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dromney - I will quote Ralph Karsten from

192.5.202.6

In Reply to: RE: Maggies and Passive Preamps posted by dromney on February 14, 2011 at 10:57:03

Atmasphere:

"In order to produce proper bass, there can be no phase shift above 20Hz... The loss of bass is unique to passives.

To eliminate phase shift at 20Hz requires a cutoff frequency of 2Hz. IOW, the cutoff has to be about 1/10th the frequency to be amplified. So if the cutoff has risen to 5 Hz, effects will be heard at 50Hz- audible on most speakers. This low frequency phase shift is interpreted by the human ear as a loss of bass impact.

Note that the phenomena will not be had if the source has no output coupling cap. ... But the vast majority of sources *do* have output coupling caps... So any passive used in this case will manifest the loss-of-bass problem."

Follow Ups:

RE: dromney - I will quote Ralph Karsten from - dromney 15:42:52 02/14/11 (6)
Per the schematic I put the link to, there is no output coupling cap. It is a purely resistive circuit in the signal path, hence it has no signal/phase altering characteristics. That is because the LaPlace transform is R(s)=r, which has no time domain shifts. The link you cite is correct only for circuits with capacitance.

Unless the variable resistors have capacitance or inductance they should not affect the signal at all, other than to attenuate it.
- RE: dromney - I will quote Ralph Karsten from - Davey 16:18:29 02/14/11 (5)
  Clearly, he's talking about an output capacitor in the source. Adding a passive attenuator between there and the power amp input may indeed change the cutoff of a HP filter created by that source and this new attenuator.
  
  Although he's certainly wrong about "loss of bass being unique to passive" preamps.
  
  In this particular case the loss of bass may be real, but it's certainly explainable. Unfortunately, there's too much speculation in this thread and not the right questions being asked. Power amp input impedance? Source output impedance? Cable capacitance?
  
  The LDR devices are known for measurable distortion.....certainly higher percentages than would be yielded using a "conventional" passive attenuator. This may be the reason for the subjective improvement in sound.
  
  Cheers,
  
  Dave.
  - RE: dromney - I will quote Ralph Karsten from - dromney 12:50:33 02/15/11 (0)
    Maybe I misread his quote. I was reading it to mean the output of the preamp was cap coupled to the amp to protect against DC voltages.
    
    If the signal source (DAC or whatever) has some capacitence then the formed RC circuit could act as a filter when coupled and could affect bass in that case, DEPENDING on its tuned frequency. In fact it could cause some circuits to go into oscillation - ugh! So it is not a given, but I see your point.
  - Yes, Karsten was talking about the - jult52 05:31:01 02/15/11 (3)
    output capacitor in the DAC being the issue.
    
    Specs: DAC output impedance 100 ohms / voltage 1.75v
    Power amp has input impedance of 62kohms and max input voltage (sometimes called sensitivity) of 2.1v. I was concerned about that voltage mismatch more than anything but several people assured me it is not a problem. ICs are short. Speakers are 88 or 89db sensitive.
    - RE: Yes, Karsten was talking about the - Davey 08:30:41 02/15/11 (2)
      What type of attenuator configuration is this? Series? Shunt? Do you know what the value of the output capacitor is in your DAC? What resistance value is the attenuator?
      
      I don't see anything in your specs (at this point) that would account for a loss of bass. Also, consider the possibility that your previous preamp was accentuating the bass response to begin with.
      
      Cheers,
      
      Dave.
      - RE: Yes, Karsten was talking about the - jult52 10:48:58 02/15/11 (1)
        
        In Reply to: RE: Yes, Karsten was talking about the posted by Davey on February 15, 2011 at 08:30:41
        
        "What type of attenuator configuration is this? Series? Shunt? What resistance value is the attenuator?"
        The LSA uses a series/shunt. I am (hopefully) linking to a diagram of the device. The resistors are 100ohm and I believe there are two of them.
        
        "Do you know what the value of the output capacitor is in your DAC?"
        No. I use a Nixon TubeDAC which I believe uses Black Gate non-polar caps but that isn't enough information.
        
        "Also, consider the possibility that your previous preamp was accentuating the bass response to begin with."
        Interesting thought.
        
        Thanks for the response.
        
        RE: Yes, Karsten was talking about the - Davey 20:45:17 02/15/11 (0)
        
        In Reply to: RE: Yes, Karsten was talking about the posted by jult52 on February 15, 2011 at 10:48:58
        
        Well, it's tricky to analyze without making some actual measurements. You've got a tubeDAC which probably has highish output impedance....coupled with a tricky "passive" attenuator that changes load resistance and source resistance depending upon setting....with some distortion characteristics that change quite a bit with photocell light levels. Lot of variables there. :)
        
        Cheers,
        
        Dave.
That's interesting. - RickeyM 14:24:02 02/14/11 (0)
Now I'm wondering about an experiment to try with my much modded CDP to wire in a second set of outputs bypassing the output coupling caps to use with my passive pre.

And yes, most of us know it's not technically a pre-amp :)

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