# Discrete I/V converter design



## cetoole

I thought I would post my schematic for a discrete I/V converter for use on the output of an audio DAC here, to see if anyone had any comments or suggestions as to how I could improve it. 

 This is a discrete open loop DC coupled folded cascode common base amplifier, which steers the current output of a DAC through a pair of common base amplifiers, first an NPN, which is followed by a PNP folded cascode, which, among other things, acts as a level shifter, to get the DC output near ground. Adding an inverting opamp integrator to act as a servo by modulating the CCS of the cascode transistor, a DC coupled output should be relatively easy to realize. This current is then converted to a voltage by R1, in the same fashion as the resistor dump passive I/V solution some people are in favor of, but without the massive loading of the output of the DAC, and all of the problems that entail.

 Input impedance depends on IC of the input transistor, which I intend to run at 10mA, so should be about 2-3ohm. BJTs will most likely be 2SC2240BL/2SA970BL parts, due to their extremely high hFE which remains quite linear with regards to IC for the values of IC I am interested in, as well as the low noise and capacitance of this part. Actually I am really using it because that is what Jocko Homo, whose postings have had the greatest influence on this schematic, suggested, and I havnt been able to find any other transistors which look as good.

 I plan to pair this converter with AD1862N-J DACs, though it will work with other current out designs. I currently own four of these chips, and am somewhat torn between designing the DAC to have true differential outputs by creating an inverted digital signal with a pair of XORs, or doing phase splitting with a differential pair in the analog domain. There are positive and negative aspects to both options, but I am currently leaning towards going completely differential. These chips arnt exactly as cheap as chips 
	

	
	
		
		

		
			





.

 Since it is an open loop design, PSRR is a concern, but due to the heavy use of current sources, and the folded cascode topology, I believe that with decent rail regulation and filtration, this will be a non issue.

 There are some changes I am currently evaluating for this design. First is the addition of cascodes on all current sources, though this should be most beneficial for the one based around T8, but hey, cascodes are like cookies, you cant just stop at one. I am also contemplating replacing the current base biasing solutions with emitter-followers, connected for base current error correction as shown by Malcolm Hawksford in his paper "Current-Steering Transimpedance Amplifiers for High-Resolution Digital-to-Analogue Converters". Finally, I am thinking about replacing the open loop JFET buffer with a discrete opamp, since the RC low-pass filter will drastically reduce the extreme high frequency output of the DAC, which is the primary reason for the converter being open loop in the first place. If I do this, however, I would be greatly inclined to make this opamp my headphone amplifier, since I dont like the idea of using two opamps in the signal path when unneccessary. By adding a high current discrete buffer, either diamond or some MOSFET topology, after the VAS of the opamp, I could have a very nice headphone amplifier. I dont really care for the idea of having my headphone amp integrated into my DAC, and it presents some issues with how to impliment the volume control, as there really should be a buffer between the I/V resistor and volume attenuator. If I decide on doing the phase splitting in the analog domain however, I will replace this buffer with a differential pair to provide the phase split, which will take care of the need to buffer the output.

 I would appreciate any feedback people here can provide (though not globally in the I/V). For those of you who managed to stay awake to this point, thanks for putting up with my rambling. Since it is not specified in the schematic, VR1 is a TL431.


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## Rescue Toaster

I'm still not convinced that the low-pass filter on an opamp does much given that the gain is usually pretty low in the 10+ mhz region where you really need it.

 Have you simulated this with parasitics? Try feeding signals in the 20-50 mhz range. EDIT: nm the idea I had still requires the negative supply to carry the full noise.

 Would be super to keep the noise current out of the supply but I'm not sure that'll even be possible with active I/V.


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## cetoole

Quote:


  Originally Posted by *Rescue Toaster* /img/forum/go_quote.gif 
_I'm still not convinced that the low-pass filter on an opamp does much given that the gain is usually pretty low in the 10+ mhz region where you really need it._

 

Are you referring to an opamp configured as a transimpedance amplifier, using a cap in the feedback loop to reduce HF noise, or are you referring to the low pass filter created by C1 in this circuit not being effective in reducing HF sufficiently? I agree that with a NOS DAC, this would be an issue unless the filter was operating at such a low frequency so as to audibly roll off the highs significantly, but with the OS I plan to use, likely in the form of a SM5847, unless I can get my hands on a PMD100, should let a first order filter be sufficient I think.
  Quote:


 Have you simulated this with parasitics? Try feeding signals in the 20-50 mhz range. EDIT: nm the idea I had still requires the negative supply to carry the full noise.

