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Listening Impressions: PP Series- vs. Parallel-Feed

71.168.199.38
Posted on October 24, 2009 at 12:36:57
nullspace
Audiophile

Posts: 203
Location: Philly, PA
Joined: June 18, 2004
 Hi All,

So, over the last couple of days I've had a chance to compare series- vs. parallel-feed w/ the 2A3 pushpull amps I recently finished. One channel remained wired in parafeed and I converted the other to series-feed. I also set the source (PC-based) to downconvert to a mono signal.

Differences were pretty subtle. I think the parafeed brings a bit more transparency/ clarity, handled transients a bit better/ slightly more articulate. I thought this was particularly evident when listening to pianos and the leading edges of notes. The parafeed seemed to have an ever so slightly more extended freq response. I didn't notice much of a timbral difference, except that perhaps the series-feed seemed a bit more warm, which I attributed to the rounder, less-definied sound.

The one drawback to parafeed is what I perceived as coloration from the coupling cap; there was from time-to-time a bit of edginess, maybe almost stridency with voices. I didn't spend much on the caps in there now, they're from Erse. One way or another, I'll need to do something about that.

Overall, I'm not sure it makes sense to go the parafeed PP route. I think there is certainly some performace gains, but it's not a small cost for two pairs of plate chokes plus four nice parafeed coupling caps. If I had to do it over, I'd probably go series-feed. That said, the chokes are a sunk cost for me now, so I'll be sticking with parafeed, FWIW.

Regards,
John

Follow-Up, posted on October 25, 2009 at 10:11:48
nullspace
Audiophile

Posts: 203
Location: Philly, PA
Joined: June 18, 2004
 Today I had some time, so I hooked up one channel in parafeed, no parafeed DC-blocking caps, and lifted the center-tap of the output transformer. I understand that's not ideal and that there would be some unbalanced DC on the transformer, but it shouldn't be much. The EH 2A3s I bought are new and ostensibly 'matched', so I think it shouldn't be more than a couple of mA.

In a word, wow. Really, really nice. Top to bottom, an improvement over the channel with DC-blocking caps and the center-tap grounded. Definitely this is an improvement worthy of making the investment for the plate chokes.

I'm going to hook both channels up in 'normal' parafeed for now, just for caution's sake I suppose, but over the next couple of weeks I'll be heading down one of the differential paths outlined below in the previous thread. I'll post updates.

Regards,
John

RE: Follow-Up, posted on October 29, 2009 at 16:32:18
Tre'
Industry Professional

Posts: 4845
Joined: February 9, 2002
 "Today I had some time, so I hooked up one channel in parafeed, no parafeed DC-blocking caps, and lifted the center-tap of the output transformer. I understand that's not ideal and that there would be some unbalanced DC on the transformer"

I don't think there will be anymore unbalanced DC across the transformer than when it was series connected. It should be the same.


Tre'


Have Fun and Enjoy the Music
"Still Working the Problem"
RE: Follow-Up, posted on October 30, 2009 at 06:31:23
nullspace
Audiophile

Posts: 203
Location: Philly, PA
Joined: June 18, 2004
 Yes, that's correct. One aspect of the allure of parafeed'ing a pushpull output is zero, rather than very little, DC unbalance across the transformer. Mike posted his thoughts here. I've since measured both channels, and the worst of the two had DC-unbalance of <1mA, so not much here of concern.

That said, I've since put both DC-blocking caps back in, with the center-tap still lifted from ground, and it still sounds awfully good. It would seem a lot of the difference I heard is attributable to whether or not the center tap of the output transformer's primary is grounded.

Regards,
John

RE: Follow-Up, posted on October 30, 2009 at 09:00:28
Tre'
Industry Professional

Posts: 4845
Joined: February 9, 2002
 With the center tap lifted, you only need one parafeed cap. Either end, will do.

You will have no DC through the primary.

I don't see 1ma of DC doing much, if any, harm. But the cap will, sonically.

At least in theory.

