Yes, I have done several experiments, first with separate class D amplifiers and indeed they suffered from long term stress on the voltages. So I went for something safer. The system works perfectly and does not need resistors or anything. I connect the R and L jack output to the two excellent quality Sony passive filters. The left bass and treble go out in the front channel and the right bass and treble outputs in the rear channel. The AERON AV-2800 is an effective 4x100 watt and gives me safety. Of course do some tests at low volume and go up. I never had any unexpected discharges and the sound is special. You have to try it. Of course I use the output of the SA-9500 II (which is an old fashioned 80 watt x 2 class AB beast that you can find on ebay for 1600 euros today)To blend the two styles of audio that complement each other into one magical audio reproduction.. I have already opened a patent on ZENODO for the details.
https://zenodo.org/records/15335095
Motivation Behind the Inverted Active System
My decision to build this system was initially born out of
practical and economic considerations. I didn’t want to invest in an expensive active preamplifier — not just because of the cost, but because such units often
constrain the system’s architecture and impose a specific sonic signature. I wanted more freedom to experiment with my own ideas and speaker designs. Over the years, I’ve constructed several pairs of loudspeakers, so I became curious:
What if I built a fully active system — but inverted — using components I already had?
Instead of sending a standard low-level signal from a preamp, I decided to
use the headphone jack of Amplifier A as my signal source, essentially repurposing it as a "preamp." This signal then passes through
a high-quality passive filter and feeds directly into the RCA inputs of a
multichannel surround amplifier, which drives the speaker elements directly, without any additional filtering stages.
Initial Doubts and Unexpected Success
At first, I was almost certain the system would fail — I even feared I might
damage the RCA inputs of the amplifier due to the higher voltage and current from the headphone output. To my surprise,
everything worked perfectly from the very beginning.
This led me to refine and further test the system. The elements that convinced me to continue and ultimately document the setup were the following:
- Exceptional clarity: The sound is impressively clean, without the slight "halo" or micro-reverberation effect that often merges frequencies in traditional passive or semi-active systems.
- Superior stereo image: Channel separation is remarkably precise. Instruments and voices appear in space with greater localization and spatial definition.
- Midrange retention: Despite skipping active preamplification, midrange fidelity remains strong, without the typical loss of body or presence.
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 |
I forgot to say that I've been using it for about 2 years but I never got around to publishing anyth