 Would be super to keep the noise current out of the supply but I'm not sure that'll even be possible with active I/V. 
 

I havnt simulated this, and I dont have any skill with the simulator, so I am afraid the results I would get right now would be worth about the cost I would spend on parts to run the simulation. The other issue with your previous suggestion of replacing the T7 CCS with a resistor is that I think it would significantly increase distortion. My plan is to have this running off it's own regulated supply, somewhere above +/-15v. The folded cascode class A means current draw will truly be constant, which helps us some here. Current draw wont be real high, so a nice ccs fed shunt reg is quite doable to power it.


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## Rescue Toaster

I was referring to your comment on using a discrete opamp for I/V. The slew rate on even multibit converters generates images at full power up to 20 mhz or so, then rolling off around 50mhz. (measurements done on a PCM63). The multibit sigma-deltas are almost certainly faster. Unless your discrete opamp is at least as fast as say, an LM6171 then I'd avoid it.


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## cetoole

Quote:


  Originally Posted by *Rescue Toaster* /img/forum/go_quote.gif 
_I was referring to your comment on using a discrete opamp for I/V. The slew rate on even multibit converters generates images at full power up to 20 mhz or so, then rolling off around 50mhz. (measurements done on a PCM63). The multibit sigma-deltas are almost certainly faster. Unless your discrete opamp is at least as fast as say, an LM6171 then I'd avoid it._

 

Sorry, I wasnt suggesting using a discrete opamp for I/V, but for the output buffer, to replace the JFET buffer seen in the schematic. It would probably be a JFET input, with low gain in the input pair, folded cascode VAS, and I am undecided between MOSFET or BJT output. The whole opamp is likely to be complimentary. The whole point of this open loop discrete design is to prevent the issues present when using an opamp for I/V conversion. This discrete opamp would come after R16 in the circuit, so would be operating with voltage input that has been already filtered. Anyways, the AD1862N-J I have is a multibit R2R DAC.


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## Garbz

Indeed I like it. Opamps are just fine for basic amplification they only really become an issue in I/V. Although looking at the complexity of the I/V circuit it would be a waste to go so far with that many components and then just flop an opamp on the output.

 Plus you don't get the bragging rights of being all discrete


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## cetoole

Quote:


  Originally Posted by *Garbz* /img/forum/go_quote.gif 
_Indeed I like it. Opamps are just fine for basic amplification they only really become an issue in I/V. Although looking at the complexity of the I/V circuit it would be a waste to go so far with that many components and then just flop an opamp on the output.

 Plus you don't get the bragging rights of being all discrete 
	

	
	
		
		

		
		
	


	


_

 

I wasnt talking about using a monolithic opamp as a buffer, but instead a discrete one. The only monolithic opamp here will be the servo. Those all discrete bragging rights are quite important too, and contribute greatly to the sound quality, kind of like how those big VTEC stickers add 3-5hp when applied to a car.

 Component count is rather high with this, with nearly 50 parts per channel.


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## Filburt

I think you need to make sure to spec in shiny parts as that's good for at least an extra 15 of dynamic range, -10db of THD+N, and 1200 audiophile points.


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## cetoole

Did a quick PCB layout for this board, to see how it would work out. This will really be integrated onto the same PCB as the DAC, but I wanted to do a separate board as well. This would work fine for upgrading a current current out DAC. It would be pretty easy to change this to use SMD parts, at least partially. None of the smd transistors I have found are nearly as good for this purpose as TO-92 parts such as 2SC2240BL/2SA970BL. 1206 resistors can easily replace the current through hole parts and some of the caps can be replaced by SMD ones, but I dont really see it being too much of a benefit to do this, unless I really need to shrink the board. It currently measures 69.85x50.80mm, or 2.75x2", and this is only a single channel. Bit bigger than just an opamp, eh? The actual signal path is pretty short; most of the board is taken up by the current sources, biasing parts, and servo.