I would listen, long term, both ways before making up your mind.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
RE: Follow-Up, posted on October 26, 2009 at 10:03:14
nullspace
Audiophile

Posts: 203
Location: Philly, PA
Joined: June 18, 2004
 Here are Mike's comments on how much unbalanced DC the S-240-A can handle: link. So, unless someone objects, I feel pretty comfortable that I have less than 9mA and I'm probably going to keep them hooked up this way until I put something together as a means to balance the DC. I think the first-pass will be ripping off Phil Sieg and just using a 50ohm wirewound pot with the Rk/Ck connected from wiper to ground. Then, replace the Rk/Ck with a CCS, and so forth and so on...

Regards,
John

RE: Follow-Up, posted on October 26, 2009 at 15:41:12
nullspace
Audiophile

Posts: 203
Location: Philly, PA
Joined: June 18, 2004
 Just add some details, measuring the DC voltage across the four EXO-003 plate-loading chokes w/ the parafeed DC-blocking caps in place, one channel measured .35mA DC-unbalance and the other .46mA. So, I'm pretty comfortable taking out caps and running the amps without them in place. Better to be lucky than good, I suppose.

EDIT: I'm guessing that the way to calculate the DC-unbalance is the difference in DC current draw of the two 2A3s. Should I instead, w/ the DC-blocking caps removed, measure the DC voltage from end-to-end of the S240A primary, then w/ DCR of the winding calculate DC-unbalance that way? A little help here would be appreciated.

Regards,
John

RE: Follow-Up, posted on October 31, 2009 at 05:36:18
Colonel Sanders
Audiophile

Posts: 3009
Location: Drinking wine
Joined: October 30, 2003
 I personally think that the ''complexity'' required to avoid DC unbalance defeats the purpose and kills the kill the beauty of the SA240.

I have lived with my perfectly unperfect Seth for quite a while now. I think I found happiness. I designed the Seth in 2004 after that, the obsession that started as hobby came to a halt.
Sounds like chicken!
RE: Follow-Up, posted on November 2, 2009 at 13:39:40
mqracing
Manufacturer

Posts: 3314
Joined: June 29, 2000
Contributor
  Since: May 7, 2004
 if it smells like... err... sounds like chicken... then the Colonel must have built it!!!

Still "down under"... got any silver to send me yet???

my initial attraction to the idea of parafeed PP was largely the product of wondering what effect if any having separate paths for the AC and DC paths would have... and if the resultant "improvement" in PSRR would make a difference...

like I said in an earlier post... I might have gone with SUPER plate chokes (i.e., much higher L's)... though this is not meant to fault the good work of nullspace...

and my other thought... that I was going to mention in a response to one of nullspace's posts... I'll say it here...

I think nullspace has only tried one brand (and one value?) of blocking cap... I would assemble a handful of candidates of varying constructions and give try out different brands, values, and cap materials.

Motor runs, Jensen oil\copper caps (available through K&K audio), old (i.e., vintage) oilers, etc.

Caps do have voices... but so don't transformers... and tubes... and perhaps even the choice of solder... so I don't interpret this as a "fault" or shortcoming of capacitors (no capcitoritis here)...

but it is worth exploring the variations... just as various brands and materials of resistors can make subjective differences in sound....

ya gotta try a few to hone in on the guys who are going to work best for your application and your ears.


cheers all,

Mike





Builder of MagneQuest™ & Peerless™ transformers since 1989
RE: Follow-Up, posted on November 5, 2009 at 06:49:13
Colonel Sanders
Audiophile

Posts: 3009
Location: Drinking wine
Joined: October 30, 2003
 Yep, still down under.

And I'll be here for a while as I just got my permanent residency.

The weather is fantastic around here.
Sounds like chicken!
RE: Follow-Up, posted on November 2, 2009 at 14:48:36
nullspace
Audiophile

Posts: 203
Location: Philly, PA
Joined: June 18, 2004
 Hi Mike,

I meant to post a follow-up vis-a-vis the caps... Currently, I'm listening to the amp set up PP parafeed with the DC-blocking caps in circuit, but with the primary's center-tap lifted off of ground, and am hearing none of 'the edginess, maybe almost stridency with voices' I mentioned earlier. So, more a consequence of the circuit and less the capacitor's fault, I suppose.

