.pdf

  Quote:


 Or even better how about a discrete analog stage for the EZDAC. Again the differential to single end and the high p-p current out of the PCM1794 make plugging in a Jocko, Ctool, or D1 difficult for me 
 

I don't see why you couldn't use those... just change the value of the I/V resistor to get the desired voltage out.


----------



## regal

I meant a discrete active I/V for the EZDAC I am not a fan of passive I/V.


----------



## 00940

That's what I mean too... There is a resistor in both Jocko and D1 I/V stages to set how much voltage you get at the output.


----------



## regal

Quote:


  Originally Posted by *00940* /img/forum/go_quote.gif 
_That's what I mean too... There is a resistor in both Jocko and D1 I/V stages to set how much voltage you get at the output._

 

Ok, I'll research that. I imagine you would need a separate D1 I/V for each of the differential outs, so 4 in total. Then send this to the SACD enhancer buffer to transfer to single ended.

 The D1 could be set to out put 5.6V pp as that is what the sacdenhancer buffer is set up for.


----------



## 00940

In Jocko's, it's R1:







 In the D1; it's R27 and R34:







 A side note : Jocko only considered that "easy to build" I/V stage as a concept that needed to be improved on, not as the "best thing out there". For improved variations on this concept, search Rbroer's I/V on Diyaudio (as Pars did...)


----------



## TzeYang

Quote:


  Originally Posted by *TzeYang* /img/forum/go_quote.gif 
_thanks. So which means if i want to modify the zhaolu's output stage, a buffer that takes differential input will work?_

 


 I think my question got missed. Anyone mine answering it? Thanks


----------



## BrianDonegan

Yes, a buffer would work.


----------



## joe_cool

Quote:


  Originally Posted by *TzeYang* /img/forum/go_quote.gif 
_So if i want to modify the zhaolu's output stage, a buffer that takes differential input will work?_

 

Yes, the best accuracy (lowest distortion) will come from a circuit which compares the difference in the DAC outputs. Also a 2- or 3-pole low pass filter at about 75kHz will minimize audible artifacts.

 The Zhaolu D2.5 circuit is very similar to the AD circuit referenced in post #15, except that C10 and C12 are missing on the Zhaolu PCB. On the particular DAC which I studied, the correct value would be 470pf as the values are all shifted down a bit (R9 for example is 1K ohms).

 My conclusion was that replacing the op-amps, bypassing the coupling caps and installing the 470pf caps for C10 and C12 were the best practical modifications to that circuit. Further improvement would require topolgy changes, for example the Ori or Zap modifications.


----------



## 00940

I was playing with ltspice yesterday and remembered this post... Here's Jocko I/V adapted for the pcm1794 and BC547C/BC557C. I switched the reference from the positive to the negative rail, it's easier to deal with the offset. With a 750R I/V resistor, you have a 3V peak to peak output (so that you get 2Vrms standard output). But it goes at ease up to 4V p2p.

 R5 and R3 are trimmers. Adjust R5 for 0V at the output of the DAC and R3 to get around -5V in between Q1 and Q3.


----------



## Cauhtemoc

Are you planning on building it? I have a better version if you want something to compare it to. THD is -85 dB compared of -70 dB, input impedance is 1 ohm compared to 5 ohm, and there's no capacitor in the signal path.


----------



## regal

Quote:


  Originally Posted by *00940* /img/forum/go_quote.gif 
_I was playing with ltspice yesterday and remembered this post... Here's Jocko I/V adapted for the pcm1794 and BC547C/BC557C. I switched the reference from the positive to the negative rail, it's easier to deal with the offset. With a 750R I/V resistor, you have a 3V peak to peak output (so that you get 2Vrms standard output). But it goes at ease up to 4V p2p.

 R5 and R3 are trimmers. Adjust R5 for 0V at the output of the DAC and R3 to get around -5V in between Q1 and Q3.
_

 

Thanks for sharing, I want to build this for an EZDAC.

 I can't make out what is connected to the emmitter of Q4, is it ground ?

 So AC1 gets Iout+ and Iref goes to grond. Where are you connecting Iout- ?


----------



## 00940

Quote:


  Originally Posted by *Cauhtemoc* /img/forum/go_quote.gif 
_Are you planning on building it? I have a better version if you want something to compare it to. THD is -85 dB compared of -70 dB, input impedance is 1 ohm compared to 5 ohm, and there's no capacitor in the signal path._

 

To be honnest ? No. It's just an example to show how to quickly adapt a discrete I/V to the requirements of the pcm1794 (hot output and high negative current offset). 

 I plan to test something along this line but with a folded cascode (as per Rbroer's ideas, nihil novi sub sole). The caps will go away since the offset will be nulled and servoed at the balanced to single ended stage. The CCS will probably keep the BC547-557C (or 560C-550C) but the other transistors will be changed to 2sc2240 and 2sa970. I wonder if they're anything to be gained by putting a CCS instead of R5. Since I'm designing with the pcm1798 in mind, the idle currents might also be a tad lower. And of course, the schematic shown lacks analog filters. 

 But if you have some better ideas, I'm all ears 
	

	
	
		
		

		
		
	


	





 Regal: you need to build 4 of those stages for a pcm1794. So each Iout goes to a different stage. All the "triangle symbols" are ground, even if they're going up, I was just lazy drawing the schematic.


----------



## regal

Quote:


  Originally Posted by *00940* /img/forum/go_quote.gif 
_
 Regal: you need to build 4 of those stages for a pcm1794. So each Iout goes to a different stage. All the "triangle symbols" are ground, even if they're going up, I was just lazy drawing the schematic._

 

But thats the issue with these diff out DAC's, you have to somehow do a balanced to unbalanced conversion to get the SNR benefits for single ended operation

 Twisted Pair audio is working on a discrete stage for this:

diyAudio Forums - Discrete Super Symmetric(I think) Opamp for I/V Etc...


----------



## Cauhtemoc

Quote:


  Originally Posted by *00940* 
_I wonder if they're anything to be gained by putting a CCS instead of R5._

 

Not really. All Q4 does is to shift the voltage at the base of Q3 by whatever voltage is developed across its BE junction. This is normally said to be around 0.65V, but as the voltage developed across a silicon PN junction varies somewhat depending on the current, you can adjust this voltage by adjusting R5.

  Quote:


  Originally Posted by *00940* 
_But if you have some better ideas, I'm all ears 
	

	
	
		
		

		
		
	


	


_

 

The folded cascode is a good start. There are a couple of more things you could do, but I'll let you try on your own first. 
	

	
	
		
		

		
		
	


	




 When you have built this I/V converter, feel free to drop me a PM and I will send you my improved version.


