Differential I/V stage? And RX input

[guzzler]

I'm looking to build a DAC based around a SINGLE PCM179*, and I'm having trouble finding a decent I/V stage, as the the output is differential. I don't need or want balanced outputs, so the one of the datasheet isn't much use.

The only schematic I've seen is a 3rd order Bessel filter done by Aos

http://www.aoselectronics.com/gfx/ally/ppa_db_v1.1.gif

However, it looks a little too complicated for my taste, and I'd prefer a simpler discrete output stage so I can tweak the sound a little more than by rolling opamps...

Any suggestions MOST welcome

Also, regards input to a CS8416, I've been reading around on the forums about both transformer inputs, and differential inputs using logic buffers. What is the general consensus of opinion on that these days?

 

[rbroer]

There a several I/V circuits on this forum, you need two of the single ended I/V's.

To create a single ended buffered output signal, you can get some inspiration here:

http://ultranalog.com/cdenhancer/sacdenhancer/


I'm puzzled though why you need an I/V for your PCM1793

I would suggest sticking to opamp rolling

 

[jan.didden]

I'm looking to build a DAC based around a SINGLE PCM179*, and I'm having trouble finding a decent I/V stage, as the the output is differential. I don't need or want balanced outputs, so the one of the datasheet isn't much use.

The only schematic I've seen is a 3rd order Bessel filter done by Aos

http://www.aoselectronics.com/gfx/ally/ppa_db_v1.1.gif

However, it looks a little too complicated for my taste, and I'd prefer a simpler discrete output stage so I can tweak the sound a little more than by rolling opamps...

Any suggestions MOST welcome

Also, regards input to a CS8416, I've been reading around on the forums about both transformer inputs, and differential inputs using logic buffers. What is the general consensus of opinion on that these days?

Hi Guzzler,

Note that the 1793 has internal opamps and provides voltage outputs. There is no I to convert to a V. This is clearly shown in the data sheet.

If you want to convert the balanced output signal to single-ended, one option (passive) is a 1:1 transformer, with the primary across the outputs and the secondary grounded at one terminal and the other to the RCA center pin.

Jan Didden

 

[guzzler]

Sorry, my fault not explaining clearly enough. I'm not looking at the PCM1793, rather the PCM1794/PCM1798. Thanks for the replies anyway

 

[guzzler]

I’ve had a look a the RAKK DAC schematic, looks interesting. However, I’ve never been a huge fan of transformers in non-digital signal positions (personal preference, sound technical reasons, don’t want to go into the argument about which sounds better etc).

Ideally, what I’d like would be a solid-state discrete stage powered from a bi-polar supply to remove the output capacitor. Looking at Elso Kwaks schematic, he’s got the o/p capacitor (which I’m not really adverse to, but would prefer to be without). For the input, would it be as simple as in the RAKK DAC with a resistor between I+ and I-?

 

[guzzler]

anyone?

 

[jan.didden]

anyone?

Well if you want to go from balanced to single ended you need either a passive devise (transformer) which is straightforward, or an active device and you can either brew something yourself or use any of the audio balanced receiver chips like those from TI (e.g. OPA1632 or the INA134/137 series). Alternatively, why not use just ONE output into your preamp/power amp and forget about the other? You may have some more 2nd harmonic distortion, but that may even be an advantage as it will help to mask 3rd hasrmonic etc.

Jan Didden

 

[00940]

The easiest way, but not discrete, is to do it like Bel Canto did it for the DAC2. Opa1632 (or ths4130) for balanced I/V followed by a INA134 to convert to single ended.

Or if you can accept complicated and expensive, there was this : http://www.diyaudio.com/forums/showthread.php?s=&threadid=9910&highlight=

But it gives balance output, so you will need to use an ina134 afterwards.

 

[jan.didden]

The easiest way, but not discrete, is to do it like Bel Canto did it for the DAC2. Opa1632 (or ths4130) for balanced I/V followed by a INA134 to convert to single ended.