You've said a couple times that you would've recommended more H in the plate chokes; could you speak a bit more about this? I would've thought that if the EXO-003 is good enough for a 2A3 SE circuit, it would at least be in the ballpark for a PP application. I get the feeling that you think the EXO-003 isn't nearly enough here...

Regards,
John

RE: Follow-Up, posted on November 2, 2009 at 18:44:31
mqracing
Manufacturer

Posts: 3314
Joined: June 29, 2000
Contributor
  Since: May 7, 2004
 Hi John:

My sense of perhaps wanting to see a larger L plate choke has to do with that as the impedance of the plate choke becomes larger and larger it will have less and less effect on the effective impedance of the output transformer.

The L of the plate choke is going to be in most cases the limiting factor and even with our larger plate chokes it will most likely be the smallest of the paralleled impedances... hence it "limits" the net impedance that the anode of the tube will see.

But with more thought I am not now so sure that I would strictly urge you to abandon your 003's and go with something larger... the 003's themselves as you rightly point out should have enough L to adequately load a 2A3.

So allow me to modify my advice... and carry on forward with your present game plan.


Mike



Builder of MagneQuest™ & Peerless™ transformers since 1989
RE: Follow-Up, posted on October 27, 2009 at 09:32:41
danlaudionut
Audiophile

Posts: 3564
Location: Upstate NY
Joined: June 6, 2002
 John

Since you want to know the current draw
across the S240A primary then the method
you said about measuring the DC across
the primary would be the way to go.

DanL
RE: Follow-Up, posted on October 27, 2009 at 09:55:46
nullspace
Audiophile

Posts: 203
Location: Philly, PA
Joined: June 18, 2004
 Thanks Dan. The more I thought about it last night, the more it seemed obvious that is the relevant measurement.

Regards,
John

RE: Follow-Up, posted on October 25, 2009 at 18:17:40
gluca
Audiophile

Posts: 749
Location: SomeWhere Islands
Joined: February 17, 2005
 Not sure I got your idea properly. What did you lift?

RE: Follow-Up, posted on October 26, 2009 at 04:32:34
nullspace
Audiophile

Posts: 203
Location: Philly, PA
Joined: June 18, 2004

Here's what I did, Gianluca.

RE: Follow-Up, posted on October 26, 2009 at 12:53:58
gluca
Audiophile

Posts: 749
Location: SomeWhere Islands
Joined: February 17, 2005
 You really need a cap at the OPT center tap as discussed a few days ago. Keep us posted, interesting subject.

Gianluca

RE: Follow-Up, posted on October 27, 2009 at 15:14:50
Tre'
Industry Professional

Posts: 4845
Joined: February 9, 2002
 Why does the center tap need to be connected to anything?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
RE: Listening Impressions: PP Series- vs. Parallel-Feed, posted on October 25, 2009 at 09:34:43
braubeat
Audiophile

Posts: 56
Location: San Diego
Joined: May 20, 2005
 Hi
I am not sure what circuit you are using but normally you do not really need papafeed blocking caps in a PP circuit.
Michael

RE: Listening Impressions: PP Series- vs. Parallel-Feed, posted on October 25, 2009 at 09:57:50
nullspace
Audiophile

Posts: 203
Location: Philly, PA
Joined: June 18, 2004
 The project is discussed here: 6EW7_2A3 Parafeed PP.

Erse caps, posted on October 24, 2009 at 19:13:19
tweakydee
Audiophile

Posts: 211
Location: Gainesville Virginia
Joined: February 27, 2004
 Was it your coupling caps that were Erse, or the parafeed caps?. Or did you use Erse for all of them?

RE: Erse caps, posted on October 25, 2009 at 07:02:31
nullspace
Audiophile

Posts: 203
Location: Philly, PA
Joined: June 18, 2004
 Both coupling and parafeed caps are Erse.

Regards,
John

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