----------



## Cauhtemoc

Quote:


  Originally Posted by *regal* 
_But thats the issue with these diff out DAC's, you have to somehow do a balanced to unbalanced conversion to get the SNR benefits for single ended operation

 Twisted Pair audio is working on a discrete stage for this:

diyAudio Forums - Discrete Super Symmetric(I think) Opamp for I/V Etc..._

 

That's nothing more than a balanced opamp (a discrete one, but an opamp none the less). It relies on the use of feedback to do the I/V conversion. What we have posted here are proper I/V converters.


----------



## regal

Quote:


  Originally Posted by *Cauhtemoc* /img/forum/go_quote.gif 
_That's nothing more than a balanced opamp (a discrete one, but an opamp none the less). It relies on the use of feedback to do the I/V conversion. What we have posted here are proper I/V converters._

 

its better than nothing, and nothing is what have seen posted elsewhere to do I/V for a PCM1794 to give a single ended output.


 I suspect that commercial products like CI VDA2 are just leaving -Iout open for single ended operation.


----------



## Cauhtemoc

Quote:


  Originally Posted by *regal* 
_its better than nothing, and nothing is what have seen posted elsewhere to do I/V for a PCM1794 to give a single ended output._

 

If you want a single ended output from a PCM1794 you can just do what is suggested in the datasheets. Check page 20 and 21.


----------



## philodox

Given that the zapfilter can take voltage or current and unbalanced or balanced and output unbalanced and balanced with all discrete circuitry, I consider it a pretty good value. That's not to say it is the only way to do an output stage, but it certainly is versatile.


----------



## regal

Quote:


  Originally Posted by *Cauhtemoc* /img/forum/go_quote.gif 
_If you want a single ended output from a PCM1794 you can just do what is suggested in the datasheets. Check page 20 and 21._

 


 If I wanted IC opamps I would just buy a DAC.


 This is the point I am making about Twisted Pairs solution. With the PCM1794 (all new DAC's really) you are going to resort to feedback in the analog stage if you want a single ended output.


----------



## regal

Quote:


  Originally Posted by *philodox* /img/forum/go_quote.gif 
_Given that the zapfilter can take voltage or current and unbalanced or balanced and output unbalanced and balanced with all discrete circuitry, I consider it a pretty good value. That's not to say it is the only way to do an output stage, but it certainly is versatile. 
	

	
	
		
		

		
		
	


	


_

 

When I contacted them they said there is no way to adjust the output Vrms (gain). So if it works for all DAC's like they say you could end up with one hell of a hot signal with something like the PCM1794 (+_8ma Iout !) I passed.


----------



## 00940

Quote:


  Originally Posted by *regal* /img/forum/go_quote.gif 
_But thats the issue with these diff out DAC's, you have to somehow do a balanced to unbalanced conversion to get the SNR benefits for single ended operation

 Twisted Pair audio is working on a discrete stage for this:

diyAudio Forums - Discrete Super Symmetric(I think) Opamp for I/V Etc..._

 

Of course you need that conversion. TI's engineers want you to do it. 
	

	
	
		
		

		
		
	


	




 An opamp will be hunting on its grounds doing it. 

 The symmetric opamp Brian is working on won't btw convert the balanced output to single ended either. I'm also not quite certain it's worth going discrete to build symmetric opamps while the opa1632 is already such a good beast. Te proof will be in the pudding... sorry the hearing


----------



## Pars

Quote:


  Originally Posted by *Cauhtemoc* /img/forum/go_quote.gif 
_Are you planning on building it? I have a better version if you want something to compare it to. THD is -85 dB compared of -70 dB, input impedance is 1 ohm compared to 5 ohm, and there's no capacitor in the signal path._

 

Just checking to make sure I understand these to a rudimentary degree...

 The input impedence in Jocko's would be the impedence looking into the emitter of Q2, which is given by re = 25 / Ic (page 81 of AoE), so assuming ~1.8V LED, would be 25/4.8mA or ~5.2 ohms? Similarly, the circuit posted by 00940 would be the impedence looking into the emitter of Q3, or 25 / 5.4mA or 4.6 ohms? Or am I missing something? If that is correct, then your circuit (Cauhtemoc) is running on the order of 25mA thru it? Thanks.

 Chris


----------



## 00940

it's only sims but... a cfp seems to do wonders to the input impedance .


----------



## Pars

cfp?


----------



## regal

Quote:


  Originally Posted by *Pars* /img/forum/go_quote.gif 
_cfp?_

 




diyAudio Forums - Easy-to-build I/V stage


----------



## 00940

Sorry. *C*omplementary *f*eedback *p*air. It seems to have been discussed quite a bit on diyaudio (I just searched and found this: diyAudio Forums - "Super-Pair" I/V for TDA1543 )

 And Douglas Self discuss it here : Design with discrete transistors.

 The great thing is that it would allow me to run the 2sc2240 with high currents while the weaker 2sa970 would receive almost no loading. 

 edit: I said wonders because my sims jumped from 4ohms input impedance to 0.4ohms.


----------



## BrianDonegan

Quote:


 I'm also not quite certain it's worth going discrete to build symmetric opamps while the opa1632 is already such a good beast 
 

We have been using the THS4131 (TXD without inst amp) on the bench and it is impressive (similar to the 1632 but a bit more versatile).


----------



## regal

unless the slew rate of these opamps is 1000V/uSec, forget a constant virtual ground, probably better with passive I/V depending on the chip.

 Plus if you resort to opamps there are hundreds of DAC's you can buy and have instant gratification which IS better than spending hundreds of hours finding the right discrete circuit.

 I am thinking I/V is best left to a transformer. The solution just seems so simple and straightfoward. With an open discrete I/V there are just too many things to get wrong. And odds are you will need a big coupling capacitor which is going to color the sound as much as a transformer. Cost wise a couple transformers for a couple hundred bucks isn't that bad vs. man-hours.


----------



## Pars

The rbroer stage I built is dc coupled, and works great... almost stable enough to forgo the DC servo... cetoole's stage is similar, and also dc coupled.

 I have heard very good things about the THS4031/THS4032 for use as I/V opamps, although they require careful bypassing and design. If I were to pursue opamps, these were going to be the next ones I tried...


----------



## regal

what do you think about a transformer like the Sowters?


----------



## Pars

I'm guessing that I would think they are damn expensive? And subject to the non-linearities of most transformers?


----------



## cetoole

Quote:


  Originally Posted by *00940* /img/forum/go_quote.gif 
_it's only sims but... a cfp seems to do wonders to the input impedance ._

 

Yeah, up to about 2MHz. Havnt been able to keep it from going inductive above that, except by adding capacitance to the input, which will increase distortion. Maybe with different transistors than I have tried it would work nicely.

