Tweaker's Asylum

• some i/v options for current output DAC chips (technical sugestions) - jcox 16:18:02 03/19/02 (6)
  In Reply to: i/v options for current output DAC chips (technical question) posted by MBK on March 18, 2002 at 08:53:34:

  
  i've been looking at dac i/v circuits as a fascinating and difficult circuit problem since tda1541 tweaking articles starting appearing nearly 15 yrs ago

  the tda1541 data sheet gives a +/- 25 mV ac compliance range, requiring a i/v converter input Z < ~12 ohms

  while not difficult for op amps over audio range, ideally the i/v circuit would have low input Z into the MHz range to "soak up" switching glitch spikes, preventing possible intermodulation distortion

  i feel a common misunderstanding of op amp i/v requirements leads to tweakers often calling for ultra hi slew rate op amps, if at least one pole of the image rejection filter is incorporated in the i/v stage there is little need for high slew rate -- lose the glitches don't reproduce them

  in voltage feedback op amp i/v with a feedback cap the op amp requires low output Z at high frequencies where the feedback cap is a short circuit conducting dac glitches directly to the op amp output, decades ago output buffering in the feedback loop was recomended with i out dacs

  analog devices has shown a "noise gain" op amp i/v connection that places a cap from input to ground which reduces impedance at high frequencies where op amp gain ( and finite op amp output Z ) cannot handle glitch energy see AD745, AD797 data sheet app circuits

  i am interested in 2nd order multiple feedback filter i/v options although keeping Zin max < 12 Ohm at filter resonant freq seems difficult (in multiple feedback 2nd order low pass, at filter resonant freq current through feedback R cancels input C due to 180 degree phase shift from inverting integrator causing input Z to peak)

  please provide BB refs

  obtaining low input Z with cascode circuits would seem to point in the direction of bipolar transistors

  for an integrated option, consider OPA660 diamond transistor as a symetrical casocode device, the output conductance/distortion specs aren't very good though

  AD811 has good current transfer specs but you would need to parallel 3-4 devices to handle tda1541's +/- 2 mA iout (using current mode amp as cascode by taking hi Z current mirror output from comp pin)

  to reduce discrete transistor cascode distortion consider (Baxendall's)"super-pair":

  my favorite obscure bipolar trick... ,
  Re: FET versus bipolar in MC input cascode ]

  For some reason I have never seen a “super-pair” in any discrete audio circuit design, anybody got a reference?

  Theoretical Audiophile, Obsessive circuit designer
  
  
  
  

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  • Re: some i/v options for current output DAC chips (technical sugestions) - MBK 18:14:00 03/19/02 (5)
    In Reply to: some i/v options for current output DAC chips (technical sugestions) posted by jcox on March 19, 2002 at 16:18:02:

    > while not difficult for op amps over audio range, ideally the i/v circuit would have low input Z into the MHz range to "soak up" switching glitch spikes, preventing possible intermodulation distortion

    > i feel a common misunderstanding of op amp i/v requirements leads to tweakers often calling for ultra hi slew rate op amps, if at least one pole of the image rejection filter is incorporated in the i/v stage there is little need for high slew rate -- lose the glitches don't reproduce them

    That was my reasoning too. The circuit lifted from BB appears in app note SBOA046 "IMPLEMENTATION AND APPLICATIONS OF
    CURRENT SOURCES AND CURRENT RECEIVERS", page 27, Figure 61; I recalculated for 22 kHz cutoff and the final implementation with values I had handy was: R2=511R; R3=20R; C1=0.01µ; C2=0.22µ

    SBOA046 also notes noise gain and the peaking you mentioned, but still eventually a good cutoff at HF.

    Now sound wise: I used parts I had lying around: OPA2134 SMD, carbon R, ceramic caps and RS polypropylene caps, haha, (though for HF filtering to GND why not) and it sounds decent, very dynamic with very good bass BUT a bit "bland"; then again downstream sits a Rotel integrated amp used as a pre whose headphone out feeds my active speakers; so maybe that solution achieves a resolution I am unable to appreciate with the compromised "pre" solution. The simple resistor solution, albeit with 20R, so outside compliance V, had sweet imaging and microdynamics but awful transients, no highs and no bass.

