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Upgrading coupling caps - which ones first?

I have been experimenting with different coupling caps on the output of my amp.  I now want to try some more expensive caps (Vcap, etc), but cannot afford to replace every coupling capacitor in my system.  Therefore I want to place the best capacitors where they will make the most difference.

For each channel of my amp, I have one coupling cap between the driver and phase splitter stages, and a pair between the phase splitter and output tubes.  There are also output coupling caps on my DAC that feeds the amp which I am thinking about upgrading as well.  Which ones benefit the most from an upgrade that should be targeted first?
solman989
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georgehifi
4,966 posts
 

Music in your system, it all starts with the source, so you may as well start there, and work forward, this way you may get an idea what type colourations each one is contributing.

Give me dc coupled all the way through any day, no questions ask then.

Cheers George

ultra_fi
24 posts
 
I concur the furtherest up the signal chain.

That in itself isn't much help though.  Perhaps, think of it like this: the capacitor is a bell and music signal is a hammer.  The further up the chain the smaller the hammer and the less the bell will ring imparting its artifacts to the signal.

So, as you move from small to large signal the capacitor needs to have more and more dampening effect.

almarg's avatar
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almarg
8,673 posts
 
It may be helpful if you could let us know the capacitance values and the voltage ratings of the three pairs of capacitors, and also the input impedance of the amp. If the amp has lowish input impedance (although it probably doesn’t, given that it is a tube amp) it would tend to increase the criticality of the coupling cap at the output of the DAC. And I’m thinking that as a rough rule of thumb, and everything else being equal, it is probably reasonable to expect that higher values of capacitance and voltage rating will tend to correlate with less ideal behavior of a capacitor.

Regards,
-- Al

solman989 OP
42 posts
 
The caps in the amp are .1 and .47 uF at 600V, and the output caps in the DAC are 3.3uF 200V. 

it is probably reasonable to expect that higher values of capacitance and voltage rating will tend to correlate with less ideal behavior of a capacitor.

My thoughts also.  Amp input impedance is about 70k which means I could get by with a much smaller ~1 uF cap on the DAC without any frequency rolloff.
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almarg
8,673 posts
 
Sounds good re the change to 1 uF.  As you may already have calculated, in conjunction with the amp's 70K input impedance that will result in a 3 db bandwidth extending down to 2.3 Hz, and only about 6.5 degrees of phase shift at 20 Hz.

Based on that and on the values you supplied, and without knowing anything further about the design of the amp, what seems likely to be the most optimal prioritization would be doing that pair of caps first, then the 0.47 uF caps, then the 0.1 uF caps.

Good luck.  Regards,
-- Al
  
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