 Regal, at HF, where the transformer isnt able to act as a transformer anymore, and where there is all of that HF noise, the DAC will see some nasty reflections on it's output. Yes, it may filter the HF noise from downstream components, but it will also hurt DAC linearity. For your purposes (dc coupled single ended output from PCM1794A), try a folded cascode I/V, with passive filtering, and a fully differential opamp (can be discrete) on the output of that, acting as a buffer and summing the two phases.

 Here is where I currently am with regards to discrete I/V:


----------



## Russ White

Quote:


  Originally Posted by *Cauhtemoc* /img/forum/go_quote.gif 
_That's nothing more than a balanced opamp (a discrete one, but an opamp none the less). It relies on the use of feedback to do the I/V conversion. What we have posted here are proper I/V converters._

 

Proper.... hmmm what exactly is "proper"

 You say my design is simply another opamp... Its actually quite a bit more than that.
	

	
	
		
		

		
		
	


	




 The version I am working on is super symmetric with twist, and does not rely on conventional negative feedback, but is truly super symmetrical(what I mean is it not only X'd but also has symmetrical pairs). What is more, it utilizes a clever method of nulling any common mode current/voltage.... and Errors!!!. 

 There simply is nothing like it that I know of. The THS40xx is close, but it lacks the symmetrical folded cascodes. My design also has orders of magnitude lower distortion than simple passive I/V (resistor or transformer) + buffer approach (like the zapfilter).

 Lets make it clear, there are only two ways I know of to do I/V conversion. Ok really only one, but I won't get into that...

 You can do it actively by utilizing feedback, or you can do it passively by presenting the current to an impedance(not necessarily a resistance). The impedance can be generated any number of ways, but its still just an impedance. Both have there strong points, and I use both on my DACs.

 The PCM1794 board I designed allows both passive and active options. And both stereo and mono use as well.

 The wonderful thing about feedback is, it makes the output of the DAC have to swing less around its stasis point. What I mean is, if you look at the voltage across the ouput of the DAC it will barely move with most active (feedback) I/V stage. With passive I/V (any type) the ouput of the DAC has to follow the output of the stage closer to the supplies (0 and 5V for example). This makes the output much more prone to capacitive and inductive effects of both the DAC, and the ccts down stream (even on the same PCB).

 So I would not be too quick to dismiss active I/V. It can be stellar 
	

	
	
		
		

		
		
	


	




 Cheers!
 Russ


----------



## Pars

Quote:


  Originally Posted by *Russ White* /img/forum/go_quote.gif 
_Proper.... hmmm what exactly is "proper" <snipped>
 Lets make it clear, there are only two ways I know of to do I/V conversion. Ok really only one, but I won't get into that...

 You can do it actively by utilizing feedback, or you can do it passively by presenting the current to an impedance(not necessarily a resistance). The impedance can be generated any number of ways, but its still just an impedance. Both have there strong points, and I use both on my DACs.
 <snipped>_

 

The stages being discussed here are *active* and *do not use feedback* (3rd case?). I know you hang out on diyaudio, and I'm sure you are well aware of Jocko's, etc. discussions of I/V (or _proper_ I/V if you will). Your discrete opamp uses feedback... what is its performance and more importantly, input impedance, as the frequency goes up? I'm sure with feedback it is quite low at DC. And how does it compare to other discrete opamps, such as the Bursons?

 I'm sure you have done much more work in this area than I have (I'm a neophyte comparatively, to many in this discussion BTW)


----------



## Russ White

Quote:


  Originally Posted by *Pars* /img/forum/go_quote.gif 
_The stages being discussed here are *active* and *do not use feedback* (3rd case?).

 I'm sure you have done much more work in this area than I have (I'm a neophyte comparatively, to many in this discussion BTW) 
	

	
	
		
		

		
		
	


	


_

 


 I have no idea on your last point.

 In my opinion "buffered" is != to "active". Most of the designs you mentioned, are just using one form of buffer or another if they do not use feedback. The DAC still has to swing the full output voltage. I don't like that personally. In my opinion, the DAc sounds best when its output voltage stays as close to steady as possible.

 What I do know is, there are many "active" designs which simply utilize the mundane act of passing a current through an impedance (resistor in most cases). This while simple, is not exactly elegant... Now I understand, even in a feedback scheme, there is still an impedance that sets voltage gain, but not exactly in the same way.

 Now I am not going to say those stages are not good or even great, I can't I have not heard them all. What I can say, is my opinion is that they are not optimal.

 Cheers!
 Russ


----------



## cetoole

Russ, if you look at the schematic I posted above, it buffers the DAC from the voltage swing. Sure, input impedance is ~2.6ohm, but resistive out to 10MHz, then goes capacitive. At no point is the input impedance, which is the impedance seen by the DAC, inductive. Yeah, the DAC will see a small voltage swing on it's output, but not very much, and this voltage will not be much higher at HF. Also, there will not be the issue of the amplifiers slewing. I/V conversion is performed by R16, which is fully isolated from the DAC output.


----------



## Pars

Russ,

 I started a reply, but Colin answered you much more effectively than I could...

 Regards,

 Chris


----------



## Russ White

One problem I have, is I have only simulated my cct so far. I have not yet tested it yet. I will soon (still laying it out). I also do not possess a lot of testing gear. So I have to rely on simulations to tell me what it might do. One thing I have difficulty with is that because it uses symmetrical feedback I cannot use the normal open loop gain measurements fixtures provided with LTSpice. At least I do not know enough yet to know how to modify them to work in a fully symmetrical circuit.

 Honestly I am not a guru. I am just a very avid hobbyist.

 Cheers!
 Russ


----------



## Russ White

Quote:


  Originally Posted by *cetoole* /img/forum/go_quote.gif 
_Russ, if you look at the schematic I posted above, it buffers the DAC from the voltage swing. Sure, input impedance is ~2.6ohm, but resistive out to 10MHz, then goes capacitive. At no point is the input impedance, which is the impedance seen by the DAC, inductive. Yeah, the DAC will see a small voltage swing on it's output, but not very much, and this voltage will not be much higher at HF. Also, there will not be the issue of the amplifiers slewing. I/V conversion is performed by R16, which is fully isolated from the DAC output._

 


 Hi, I actually was not speaking to your design, I have not had a chance to fully digest it. 
	

	
	
		
		

		
		
	


	




 It does look interesting and very well thought out.

 Still I believe there is some merit to the fully symmetric nature of my approach as well. Especially since all my gear is balanced.

 Cheers!
 Russ


----------



## Russ White

Quote:


  Originally Posted by *cetoole* /img/forum/go_quote.gif 
_Russ, if you look at the schematic I posted above._

 



 I thought we were talking about I/V stages? Looking at the simulation you posted, where is the current input? I see a voltage input (V8), but no current.

 I could be missing something obvious. Sorry if I did.

