Pathos InPoL Legacy

Pathos was unofficially born in 1994, thanks to the Twin Tower, the first amplifier with the innovative InPoL circuit (called TT after 2001). Engineer Gianni Borinato was working on an electromechanical design, and he realized that the circuit not designed for the audio field would sound.

He asked his friend Gaetano Zanini, an audio dealer of high-end products, to listen to it; Gaetano was surprised by the sound quality, the transparency, and definition of the sound, and the natural unforced dynamics, which equaled and even exceeded the characteristics of other quality amplifiers.

A comparison with some hi-fi gurus confirmed what Gaetano had already heard.

Gianni Borinato and Gaetano Zanini involved the designer Paolo Andriolo to produce this new amplifier. They also found Pathos Acoustics in 1995.

Participation first at Top Audio for the Italian market, and then at CES in Las Vegas for the world market, immediately procured orders all over the world, sanctioning the success of the Twin Tower and the innovative InPoL circuit.

InPoL: don’t call it hybrid

Pathos aimed to make an amplifier that would preserve the “natural sound” of tube amplifiers, which was not, however, subject to the final tube calibrations and high feedback rates. In addition, Pathos aimed to achieve a higher current delivery capability than a pure tube amplifier.

The partners decided to make a tube preamplifier stage that would provide all the necessary voltage amplification (gain) followed by a unity-gain buffer, using a MOSFET that copies like a pantograph the output signal of the preamplifier stage by supplying all the current required by the load (loudspeaker).

Thus, they realized an all-class A amplifier that employs a minimum number of components operating on the audio signal (only two tubes and a MOSFET) without the use of output-input feedback. (pic 1)

At rest, the current flowing through the MOSFET is equal to the constant current generator one. Applying Kirchhoff's first principle to node P, we have that the current 𝐼𝑐 on the speaker is 𝐼𝑀−πΌπ‘œ =0

When the positive half-wave of the audio signal arrives at the gate of the MOSFET, the MOSFET increases the conduction and the IM current increases as well (assuming it is now 𝐼0 + 𝐼𝐼𝑁𝐢).

The generator current is always I0 so the current on the speaker is 𝐼𝐢 =𝐼𝑀 −𝐼0 =𝐼0 +𝐼𝐼𝑁𝐢 −𝐼𝑂 =𝐼𝐼𝑁𝐢 that is, it is equal to the current increase in the MOSFET.
Similarly, when the negative half-wave of the signal occurs, the current on the MOSFET decreases 𝐼𝑀 =𝐼0 −𝐼𝐼𝑁𝐢 and the current on the speaker will be 𝐼𝐢 =𝐼𝑀 −𝐼0 =𝐼0 −𝐼𝐼𝑁𝐢 −𝐼𝑂 = −𝐼𝐼𝑁𝐢 𝑉𝐼η = 𝑃 𝑐 · 1 0 0 = 2𝐴 · 02 = 5 0 %

Whileworking,theinductormakestheMOSFETworkinnegativevoltagesupto− 𝑉𝐴. This is the so-called InPoL (linear pump follower) circuit. (Pic 2)

In this circuit, the power supply is no longer dual, and if the inductor was ideal, the theoretical maximum efficiency would be  π‘ƒπ· 𝑉𝐴·πΌ0 that is, twice as much as the circuit in Figure 1, which uses an active component to make the current generator.

This value is the double of traditional Class A amplifier, which has an efficiency of the 25%.

The capacitor C is used to eliminate the DC component on the load. This circuit allows designing amplifiers of not excessive size with power ratings up to 30-40 W.

The inductor in the InPoL circuit must have an adequate value because it must store the energy necessary to supply the current on the load during the negative half-wave, even at very low frequencies, and it can thus be compared to the ideal current generator of FIG 1.

This fact implies that the inductors must have high inductance values. Furthermore, in the InPoL circuit 
the BIAS current flows through the inductor, and the inductor must be of such a size as to avoid saturation of the core.

Amplifiers with higher powers would need huge inductors.

To avoid this and to increase the output power, Pathos designed the "double InPoL" circuit, which consists of two InPoL circuits bridged together with the two inputs driven in counterphase. (Pic 3)

In this circuit, the two inductors are wound in a single core and mounted in opposition so that the magnetic fields generated by the BIAS currents 𝐼0 cancel out.

