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Gamma 2 output coupling caps

post #1 of 2
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I have a quick question regarding the output caps. it is stated that for the gamma 2 Full++ config, using 470uF caps is recommended when driving headphones directly, and 22uF is recommended when feeding to an amplifier.

I have the 470uF on there now and am awaiting shipment of 22uF caps. What would happen if I plug into an amp as it is? Will there be too much low frequency response or some kind of roll off?
post #2 of 2
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Originally Posted by shrimants View Post

I have a quick question regarding the output caps. it is stated that for the gamma 2 Full++ config, using 470uF caps is recommended when driving headphones directly, and 22uF is recommended when feeding to an amplifier.

I have the 470uF on there now and am awaiting shipment of 22uF caps. What would happen if I plug into an amp as it is? Will there be too much low frequency response or some kind of roll off?

The rolloff is controlled by the uf of the output caps plus the output load.  You can read all about it here: http://www.diyforums.org/BantamDAC/BantamDACf-cutoff.php

 

As the resistance of the load goes up very high, the size of the caps matter less.  Amplifier input impedance is usually controlled by the volume pot - 10,000 ohms, 50,000 ohms, 100,000 ohms, etc.  Whereas headphones are 32, 60, 300 - that sort of range.  At the same time, electrolytic caps do not sound as good as film caps.  In actual practice with most amps, 100% of the signal is simply going through the 1uf film bypass caps.  You could probably delete the electrolytic caps entirely for a better sound.*

 

There is no extra bass created if the caps are very large - only more distortion and loss of sound quality because electrolytics are not as good as film caps.

 

When using headphones directly, though, even 470uf will result in some bass cutoff with 32 ohm headphones and lower.  The equation shown on the webpage referenced above will give you the -3db cutoff.  With 32 ohms and 470uf, that results in about 10 Hz.  However, the frequency curve is bending down by -1db at about 20 Hz.

 

That's using simplistic calculations, but close enough to illustrate what's going on.  There are some resistors on the output of the Gamma that would also have to count in the equation (1/Rtotal = 1/Rheadphone + 1/Routput resistor) and so would the film bypass (492 uf total).

 

 

* The graphs on the referenced webpage indicate some bass rolloff with a 10K pot and 1uf output capacitance.  If you're feeding a portable amp, which often use an ALPS RK097, those are typically sized at 10K.  A desktop using a 50K RK27 would have no issue.


Edited by tomb - 2/10/13 at 1:33am
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