 Cheers!
 Russ


----------



## Pars

Quote:


  Originally Posted by *Russ White* /img/forum/go_quote.gif 
_I thought we were talking about I/V stages? Looking at the simulation you posted, where is the current input? I see a voltage input (V8), but no current.

 I could be missing something obvious. Sorry if I did.

 Cheers!
 Russ_

 

He's developing the current across the series resistor (probably for simulation purposes?). I know when I did preliminary listening tests of one of the stages I built, I used a 1K resistor in series with a discman output to simulate a current in...


----------



## Russ White

Quote:


  Originally Posted by *Pars* /img/forum/go_quote.gif 
_He's developing the current across the series resistor (probably for simulation purposes?). I know when I did preliminary listening tests of one of the stages I built, I used a 1K resistor in series with a discman output to simulate a current in..._

 

Hmmm....

 That does not make much sense to me(once again, could be my mistake). In simulation it would seem you would want to model close to an ideal current source. A voltage source across a resistor is not a good current source. The impedance will not even be close to the actual output impedance of the DAC.

 Cheers!
 Russ


----------



## Russ White

Here is my cct with differential current sources for input.

 about 470 ohms input Z. The same as the feeback R.

 The impedance about 1mhz is dictated by the filter caps, and the pole (compensation) caps.

 Cheers!
 Russ


----------



## Russ White

And here is the response of the same cct.


----------



## Cauhtemoc

Quote:


  Originally Posted by *Russ White* 
_You say my design is simply another opamp... Its actually quite a bit more than that.
	

	
	
		
		

		
		
	


	




 The version I am working on is super symmetric with twist, and does not rely on conventional negative feedback, but is truly super symmetrical(what I mean is it not only X'd but also has symmetrical pairs)._

 

Alright, so it's a super symmetric balanced opamp. 
	

	
	
		
		

		
		
	


	




 There was also a paper written on this subject by Bruno Putzeys:

http://www.grimmaudio.com/whitepaper...g%20blocks.pdf

 And the resulting amplifier:

ExtremA, a reference class-A DIY amplifier - Introduction

  Quote:


  Originally Posted by *Russ White* 
_What is more, it utilizes a clever method of nulling any common mode current/voltage.... and Errors!!!._

 

The common mode control loop, I noticed. I also noticed that you do not have a current mirror here, you should seriously consider adding one. The common mode control loop plays a crucial role in minimizing THD when using only one output (referenced to ground instead of differentially between the two outputs).

  Quote:


  Originally Posted by *Russ White* 
_My design also has orders of magnitude lower distortion than simple passive I/V (resistor or transformer) + buffer approach (like the zapfilter)._

 

Ah, but then distortion isn't everything.

  Quote:


  Originally Posted by *Russ White* 
_The wonderful thing about feedback is, it makes the output of the DAC have to swing less around its stasis point. What I mean is, if you look at the voltage across the ouput of the DAC it will barely move with most active (feedback) I/V stage. With passive I/V (any type) the ouput of the DAC has to follow the output of the stage closer to the supplies (0 and 5V for example). This makes the output much more prone to capacitive and inductive effects of both the DAC, and the ccts down stream (even on the same PCB)._

 

The design posted by cetoole above has an input impedance of 2.5 ohms, and meaning that the output of a PCM1794 swings some 10 mV. This is hardly enough to generate noticable distortion.


----------



## Cauhtemoc

Quote:


  Originally Posted by *Russ White* 
_Hmmm....

 That does not make much sense to me(once again, could be my mistake). In simulation it would seem you would want to model close to an ideal current source. A voltage source across a resistor is not a good current source. The impedance will not even be close to the actual output impedance of the DAC.

 Cheers!
 Russ_

 

Pars is correct. A voltage through a resistor gives a current, just as a current through a resistor gives a voltage. It's Ohm's law.

 You do not want to model an ideal current source because a DAC is not an ideal current source. 1k is a good simulation value, and about the worse case you will encounter when it comes to real DACs.


----------



## 00940

Quote:


  Originally Posted by *cetoole* /img/forum/go_quote.gif 
_Yeah, up to about 2MHz. Havnt been able to keep it from going inductive above that, except by adding capacitance to the input, which will increase distortion. Maybe with different transistors than I have tried it would work nicely._

 

You're right. This cfp thing goes completly crazy at high frequencies. I've tried a few transistors in ltspice and nothing changes much. 
	

	
	
		
		

		
			





 This being said... The digital filter inside the pcm1798 achieves a 120dB attenuation at 4fs. For 44.1KHz, wouldn't it mean that there isn't too much junk at 2MHz ?

 Here's what I'm having right now in ltspice:


----------



## Cauhtemoc

Quote:


  Originally Posted by *00940* 
_You're right. This cfp thing goes completly crazy at high frequencies. I've tried a few transistors in ltspice and nothing changes much. 
	

	
	
		
		

		
		
	


	




 This being said... The digital filter inside the pcm1798 achieves a 120dB attenuation at 4fs. For 44.1KHz, wouldn't it mean that there isn't too much junk at 2MHz ?_

 

Sigma-delta DACs rely on extreme noise shaping and so have a lot of noise past 100 kHz, this can cause problems if the input impedance change.

 R2R DACs don't have this problem, but they have very fast settling times, in the order of 0.1 us, which again causes problems. Feed it with a pulse with a rise time of 0.1 us and watch the input and you will see what I mean.

  Quote:


  Originally Posted by *00940* 
_Here's what I'm having right now in ltspice:_

 

You should use a non-ideal current source to simulate a real DAC, a voltage source with a 1k resistor is a good start.


----------



## Russ White

Quote:


  Originally Posted by *Cauhtemoc* /img/forum/go_quote.gif 
_Alright, so it's a super symmetric balanced opamp. 
	

	
	
		
		

		
		
	


	




 The common mode control loop, I noticed. I also noticed that you do not have a current mirror here, you should seriously consider adding one. The common mode control loop plays a crucial role in minimizing THD when using only one output (referenced to ground instead of differentially between the two outputs).

 Ah, but then distortion isn't everything.



 The design posted by cetoole above has an input impedance of 2.5 ohms, and meaning that the output of a PCM1794 swings some 10 mV. This is hardly enough to generate noticable distortion._

 

I think you are looking at my first design, my second can be found here:

 Notice there are definitely current mirrors for the corrector. 
	

	
	
		
		

		
		
	


	




diyAudio Forums - Discrete Super Symmetric(I think) Opamp for I/V Etc...

 Now here is the main point. With my I/V stage the voltage at the input is only... 300uv at full scale output from DC to 1mhz. thats one third an mv. 
	

	
	
		
		

		
		
	


	




 and yes I think that can make a difference.

