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.

 

 

Theoretical Signal Path: Voltage and Current Flow in the Inverted Audio System

1. Headphone Jack Output from Amplifier A

The headphone jack output (typically a 6.3mm or 3.5mm jack) on Amplifier A is not a standard line-out, but rather an amplified output designed to drive headphones with impedances from 32 to 600 ohms.

 Typical characteristics:





Output Voltage (RMS): between 0.3V and 2V RMS, sometimes up to 3V RMS on powerful amps




Current Capability: capable of delivering 50 to 100 mA, much higher than line-level RCA outputs (which are usually <1 mA).




Output Impedance: typically less than 10 ohms, allowing it to drive low-impedance headphones effectively.



So while the headphone output is stronger than a standard preamp or RCA signal, it's still within safe ranges for many line-level devices if the current is properly limited downstream.

2. Through the Passive Filter

Next, the signal passes through a high-quality passive filter, typically composed of resistors, capacitors, and/or inductors. This filter performs two crucial roles:

 Signal Conditioning





It attenuates and shapes the frequency spectrum, splitting bands (e.g., bass vs treble).




It reduces the amplitude slightly, depending on filter topology (e.g., low-pass, band-pass).



Current Limiting





Because it is passive and resistive, the filter acts as a natural current limiter.




For example, a simple RC low-pass filter introduces series resistance (e.g., 1–10 kΩ), which significantly limits the current reaching the RCA input of the surround amplifier.



Example:

If the headphone output produces 2V RMS and encounters a 10 kΩ resistor:



Current = V / R = 2V / 10kΩ = 0.2 mA, which is well within safe limits.


3. Into the RCA Inputs of the Surround Amplifier

The surround amplifier is designed to receive line-level signals through RCA:





Expected input voltage: around 0.5–2V RMS




Input impedance: typically 10 kΩ to 100 kΩ, which ensures very low current draw



Why doesn’t it burn out?



The passive filter protects the RCA input by attenuating current and conditioning voltage.



Although the headphone jack can deliver higher current, the input stage of the amp only draws what it needs — it’s like putting a small load across a powerful source, but with a resistor in between.




No short circuit occurs, and the signal level is within range.



Summary of Voltages & Currents

 








Stage
Voltage (RMS)
Current (typical)
Notes




Headphone Output (Jack)
0.5–2.5V
up to 100 mA
Amplified signal, low impedance


After Passive Filter
0.3–1.5V
< 1 mA
Current-limited, signal-shaped


RCA Input of Amp
0.3–1.5V
few μA to mA
High-impedance input