Audio Research Reference 5SE modifications/bypassing output caps

I agree with Atmasphere that bypassing a coupling cap results in smearing and a generally incoherent sound.  This is especially true when the coupling cap is a high quality cap like a V-Cap CuTF (my favorite).  And this is also true even when the manufacturer used bypass caps in the original design.  For example, for a long time ARC used a cocktail consisting of a large value film cap (either a Rel-Cap or MIT) bypassed with one or even two .01uf InfiniCaps.  You would think an established company like ARC would know what they were doing but not in this case.  For each of my ARC preamps, I was able to improve the sound quality by removing the bypass cap and letting the base cap (especially the 5uf MIT base caps) do all the work.  The InfiniCaps always added an unnatural sheen and spotlighting of the treble along with making the midbass too thin and anemic.  With today's better film caps like the copper V-Caps this would be even more true.

I do take exception to Atmasphere's comments about using a bypass cap across power supply filter caps.  My experience with bypass caps in a PS is exactly the same as with coupling caps.  I can always hear the effect of the bypass cap and it causes a loss of coherency and usually an artificial brightness and thinning of the midbass.  In my opinion it is always the better course to use a single very high quality cap rather than a combination of caps.

It is logical that using even a high quality cap of a different type to bypass a large coupling cap could produce timing differences as well as other effects due to differing dielectrics.  In the past I have replaced ARC mylar and polypropylene coupling caps with larger polystyrene caps which I also would bypass with smaller polystyrene caps of the same manufacture.  This did improve the sound of both amps and preamps. As I do not have access to smaller value caps of the same type and manufacture as the larger coupling caps in the ARC Ref 5 SE I shall not bypass them.  Thanks for all the comments. 

I do take exception to Atmasphere's comments about using a bypass cap across power supply filter caps.  My experience with bypass caps in a PS is exactly the same as with coupling caps.  I can always hear the effect of the bypass cap and it causes a loss of coherency and usually an artificial brightness and thinning of the midbass.  In my opinion it is always the better course to use a single very high quality cap rather than a combination of caps.

I'm not going to take issue with this- with most power supplies for single-ended circuits this is probably true. I do most of my work with fully differential circuits and often with high capacitance values in our power amplifiers- in the latter case its often helpful to bypass the electrolytic right at its terminals to reduce ripple current problems, which prolongs the life of the part. In our MP-1 which has a five foot umbilical connection, we have found it helpful to bypass the power supply connections inside the preamp, which helps reduce noise.