DC blocking caps

[Andy Peters]

I had that discussion with Gary Hebert at THAT; he sent me a SPICE model for simulation that showed the issues when the source Z is not zero. CMRR is all over the place. However, if the receiving end has good rejection, it may go unnoticed.

CMRR is a function of an input. But what is all over the place is the output swing gets unbalanced. Instead of the swing on + and - being the same except for polarity, the amplitudes differ, and that difference is a function of the imbalance in the source impedance.

 

[CJ]

put a transformer in there,  and one on the output, maybe add one to the feedback path,

 

[KrIVIUM2323]

John Roberts,

thank you for the answer, i wasn't sure about the statement you made (and because the That line driver can be configured for dc servo operation -not sure for 1646 but for 1606 i am). Now it's clear.

Abbey Road d'enfer,

Thank you too. Most of you explained i wasn't aware of or hadn't think about it enough but it make perfect sense (tolerance, audibility/deterioration of sonic of cap related to voltage thru them).

About 105°c range, i had to recap some AMS/Neve VR and SSL in the past and each time the studio owner made the choice of 85°c (for cost) they regrets it afterward.
AMS did made the choice to use 105°c (probably military speced  and 125°c but i'm not 100% sure about that- the one in metal can looking like el caps you sometime find on high quality PC motherboards) for the stock VX which is the last iteration of the V range and for the 2 desk i've seen and used in France they never  had to change one of those caps ( and there is high ambient temp into this chassis).

Ian,
there is a lot of debat in the monitor/loudspeaker field about the audibility of phase (for the low end especially). I'll try to find the last serious study i have (or refrence) and point them to you but it could take some time as i have them burried in one of my HDD and i'm short of time at the moment.
I agree with Abbey road in that even FIR in the low end can be audible as i witnessed when seting my monitoring system (Filtered using a mix of IIR and FIR with a Lake processor). But i must confess that it could have been something else that the processor by itself as many variable take place and i havent made it scientific when comparing (no abx, quick and dirty analysis with  barely tolerable analysis gear -mic and preamp especially).

Personnaly i will stick to the definition of a sound i've learned: a sound is a relationship of frequency, amplitude and phase.
Modify one of the parameter and you introduce distortion.

And now i've got historical and engineering answer for the how and why, i'am less dumb about this now, applying 'recipes' i don't know why.
  Thank you for that.

 

[abbey road d enfer]

Interesting statement. What evidence is there for this?

Cheers

Ian

There have been numerous experiments conducted on the subject, particularly with renewed interest as FIR filters are now becoming more and more common in so-called loudspeaker management systems.
You can start there
http://www.silcom.com/~aludwig/Phase_audibility.htm
Googling "phase audibility" will open a large can o' worms  ;D
Some articles leave no doubt about the fact that an all-pass filter can be heard because it changes the waveform's symmetry (or lack of) and crest factor; a linear-phase EQ or an FIR filter can be decomposed as a product of a minimum-phase filter and an all-pass filter, thus they will be audible for the same reason.

 

[abbey road d enfer]

CMRR is a function of an input.

Actually CMRR is a function of a connection, but it doesn't prevent the AES and several other organizations to publish specifications for the measurement of output and input CMRR. As an example, THAT indicates 65dB output CMRR for their 1606/1646 line drivers.