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Aikido Parafeed  2010

 

The Aikido schematic for a linestage/headphone amplifier by John Broskie has been around for years and has gone through several iterations. It always consists of a totem pole configuration of two double triodes for the first stage followed by a WCF (White Cathode Follower) for the second stage (in case you are using the schematic as headphone amplifier... the WCF is able to deliver a higher current swing needed to push low impedance headphones). There is extensive documentation on it available with nice tables of all relevant parts values for a wide variety of tubes.

  

 

 

 

I choose the 6CG7 for the first stage and the russian 6H30Pi for the second stage. Having built two OTL amplifiers before I wanted to avoid the largish output capacitor and planned to build it as Parafeed amplifier. Luckily I found some information about an Aikido Parafeed on the internet and, for the first time, I had the amplifier on a breadboard first for testing and evaluating! In fact I definitely recommend to always build a schematic on a breadboard first because it makes changing parts and troubleshooting much easier. I got the noval boards from Broskie but found out the hard way that they are not mirrored so they cannot be used face to face!! Yowzers!

That was not to my liking at all since I definitely wanted a symmetrical top plate, so after thorough consideration I began to rewire one board to basically use the bottom side of the board. In the end all the rewiring was quite easy and has not caused me any trouble ever since! This is probably the worlds only mirrored build using the original noval mono boards ;).

 

This was also my first amplifier with a regulated power supply. For a very short time I was testing it with a Maida-style regulator but soon replaced it with my first Salas shunt regulator, the V1.0 wired P2P on veroboard . I later "upgraded" the reg (after some stability issues probably caused by my layout or the semiconductors used) to the Salas shunt V2.0 with better layout but still P2P. The power comes from a generic 55V toroid with the secondaries wired in series and followed by a voltage doubler and then followed by the prefilter section. Another toroid takes care of heater duties done via an LT1084 regulated supply delivering a hefty 2800 mA of heater power.

On the breadboard I was using cheap Hammond line transformers (119DA) that worked surprisingly well even in the bass department. For the final build those were exchanged to custom wound 10k : 300 Parafeed transformers from AE Europe in the Netherlands. The case was fabricated with CNC cut panels bolted together with custom M8 Allen bolts and then painted in the matte grey body colour of a Mercedes SLS.

 

Sonically this was the first self built amp that delivered what I expected from a really good amp! I would describe it sounding tonally balanced with very good resolution even of finest details, a wide soundstage, natural timbre and tight bass. It is a monster of a head amp with its external psu and I will probably not build something big like this again... but you never know in this hobby ;)  

 

                                                                                 

 

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