Inverted Signal Path for Hi-Fi Audio Using Passive Filtering and Multichannel Amplification
Creators: name: Nania, Francesco Antonio
Publisher: name: Zenodo
Year: 2025

description: My Inverted Audio Setup – Surprising Results with Passive Filtering and Multi-Channel Amplification

Let’s call it an “inverted system” compared to standard setups. Here’s what I do:





I use the jack output of Amplifier A (which acts only as a preamplifier) as my source.




The signal then passes through a high-quality passive filter – no active crossover or DSP.




From there, the signal is sent via RCA cables to a five-channel hi-fi amplifier, typically used for surround setups.




I use this amp to power the low and high frequencies for the left channel and the low and high frequencies for the right channel separately.




The signal then goes directly to the speakers without any additional filtering stages.



The result? In my opinion, the sound is extremely clean, with excellent stereo imaging and a surprising presence in the midrange, which I was initially concerned about. The overall balance feels more natural, and the system avoids some of the colorations I’ve heard in more traditional setups.

The element that makes it an invention is that it greatly reduces the costs compared to a traditional system with an active amplifier since the amplifier A can be any headphone source.

Theoretical Simulation of an Inverted Hi-Fi Audio Path Using Passive Filtering and Multichannel Amplification

Abstract

This document explores a theoretical and experimental configuration for high-fidelity audio routing, in which a reversed audio signal path is proposed. The output from a headphone jack is used as a line-level source, feeding a passive filter stage before distribution into a multichannel amplifier typically intended for surround sound systems. The configuration enables direct speaker drive without further crossovers, promoting signal purity and stereo coherence.



1. System Model and Signal Flow

Let s(t)s(t)s(t) represent the analog audio signal from the headphone jack, modeled as a continuous voltage waveform:

s(t)∈R,−Vmax≤s(t)≤Vmax,where Vmax≈1 Vs(t) \in \mathbb{R}, \quad -V_{\text{max}} \leq s(t) \leq V_{\text{max}}, \quad \text{where } V_{\text{max}} \approx 1\,\text{V}s(t)∈R,−Vmax≤s(t)≤Vmax,where Vmax≈1V

This signal passes through a passive linear filter H(f)H(f)H(f), characterized by a frequency-dependent transfer function. In the frequency domain:

Sfiltered(f)=H(f)⋅S(f)S_{\text{filtered}}(f) = H(f) \cdot S(f)Sfiltered(f)=H(f)⋅S(f)

where S(f)S(f)S(f) is the Fourier transform of s(t)s(t)s(t).

Assuming a two-way passive filter, we obtain:





Low-pass component:



L(t)=F−1[Hlow(f)⋅S(f)]L(t) = \mathcal{F}^{-1}\left[H_{\text{low}}(f) \cdot S(f)\right]L(t)=F−1[Hlow(f)⋅S(f)]





High-pass component:



H(t)=F−1[Hhigh(f)⋅S(f)]H(t) = \mathcal{F}^{-1}\left[H_{\text{high}}(f) \cdot S(f)\right]H(t)=F−1[Hhigh(f)⋅S(f)]

These components are sent to separate amplifier channels, forming a 4-channel system: left low, left high, right low, right high.



2. Advantages and Hypothesis

By removing digital signal processing (DSP) and active crossovers, the system exhibits reduced phase distortion. The cumulative phase response is primarily defined by the passive components:

Φtotal(f)≈Φpassive(f)\Phi_{\text{total}}(f) \approx \Phi_{\text{passive}}(f)Φtotal(f)≈Φpassive(f)

which tends to be smoother than phase responses introduced by digital filters.

Additionally, harmonic distortion is minimized:

THDtotal≈THDamp+THDpassive\text{THD}_{\text{total}} \approx \text{THD}_{\text{amp}} + \text{THD}_{\text{passive}}THDtotal≈THDamp+THDpassive

compared to traditional systems:

THDtotal≈THDpreamp+THDcrossover+THDamp\text{THD}_{\text{total}} \approx \text{THD}_{\text{preamp}} + \text{THD}_{\text{crossover}} + \text{THD}_{\text{amp}}THDtotal≈THDpreamp+THDcrossover+THDamp

This implies reduced signal coloration, better transient response, and improved stereo imaging.



3. Conclusion and Future Work

This configuration offers a minimalist and purist approach to high-fidelity audio signal routing. Potential future work includes:





Empirical frequency response measurements of the passive filter




Phase coherence validation using impulse and sine sweep tests




Real-world THD measurements under load conditions



The system presents a compelling alternative for audiophiles who prefer analog integrity over complex DSP chains.