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uf value change - coupling caps

71.62.34.73
Posted on August 18, 2015 at 10:08:59
Devils Advocate
 Everything else remaining the same, what effect does a change in a coupling capacitor's farad value (0.22 vs 0.33 vs 0.47...) have on the amp's sonic quality and stability? Looking into swapping out the coupling caps in my Cary 300sei, and curious what the benefits (if any) between caps of same brand, type and voltage. Good idea, bad idea, waste of time? Thanks.

 

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RE: uf value change - coupling caps, posted on August 18, 2015 at 11:54:20
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 When the amp is overloaded, so that grid current is drawn, the coupling cap charges which shifts the operating point. The time to recover depends on the RC value of the grid to ground resistor and the coupling capacitor.

I have not found Cary's circuit, so I don't know the 300B grid stopper resistance (it limits the grid current charging), or the grid to ground resistance, so no examples or recommendations are possible.

 

RE: uf value change - coupling caps, posted on August 18, 2015 at 13:23:58
Blackdog
Manufacturer

Posts: 1505
Location: Ontario
Joined: March 20, 2006
 There are three different versions of your amp. Two with fixed bias, one with cathode bias. All have different grid to ground resistance after the cap.
Given that, I would leave well enough alone.


Dan Santoni

 

Bigger Ain't Always Better, posted on August 19, 2015 at 15:54:58
Chip647
Audiophile

Posts: 2651
Location: The South
Joined: December 24, 2012
 The right size is calculated by the engineer. Too big and you have the grid bias problems as well as potentially providing more subsonic garbage to rob power. Too small and you have low frequency cut off and phase anomalies to contend with. The size of the coupling cap is a trade off Denis made. If you think that he chose the wrong value and that the amp can be improved by a simple change in the coupling cap value leaving the grid resistor the same, you are deluding yourself.

 

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