Dual Mono 4-way Crossover-Equalizer

Circuit :  Roberto Grilli - Ancona, Italy
Email :
Web   : Volterra Elia

Below is the diagram for a dual mono multi-amplification with equalized four-way analog crossover, designed by Roberto Grilli from Ancona, Italy. The idea is that each channel (left and right) has an independent amplification for each speaker; with the aim of reducing intermodulation distortion.
The dual mono multi-amplification fascinates me because it recalls the arrangement of the instruments in classical or modern concerts:
Each musician has his own score to perform, distinct but in harmony with the others.
The dual mono multi-amplification (all double and all separate) is the professional setting of excellence to have the optimal separation of the signal from the source to the speakers.
According to the radical canons of the "sound purists", the Hi.Fi. needs to:
(1) be as simple as possible and with the fewest components in order not to alter the original information;
(2) add nothing: selective amplifications / attenuations and even less distortions

Of course, I will consider the "sound purists" with reasonable compromises.
The block diagram shows the general structure.
Comments on the Block Diagram
For each channel (left and right) I would use:
• a toroidal transformer with 1 primary and 4 secondary for the power amplifiers: please indicate the power;
• a toroidal transformer with 1 primary and 2 secondary for preamplifier and V-meter LED: indicate the power.

I would add the LED spectrum analyzer because it recalls the symmetrical architectures of the organ pipes, even if this is not appreciated by audiophiles.

General Specifications
1. Input selector (ideal):
to connect: CD, DVD, radio-TV tuner,
- must include RCA connectors, optical fiber, USB and HDMI ports;

2. Preamplifier (ideal):
- in differential input configuration with stabilized current;
- work class: A;
- linear frequency response on the 20 - 20 KHz band;
- THD distortion of the order of 0.001%

3. Four-way Cross Over (ideal):
This block is not appreciated by "Hi-Fi purists" who prefer the wideband speaker to avoid any filter;
of course, common sense tells us that in practice it is not technically possible;
the speaker cannot perform excellently over an extended frequency range.
Loudspeakers, electro-acoustic actuators, respond to electromagnetic and mechanical principles and, themselves, can generate resonance phenomena.
The number of crossover channels defines how many areas the audio band will be divided into.

In two-way cross-over, each speaker must respond in a wide frequency band, about half of the audio band.
A three-way crossover with high and low pass filter (shelf filters) and band pass filter (bell filter) allows definition of low, medium, high frequencies. The four-way crossover, yet to be tested, would offer:
- theoretically subdivision of the audio range into about four parts;
- would allow a greater reconstruction of the musical scene (classical or modern concert);
- less intermodulation distortion.
Looking at the Linkwitz-Riley parameters, the most linear response is obtained.
Regardless of the number of crossover routes,
the selectivity of the filters is visible in their characteristic curves:
For example, in a three-way crossover, with low slope filters and high signal level, there are many frequencies in common:
- after the lower "cutoff frequency" we have many low-pass frequencies in the band-pass area;
- before the "lower cut frequency" of the band pass we have many frequencies in the low pass area;
- after the "upper cut frequency" of the band pass we have many frequencies in the high pass area;
- before the high pass "cutoff frequency" we have many frequencies in the band pass area.

dB/Octave Reactive Components Tilt in response or Slope I/O Phase Shift
6 1 Low 90°
12 2 Moderate 180°
18 3 Great 270°
24 4 High 360°

4. Four way analog cut/boost shelving equalizer (ideal):
The equalizer should adapt the non-linearity of the speakers and compensate the listening environment, however, this part of the circuit is not recognized by the canons of Hi-Fi "purists".
As opposed to a graphic equalizer, with numerous very selective filters but which can introduce distortions, we choose a limited number of analog filters, each with a greater bandwidth but with limited resonance phenomena.
With this compromise, the smaller number of components would ensure greater fidelity of the original information.
Since the corrections often do not affect the center of the band, the crossover / equalizer group should be two (low and high) or four (low, medium-low, medium-high and high).
With this type of equalization, it is possible to intervene only on the output level of the cross-over which will have 4 potentiometers for the volume of 4 speakers for each channel.
Many professional equalizers have, for each filter, the asymmetrical gain potentiometer, with zero not central, so that the attenuation is greater than the amplification: this, beyond the purist concept, would serve to reduce the distortion of the active filter.

5. Final amps (ideal):
- active element: BJT, Mosfet or IGBT;
- complementary Push-Pull configuration;
- frequency response: linear on the 20 to 20 KHz band;
- working class: AB;
- THD distortion of the order of 0.01%
The four power amplifiers should all be schematically the same to avoid delays or phase shifts of the input signal.
Having four different supply voltages, four power levels could be obtained:
from the minimum power for the tweeter to the maximum for the woofer.
Considering that the maximum power refers to the output sliders of the filter positioned at maximum exaltation (+3dB); example:
30 Watt for the Woofer;
15 Watt for the Mid-Woofer;
10 Watt for the Mid-Tweeter
; 5 Watt for the Tweeter.

6. speakers (ideal),
Since the ideal shape is the sphere,
it would be interesting to try oak barrels:
even if the image will be spartan, the goal is to create something original and possibly with good theoretical-technical characteristics.
Hypothetical wiring diagram of analog apparatus for a four-way crossover and cut / boost shelving equalizerr
Today, many people consider online music as the best Hi.Fi. source.
Since uncompressed music files take up a lot of space, an adequate storage medium must be provided.
A computer is required to manage archived files and the audio-board.
Mid-to-high level audio-board have digital crossovers and equalizers, fully parametric.
Here you can modify the “cut frequencies” of all filters and their gain;
this would increase the number of components, with all the dissensions of the sound purists.
The audio-board have integrated a DAC (Digital-Analogic-Converter) generally at 24 bit;
the outputs of these audio-board can be from stereo type (left and right) up to the Dolby Digital 7.1 configuratio
n Many audio-board models have preamp outputs for connecting power amps.
The cooling system in the motherboard must be absolutely silent for this reason fanless radiant panels are provided.
However, there is no multi-amplification, and for the nostalgic of the welder here there is little to assemble!
Or you can choose an external 32-bit / 384 kHz DAC with outputs to connect to external preamplifiers, and continue with the traditional system.
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