 To me, while not everything distortion is pretty critical.

 I will experiment with using a resistor in series with a voltage as a current source. I have just never done it that way.

 Cheers!
 Russ


----------



## Russ White

Ok I changed my simulation to module the current in as a voltage in series with a resistor. I used the PCM1794A DS to calculate a series resistor of about 362 ohms.

 Simulated as such I actually got better numbers then I had before. 
	

	
	
		
		

		
		
	


	




 And this into a 300ohm load with 400pf of capacitance in parallel.

 Really I wanted my amp to be able to drive headphones etc directly.

 Cheers!

 Fourier components of V(out+,out-)
 DC component:5.79172e-008

 HarmonicFrequency Fourier Normalized Phase Normalized
 Number [Hz] Component Component[degree]Phase [deg]
 1 1.000e+033.687e+001.000e+00 -0.02° 0.00°
 2 2.000e+031.150e-073.118e-08 90.06° 90.08°
 3 3.000e+031.328e-073.602e-08 73.27° 73.29°
 4 4.000e+031.144e-073.102e-08 89.92° 89.94°
 5 5.000e+031.165e-073.160e-08 96.83° 96.85°
 6 6.000e+031.163e-073.155e-08 90.29° 90.31°
 7 7.000e+031.156e-073.135e-08 84.15° 84.17°
 8 8.000e+031.147e-073.110e-08 89.76° 89.78°
 9 9.000e+031.147e-073.110e-08 88.52° 88.54°
 Total Harmonic Distortion: 0.000009%


 .step f=10000
 Fourier components of V(out+,out-)
 DC component:1.31338e-009

 HarmonicFrequency Fourier Normalized Phase Normalized
 Number [Hz] Component Component[degree]Phase [deg]
 1 1.000e+043.687e+001.000e+00 -0.21° 0.00°
 2 2.000e+043.152e-098.548e-10 -56.24° -56.04°
 3 3.000e+041.294e-073.509e-08 84.26° 84.46°
 4 4.000e+044.319e-101.171e-10 115.13° 115.34°
 5 5.000e+041.128e-083.060e-09 42.66° 42.86°
 6 6.000e+043.642e-099.879e-10 10.91° 11.12°
 7 7.000e+041.724e-094.676e-10 -115.59° -115.39°
 8 8.000e+043.552e-099.634e-10 26.33° 26.54°
 9 9.000e+042.603e-097.059e-10 119.28° 119.48°
 Total Harmonic Distortion: 0.000004%

 .step f=20000
 Fourier components of V(out+,out-)
 DC component:-7.28242e-010

 HarmonicFrequency Fourier Normalized Phase Normalized
 Number [Hz] Component Component[degree]Phase [deg]
 1 2.000e+043.687e+001.000e+00 -0.41° 0.00°
 2 4.000e+041.454e-083.943e-09 -20.90° -20.49°
 3 6.000e+042.551e-076.920e-08 96.55° 96.96°
 4 8.000e+042.279e-086.182e-09 -4.74° -4.33°
 5 1.000e+052.984e-088.093e-09 -5.85° -5.44°
 6 1.200e+053.353e-089.094e-09 -2.14° -1.72°
 7 1.400e+054.591e-081.245e-08 2.55° 2.96°
 8 1.600e+054.810e-081.305e-08 -6.98° -6.57°
 9 1.800e+055.488e-081.489e-08 -2.66° -2.25°
 Total Harmonic Distortion: 0.000007%


----------



## 00940

Input impedance down to 1.75 ohm. But the 2sa970 is hitting the limit of its linear range. There is 16ma and 9ma iddle accross the 2sc2240/2sa970.


----------



## 00940

I just saw this... amazingly low input impedance (less than 0.5R). Have a look at the last schematic.

homecinema-fr.com : Voir le sujet - Etage IV sans condo de liaison , sommation de sortie de dac


----------



## cetoole

Quote:


  Originally Posted by *00940* /img/forum/go_quote.gif 
_I just saw this... amazingly low input impedance (less than 0.5R). Have a look at the last schematic.

homecinema-fr.com : Voir le sujet - Etage IV sans condo de liaison , sommation de sortie de dac_

 

Yeah, via a feedback mechanism, a la Hawksford. Have you guys seen his paper Current Steering Transimpedance Amplifiers for High-Resolution Digital-to-Analogue Converters? It uses the same current mirror feedback to the bias transistor as shown in that schematic. Good, but also has the problematic rising impedance at HF.

 Here is a simulation showing the input impedance for my I/V stage using CFPs. Hard to tell, but the impedance is 260mohm, with the first rising step at 2.37MHz. Also, Pars was correct, I am using a voltage source followed by a series 2.1kohm resistor to simulate the nonideal nature of the DAC current output. I am designing for use with the AD1862N-J DAC, of which I have four.


----------



## Pars

Quote:


  Originally Posted by *00940* /img/forum/go_quote.gif 
_Input impedance down to 1.75 ohm. But the 2sa970 is hitting the limit of its linear range. There is 16ma and 9ma iddle accross the 2sc2240/2sa970._

 

Yeah, when building these, when you get up around 10mA the SA970/SC2240s get touchy. I had to reduce the current in both rbroer's and cetoole's stages to get them stable at all with these transistors. Small air currents, etc. would cause alot of drift.

 This is what I wound up with for the rbroer stage:



  Quote:


  Originally Posted by *00940* /img/forum/go_quote.gif 
_I just saw this... amazingly low input impedance (less than 0.5R). Have a look at the last schematic.

homecinema-fr.com : Voir le sujet - Etage IV sans condo de liaison , sommation de sortie de dac_

 

Interesting... I assume you mean the last one on the page?


----------



## 00940

Pars : yes, the last one. Combine it with this one : 






 And you get something quite clever perfectly suited to balanced out DAC (in mono mode). The only thing I don't like is that the simulation shows dissimilar input impedances with the 2sa970-2sc2240. 0.02R (!) vs 0.1R @20KHz. At 2Mhz, we're back to 3R for both inputs.

 Another good thing in that design is that you get offset nulling at both inputs and output with only three trimmers. Regal will be happy. 
	

	
	
		
		

		
		
	


	




 No need for balanced to unbalanced conversion.


 Cetoole: yes, impedance rises with frequency too. 
	

	
	
		
		

		
		
	


	




 If I might ask, what command do you use in ltspice to plot the input impedance vs frequency ?


----------



## cetoole

00940, as shown in my simulations, though difficult to see, I use ".ac oct 100 1 100000000", and then to get it to show impedance instead of voltage, just divide the voltage at the input by the current through the resistor.


----------



## Russ White

Ok if I measure input impedance at each input, this is what I get:

 Yes, that's 38 mOhms at 20Khz. And it stays pretty flat up through 5Mhz.






