Inside the MLA-30 Active Loop Antenna

You’ve probably seen the MLA-30 Active Loop Antenna on Ebay. It can be had for a bargain price of around £35 and works great for the price. There’s only one problem in my book; the PCB is covered in epoxy potting and you can’t see the design or repair/modify it! Something that I’ll have to fix!

What follows is the results of 45 minutes in the kitchen with a pan of boiling water and a pair of tweezers. Behold, the MLA-30 in all its naked glory!

A single screw hidden beneath the epoxy holds the PCB into the case.

The PCB removed.

Underside of the PCB.

Boiling the PCB to remove the epoxy. Note to self: Don’t use the wifes best pan!

Slowly removing the epoxy. Once it reaches it’s plasticizing temperature, it just peels off like putty.

Not bad. Only 2 small inductors were damaged. I’ll replace these and see if the PCB still works once I’m done drawing the schematic.

And now, the moment we’ve all been waiting for. The naked MLA-30 PCB!

So, it looks like a single chip amplifier design and it’s pretty obvious to see that the Pot is a simple gain control. I hope that answers some questions about this interesting little loop.

21 Replies to “Inside the MLA-30 Active Loop Antenna”

  1. Excellent post! I’ll be interested to see that schematic when you’re done. Nice work!

  2. Thanks for the dissection and your efforts in helping out. If possible, please email me when you have your schematic done.

  3. Due to the Layout of the PCB and the Pinout of the OPAMP i guess that a differential Input OpAmp/Line Driver like an LMH6550 is used. It would perfectly fit to the Purpose and ists Pinout fits to the rest of the components.

  4. From what I’ve seen so far, you could be bang on the money there Fred. Nice work!

  5. HI All,

    Matt has kindly sent me the PCB to play with.

    The IC is not a LMH6550.

    It’s some sort of fully differential amplifier with a resistor controlled gain adjustment .

    SOIC-8 package
    Inputs on pins 1 & 8
    Outputs on pins 4 & 5
    Gain control on pin 7

    Any suggestions as to type ?

  6. Hi all,
    Maybe TL 592 or NE 592, depending on production factory. ( both originally video amplifier) The same is a LM 733 but has another one pinout.
    TL592 me used in construction of hula loop antenna homebrew. Works fine!

  7. Ignore previous message: I am pretty sure it is the NE592 (very cheap in quantities) and gain can be adjusted on pin 7 (in fact between pin 2 and 7)
    Pin 1 — In1
    Pin 2 — Gain1
    Pin 3 — V-
    Pin 4 — Out1
    Pin 5 — Out2
    Pin 6 — V+
    Pin 7 — Gain2
    Pin 8 — In2

  8. The IC seems to be NE592 (NE592D8R2G SOIC8). Used in many active loop amplifiers.

  9. Is it really a simple gain potmeter? Or is it a balance adjustment between two amps?

  10. Thanks for the link Matt! I found the schematics.
    73 PD3LK, Leon

  11. the main issue with the MLA-30 (I got an MLA-30+ last week) is the noisy bias tee, bypassing the voltage converter and feeding it with 12VDC brings the noise down to normal levels, a mod which may be worth trying to improve the preamp linearity could be placing a 1:N transformer (1 to antenna) between the loop and the preamp, that could be done w/o unpotting the circuit and may possibly improve the I/O impedance matching for the lowpass filter placed at the preamp input, didn’t try it yet, but the idea is to wind two turns to the loop and 8 turns to the preamp on a binocular core to start, and then try with other ratios too (the above would be a 1:16 transformer)

  12. Interesting idea Andrew. Let us know if you experiment with the transformer.

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