    MBK
    
    

    • BB application note SBOA046

    
    

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    • turn op amp around in i/v circuit - jcox 10:13:12 03/21/02 (4)
      In Reply to: Re: some i/v options for current output DAC chips (technical sugestions) posted by MBK on March 19, 2002 at 18:14:00:

      the photodiode transimpedance amp's 2nd order low pass filter topology you've used doesn't limit switching glitches at op amp input as effectively as the "other" multiple feedback biquad formed from the same feedback network with the op amp turned around (swapping overall circuit in/out too)

      now the large cap directly shorts dac Iout to ground at high frequencies

      the input Z peaks to ~20 Ohm @ Fc (with your values) but is reduced at lower freq by feedback and higher frequencies by input cap to ground

      this approach may be acceptable with the tda1541 as many find it usable with <100 Ohm passive i/v -- and one could argue that since "reasonable" music has a power bandwidth of ~ 4 KHz, the dac wouldn't see greater than the 25mV AC spec

      the circuit has rising noise gain with frequency, peaking @ Fc, leading me to the recomendation of low noise bipolar op amps or prehaps AD745

      
      ( your "junk box" circuit values give ~33KHz Fc)
      
      

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      • Re: turn op amp around in i/v circuit. Actually... - MBK 03:02:24 03/22/02 (0)
        In Reply to: turn op amp around in i/v circuit posted by jcox on March 21, 2002 at 10:13:12:

        ... since we're at it, one could put a 2 pole filter right at Iout, no? Also, what we have here in this circuit, isn't in an embryonic balanced output: since we already have created a virtual ground above actual ground, then we have actual ground, and an active (-) output - couldn't these 3 potentials be interpreted as a balanced output?

        MBK
        
        

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      • Re: turn op amp around in i/v circuit - Just did - MBK 02:35:58 03/22/02 (2)
        In Reply to: turn op amp around in i/v circuit posted by jcox on March 21, 2002 at 10:13:12:

        OK, I just tried that, switching positions. Silly me, I use a similar topology for my power op amp chips, I could have noticed that before.

        Anyhow, the result: much nicer already! It removes the hard edge without restricting dynamics or frequency response, or at least, not too audibly. It's smoother but still with sparkle. That is a candidate for building with quality components now. But I still have to try the cascode as well.

        I am aware that my Fc is off from the intended value of 22 kHz, however, I couldn't manage to rearrange the formulas to calculate the Z seen by the DAC. I assume (-)in is held at virtual ground: the op amp inverts and amplifies in such a way that (-) and (+) are equal potential, manipulated by the feedback. So the op amp equates U across the large C with minus U across the feedback network, while the DAC sees this apparent Z created by the op amp in parallel with the Z from the large capacitor. However I didn't make it in the math department to combine this into a formula for Zout(DAC), sadly :-)

        MBK
        
        

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        • Happy vacations - MBK 20:48:20 03/27/02 (0)
          In Reply to: Re: turn op amp around in i/v circuit - Just did posted by MBK on March 22, 2002 at 02:35:58:

          ... no rush, this is a game, but thanks!

          MBK
          
          

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        • vacationing, can send eqs after april 2 - jcox 11:20:13 03/24/02 (0)
          In Reply to: Re: turn op amp around in i/v circuit - Just did posted by MBK on March 22, 2002 at 02:35:58:

          unless we get a really long rest day
          
          

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  Similar findings - Dave I 12:30:31 03/18/02 (0)

  In Reply to: i/v options for current output DAC chips (technical question) posted by MBK on March 18, 2002 at 08:53:34:

  http://www.audioasylum.com/audio/tweaks/messages/33578.html

  Using a simple two transistor current mirror (preferably of the no feedback type) and a discrete current source (from a couple of the same transistors) and a good quality resistor will get you much better results than any resistor or opamp based I/V convertor, based on my experience. I posted a simplified schematic of my DAC I/V stage at one time but it's not available online anymore. E-mail me if you would like to talk about it more.

  Dave
  

  • similar findings

  
  

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