Modification |
Two loudspeakers for 140 euro...
The first listening test is disappointing...
- The bas range is ok for that money...
- The midrange is spongy, unclear and blurred
- The tweeter is to loud ( +3dB )
Looking to the crossover network, i understond why.
Original Network Circuit
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Original network
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3 coils and 1 capacitor for 3 drivers?
- 1.51 mH woofer ( 6 dB )
- 0.21 mH midrange ( 6 dB )
- 0.53 mH and 4.7 uF tweeter ( 12 dB )
No impedance correction circuit?
Next step: measuring of the impedance of the loudspeaker.
I decided to add a RC circuit to the woofer and midrange driver.
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Concept 500 Impedance Curve measured with ARTA (Limp)
Impedance without compensation, Impedance with RC-compensation
The irregularity at 220 Hz is caused by enclosure resonances ( Enclosure height 80cm )
Here the calculation:
344 m/s divided by Frequency Hz = Wavelength in meters
(344 / 220) / 2 = 0.78 meter or 78 cm
Calculate here Enclosure Resonances
Here the picture of the modified crossover network with Zobel circuits for bass and midrange.
| Crossover frequencies |
| Lowpass woofer: |
750Hz ( 6dB Bessel alignment ) |
Inductor: 1.5mH |
| Lowpass midrange: |
4200Hz ( 6dB Bessel alignment ) |
Inductor: 0.21mH |
| Highpass tweeter: |
4200Hz ( 12dB Bessel alignment ) |
Inductor: 0.52mH, Capacitor: 4.7uF |
Impedance equalization
- Resistor : 4.7R 10 watts
- Capacitor : 33uF bipolar 50VAC
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Tweeter attenuation
- Resistor serial : 1R 5 watts
- Resistor parallel : 15R 5 watts
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Until now i didn't measure the Frequency Response, because i was curious if it's possible to tune the network by ear...
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Frequency Response: original Crossover Network
Port: closed - open
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Frequency Response: new Crossover Network
Port: open
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** note the different resolutions of the Frequency Response Plots (left side: 5dB, right side: 10dB)
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The aim of the crossover optimization was to optimise with as few resources which should also fit on the existing crossover board.
The enclosure also leads despite 2 ring stiffeners due to the small thickness of 13 mm a busy life of its own.
But for a handful of dollars ( 10$ ) are you able to optimise this loudspeaker in the good direction
A listening test will come soon...
PC Loudspeaker
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Material:
MDF 18mm |
| 4 pieces | 180 x 320mm |
| 4 pieces | 156 x 320mm |
| 4 pieces | 120 x 180mm |
| Inside measurements: |
| 12cm x 18cm x 24cm | = | 5.18ltr |
| Vb = (5.18ltr - DrviverVol = 0.3ltr) | = | 4.88 ltr |
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New Network: 12dB Crossoverfrequency (acoustical: 2.5kHz, electrical: 1.7kHz)
The network is totaly recalculated. One capacitor is not working...
Now it's a 12db network with impedance compensation for Midwoofer and Tweeter. There are problems at 6.5 kHz and 10kHz.
This is caused by the Tweeter...
But i don't change this because the network components would be 4 times the money as the both drivers...
And, the speaker is used as PC loudspeaker and not as HiFi loudspeaker.
Calculated and optimized Frequency-Response
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to be continued...
with Impedance-, Frequency-measurements and listening tests...
JX92 Horn
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My plan is to add a Tweeter to the JX92-Horn.
Looking for a suitable Tweeter, i found the Swans TN-28.
The absence of a mounting flange on the TN-28 allows flexible placement and diverse cabinet design. The TN-28 can be placed on top of a cabinet to reduce diffraction effects and align acoustical centers of transducers.
The solid aluminum case enclosing the magnet system provides effective cooling.
The special weave of the fabric results in better axial symmetry of the entire moving system.
The tweeter has very smooth on-axis and off-axis frequency response delivering excellent acoustic power dispersion. The TN28 creates an accurate imaging source and is distinguished by exceptionally transparent and balanced sound.
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Frequency response of the Swans TN-28
The crossover frequency will be around 3000Hz, because the JX92 response falls back a bit at thet frequency.
So, my idea is to use a 12dB lowpass filter and a 6dB highpass filter.
At the moment i'm busy to measrue both drivers, impedance curve and frequency response. This data i need to
design the crossover network and the impedance equalization for the JX92.
If you choose a crossover point in a range where the driver's frequency response is changing rapidly off-axis, the off-axis response will have large response anomalies which degrade the power response the listener perceives.
Frequency response at listening position of the original construcuion without filters.
Measured with ARTA
20 Hz - 20 kHz ± 12.5dB - too sad for words...
The crossover network is calculated. It's a lowpass filter with 12dB and a highpass filter with 6dB.
The choosen crossover frequency is 4800Hz and not 3000Hz as planned.
This is done because:
- the resonant frequency of the tweeter is 1700Hz, to low to cut at 3000Hz
- the power response of the JX92 fullrange driver fits beter at 4800Hz
Lowpass 12dB, highpass 6dB crossover filter (4800Hz) and -4 dB attenuator for tweeter.
The rising impedance of the JX92 is compensated with a Zoble circuit (5.6ohm and 6.8uF)
Impedance- and phase of the complete loudspeaker
Now i will listening a lot of my CD's and LP's.
Perhaps i have to fine-tuning the
- crossover network...
- or the Horn damping...
- or the acoustical properties of my listening room...
- perhaps all?
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Projects ( planned or in progress )