By eliminating core saturation problems, the size of the inductors is acceptable and it is possible to design class-A amplifiers with considerable power ratings.

The bridged configuration of the double InPoL circuit allows eliminating the output capacitor and designing amplifiers with fully balanced circuitry from input to output, reducing noise and distortion. With such a "double InPoL" configuration, Pathos produces the following amplifiers:

- InPoL Remix MkII
- InPoL2 MkI and MkII
- Heritage
- InPower
- Adrenalin

These amplifiers reach powers up to 200W in class A.

InPoL Legacy

Pathos introduces its new integrated amplifier with world-patented InPoL technology: InPoL Legacy. As Heritage was a tribute to the work of Gianni Borinato, co-founder of Pathos, a heartfelt acknowledgment of his innovations and his contribution to the world of audiophiles, similarly InPoL Legacy represents the ideal legacy of Gaetano Zanini, co-founder, and owner of Pathos.

An ambitious project, the non plus ultra of Pathos technology is encased in an integrated amplifier, which takes the listening philosophy developed over 25 years by the company to the extreme, both in terms of component choice and the absolute performance of this masterpiece.


InPoL Heritage demonstrated that the dual InPoL circuit was still improvable, especially with regard to the power supply. If in previous InPoL amplifiers we made sure that the power supply was adequate, especially not to overdo the weight, in the case of the Heritage the power supply is generous. It made the Pathos engineers realize that they could dare even more.

The natural consequence of the innovations introduced in InPoL Heritage was the new version of the InPower monophonic.

The presence on the market of amplifiers weighing over 100 kg made us put aside our qualms for good: we designed a new amplifier according to the features we wanted, without setting any limits. Various power supply and inductance tests then led to the Legacy configuration.

Our research prompted us to test and compare different components; as happens in other fields, from automobiles to surgical instruments, even in an amplifier the smallest detail can make a difference. For Legacy, we chose only the best and best-performing components.

InPoL Legacy was born as dual mono; all power supplies are dual, one per channel. The amplifier is fully balanced from start to finish.

700VA would have been sufficient for the power supply circuit, but we chose to provide 1800VA per channel so that the transformer would not have the slightest hesitation. Twice as much power as necessary ensures that the transformer does not go into saturation and that it has no disturbances; the cables used were sized to ensure a faster power supply.

The power supply is inductive-capacitive, with outcomes comparable to stabilized, because it does not stress the capacitors and diodes, which thus give maximum performance.

The tube stage may be comparable to that of the Adrenalin: each channel has two Tung-Sol ECC803 tubes, dedicated to voltage amplification, and 1 6H30 Sovtek for the buffer.

MOSFETs are carefully selected: placed in a device that biases them uniformly, the current of each individual MOSFET is measured and grouped with others of the same value. In the InPoL Legacy, MOSFETs are supernumerary to actual requirements to avoid stress.

InPoL Legacy could compete with the Synapse and Adrenalin combo, to which it is inferior only in power; it can boast, on the other hand, a more effective electronic spring than Adrenalin, with an overall improvement in performance.

The amplifier delivers 100W at 8 Ξ© and 170W at 4 Ξ©; due to the inherent characteristics of the InPoL circuit, the power does not go double as the impedance is halved. This design choice is mainly related to the thermal Ξ” limits granted by the standards.


The lines of the InPoL Legacy are reminiscent of those of the already well-known InPoL2 MkII; the dimensions are significantly larger to hold all of its 100W. In the center of the front panel stands the infinite-turn knob with a LED display; the circuit was designed in Pathos labs. Behind it, the tubes open like a curtain to show the perspective of the six capacitors.

The heatsinks are now known as a Pathos trademark, with the profile modeled after the company logo. In the Legacy, they are larger to ensure adequate dissipation for the 100W Class A power.


The Pathos sound philosophy, well understood from the beginning and developed over more than 25 years, wants the least manipulation of the signal by transistors, preferring the sound of tubes, which has an unmistakable naturalness, a smooth, clean sound.

In designing the Legacy, we chose the ultimate: the technical features, choice of components, power, and oversized transformer, make the Legacy sound the best InPoL sound ever.

Harmonic, defined, spatial, natural...these are adjectives that are not enough to describe it. You have to listen to it.

Closing your eyes feels like being in a concert hall, with every instrument in its place, a sharpness of detail you've never heard.