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Okkay, so yesterday I'm doing my yearly once-over on my gear, and I get into my ARC sp6a preamp. I've had this unit for almost a year, and am quite satisfied with the sound. I'm looking at the circuit board, and I see many coupling caps replaced with angela / scr metalized polyprops. No biggie, all values agree with the runes.Then I get to the last tube on the linestage, the one leading out to the power amp. These output couping caps caught my eye because they're not Angela/SCR, they're 'Aeon' or something like that, and instead of the 15mfd specified in the runes, it's 5.6. Previous owner obviously performed a 'mod'.
My question: What would this do to the sound? Was this in a quest for 'more bass'? 'less bass?' I'm familiar with using coupling caps to 'tune' a stage's sound, but I've never dealt with cathode followers before. The cap in question is circled in red in the schematic snippet below. It comes out of the cathode follower's cathode and leads to the power amp output.
When I got this preamp, I had Klipsch SF-2. My first impression, vis a vis my Sansui CA2000 sand preamp was "this pre is a bit thin on bass". Then I got my Fortes, and I have tons of bass. (The SF-2, in all honesty is quite thin from the low midrange and down, which is why they're banished from my system.)
I'm ordering the correct values. I'm interested to hear the results of bringing it in line with the original designs. I'm just curious as to the asylum's thoughts on this pre-existing 'mod'.
Please pardon the bad image quality on the scan. The cap in question reads "C10 15" (mf) and then a voltage rating.
Follow Ups:
Following the other comments that Aeon is a disguised Solen... I'd go for a smaller cap with better quality. I've tried a Solen on the output of my preamp and was amazed how much sonic degradation it caused vs. a Kimber (also metalized) and Vit-Q (paper-in-oil).
The 5.6uF has an impedance around 1.4K ohm at 20 Hz, so it is mostly not a limiting factor with your typical tube amp's input impedance. Dynaco MkIII has Zin around 470K, so a cap as small as 0.22uF may be used with not much ill effect. This enables you to invest some money in a decent NOS Sprague Vit-Q or a new copper/foil cap.
The result would be a definte drop in LF response with the 5.6.
...I'll have to move the fortes further out from the wall.. I'm *borderline* "too much bass" as is ;o)I wonder if the previous owner then had a problem with too much bass and tried to 'correct' it by doing this 'mod.' I also can't help but wonder what that's doing to the rest of the circuit.
it determines a low hertz roll off. In part the value of which is dependent on the power amps input impedance. If you were driving an usually low input impedance solid state amp it might prove to be a problem. But with the Dynaco you are fine and could likely go to an even lower uF cap without problems. As you report, there is no bass problem. If it makes you any happier I only use a 0.47 uF output cap in my preamp:) So really, don't stress over it. And BTW, Aeon and SCR are the same brand caps.....as is solen...all made by SCR.
Just that I tend to view value changes in a ckt with skepticism. The bigger the change, the higher up my eyebrow goes. =o)The dyna has a rep for being rock-solid under any conditions.. so nothing's at risk. The bass is OK but I bet I can even out the response curve some.
The angelas (angela cap and the tinfoils) are scrs too. Seems SCR is the french sprague =o)
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