 Cheers!
 Russ


----------



## Rescue Toaster

Quote:


  Originally Posted by *cetoole* /img/forum/go_quote.gif 
_00940, as shown in my simulations, though difficult to see, I use ".ac oct 100 1 100000000", and then to get it to show impedance instead of voltage, just divide the voltage at the input by the current through the resistor._

 

cteoole, what is the limiting factor for max impedance at high freqs? Hitting 5 ohms at ~105 Mhz isn't bad or anything I'm just wondering what is the dominant effect above 20Mhz or so. And how much AC current do you experience in the supply at that range?


----------



## Cauhtemoc

Quote:


  Originally Posted by *Russ White* 
_I think you are looking at my first design, my second can be found here:

 Notice there are definitely current mirrors for the corrector. 
	

	
	
		
		

		
		
	


	




diyAudio Forums - Discrete Super Symmetric(I think) Opamp for I/V Etc..._

 

Yes, my apologies.

  Quote:


  Originally Posted by *Russ White* 
_Ok I changed my simulation to module the current in as a voltage in series with a resistor. I used the PCM1794A DS to calculate a series resistor of about 362 ohms.

 Simulated as such I actually got better numbers then I had before. 
	

	
	
		
		

		
		
	


	




 And this into a 300ohm load with 400pf of capacitance in parallel.

 Really I wanted my amp to be able to drive headphones etc directly.

 Cheers!_

 

Just to prove that I also can play the feedback game, I took the famous JE-990 discrete opamp and put it to I/V conversion duty. The result was some 0.000001% THD.

 The JE-990 uses some 10 transistors. How many do you use?


----------



## Russ White

Cauhtemoc. No apologies required.

 I am not playing any games at all, feedback or otherwise. I am Only trying to explain why I like I/V stages that are low distortion. 
	

	
	
		
		

		
		
	


	




 And yes if I omit the filter caps in two spots (which does not effect the amp's stability, just its linearity and bandwidth) I can achieve a zero distortion number at 20K too. Thats really not the point. 
	

	
	
		
		

		
		
	


	




 I have a very similar single ended version of this opamp which also uses only 10Qs and gets the same THD numbers. 
	

	
	
		
		

		
		
	


	




 The only difference between you and I is I have shown my schematic for critical review. 
	

	
	
		
		

		
		
	


	




 Cheers!
 Russ


----------



## Russ White

For example, here is 2V RMS at 20Khz into same load with no filter caps.
	

	
	
		
		

		
		
	


	




 Fourier components of V(out+,out-)
 DC component:-7.26524e-014

 HarmonicFrequency Fourier Normalized Phase Normalized
 Number [Hz] Component Component[degree]Phase [deg]
 1 2.000e+049.945e-011.000e+00 -0.05° 0.00°
 2 4.000e+043.141e-123.159e-12 -2.68° -2.63°
 3 6.000e+043.296e-093.314e-09 74.97° 75.02°
 4 8.000e+046.279e-126.314e-12 -1.26° -1.21°
 5 1.000e+055.622e-105.653e-10 105.90° 105.95°
 6 1.200e+059.416e-129.469e-12 -0.75° -0.70°
 7 1.400e+053.094e-103.111e-10 -151.45° -151.40°
 8 1.600e+051.255e-111.262e-11 -0.48° -0.43°
 9 1.800e+053.362e-103.380e-10 22.37° 22.42°
 Total Harmonic Distortion: 0.000000%


----------



## Cauhtemoc

Quote:


  Originally Posted by *Russ White* 
_I am not playing any games at all, feedback or otherwise._

 

Of course not. My point was that designs with feedback will always achieve very good THD figures, but as I said before, THD isn't everything. There is a reason for why Wadia uses a non feedback I/V converter in their $10'000 581i CD player.


----------



## 00940

Of course 0.00000000000000000000000 sounds more impressive than :

 HarmonicFrequency Fourier Normalized Phase Normalized
 Number [Hz] Component Component[degree]Phase [deg]
 1 2.000e+041.451e+001.000e+00 161.09° 0.00°
 2 4.000e+041.861e-041.282e-04 59.57° -101.53°
 3 6.000e+045.511e-063.797e-06 135.82° -25.27°
 4 8.000e+041.955e-071.347e-07 -142.05° -303.14°
 5 1.000e+051.474e-081.016e-08 -76.23° -237.32°
 6 1.200e+051.418e-099.772e-10 -73.68° -234.78°
 7 1.400e+052.510e-091.729e-09 29.32° -131.78°
 8 1.600e+058.599e-105.924e-10 -168.82° -329.91°
 9 1.800e+052.275e-091.567e-09 -156.77° -317.87°

 Total Harmonic Distortion: 0.012827% (it's for the I/V I posted above)

 or

 HarmonicFrequency Fourier Normalized Phase Normalized
 Number [Hz] Component Component[degree]Phase [deg]
 1 2.000e+042.951e+001.000e+00 -7.27° 0.00°
 2 4.000e+046.286e-052.131e-05 -167.13° -159.86°
 3 6.000e+047.141e-052.420e-05 0.03° 7.30°
 4 8.000e+041.243e-064.214e-07 8.25° 15.52°
 5 1.000e+055.357e-071.816e-07 -141.65° -134.38°
 6 1.200e+052.703e-089.162e-09 148.32° 155.59°
 7 1.400e+058.763e-092.970e-09 25.50° 32.77°
 8 1.600e+056.814e-102.310e-10 -60.95° -53.68°
 9 1.800e+053.674e-091.245e-09 102.98° 110.25°

 Total Harmonic Distortion: 0.003225% (for the stuff with current mirror found on homecinema-fr.com : Source de Passion)

 Both at 2Vrms, 20khz of course


----------



## Russ White

The bottom line is, how good does it sound? And this is always between any two decent designs 100% subjective
	

	
	
		
		

		
		
	


	




 I have not built my circuit yet so I don't know. But it seems interesting.

 I have nothing against no feedback designs. If you think they sound nice, then use them. I have and do!! 
	

	
	
		
		

		
		
	


	




 Certainly there is a niche there, so perhaps I could make one just to please the masses. 
	

	
	
		
		

		
		
	


	




 I just happen to believe that a very linear feedback design can sound better! That's just my personal observation.

 I guess part of me thinks that if your going to use a DAC capable of .0004% THD, you should at least "try" to create an output stage that can match it. 
	

	
	
		
		

		
		
	


	




 I know in reality it is not likely that a simulated circuit will live up to its promises when you get to building it. But hey its all good fun. 
	

	
	
		
		

		
		
	


	




 Also I have known for quite a long time now that the cost of a component sometimes has nothing to do with its performance. Its all about perception. 
	

