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Custom Amplifier Experiment

edited August 2017 in DIY
Decided to make Active 2-way bookshelf speakers based on a Class AB chipamp.

Narrowed down to -
Amplifier kit - 2x parallel LM3886 from eightaudio.
Speakers - Dayton RS180-4 Midrange & Pyramid TW18 Tweeter.

Speaker enclosure fabricated by diyaudiocart.

For power supply tried first with 2x 12v 5A separate SMPS. Internal they were modified to reduce DC line noise. Works fine individually (mono)but as soon soon as the next speaker (stereo) is added it created a slight high frequency buzz - ground loop issues. The tweeter being a highly sensitive (high SPL 100db+) added to the woes. Fixing a capacitor-resistor ground loop isolator could not solve the issue.

Then removed it & fixed toroidal 12-0-12 160VA transformer with 6800uf x4 per rail capacitor bank. Noise issue resolved. One of the major reasons running low voltage & higher amps is LM3886 runs much cooler.


  • edited August 2017
    Got the passive crossovers from eightaudio.

    Initially had 12db Butterworth crossover at 2500Hz. Then changed it to 12db Linkwitz Riley crossover at 3100Hz.
    Left Linkwitz Riley - smaller capacitors & bigger coils vs. Right Butterworth - bigger capacitor & smaller coils.

    Tweeter polarity is reversed compensating for phase shift.

    Rear of the Speaker enclosure has audio RCA connector (MX Gold type), 230v AC EMI line filter, 2x fuse holders & 2x LED for monitoring rail voltage.

    Used a sheet of yellow glasswool between the speakers & rear wall. It reduces the boominess & give bass a tighter punch. Outer dimensions are : 8.5"W x 18.25"H x 9.5"D = 1474 cubic inches total volume. Wood is MDF.
    Weighed it - 12.3 Kgs per box ! - with everything inside. The wooden enclosures are double walled in the front to compensate for the recess cut for speakers to be flush mount to the surface. With that and all the contraption inside to lift the two speakers one should be a regular at the gym ! - so heavy.

    Finished bullet speaker project without \ with grill. The front gill guide fitment is perfect such that right grill fits on left speaker & vice versa. Surface is polished and lacquer coated. Ports are Jantzen-audio 2".

  • Specification sheets of the drivers -

    Dayton RS180-4

    Pyramid TW18-4

    Modulating Mid-range & Taming Tweeter :
    Initially I had the RS180 (SPL 89dB) crossed below 2.5kHz. I found the Pyramid TW18 (SPL 100dB+ - specs say 96dB but its way higher) overpowering even after using 15Ω 5W resistors before the passive high pass network giving attenuation of 13.5dB. Between 2kHz - 8kHz the tweeter is so powerful that off-axis bass was literally absent. What impressed me about the tweeter was its performance beyond 10kHz - exceptional crisp treble.

    To reduce this effect later I raised the crossover point to 3.1kHz. RS180 with the solid fixed metal cone (non-moving fixed onto frame) is known to exhibit a ringing\beaming effect if crossed above 2.5kHz 12dB/octave. Hence made an active high-pass pre-amp filter above 70Hz. This worked out for the mid-range (70Hz - 3.1kHz). Am planning to add a subwoofer later to take of lower <80Hz frequencies. However still the tweeter was still giving me grief - the 'sibilance' in the instruments\vocals especially artists who use autotune was hitting the ears.

    For a solution to tame the tweeter decided to implement a parallel notch filter.
  • edited August 2017
    Designed own notch filter on a universal line PCB.
    Had few 0.5mH 3A toroidal iron core coils lying with me - made the best use of it. Know air cores are better for audio but anyway wanted to know the difference after inserting these filters.

    Values based on this formulae:

    I added the parallel notch filter few days back.
    All I can say I am amazed by the transformation in the tweeter's response ! Till about 8kHz where the peaks were prominent its absent now. The tweeter's mid-frequency response is closely in-line with the midrange's response.
  • Great initiative and build.
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