The Horus 2A3 Silver
Parafeed Monoblock Amplifier
by Jean-François Lessard
This parafeed amp was initially designed as a trial to uncover the merits of the parafeed output transformer. All the raving I read on the net finally got me curious enough to give it a try. I decided to go the Full Monty when I bought a custom MagneQuest EXO-35 wound with pure copper Teflon coated wire at the primary and silver wire at the secondary. Following some judicious advice by Gordon Rankin, I decided to take a shot at using a nickel grid choke for the power tube. The result after much cutting, metal punching, filing, and soldering, was this outstanding pair of monoblocks that really had me rethinking about the limitation of SETs (underdamped bass, etc).
Construction [click above graphic for full size 800px]
I decided to use the two stages of a 5687 as an input and a driver. I heard this tube in a really nice 45 amplifier I had lived with for a while and decided to try it with the 2A3. The input and driver stage were designed by Christian Phaneuf from Experiences Sonores (I must add my most sincere thanks to this kind man who taught me so much).
Actually, I initially wanted to use a VT52 instead of the 2A3, but lack of space in the chassis prevented me from adding another 6.3V transformer without inducing too much hum in the two filtering chokes. So out with the VT52 and in with the 2A3. I had some NOS RCA lying around, doing nothing but collecting dust. They did a credible job but have since been replaced by the AVVT 2A3 Mesh Plate, which sounded much better in this amplifier in my opinion. In addition, the only rectifier I can recommend for this amplifier is a Mullard GZ34. I tried the 5U4G, GZ33, GZ37, and 5R4 potato masher. In the end, the clear winner was the GZ34 in this application.
The circuit is actually a really classic one with tube rectification as mentioned above. The only extraordinary things (if I can describe it like this) are the grid choke and the custom EXO-35. If you build it with the specified parts you can call it my design (Any substitution and it will become yours, fair enough?). I know I am paraphrasing Herb Reichert here but with a simplistic circuit like this one, every swap of parts leads to important changes in the sound. I find that this amp sounds outstanding with the parts specified. I cannot say for sure how others will affect the sound. For sure, the heart and soul of the amp is the nickel grid choke and the custom EXO-35.
You should consider using a 1/8 inch thick copper top plate, a 10 AWG ground buss, and ground it at one point only on the copper plate (I grounded it at the RCA input jack). I also preferred ceramic sockets for performance as well as appearance. I ended up only using Teflon coated silver plated copper wire in this amplifier. I used my traditional wood base which complimented the copper plate top very well. The top plate was sprayed with a scratch resistant paint by a specialized shop to give it protection from fingerprints and the elements. I used DC on the 5687 heaters and AC for the 2A3. I tried to put as much space as possible between the output transformer, the plate choke, and the power transformer to minimize interaction. I managed to have a 2mV residual hum which was OK in my book.
This amp, as described, will give you outstanding bass response and will be lightning fast. It was tuned on my Lamhorn 1.8's from RL Acoustique with the AER MK-1 full range driver. I can't say enough good things about this driver and the Lamhorn 1.8 enclosure. These speakers have brought me to the next level of sonic bliss, offering a direct communication to the soul and essence of the music itself. I have auditioned many horns and most are horribly colored; the Lamhorn 1.8, however, belong in the top echelon of speakers designs - and the AER driver is an instant classic.
I do not recommend using this amplifier on any speakers with an efficiency under 96 dB (99-100dB better yet). Doing so would probably loose the fantastic dynamic and transient handling a grid choke and parafeed provides thus defeating the very objective of this fine amp.
The dynamics and transient handling on these amps are on a par with a really good OTL. You actually feel the drummer right with you in the room! The attack on any note is right on. Having a listen to Dave Brubeck's "Take Five" on Time Out is a treat now. You hear all those small details within the recording without superseding the music in any way. In addition, ordinary and even poor recordings can be played with this amplifier without you wanting to run out of the room.
This is the SET for those like me who do not wish to spend the rest of life listening to 99% jazz and chamber music. With these amplifiers (providing you have the right speakers), you'll finally be able to listen to other things than Patricia Barber and Sarah K. Be prepared to get that dusty Led Zeppelin II album out of the closet and start to play "air guitar" on Heartbreaker (my wife nearly gave me a heart attack when she came up behind me while I was doing an air drum solo with my closed eyes, and screamed BOOO!). One of the strong points of this amplifier is that you loose the halo around the vocals usually wrongly associated with warmth. You no longer feel that the singer's voice is coming from a giant mouth three feet wide! The sound is still warm but not larger than life; in other words, instruments and the human voice take on their proper proportions.
In short, use this amplifier with horns and it should put a big smile on your face. Quite frankly, I have to say this is one of the best SET amps I have ever heard, bar none. On the other hand, if you use it on low efficiency loudspeakers…don't expect to get these same results. However, if you choose to go this route, do not even bother to email me to tell me "this amplifier stinks" since you will have been warned! I will be glad to hear about happy or unhappy builders (providing they have appropriate speakers). Please let me hear from you at my email below. I would also like to hear about any tweaks, modifications, and improvements you have found work well. As in all DIY projects, it's a learning experience, and all circuits can be optimized. As is, I feel I have hit on a totally unique amplifier that brings out the best qualities of the SET amplifier, without many of the weaknesses.
I hope you'll enjoy this amplifier as much as I do.
May 9th 2001
NOTE: Of course, there had to be the usual disclaimer; if you are clumsy and do not know anything about electricity please stay away from playing with tubes circuits and high voltage without learning the basics and being cautious... carelessness may kill you!
[Jean-François Lessard's "hand-drawn" schematic]
T1 MagneQuest MQR 5500 Universal power transformer
T2,3 Hammond choke 3 H 100 mA (MagneQuest would be a good substitute here)
T4 Custom made MagneQuest EXO-35 with silver wound secondary
T5 MagneQuest EXO-03 Plate Choke
T6 MagneQuest BCP-16Ni Grid Choke
R1 Allen-Bradley 90 ohms 2W
R2 Allen-Bradley 470 ohms 2W
R3 Dale wirewound 2.7K 10W
R4 Dale wirewound 13K 10W
R5 Dale wirewound 6K 10W
R6 Dale 100K 10W
R7 NEI non inductive wirewound 1K 50W
R8 Holco 330K 1/2W
R9 Any 10 ohms 1/4W
R10 Alps 100K Black Beauty pot
C1 Solen Polypropylene 1.5uF 630V
C2a&b Elna Cerafine 100uF+100uF 500V
C3 Solen Polypropylene 47uF 630V
C4 Black Gate electrolytic 160uF 100V
C5 Anstron tin-foil-in oil 0.1uF 400V (Jensen or AudioNote copper in foil are good substitutes)
C6 Two Anstron tin-foil-in oil 0.1uF 400V in parallel (Jensen/AN copper in foil are good substitutes)
C7 Two SCR Tin foil and film 1.5uF 630V in parallel (Mike Lafevre suggests between 4 and 5uF)
C8 Solen Metallized Polypropylene 10 uF 630V
C9 Any small value ceramic disk capacitor (approx 0.01 uF)
C10 Any electrolytic 7000 uF with the appropriate voltage rating
V1 1/2 5687
V2 1/2 5687
V3 Mullard GZ34
V4 AVVT 2A3 Mesh Plate