Preamp Output Capacitor: Mundorf Supreme vs. Supreme Silver Oil

Could someone explain why adding a 0.01uF capacitor in parallel/bypass with a 2.2uF capacitor improves sound quality?

@kalali   It doesn't! Any capacitor has a certain 'speed' defined by a  combination of its dielectric constant and series resistance. This varies from cap to cap (inductance plays a role too; this is caused by the fact that the cap is rolled in a spiral fashion) and even from value to value of caps of similar construction.

BTW this is quite measurable (notwithstanding the known effects of the specifications on paper :) ) and one way to do it is to measure bandwidth. Some caps roll off the signal a lot quicker than others. The bigger the capacitor, the more profoundly its affected by these aspects.


Since smaller caps tend to be faster than larger caps, essentially a phase shift is introduced by the inclusion of a smaller cap in parallel with a larger cap. In a power supply this is advantageous, but as a coupling cap in a nutshell it smears the signal (and being real, the signal is messed with by the cap regardless). The more transparent your setup, the easier it is to hear this!


If this is done in a circuit that employs loop feedback, the phase shift introduced can interfere with the phase margin of the circuit; YIKES! this could lead to oscillation.


So as a result, unless you are compensating for a deficiency elsewhere in the circuit or system, the best approach is to give it your best shot and simply install the best capacitor you can and keep it as low a value as is practical.


This is also why increasing the value of a coupling cap (for example to extend bass response) isn't always a good idea. Circuits behave according to math  and the use of coupling caps is a really good example.

@atmasphere You keep slowly (and totally indirectly) convincing me to try your direct coupled pre. Unfortunately I'm quitting my job in May (and applying for new ones now), or I'd be saving up for a trial. 

All I can say is it sounds better in my highly resolving system. It just does. Experience and try for yourself.  This is the only way to know for you . 

Experience and try for yourself.  This is the only way to know for you .

@grannyring  A lot of people are of this opinion. I've just found that the more transparent the system is, the more you hear that paralleled caps  don't have the desired effect. But I'm sure neither of us want to argue about it- in fact I suspect my viewpoint on this topic could easily be unpopular.

People tend to think that since we make some pretty esoteric products like our fully balanced triode zero feedback OTLs, that we might be doing more listening than measurement or something like that. But to get to where we are takes engineering- and that requires math. If I can't make it work on paper, I have trouble with the idea that it will work in practice. I don't like it when things work and I don't have a good explanation- I remember the first time I heard a power cord make a difference and it really bothered me until I sorted out why. I'd love to be proven wrong on this capacitor thing but so far that hasn't worked out- we certainly *have* tried it and more than once with exactly the same results, 22 years apart. It doesn't work out on paper either, so I've maintained this position for a very long time.


One area that might be a variable is output transformers. They have capacitive effects of their own (both inter-winding and to ground) so it may be that in a transformer-coupled amp there is a benefit as one is compensating for a transformer. I've not worked that out as I try to stay away from transformers if I can.

Thank you for taking the time to answer the question. I'm just glad to see some folks take the time and the expense to experiment and share their results. Given that the results are system/circuit dependent, I guess I'll need to perform my own experiment, albeit an expensive one, and see how it goes.   
P.S. If I ever decide to get an MHDT Orchid, I know who to contact for advice or how to make it sound better. 

Ralph is far smarter than me and I would never argue with him 🙂.

A quality, educational discussion... Thanks. 

Audio research uses bypass caps and their products sound amazing. It must come down to personal preference. 

With my Rogue audio m180 darks they are traditional push pull transformer based tube  amps. I have tried both options, with and without bypasses. The coupling caps in my amps are 2.2 value and sound more open with greater extension and air with .22 value bypass caps added to the circuit. Without them it is more midrange oreintated and darker sounding. The treble stills sound clear and clean with good recordings, on my Revel 228 be speakers on the Chet baker "Chet" reissue you can clearly hear front  to back separation and his lips pressing against his horn without smearing, you can literally hear the brass instrument. Very clean and high resolution.  

If I am going to be in a minority, I don’t mind having Ralph from Atmasphere in my corner.  Over the years I have noticed that Ralph’s observations often agree with my own.  
That said, I am not sure I agree with Ralph’s implication that problems with bypass caps are more of a problem on highly resolving systems.  As I said in my post on the previous page, I have experimented with bypass caps for 40 years and my gear from way back then would not be considered highly resolving by today’s standards, yet the sonic problems with bypass caps were evident in all of my systems.

I think the different reactions to bypass caps are more related to different listening priorities.  To my ears the positive things that bypass caps can sometimes add can be described as “Hifi spectacular” artifacts—-bigger soundstage, more specific imaging and seemingly more inner detail.  Certainly I am not opposed to hearing more detail, but for me the “improvements” caused by bypass caps are artificial.  They come at the expense of a skewed tonal balance where the HFs are emphasized and have an artificial sheen or glaze, and the upper bass/lower midrange becomes lean and lacking in natural warmth.  The degree of skewed balance changes with different caps but the direction is always the same.  
In addition bypass caps make the sound less coherent: music sounds like it’s coming from a 3-way speaker rather than a single driver.
Furthermore, with a truly high resolving system I find I hear just as much inner detail without bypass caps.  The detail is just less hyped.

Some people hear and like the “Hifi spectacular” sound and perhaps don’t notice or aren’t bothered by the negatives.  Other people find the more spectacular sound artificial and find these qualities make the music less natural.  
At the end of the day, if you like the sound with bypass caps then go for it.  But you really should try removing the bypass caps at some point after they are fully broken in and you are fully used to the sound.  You might just find you actually prefer the sound without the bypass caps.

And that's why it's a personal preference, in my system bypasses made my system sound more real, not less. It has nothing to do with fake hyped detail. 

I tried the Duelund .01uF tinned copper cast bypass caps in three positions and came to different conclusions each time. 

I liked them on the .47uF interstage coupling caps of my Coincident Frankenstein 300b monos.  Thought the sound got more real and natural with them in the chain.  Couldn't hear them at all over the 1uF Miflex output capacitor on my Don Sach's 6SN7 preamp.  And heard the smearing effect when I put them on the 4.7uF cap (Clarity at the time) used as a high pass on my Coincident PRE's tweeter.  

I forgot to add that IMO part of the issue with bypass caps has to do with the size of the coupling cap you are bypassing. Generally speaking I've not liked what larger coupling caps do as they impose more coloration. But in some cases they have to be used- such as at the output of many tube preamps and also driving output tubes in a power amp. For this reason I've avoided coupling caps in these specific areas (instead I employ direct-coupling) as this allows me to use much smaller values overall. I can easily see resorting to bypass caps in the case of larger caps- 1uf and above in particular.

Exactly Ralph, my Amps use four 2.2 value caps per monoblock, one for each tube. When running just a single large cap the sound is definitely shelved off in the highs, scymbals sound rounded off for an example. Adding .22 value caps opens the amps up, while still sounding very clear and clean, it just adds some sparkle up top making the system sound less dark.