	
	
		
		

		
		
	


	




 There is no good or bad in that. Things go in and out of fashon all the time.

 Cheers!
 Russ


----------



## Cauhtemoc

Try this on for size:

 THD is 0.0006% mainly second harmonic, input impedance is 0.003 ohm.

 Long story short, I was playing with current conveyors and came up with the above. It has low enough THD and input impedance to sway even the most stuborn feedback user (like Russ above! 
	

	
	
		
		

		
		
	


	




).

 I mentioned Wadia earlier, this is something along the lines of what they use.


----------



## 00940

This is very impressive. Out of curiosity, how does the input impedance change wrt frequency ?

 With this more common stage, I'm now at around 0.008% (real life won't be as good of course...).


----------



## Russ White

Quote:


  Originally Posted by *Cauhtemoc* /img/forum/go_quote.gif 
_Try this on for size:

 THD is 0.0006% mainly second harmonic, input impedance is 0.003 ohm.

 Long story short, I was playing with current conveyors and came up with the above. It has low enough THD and input impedance to sway even the most stuborn feedback user (like Russ above! 
	

	
	
		
		

		
		
	


	




).

 I mentioned Wadia earlier, this is something along the lines of what they use._

 

Now that is very interesting! 
	

	
	
		
		

		
		
	


	




 I actually have looked at the current conveyor before too. But not for a while. It does seem promising. Why the choice of Q types?

 Now just to clear the air, its not that I would only ever choose to use global feedback excluding all else, its just that I am not adverse to using it either rhetorically or electrically. 
	

	
	
		
		

		
		
	


	




 I am learning quite a lot. So thanks for the healthy and respectful conversation.

 Cheers!
 Russ


----------



## Cauhtemoc

Quote:


  Originally Posted by *00940* 
_This is very impressive. Out of curiosity, how does the input impedance change wrt frequency ?_

 

The input impedance does rise with frequency, up to about 4.5 ohm at 100 MHz before falling of again. It uses feedback, albeit not global feedback, so this is to be expected.


----------



## Cauhtemoc

Quote:


  Originally Posted by *Russ White* 
_Now that is very interesting! 
	

	
	
		
		

		
		
	


	




 I actually have looked at the current conveyor before too. But not for a while. It does seem promising._

 

Feel free to build it and compare to your balanced opamp converter.

  Quote:


  Originally Posted by *Russ White* 
_Why the choice of Q types?_

 

The 2SC2240/2SA970 is a very low noise general purpose transistor with a very linear hfe in the region of 1 mA to 10 mA. I use it for many of my designs, as do a lot of people. You can use other small signal transistors here if you want, performance just won't be as good. Some examples would be BC550/BC560 or 2N5087/2N5089.


----------



## regal

Quote:


  Originally Posted by *Russ White* /img/forum/go_quote.gif 
_Proper.... hmmm what exactly is "proper"

 My design also has orders of magnitude lower distortion than simple passive I/V (resistor or transformer) + buffer approach (like the zapfilter).

 Cheers!
 Russ_

 


 I looked at the zapfilter and the best I can tell is it is active I/V. Unless they tell you after you buy it that you have to add a resistor to ground? Or am I missing someting?






 Yours should be much better since it is configurable to the PCM1794. But how is the performance single ended? Does it have the common rejection that a balanced to unbalanced IC opamp conversion would have?


----------



## Russ White

I was not really trying to say it was not active. I was simply pointing out the difference between it and a feedback type approach.

 I actually think its a clever design.

 Cheers!
 Russ


----------



## regal

Quote:


  Originally Posted by *Russ White* /img/forum/go_quote.gif 
_I was not really trying to say it was not active. I was simply pointing out the difference between it and a feedback type approach.

 I actually think its a clever design.

 Cheers!
 Russ_

 


 Jeff,

 Have you considered adding something like this to a balance to single transfer (minus the filter) ?


http://home.quicknet.nl/qn/prive/ra....tstage_PCB.pdf


----------



## 00940

Quote:


  Originally Posted by *Cauhtemoc* /img/forum/go_quote.gif 
_The input impedance does rise with frequency, up to about 4.5 ohm at 100 MHz before falling of again. It uses feedback, albeit not global feedback, so this is to be expected._

 

It's still a very smooth curve. it's not worse that a cfp or a folded cascode with high current transistor.

 To balance the possible input offset of a dac, I however had to play with the value of R6, which reduces the current going through the stage and slightly worsens the distortion figures.


----------



## Cauhtemoc

R6 is set to allow about 10 mA through the Q9. If you go below this the distortion increases, above this and the 2SC2240/2SA970 will start to loose their linearity.

 You should adjust R5 to get 10 mA through Q6 _including_ the DAC offset. For example, if you are using the TDA1541, which has 2 mA offset, you should set R5 to provide 8 mA. This gives a total of 10 mA through Q6 (8 mA from the current source plus 2 mA from the DAC). This is the same as the current provided by R6, so no current goes through the output resistor R7, and thus there is output voltage.

 Adjusting this way means that there will always be 10 mA through the stage, which will help to keep distortion down.


----------



## Pars

Quote:


  Originally Posted by *regal* /img/forum/go_quote.gif 
_I looked at the zapfilter and the best I can tell is it is active I/V. Unless they tell you after you buy it that you have to add a resistor to ground? Or am I missing someting?_

 

I would say the 4.02K resistors and the 470pf caps are the I/V resistor(s) and low pass filter in this design. I'd have to look at their instructions (also online, IIRC), but the solder bubbles at the input (1-6) are how you configure this for current out or voltage out DACs and single ended or balanced input. I'm guessing that solder bubble 3 is the one to bridge for current in (I/V) single-ended stage, and 3 and 5 for balanced I-in.


----------



## cetoole

Quote:


  Originally Posted by *Pars* /img/forum/go_quote.gif 
_I would say the 4.02K resistors and the 470pf caps are the I/V resistor(s) and low pass filter in this design. I'd have to look at their instructions (also online, IIRC), but the solder bubbles at the input (1-6) are how you configure this for current out or voltage out DACs and single ended or balanced input. I'm guessing that solder bubble 3 is the one to bridge for current in (I/V) single-ended stage, and 3 and 5 for balanced I-in._

 

I would think I/V is actually performed by the two 300ohm resistors going to v- through that zener diode.


----------



## regal

Quote:


  Originally Posted by *cetoole* /img/forum/go_quote.gif 
_I would think I/V is actually performed by the two 300ohm resistors going to v- through that zener diode._

 

So its passive + a buffer with the DAC seeing a very high 300R load ?


 Looking at the other designs posted here I am thinking only Twisted Pairs would handle the high DC offset of the new PCM179x chips. This is what I ran into with the Borbely I/V stage. Works great with my AD1862 but when I tried to work it with my EZDAC (PCM1794) it couldn't deal with the high DC offset (I remember volts not mV.) This is because the new chips are single rail supplied.


