Some of Bruces thoughts

[Ski Bum]

Not in agreement on the idea that parafeed (as Doc B. used the term) do not have the clarity of traditional SET. Quite the contrary in my experience, actually. I built Lessard's Horus....what was it, something like ten years or so ago now? And discussed my observations about it's absolutely stellar performance in terms of those very elements - extreme clarity compared to the other SET amps I had been using before I built the Horus -- and then a modified version of the same circuit. The other amps were the classic Welborne Labs Moondogs, which I still own. I also bought the Moth Audi si2a3 from Moth Audio's Craig U. (now of Eddie Current) in the form of a chassis, transformers, and box of parts. The Moth is direct coupled, and came closer to the sound of an OTL, but not so much as the Parallel feed Horus.

DRDs are also very fine, and I built both the 2a3 and 45 versions for users in other countries. DRD combines direct and interstage coupling, along with the so-called Ultrapath connection on the output. That was designer Jack Eliano's term for it, but it is not new (nor is parafeed, for that matter). Both go back to the very early years of tube audio. I mentioned all of this years ago here, when writing under another handle.

Sorry Eric, I was referring to the Vaughn authored pieces, mainly the second one where he goes into some detail about the benefits and limitations of the various types of opt for pp vs. se amps.  The inference (which I drew) was that the parafeed type would suffer the same sort of zero-point hysteresis as pp types.  Happy to be wrong about the sound in that case.  As to the terms "kooky clarity" and "better" and such, I'm in full agreement.  Those terms could very well be referring to high distortion, given my preferences and what I've seen on the scope!  I do prefer SE types, but the desire for stronger bass has me wondering about the parafeed approach (larger room these days).

 

Do you still have all the amps you've built?  Care to sell the Horus to a lazy bum?  (Just kidding.  Ok, half serious.)  There may not be much discussion here, but when Maynard and Eric are contributing, it's quality.  Thanks, dudes! 

[erik2A3]

How I wish I did have that amp, configured of course as monoblocks. Of the single-ended transformer coupled amps I own, it was the one, along with the Moth si2a3 close behind in terms of performance, that simply did not sound like a SET amplifier. I even experimented with a design that combined some of the best features of Eliano's DRD circuit with the Horus, which, while quite good in its own right was admittedly mildly out-performed by the other two as originally designed. However, there was one important feature of the DRD that I ultimately retained in the Horus, which I suppose transformed it into a sort of non-Horus Horus. The signal flow of this thing at the output stage became complicated, but the end result was something I won't forget because of its sound quality - for me.

The stock Horus parafeed amp is truly extraordinary, but not inexpensive to build. The MagneQuest parafeed OPTs, output stage plate choke, and 5687 input/driver grid choke are an investment, but the quality of theae critical parts is very high. I remember some criticism of parallel feed amps in general at the time, where some thought of them as inferior because of their comparatively smaller output transformers. What that revealed, however, was a lack of understanding of how these amps work. The last thing for which the Horus could be blamed is a deficit in frequency response.

I sold the amps to another forum member years ago, and to this day remains a big regret. The Horus was inspired in part by the work of other Parallel-feed designers such as Dan Schmalle (Bottlehead) and Gordon Rankin (Wavelength Audio), but the Horus without a doubt has important traits that are unique. Bottlehead sells very well-put-together kits, which make a step toward parafeed much easier. You might have a look on their website to see what they have available right now. Bottlhead also used to be a distributor for MagnaQuest, but don't think that's the case now.

Anyway! if you are interested in SET, parallel-feed designs are one of the very best ways to do it...IMHO.

Edited July 28, 2016 by erik2A3

[tube fanatic]

For those interested in a little background on what a DRD amp is, you will find some thoughts here:

 

http://www.enjoythemusic.com/magazine/equipment/0206/welborne_laboratories_300b.htm

 

The issues of which type of circuit sounds "best" can be debated until the cows come home!  I don't have a problem with circuits utilizing output xfmrs, and don't necessarily agree that no transformer is better than having one (same for capacitors in the signal path).  If selected appropriately, and used in a way which allows them to perform at their maximum capability, they are not detrimental at all.  This was highlighted by George Anderson who ran measurements of some budget opts years ago:

 

http://tubelab.com/articles/component-testing/budget-output-transformers/

 

Even budget transformers are capable of phenomenal bandwidth and very low distortion.  It's just a matter of designing a circuit which allows the transformers to "disappear."  Conventional SETs can offer very low distortion regardless of the varying speaker impedance, do not have mushy bass as some claim, and can sound really phenomenal.  Conflicting opinions are equally valid!  It's what makes doing this type of work so much fun!

 

 

Maynard 

[erik2A3]

Don't think anyone ever said OTLs are inherently or universally better except on a person by person basis. Were that not the case, I don't think designers like Ralph Karsten. Bruce Rozenblit and others would have gone to such lengths to design extremely well-reviewed OTL circuits.

There are obviously all kinds of listeners, Maynard, and we all have personal preferences. I would say that OTLs are arguably more challenging to build however. Same with caps, regardless of quality or cost. There are those designers who would just rather do without them if and where possible. Doesn't mean one is better or worse than something else; it's absolutely and entirely a personal preference sort of thing and nothing more.

Jack Eliano's very first DRD is also available in online VTV articles (I think!?) I was fascinated with it when I received that issue, and it's not difficult to build. Moreover, even with AC filaments on the output triode, the things are inherently and amazingly hum-free.

[erik2A3]

So here you go:

http://www.electra-print.com/schematics.php

[erik2A3]

Further:  The DRD topic is of interest to me, and we discussed it extensively in 2-channel so many years ago it amazes me.  I, along with a couple of members had built the DRD Welborne kits, and posted pictures of parts of the process.  All those images are on a flash drive somewhere, though maybe the posts are archived here someplace.  I also wrote a lengthy bit of text outlining the output stage of the circuit to which the designer referred to as "Ultrapath."  Far too long to repeat again, but it did show up here awhile back attached to another forum member's post who referenced it back when I was posting under my full name (beginning on this forum in about 2002).

 

An important feature of the Ultrapath connection (as it was called) was initially the use of a high quality motor run capacitor, which as chance would have it, was also used by PWK and company in the original heritage crossovers.  DRD deaigner found the motor run caps to be particulalrly effective and sensitive at lower voltages, and that they proved at the time to be more sonically transparent, allowing for transmission of small but important details.  I, as have numerous other DIYers, have also put good motor run capacitors to very satisfactory use in smoothing filters in B+ (high voltage) power supplies and output stage cathode resistor bypass positions.

 

I'll look for the pictures of the DRD kits I worked on (not for myself, I built them for others).  It's really a very clever design, and one where all sections, as Elliano explained, "shake hands with each other." 

[sunnysal]

J F Lessard made some really great amps, selling any of them is always a source for regret.  I love my 2A3 PP amp and assuming it doesn't blow up it will be my last amp!  T