Constant (or Compensating) Current musings

[TadS]

Is it just me, or does moving to a CC source attenuate the breathiness of the amp/pre-amp?  I have three BH products (Foreplay II, SEX2.1, and Quickie).  I have purchased the constant/compensating current boards for each, but I am reluctant to install them in the latter two builds based on my mixed perception of benefits in the Foreplay.  I had the FPII for about a year before I installed the C4S boards, and my impression was that the bass was much improved, cleaner and more realistic.  However, I felt like the high end lost some of its "realism," tube-iness, or breathy quality.  I hope I am describing this quality understandably.  I typically shy away from audiophile speak. 

Later when I built my SEX2.1 amp, it brought back memories of that original, articulate sound, particularly on cymbals, whispers, and the like.  It is a sort of "live" sound quality that I also get from the from the Quickie.  I have spent a fair amount of time comparing the Quickie head-to-head with my old FP and there is more "clarity" in the top end with the Quickie, but the bass seems better controlled on the FP.  Also, though the FP is not noisy, the Quickie is dead quiet.  So, my question is: will my Quickie and SEX amp lose some of the magic like the FP did?  The change was subtle, and I _did_ leave the C4S boards in the FP.   The boards remain in the FP partly because the loss was small, and gain in the bass was significant.  I know what obvious answer is: try it and see.  I have already built up the boards and probably will install them, but I don't feel strongly compelled  to.  I just thought I would ask whether others have noticed a decrease in the tubey goodness when installing CC boards.

--Tad

[Paul Birkeland]

  I just thought I would ask whether others have noticed a decrease in the tubey goodness when installing CC boards.

Hello Tad,

Yes, in a sense there is an attenuation of the "tubey-ness" of a circuit when CCS's are in place.  This is (IMO) for the general reduction in distortion brought about by the load impedance offered by the CCS. 

It's hard to say what's going on up top, I'd imagine that PJ will chime in here any minute...

[Paul Joppa]

Teomore replies while I was typing - so, with some overlap, here's my take on it:

Well, you're right about the standard answer! I can only usefully address the technical issues I know about, but in case it helps, here thy are:

The raw effects of a current-source plate load are primarily a reduction in distortion of the stage, and except for cathode followers, an increase in the isolation from power supply noises. These effects play out differently in the three designs mentioned.

In Quickie, the above effects are pretty much it. The power supply is of course not very noisy to begin with, so that's not going to be a big contributor. The distortion of triodes predominant second with monotonically decreasing higher orders) is widely thought to have either an innocuous or even a positive effect; there are several theories around for why but not any really knowledge-based consensus. Some people call it a "tubey" sound, and I think opinion is about evenly split on whether that's a good thing or a bad thing. Of course, at some point too much tubiness becomes just muddiness - it's just very difficult to know how much is to much, or where the sweet spot is.

In Foreplay, the second stage is a cathode follower, which has local negative feedback and thus creates its own distortion harmonic issues. Presumably the greater linearity of the current source load reduces those distortions, but their interaction with the first stage distortion harmonics is difficult to analyze intelligibly.

In SEX, the second-stage distortions are of similar level but opposite phase compared to the first stage. Consequently the even-order distortions are reduced by cancellation, while the odd orders are increased. Thus the harmonics do not die out monotonically. With a C4S on the first stage, the second stage is allowed to dominate the distortion spectrum, giving something closer to the single-triode character.

For the above reasons, I would not rely on C4S experience with any one of these circuits to predict the effect on another.

[aragorn723]

While I can't speak towards the technical side of this topic, breathiness is something i'm after also.. Here are some of my experiences with that, hope it is helpful.  Original system:

Audio Electronic Supply AE-3 (6SN7 tube preamp made by Cary Audio)
Rotel CD player
Inter-M R500 amp (170 X 2)
Epos ELS-3 bookshelf speakers

Current System:
Bottlehead Quickie with PJCCS
Rotel CD player
Inter-M R500 amp (170 X 2)
Definitive Technology BP6B tower speakers

The first system definitely had a lot more breathiness, but had a noisier background, due to the AC powered tubes in the preamp.  Honestly, when it comes the current system, i'm not completely convinced the Quickie is the issue as far as breathiness.  I tried using a couple of different sources, and found the Quickie to be more revealing than the AE-3, and there were 3 different configurations:

1)  Rotel CD Player as source
2)  Macbook playing itunes with an analog audio output thru a 3.5mm to rca cable (with lossless files)
3)  Macbook playing direct into a Audioengine d1 dac thru usb (with lossless files)

Changing the source imho changed the breathiness even more than the preamp (though it's probably a contributor).  Of the 3 sources, the audioengine was the best (incredible detail, breathiness, and bass definition/depth), followed by the rotel cd player, then the macbook with analog audio output.  If you have a few sources you can try, my advice is to experiment with your sources, and see if it doesn't give you the sound you want.  To me the Quickie boosts whatever signal you give it, and reveals any flaws in source material.  Hope this helps!

Dave

[TadS]

Thank you all for your input.  Sorry for the delay in me getting back to the thread I started.  As I have continued to try to describe the sound that I was missing after C4S installation into the Foreplay, I think--as PB said--it could accurately be described as a reduction in distortion.  Because it is largely an even order harmonic distortion that doesn't sound abrasive, I suppose reducing it can seem like a loss of source material in those upper registers.  Another word to describe the change I heard after installing the C4S in the FPII is "flat."  It seemed less lively, but still more lively compared to my previous cheap solid state preamp.  Anyway, as I mentioned, I left the C4S in due to the cleaner low end.  It has remained in my living room for about 12-13 years.

Perhaps, the amount of this effect that is agreeable vs when it becomes too much is dependent on other components in the system.  The system with the FPII (or now Quickie) is in a large-ish room driving some very flat, inefficient, speakers via a SE output stage MOSFET power amp.  Having a little life or breath is enjoyable in that solid state setup, though too much would start to sound muddy and bloated.

Paul's comment that the change in sound produced from a constant current source in one topology would not necessarily portend what would happen in another was enough to nudge me into installing the PJCCS in the Quickie this morning.  All I can say is "Wow!"  Top to bottom it sounds better.  Maybe a hair of that "magic" is attenuated, but it is not missed.  The cleaner top end has widened and deepened the soundstage.  Additionally, the mids and lows seem more focused and real.   Thanks for convincing me!  Looks like I will be installing the upgrade in my SEX2.1 next weekend.

--Tad