API 550 cap substitute question

[fazer]

I have a 24 ch API with 24 550 EQ's.  I'm in the process of recapping all my 550's. 

In the 50 to 15khz bandpass filter section (of the 550)  is a .47 uf  electrolytic caps. 

I'm wondering about using a .47 cog ceramic in its place instead of the electrolytic.  Anybody have any opinions about the sonic difference.  I'll be testing an eq to see if I can hear the difference but  just reaching out.  It is very crowded  so film caps are out.  It is inline with A1 to A2 main amps. 

 

[joaquins]

  Might sound obvious, but if you are going to do the comparison, do so with a recapped one with decent electrols, not with the old caps one.
  If you want to measure the difference, probably THD at the corner frequency, where it's higher. If that's a 9k1 resistor, it's 37Hz, but as it's also loaded with the rest of the circuit, depending on the A6 input impedance, it could be a bit higher. You should probably try between 40Hz and 50Hz, I think THD at that point would be the major difference between type of caps, if any. Once the frequency goes higher than the corner freq the AC signal voltage in the cap is negligible, so no much effect in the signal, lower frequency than that is attenuated by the same filter so any distortion there should have little impact. While higher harmonics could show, you can test for it, usually in an RC filter the nonlinearities are shown better around the pole frequency.

JS

 

[JohnRoberts]

COG is better, should not have any "sound" (a good thing IMO).

JR 

 

[gridcurrent]

In the 50 to 15khz bandpass filter section (of the 550)  is a .47 uf  electrolytic caps. 

I'm wondering about using a .47 cog ceramic in its place instead of the electrolytic.  Anybody have any opinions about the sonic difference.  I'll be testing an eq to see if I can hear the difference but  just reaching out.  It is very crowded  so film caps are out.

C2 is a 0.64 uF in the 550,  0.47 uF in the 550a,  only in the circuit when filter is engaged. 
so how much sonic "grit" is injected with the filter IN ?
with a little work adding wispy sounding capacitors and Quad/Eight inspired transformers,
one could really wreck the time proven qualities of an API.

 

[joaquins]

C2 is a 0.64 uF in the 550,  0.47 uF in the 550a,  only in the circuit when filter is engaged. 
so how much sonic "grit" is injected with the filter IN ?
with a little work adding wispy sounding capacitors and Quad/Eight inspired transformers,
one could really wreck the time proven qualities of an API.

With the 512a they chose not to remove the output coupling electro to not affect the potential sound artifacts it generated, while they could remove it as they added a DC servo... (the DC servo was added to deal with the popping noise of the ø switch,  IIRC)

JS

 

[Whoops]

I dont think you or anyone will notice any sonic difference, but you can replace it for a COG or any Polyester Film Type

 

[Walrus]

C2 is a 0.64 uF in the 550,  0.47 uF in the 550a.

The original API data sheets I have show that the 550A  did have variants with 6.4uF/.64uF  and 4.7uF/.47uF.
Unfortunately, we don't have any 550 data at all.

 

[fazer]

All are 550a's.  Not 550's.

with a little work adding wispy sounding capacitors

The term wispy is a sound I'm not fond of. 

I'll try a  COG to see if there is a difference but will order the .47 electro's to be original.