Or if you can accept complicated and expensive, there was this : http://www.diyaudio.com/forums/showthread.php?s=&threadid=9910&highlight=

But it gives balance output, so you will need to use an ina134 afterwards.

Well if they used THS4130 AND INA134 that's not very smart. You can just ground one output of the THS and run that SE.

Jan Didden

 

[00940]

Don't you improve THD measurements by running it fully differential ?

 

[jan.didden]

Don't you improve THD measurements by running it fully differential ?

Maybe, depends on the topology, but you got to get SE sometime. Might as well do it early on rather then keeping on adding active devices/opamps.

Jan Didden

 

[00940]

the topology of the THS4130 (opa1632, same stuff) is rather simple.

 

[rfbrw]

Ideally, what I’d like would be a solid-state discrete stage powered from a bi-polar supply to remove the output capacitor. Looking at Elso Kwaks schematic, he’s got the o/p capacitor (which I’m not really adverse to, but would prefer to be without). For the input, would it be as simple as in the RAKK DAC with a resistor between I+ and I-?

There is a schematic in post #7 of this thread,
http://www.diyaudio.com/forums/showthread.php?s=&threadid=60414
that can be used as the basis of a discrete single ended to balanced stage.

 

[guzzler]

Thanks for all the replies, will look into it

 

[rfbrw]

discrete single ended to balanced stage.

Or other way around.

 

[sidiy]

3rd order Bessel

Hi Guzzler,
After I-V conversion you may try this filter. It is a differential Bessel 3rd order with Fc=75.5kHz and unity gain. Its -3dB point is @ 53.68kHz and "bites" only ~ 0.4dB @20kHz. If you want to try this topology at another cutoff frequency, use other component values or even choose a Butterworth response just let me know. I can send/post all the design equations...

You'll get more effective results by tweaking the filter response to your taste than by swapping various OpAmps. (you can settle from the start to a good one)

And beware that many filter examples given in the manufacturer's AN for DACs aren't really Bessel responses, altough they may claim 'linear phase' somewhere in the text...

Please rotate the pages in the attachment, ...I didn't see that one coming
Cheers

 

[guzzler]

Thanks sidiy, that looks pretty nice. If you could email me over (or post if you wish) the eqns and such like, that would also be most appreciated

awjl3 at cam dot ac dot uk

 

[Porksoda]

Hey Gus,

Glassman and I worked on a differential input I/V stage a while back for use as a low open loop gain non-inverting I/V stage. Have a look-see:

An externally hosted image should be here but it was not working when we last tested it.

It's based on a single-ended inverting stage from THIS Paper by Malcolm

 

[Extreme_Boky]

There is a schematic in post #7 of this thread,
http://www.diyaudio.com/forums/show...&threadid=60414
that can be used as the basis of a discrete single ended to balanced stage.

Now, that’s one really good but simple output stage. It does differential to single ended conversion and produces excellent current drive. What I see is great opportunity to modify this stage to drive loudspeakers directly, up to 3-10W. Volume control could be done just after the NE5334’s with differential stepped-ladder attenuator. The output 0 balance should get attention to ditch the coupling cap, as should power supply implementation of lets say + and – 20V at decent current. Everything (heatsink included) could fit within the DAC (I think it’s DAC output stage) and you could get really short signal paths and great sound. This may be of interest to those single-chip-amplifier freaks. I don’t think you would have to replace any transistors to achieve this output. MPSA43 / 92 are great, as are output Darlington’s. Just larger heatsink and additional power supply to power up the output discrete stage.

Extreme_Boky

 

[andrei]

Check out my NPC SM5847/SM5865 and PCM1730 DACs. Both have differential I/V stages using THS4131 differential opamp. The differential/singe ended conversion is usually done using instrumental amplifiers in the solid state world, but I would go for output transformer instead .