[jonwb]

One of the mods that both Red Wine and Bolder do to the SB2 is to run the output leads directly from the DAC (pins 7 (VoutL) & 8 (VoutR) I presume) to the RCA jacks.  Coupling capacitors are used between the DAC and the RCA jacks.  They are both using high quality polypropylene capacitors, but what I am curious about is what value would be appropriate?

Also, if the output signal is being fed (through the cap) right to the RCA jack, would it make sense to pull the ground right from the chip as well (pin 10 (Vcom))?  Looks like TI recommends a 10uF coupling cap off the Vcom pin.

Of course connecting the (relatively) giant leads of the cap directly to the itty bitty pins on the DAC (about .25mm x .25mm) might require some reductions in my daily caffiene intake.  I guess that's why Vinnie and Wayne make the big bucks!  

PS: Were it not for Wayne's testimonials, I never would have thought to try hooking the SB2 directly to my amps; I have a pretty decent pre afterall.  When I first hooked up my SB2 through my pre I was not entirely impressed w/ the sound.  I figured it must need mods to sound "decent".  However, just yesterday I hooked it up to the amps direct and, well, that's all together better.  Clearer, more dynamic, better bass...  Of course I need my pre for other sources so I'll have to come up w/ a solution, but I'll say direct to the amp is indeed better!

[JoshK]

Hmmm....I was thinking to use 2.2uf to 3.0uf.  One can always look up the formula that will tell you what freq you will be making the cut off at at a given voltage (RMS I think).  A google search usually yields the formula that I am looking for.

By the way, very good question!  One question I had myself when I first read of mods taking the DAC output directly.  I betcha diyaudio has lots of guys with opinions on the subject of what value to use as there are inevitably tradeoffs with larger/smaller values.  You can't beat experience on these things, and pro modders like Vinnie/Wayne have probably had a lot more time to test out things such as this than I have had for instance.

[Occam]

the Pole freqency is -
f = 1 / (2*Pi* R * C)
where R is in Ohms and C is in Farads
Farads are rather large, so we may adjust the formula to microfarads and get rid of that pesky Pi at the same time

1,000,000/(2 x Pi) = 159,155

so

C = 159,155 / (R x f)
f = 159,155 / (R x C)
R = 159,155 / (C x f)
 
where C is capacitance in microfarads, uf

where R is the input impedance of the load (the amp or pre you're feeding), in Ohms

and f is the -3db down (pole) frequency in Hz (Hertz), or for you really old farts cps (cycles per second)

[mgalusha]

Since the cap is in series with the output it will act as a first order high pass filter relative to the input impedance of the load.

Say your load (preamp or amp) has an input imepdance of 47K. With a 3.0uF cap this would produce a -3dB point of 1.131 Hz and would be .1dB down at
7.4Hz.

This is of course a calculated value and doesn't take into account how well DAC chip can drive it directly or any variation in the load. The calculation assumes a strictly resistive load, which of course nothing is.

Still, I suspect for most purposes a 3.0uF or so cap will work just fine. I doubt you could fit a much larger film cap into the SB2. For that matter many of the 3.0uF caps wouldn't fit.

Mike

[mgalusha]

Ah... I see Occam posted the formulas while I was typing. I'm lazy and use DazyCalc to do the calculations. The results should be the same tho.

[jonwb]

Thanks guys... I setup a little spreadsheet using Occam's formulas and then ran the calcs using mgalusha inputs/results just to see how it worked.  

My amps actually have an input impedance of 51k, I'm not sure what my pre is at (or if I'll even use it for the SB2).  Anyway, the results are as mgalusha indicated, 3uF looks like a good value.  Basically the bigger the value the lower the -3dB value, so the limitation will likely be what'll fit.  

Amazing the price variation in 3uF caps:
Sonicap: $8.60ea
Axon True Caps: $2.06ea
Auricap: $20.20ea
ICW Clarity Cap (SA series): $10.56ea
Hovland Musicap: $22.95ea
TRT DynamiCap: $26.95

Acutally, these are the "cheap" caps... you can spend over $400/cap if you want  

[Davey]

I wouldn't call those cheap.  

I'd call these cheap:

http://www.madisound.com/cgi-bin/index.cgi?cart_id=5246379.17405&pid=1769

60 cents.

Cheers,

Davey.

[JoshK]

Those would probably work just fine too Davey!  I like SoniCap from the few times I've used them and can't say much else in comparison.