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coupling cap value selection ????

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Posted on August 21, 2014 at 14:05:41
Tweaker456
 The input impedance of my headphone amp is 50k. A .1 will start a 3 db rolloff at 318hz. Well that kinda singes my hide. There are 2 coupling cacps in this simple circuit. A suggestion by a trained electronics person is to get rid of the input cap and raise the value of the second (output) cap to .47. This will of cource get rid of the input cap effect and lower the 3db point to around 60 hz. This should improve the bass responce. Why such a low value in the first place?? See any problem with this? I can't publish the schematic, I'm double checking this info. Thanks, Tweaker

 

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RE: coupling cap value selection ????, posted on August 21, 2014 at 15:05:50
Jim McShane
Dealer

Posts: 5910
Location: Chicago
Joined: January 13, 2003
 Check your math - it's 31.8 Hz. with 50K and .1 uf. And the rolloff doesn't start there, it's down 3db there.

Unless you want to roll of the bass at the input you need to go lower. For a full range (20Hz - 20KHz) you need the cutoff at 1.5 to 2 octaves below the lowest frequency of interest. So you need a cutoff at about 5 to 7.5 Hz in that case. Remember, if the cutoff (-3db) point is at a given frequency there will be rolloff at the frequencies just above cutoff as well.

.47 uf gives you a 6.8 Hz cutoff, .33 uf gives you 9.6 Hz.

That's all for the first cap, the value of the second depends on the impedance of the 'phones you intend to use with the amp.

 

I knew something was wrong., posted on August 23, 2014 at 13:42:02
Tweaker456
 Thanks for the correction and the second cap info. What I did was use the V-cap coupling cap calculator. What is said at the bottom was "Optimal low frequenct responce 318hz" Above that was the 31.83 so I miss read it. What the 318 hz means is beyond me. Tweaker

 

RE: I knew something was wrong., posted on August 23, 2014 at 19:25:53
Tre'
Industry Professional

Posts: 17305
Location: So. Cal.
Joined: February 9, 2002
 The 318Hz is where you are finally out of the phase shift caused by the filter.

This is why when we want 20Hz flat and phase shift free we design the filters for 2Hz.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: I knew something was wrong., posted on August 26, 2014 at 12:52:25
Tweaker456
 Thanks for the info Tre'. So going to .47 from .1 on the second cap look like a good idea with no potential problem?? Just 2 coupling caps in this circuit. Thanks, Tweaker

 

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