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## Pars

Good job. Yeah, I would NOT change transistors. The SMD for the resistors would be OK, and would save some space. Some suggestions:

 1) IOUT and VOUT connectors should be flipped so that the lock tab is towards the inside of the board.

 2) I might flip the LEDs and their associated caps to put the caps towards the board inside.

 3) Have you considered doing a 2 channel board with a single power connection? Might be more useful and mono...

 Chris


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## cetoole

Quote:


  Originally Posted by *Pars* /img/forum/go_quote.gif 
_Good job. Yeah, I would NOT change transistors. The SMD for the resistors would be OK, and would save some space. Some suggestions:_

 

I actually have to change the transistor package I used for this board, because last night when I did the layout, I forgot that I was using a CBE transistor package instead of the necessary ECB, which wont work quite as nicely for the layout because of the collector being in the center of the package, but shouldnt be too hard of a change to make, and will be well worth it.
  Quote:


 1) IOUT and VOUT connectors should be flipped so that the lock tab is towards the inside of the board. 
 

Yeah, I should, though there is no IOUT connector; I suspect you mean IIN. This is a very simple change to make.
  Quote:


 2) I might flip the LEDs and their associated caps to put the caps towards the board inside. 
 

I see your reasoning for this, because of board parasitics, and while I dont really feel there will be any real difference in the performance, will look at doing this.
  Quote:


 3) Have you considered doing a 2 channel board with a single power connection? Might be more useful and mono...

 Chris 
 

I hadnt really thought about doing this, because I hadnt been planning to actually build this separate from an integrated DAC. I could and likely would do that if I was intending it for use in modding existing hardware, but I currently have no current out sources, so no need for it. This board would actually be pretty easy to build p2p on protoboard, with everything but ground assembled using just the component legs bent flat and soldered to each pad, and a star ground with insulated wire. Nevertheless, I will see what I can come up with in the way of a dual board if there is any interest, or if I get bored.

 I would especially like to get some opinions on the output buffer. Do you guys feel the current open loop FET buffer is the better solution, or should I go for a discrete opamp? I dont really see a good way of implementing a volume control before the buffer that wouldnt have interaction problems with the I/V resistor and low pass filter, unless I accepted that it could only be used fully differentially, and implemented something similar to what Nelson Pass did in his D1 DAC. If I put a simple buffer before a normal style volume pot, then I would prefer to just move the whole opamp/volume control to a separate box, implement a PIC controlled 8 bit relay based attenuator, and let the amp be my headphone amp, and able to take multiple input sources. I would prefer to keep it at least somewhat modular so I can upgrade different components (DAC, pre/headamp, power amp).


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## AndrewFischer

Interesting project. I like what I see.


 Besides already having the parts on hand, is there another reason why you are using the AD1862? It is an older obsolete part currently on Last Time buy status. Just wondering if this soon to be gone chip is special.


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## cetoole

Quote:


  Originally Posted by *AndrewFischer* /img/forum/go_quote.gif 
_Interesting project. I like what I see.


 Besides already having the parts on hand, is there another reason why you are using the AD1862? It is an older obsolete part currently on Last Time buy status. Just wondering if this soon to be gone chip is special._

 

I am using the AD1862 because it is supposed to be pretty much the best sounding DAC I can get my hands on. This isnt like people saying the TDA1543 sounds great, even though it measures poorly; the AD1862 is supposed to be something special on all levels. Sure, it is only 20bit, but I have no music recorded that high, or even above 16bit. Apparently, the reason it is discontinued (have fun finding some) was due to an extremely expensive manufacturing process, and Analog wasnt able to sell enough of them to be worth it. In some ways, it is supposed to outperform the PCM1704K, which is pretty much the only other DAC I was thinking about for this project. Either way, both are extremely good R2R DACs.


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## AndrewFischer

Quote:


  Originally Posted by *cetoole* /img/forum/go_quote.gif 
_I am using the AD1862 because it is supposed to be pretty much the best sounding DAC I can get my hands on. ... Either way, both are extremely good R2R DACs._

 


 Yeah the specs are darn good. I've got a few DVDs with Linear PCM 2-channel audio.
 I'm not sure if any of them are 24-bit. My receiver tells me the sampling rate but not the bits per sample. 

