" Permalloy is used to create transformers that are very sensitive and responsive to low level current stimulation. Delicate, small signal information that conveys color, texture, and breathy little details of tempo and space are recovered, that are completely lost in ALL 'low nickel alloy' cored transformers. If you are not interested in hearing the other 50% of the musical information that you never knew was on the disc, don't buy permalloy. Remember, what you don't know can't hurt you. Hoever, if you are curious and can afford the hefty price tag (US$430-US$800 each) don't hesitate. "

Herb Reichert in Sound Practices Magazine Issue 11

Impedance : 3K : 4/8/16ohm

Primary Inductance : 350H at 115Vrms, 50Hz, no DC Current

Frequency Response* : 10Hz - 40KHz at -3db 1Wrms/8ohm

Power Response* : 15Hz - 36Khz at -3db 10Wrms/8ohm

Recomended Tubes : 2A3, 300B, Triode connected EL34, KT88

*measured in 300B amplifier using 3.9uF parallel-feed cap and 32H Anode Choke

For those of you who are familar with Single Ended Tube amplification, it couldn't be less agree that Westerrn Electric 91B is the amplifier responsible for shooting Single Ended amplifier back to fame again. Having heard a WE91B clone using a pair of original A171 output transformer (OPT), WE310 pentode driver and WE300B, I finally understand why it was so famous. The midrange process a very seductively rich tone that draw you to music emotionally. An excellent match for playing vocal oriented music.The high is a little soft but is free from any edginess. Also, the bass is a little booming lacking definition. However we have to remember this is fair as the acceptable audio frequency range back in the olden days is only from 40Hz to 14Khz.

During our prototyping for a Parallel-Feed OPT, we made many winding design in order to get the last extension in high frequency. However, the listening sessions does not equate the nice square wave measurement that achieved. What we later found out is that the coil design that produces the best square waves is also the one that sounded the most closed-in. In general, permalloy core in parallel-feed design produces a very clean sound with emphasis more on the two frequency extreme. To achieve a more neutral tone, we started studying the WE91B which is the opposite of what we are getting. We noted that a slight attenuated in high frequency extension produces a more open soundstage with a warmer midrange. This is confirmed by our own panel of audiophile friends who listen and agree that this is a better approach than what the measurement have suggested.

Armed with this little success, we concentrated in reducing the high frequency extension to the point where it does not compromise it's electrical performance. Single-Ended design does not need super frequency extension into 100KHz range since there is no feedback loop to cause circuit instability. What we have arrived here is a Permalloy Output Transformer that is not cheap, but truly value for money at this down to earth price.

Impedance : 2.5K : 4/8/16ohm

High Frequency Limit :

30.6KHz at -3db 1Wrms/4ohm

54.7KHz at -3db 1Wrms/8ohm

71.4KHz at -3db 1Wrms/16hm

Power Response : 20Hz - 54.7Khz at -3db 3.5Wrms/8ohm

Recomended Tubes : 2A3

In ideal case, an output transformer should transform all the energy generated by the high impedance output stage to the low impedance speaker load at full bandwidth without any phase shift. However in practice, it is neither a full bandwidth nor non-phase shift device. Many output transformers have specs that defined super bandwidth and minimal phase-shift but have yet to define any standard measurement for small signal sensitivity*.

STF2.5K is designed with 2A3 triode in mind with the best balance between electrical and listening test. Being non air-gap and the inherent super high permeability of the Permalloy core material, it is very sensitivity to any small voltage variation in the amp's output stage. What you will notice is the excellent reproduction of the low-level details which low permeability silicon steel core is unable to convey despite the claim of super bandwidth.

STF2.5K is very easy to implement in a upgrading amplifier or a new project. All you need is a good parallel-feed coupling capacitor and a plate choke. You can either use the existing output transformer as a plate choke or order a purposely design plate choke from us. For further technical enquiries, please feel free to drop us an email.

*small signal is refer to actual music signal and not a sinewave. The sensitivity should cover the small signal presence in loud music passage.

Operating Impedance : 3Kohm

Operating Inductance : 25H at 94Vrms, 50Hz, 60mA

Maximum Operating Inductance : 25H at 94Vrms, 50Hz, 60mA

Typical DC Resistance : 140ohm

Net Weight* : 1.1 Kg / 2.5 lbs

*This is not the actual shipping weight

An Anode Choke is designed to provide a stable inductance over a range of direct current. It should be able to handle all the signal voltage swing that is generated across it. A power supply filter choke should not be used as an anode choke since they usually cannot handle the high signal voltage swing and their resonance usually fall inside the audio frequency range.

Every Anode Choke we designed is extensively measured and handmade in the same way we produced our Output Transformer. Audio grade silicon steel core used for Output Transformer are employed in our Anode Choke. High quality UK wire and high voltage insulation are used for the coil windings to ensure maximum safety and reliability. Split-Bobbin is used to reduce capacitance in windings and bring the resonance out of the audible range. A special in-house design equipment capable of precise inductance measurement while passing the stated direct current is used to adjust the air-gap of the Anode Choke to the desire inductance.