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AudioNemesis

pA-1M AUDIO AUDIO POWER AMPLIFIER:

Echidnas (sometimes also Ekidna) are unusual animals living in Australia and New Zealand: they are small

egg-laying (primitive) mammals that are covered with coarse hair and spines. So, it's a very "strange"

animal, it's a "hybrid". And it's very "nice" and "sweet".

We at AudioNemesis called ECHIDNA

used for the first time in this model PA-1m. Echidna puts together, in an original form, a few known circuits

and ideas, creating a very unique simple topology.

Echidna idea is simple: a triode voltage gain stage plus a bipolar current gain stage, without loop feedback.

However, this hybrid configuration is unusual from many points of view.

Triodes are the best devices for voltage gain. Nothing is more linear than a triode. It's the only device

which can be used without any kind of feedback, assuring a very low distortion. Bipolar transistors are the

best devices for current gain. They can work only using local feedback but they can allow very low

distortion also in simple topology, if well designed, and very high current.

Hybrid designs usually have triodes plus Mosfets and not bipolar transistors. New York Audio Laboratories

(NYAL) in early '80 proposed the Moscode topology and since then it seems that the only way of doing

hybrid designs is triode and Mosfets. Recently, a few examples of hybrid amps with bipolar transistors

appeared on the market (and DIY magazines).

Echidna is a very special configuration with only 3 active devices in series to the signal

passive. So, there are only 7 components in series to the signal

only 7 components in series to the signal!

only 7 components in series to the signal

The output stage is made by bipolar transistors only. It's a complementary configuration composed by the

output devices and the driver devices. As output devices we chose the best available audio power

transistors: they are made by Sanken and they are ring emitter type. These devices are able to supply 17A

DC, and up to 35A for 10ms. This outperforms any requirement in real applications.

As driver devices we chose a very linear device, with 100Mhz Ft and a very low capacitance (ensuring low

distortion in real life loads), made by Toshiba.

This output stage is unconditionally stable

unconditionally stable and it has low output impedance assuring a high damping

unconditionally stable

loudspeaker, for better bass response.

A DC servo circuit assures an output DC voltage near zero without output capacitors.

Linearity of base current of output devices is essential for the best audio performance. So the voltage gain

stage must be able to drive the output stage with high linearity of base current.

Triodes are the most linear devices for voltage gain. But usually they don't have much current capability.

The standard common cathode stage (standard gain stage) is linear if properly designed but cannot

guarantee the current we want for the best performance and proper driving of the output stage. For this

reason it is very common to use a cathode follower stage after the gain stage. This configuration is standard

and it is used really often. We don't think this stage has the right sound, so we designed a driver stage using

two triodes in a single very fast stage, able to drive the current base of the output stage in a linear way.

PRODUCT PHILOSOPHY AND DESIGN

ECHIDNA our original topology for a hybrid power amplifier, developed and

ECHIDNA

only 3 active devices in series to the signal. All other devices are

only 3 active devices in series to the signal

high damping

high damping of the

high damping

M_PA1m REV.01 ENG OCT 2009