 In any case I doubt I'll use a AD1862 for my next DAC since as you pointed out they are hard to find.

 Digikey has the PCM1704K for a mere $30, but it is not recommended for new designs. <sigh>


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## regal

I actually have a AD1862 DAC made by AA that I have been trying to find a discrete I/V+filter to replace the OPA275's. It has the PMD-100 with the -6db done in that analog domain at the low-pass filter after I/V so it makes it a little tricky to find a drop-in solution.

 Cetoole I have the complete schematics for this DAC if you are interested I can email them to you.


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## cetoole

Quote:


  Originally Posted by *AndrewFischer* /img/forum/go_quote.gif 
_Yeah the specs are darn good. I've got a few DVDs with Linear PCM 2-channel audio.
 I'm not sure if any of them are 24-bit. My receiver tells me the sampling rate but not the bits per sample. 

 In any case I doubt I'll use a AD1862 for my next DAC since as you pointed out they are hard to find.

 Digikey has the PCM1704K for a mere $30, but it is not recommended for new designs. <sigh>_

 

Yeah, I got lucky when buying mine, and would have gotten more than just the four had I had the funds for it. If your receiver is telling you the sample rate, I suspect that it is a video DVD, and the audio is just 48/16, because, AFAIK, not many devices support sending high resolution audio over the digital outs.

 If I wasnt able to get these DACs, I definitely would have chosen the PCM1704. No reason for you not to grab them while they are still being produced and available from places like Digikey. I suspect strongly that the prices for these will go up drastically once TI finally stops making them, as they have a pretty strong following. I dont really feel any of the new DACs are superior, especially as most (all?) of them are delta sigma designs.

  Quote:


  Originally Posted by *regal* /img/forum/go_quote.gif 
_I actually have a AD1862 DAC made by AA that I have been trying to find a discrete I/V+filter to replace the OPA275's. It has the PMD-100 with the -6db done in that analog domain at the low-pass filter after I/V so it makes it a little tricky to find a drop-in solution.

 Cetoole I have the complete schematics for this DAC if you are interested I can email them to you._

 

I would prefer you send me that PMD-100, though I have a pair of SM5847 digital filters on the way which should be pretty good. Cant get my hands on a PMD-100 (seen a few sell for $150 recently, no way I am paying that much, thats around double what I paid for my DACs, and I got four of them)Gotta come up with some glue logic though so it can accept the I²S output of a PCM2707. I will take a look at how to implement that -6dB analog attenuation the PMD-100 requires due to how HDCD works, but why do you really need it? I can see the volume discrepancy on some discs bothering consumers, but its not like all CDs are normalized anyways, and I am sure you have another attenuator of some sort elsewhere in your system, so it isnt like you will be unable to get the volume right. YGPM.


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## cetoole

Did some searching and thinging about how to impliment this output level scaling. I really dont like the idea of attenuating the signal, so we will stick with amplifying, though it would be easy to do either. The PMD-100 datasheet shows a simple circuit in which a transistor drives a relay, which connects a resistor connected to the inverting input of the buffer opamp to ground, increasing the gain from unity to, I assume, 2. The output buffer shown here is unity gain open loop, unlike an opamp in which the gain can be increased, so you could change the buffer to an opamp and control the gain in this mannor. We have an additional option when using this discrete circuit, and this is probably how I would impliment it.

 In this circuit, output voltage is a function of two things, current output of the DAC and the value of R1. R1 functions exactly the same way as a passive I/V converter, which is just a resistor on the output of the DAC to ground. Instead of using just one resistor for R1, use two in series, and have a relay, the Omron G5V-1-DC5 would be a good choice, short one of them out, so that when pin 5 is low, the relay shorts the second resistor, so choose the value of the first resistor to give 2v output. When pin 5 goes high, the relay powers, and the two resistors are in series. Since you want the high output signal to be 4v, have both resistors be equal in value. The signal either goes through a resistor and the unpowered relay (low) or two resistors in series.