----------



## cetoole

Quote:


  Originally Posted by *regal* /img/forum/go_quote.gif 
_So its passive + a buffer with the DAC seeing a very high 300R load ?


 Looking at the other designs posted here I am thinking only Twisted Pairs would handle the high DC offset of the new PCM179x chips. This is what I ran into with the Borbely I/V stage. Works great with my AD1862 but when I tried to work it with my EZDAC (PCM1794) it couldn't deal with the high DC offset (I remember volts not mV.) This is because the new chips are single rail supplied._

 

No, not at all. Input is a common base amplifier, each input transistor is biased to ~2.7mA, making AC input impedance ~10ohm. They did a crappy job of biasing the bases of the input transistors, shoulda used at least current mirrors, so there is significant DC offset on the input. Input impedance starts rising around 200KHz, going extremely inductive. My simulation shows a peak at ~85MHz of ~150ohm, though this isnt precise at all, since I am using 2SC2240/2SA970 spcie models. I assume this rising impedance is due to the resistor biasing of the input transistor bases, just as a guess.

 Forgot about the second part. Actually, all that is needed is to rebalance an existing design. High offset can be compensated for, you just cant slap a generic design meant for something else in and expect it to work.


----------



## Cauhtemoc

Quote:


  Originally Posted by *regal* 
_So its passive + a buffer with the DAC seeing a very high 300R load ?


 Looking at the other designs posted here I am thinking only Twisted Pairs would handle the high DC offset of the new PCM179x chips. This is what I ran into with the Borbely I/V stage. Works great with my AD1862 but when I tried to work it with my EZDAC (PCM1794) it couldn't deal with the high DC offset (I remember volts not mV.) This is because the new chips are single rail supplied._

 

The current conveyor I posted above can easily handle the -6.2 mA offset on the PCM1794. Just adjust R5 to supply 16.2 mA. That's it.


----------



## Russ White

Quote:


  Originally Posted by *Cauhtemoc* /img/forum/go_quote.gif 
_The current conveyor I posted above can easily handle the -6.2 mA offset on the PCM1794. Just adjust R5 to supply 16.2 mA. That's it._

 

That may be prone to drift, for a number of different reasons. Still it could be satisfactory.

 Cheers!
 Russ


----------



## 00940

Quote:


  Originally Posted by *Cauhtemoc* /img/forum/go_quote.gif 
_The current conveyor I posted above can easily handle the -6.2 mA offset on the PCM1794. Just adjust R5 to supply 16.2 mA. That's it._

 

Thinking of it... shouldn't one adjust R5 to supply 10ma - 6.2ma and thus 3.8ma rather than 16.2 ?


----------



## Cauhtemoc

Quote:


  Originally Posted by *00940* 
_Thinking of it... shouldn't one adjust R5 to supply 10ma - 6.2ma and thus 3.8ma rather than 16.2 ?_

 

From what I can tell from the datasheets the PCM1794 has a negative offset of 6.2 mA, meaning that it will suck up this much current at bipolar zero. So if you want 10 mA to go through the conveyor you need to increase the current source by 6.2 mA. I could be wrong of course, the offset could be positive, in which case you are correct.

 So, is anyone going to build it?


----------



## 00940

well, it's a bit complicated to me... The datasheet indeed speaks of a negative offset. On the other hand, still according to the datasheet, this offset translates after I/V conversion into a negative voltage. In ltspice, using an opamp for I/V, you need to source current in order to get that.


----------



## Russ White

I see someone else on another forum had the same concern about a voltage source in series with a resistor for a current source. 
	

	
	
		
		

		
		
	


	




 In reality most current sources are much more dynamic than that.

 I am not sure there is an ideal way to simulate a DAC current output. At least none that i know of yet. The best way would be to have in depth knowledge of the inner working of the DAC itself.

 Cheers!
 Russ


----------



## SnoopyRocks

diyAudio Forums - Discrete Super Symmetric(I think) Opamp for I/V Etc...
  Quote:


  Originally Posted by *Russ White* /img/forum/go_quote.gif 
_I see someone else on another forum had the same concern about a voltage source in series with a resistor for a current source. 
	

	
	
		
		

		
		
	


	




 In reality most current sources are much more dynamic than that.

 I am not sure there is an ideal way to simulate a DAC current output. At least none that i know of yet. The best way would be to have in depth knowledge of the inner working of the DAC itself.

 Cheers!
 Russ_

 

Russ, it's nice (refreshing really) to see a DIYer attempting to make a fully differential opamp with a propper common-mode feedback loop (CMFB). IMO your characterizations of it are overzelous to say the least. Nor is it new...I've seen all of this before. This does not take away from what you've got going here though. It is a very sophosticated design for an ametuer. If you want your design to be the best you can make it, the CMFB loop needs improvement. The primary problem is where (and how) you've chosen to feedback the CMFB error signal. Beyond that, proper compensation of the CMFB loop is just as important as the differential path, but quite a bit more difficult to accomplish. You won't find this knowledge reading this forum. If you would like some guidance, send me a pm. Nice work.


----------



## 00940

Quote:


  Originally Posted by *Russ White* /img/forum/go_quote.gif 
_I see someone else on another forum had the same concern about a voltage source in series with a resistor for a current source. 
	

	
	
		
		

		
		
	


	




 In reality most current sources are much more dynamic than that.

 I am not sure there is an ideal way to simulate a DAC current output. At least none that i know of yet. The best way would be to have in depth knowledge of the inner working of the DAC itself.

 Cheers!
 Russ_

 

You mean this ? DIYHiFi.org &bull; View topic - I/V and amount of current feedback

 I think it's safe to quote diyhifi.org, we're not on diyaudio


----------



## 00940

Quote:


  Originally Posted by *Cauhtemoc* /img/forum/go_quote.gif 
_So, is anyone going to build it? 
	

	
	
		
		

		
		
	


	


_

 

I hope to. Got to have a working pcm1798 first 
	

	
	
		
		

		
		
	


	




 It would be easy to wire p2p. Here's a version with bc560c/bc550c and output buffer. It'd also be easy to add a servo to the output buffer btw. http://www.membres.lycos.fr/sst00940/schem%20cau.PNG


----------



## Cauhtemoc

You can save some parts by using the same bias for the output emitter follower as I used for the current source in the conveyor. In other words, connect the base of the output current source transistor to the same location as the base of Q12 and Q14 in my schematic. You could also cascode it in the same way I have done with Q11 and Q13.


----------



## 00940

You're right 
	

	
	
		
		

		
		
	


	




 There's something to be said against copy-paste parts of schematics.

 I updated the pics.


----------