 This probably doesnt read very clearly, so I will draw it up in Eagle when I get home tonight from Comp Sci class (about an hour).


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## AndrewFischer

Quote:


  Originally Posted by *cetoole* /img/forum/go_quote.gif 
_ If your receiver is telling you the sample rate, I suspect that it is a video DVD, and the audio is just 48/16, because, AFAIK, not many devices support sending high resolution audio over the digital outs._

 

Now that I'm home, I went and took a look. I was surprised to find it is 48Khz/24bps with DTS 5.1 as the only other audio track.

 I apologize for the thread jack. If anyone wants to discuss this lets do it here:

http://www.head-fi.org/forums/showth...42#post2867442

 =================================


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## cetoole

Here is the updated version, for use with 2SC2240/2SA970 transistors.


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## AndrewFischer

Well done.


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## regal

cetoole,

 I think I may try to build this instead of shilling out $225 for a Zapfilter. It looks simple enough to do an a breadboard. Do you have a BOM ?


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## cetoole

Quote:


  Originally Posted by *regal* /img/forum/go_quote.gif 
_cetoole,

 I think I may try to build this instead of shilling out $225 for a Zapfilter. It looks simple enough to do an a breadboard. Do you have a BOM ?_

 

I do not have a BOM, but parts selection should be pretty simple. Choose resistor values in the CCSs to run T1 and T3 both at ~10mA. You will probably want to make the T5 and T7 CCSs adjustable. Since a current mirror isnt perfect, you will want to adjust the T5 CCS to compensate, and because of the input current from the DAC, you will want to be able to configure it during setup, with the servo opamp removed, to get it as balanced as possible. I am not sure how to calculate the ideal value for the base bias of T3, but ~10v should work pretty good. Choose R1 to get your desired output voltage, using Ohm's law, with I being the current output of your DAC. C1 is for a lowpass filter, choose it so as not to affect the audio frequencies significiantly, but filter a lot of that HF crud out. You may want to make this a 2nd order or even 3rd order filter. Look at the Gilmore DynaXX amps for ideas about servo component values, and I think Borberly shows some component values which could be used for the FET buffer. I dont remember the resistances that are ideal offhand, and have to run.


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## Pars

Have you done anymore with this design yet? It looks interesting for my Rotel...

 Chris


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## cetoole

Quote:


  Originally Posted by *Pars* /img/forum/go_quote.gif 
_Have you done anymore with this design yet? It looks interesting for my Rotel...

 Chris_

 

Pars, it depends what you mean by "done anymore with this design". I havnt changed the schematic or layout, but I have started choosing some initial parts values. Some values are dependent on the DAC you will be using it with, due to output current from the DAC, and voltage it needs to see on its output. Also you have to determine what output voltage you want from the I/V converter.


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## Pars

Quote:


  Originally Posted by *cetoole* /img/forum/go_quote.gif 
_Pars, it depends what you mean by "done anymore with this design". I havnt changed the schematic or layout, but I have started choosing some initial parts values. Some values are dependent on the DAC you will be using it with, due to output current from the DAC, and voltage it needs to see on its output. Also you have to determine what output voltage you want from the I/V converter._

 

I guess I meant built one and tried it. I would be using this with a TDA1541A, so 4mA current, with probably 1.3-1.5K for R1. I have been looking at the different Jocko flavors, and at rbroer's "less simple" variant over on diyaudio, which I like because it doesn't use coupling caps (same with yours). BTW (you may have posted this) but what is the board size on these?

 Chris


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## cetoole

Quote:


  Originally Posted by *Pars* /img/forum/go_quote.gif 
_I guess I meant built one and tried it. I would be using this with a TDA1541A, so 4mA current, with probably 1.3-1.5K for R1. I have been looking at the different Jocko flavors, and at rbroer's "less simple" variant over on diyaudio, which I like because it doesn't use coupling caps (same with yours). BTW (you may have posted this) but what is the board size on these?

 Chris_

 

No, I havnt built one. This is really something I am working on for a complete DAC design. I dont have any sources to mod with this. 

 As for the size...
  Quote:


  Originally Posted by *cetoole* 
_It currently measures 69.85x50.80mm, or 2.75x2", and this is only a single channel. Bit bigger than just an opamp, eh?_

 

Now, if you are able to etch a 2 sided board, and move some parts to smd, then you can make it a good bit smaller. I am not planning on having any of these made alone, but I will get a multiproject panel made up at some point.


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## Pars

Thanks. I currently have rbroer's more complicated design laid out on 3.7" x 2.7" for a single channel. I tried emailing him, but no reply yet, just to get his thoughts, as this is a 3-4 year old design. 2.75" x 2" sounds better, and I like the fact that yours does use a current mirror. Would you be willing to share the Eagle files for his?

 Chris


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## DigiPete

Some questions, from an unlearned guy (me):

 Would this work with a PCM-1796 current out DAC?

 Now if the DAC chips are run in dual-mono for each channel (i.e. one 2ch DAC chip per channel) do you just run both V+ ouputs from each side of the DAC to Input 1, and both V- outputs to Input 3? 

 Would the output be 2.0 vrms (standard CD unbalanced output voltage), or do you need some amplification/buffer stage for the output?


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## cetoole

Quote:


  Originally Posted by *DigiPete* /img/forum/go_quote.gif 
_Some questions, from an unlearned guy (me):

 Would this work with a PCM-1796 current out DAC?

 Now if the DAC chips are run in dual-mono for each channel (i.e. one 2ch DAC chip per channel) do you just run both V+ ouputs from each side of the DAC to Input 1, and both V- outputs to Input 3? 

 Would the output be 2.0 vrms (standard CD unbalanced output voltage), or do you need some amplification/buffer stage for the output?_

 

Why PCM1796, instead of PCM1792A? You can use this I/V with that DAC though. I would have to study the datasheet more to see how mono mode works though. Do you have two + and two - outputs per DAC, and do I/V conversion on each? If so, you can do that with this circuit.

 Output voltage depends on DAC output current, and the value of R1. Use Ohm's law to figure out the output voltage based on these two factors.


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## DigiPete

My Denon 3930CI Universal player has the 1796 Dacs.

 "Do you have two + and two - outputs per DAC, and do I/V conversion on each?"

 Each DAC has two V+(r,l) and two V-(r, l).

 Here is what a friend sent me about my player from a picture he saw of the audio board:

 The stereo DACs each run one channel, and 
 > the outputs are paralleled. The important question is whether the 
 > outputs are summed before or after the I-V opamps. From the picture, 
 > it looks like they run separate I-V opamps then sum the voltage 
 > signals from those opamps. I need to get a schematic to be sure, and 
 > I'm going to look into getting one soon. If this is the case, I might 
 > have to look into whether the current outputs can be paralleled directly or not.

 I was thinking about how to use your circuit instead of a Zapfilter, and get balanced outputs.


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## cetoole

Quote:


  Originally Posted by *DigiPete* /img/forum/go_quote.gif 
_My Denon 3930CI Universal player has the 1796 Dacs.

 "Do you have two + and two - outputs per DAC, and do I/V conversion on each?"

 Each DAC has two V+(r,l) and two V-(r, l).

 Here is what a friend sent me about my player from a picture he saw of the audio board:

 The stereo DACs each run one channel, and 
 > the outputs are paralleled. The important question is whether the 
 > outputs are summed before or after the I-V opamps. From the picture, 
 > it looks like they run separate I-V opamps then sum the voltage 
 > signals from those opamps. I need to get a schematic to be sure, and 
 > I'm going to look into getting one soon. If this is the case, I might 
 > have to look into whether the current outputs can be paralleled directly or not.

 I was thinking about how to use your circuit instead of a Zapfilter, and get balanced outputs._

 

To use this circuit to achieve balanced outputs, just build one for each output. You may want to build it to share the LEDs used to bias the current sources, which is pretty easy to do. Sounds like to use it with differential mono mode, you will have to build 4 of these per channel.


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## cetoole

Here is the latest schematic for my discrete I/V converter. As you can see, there are quite a number of significant changes, most notably to the biasing arrangement, but also to the current sources/sinks. I am working on the PCB layout for it now, as well as learning to use LT Spice, so I can simulate it.


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## cotdt

have you built it yet, cetoole?


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