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Magnus Audio MA-300 Power Amplifier Upgrade

This monster Magnus Audio MA-300 power amplifier came in for a factory power supply upgrade. Let’s take a look!

Magnus Audio is not a well-known name here in Australia, maybe not elsewhere either, but, after working on this amplifier, perhaps they should be. I’d not worked on any Magnus Audio gear until this unit came in and I can now definitively state that the Magnus Audio MA-300 amplifier is a beast.

Capable of 300 Watts per channel into 8 Ohms, the MA-300 will drive any load without trouble. It’s also one of the heaviest amplifiers I’ve ever had to move around my workshop. At 75 kg (my body weight), this unit is absolutely a two-man lift. Add superb build quality and gorgeous metalwork and you have a killer amplifier.

My customer informed me there are only four of these $12,000 USD amplifiers in Australia, no wonder I haven’t worked on one before!

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This image doesn’t convey the sheer scale and mass of this 75 kg monster amplifier.


Rated Output Power:300 Watts at 8 Ohms
600 Watts at 4 Ohms
Frequency Response:+/- 0.1 dB from 10 Hz to 20 kHz
Signal-to-Noise Ratio:-110 dB, reference level: full power output
Total Harmonic Distortion (THD):<0.02% at 1 kHz, at 300 W, 8 Ohms
<0.05% at 20 kHz, at 300 W, 8 Ohms
Gain:22 dB
Input Impedance:50k Ohms (single-ended)
100k Ohms (balanced)
Damping Factor:250, reference 8 Ohms nominal
Power Consumption:550 Watts idle, 1,800 watts maximum
Inputs:2 balanced via XLR connector (1 per channel)
2 single-ended via RCA connector (1 per channel)
Outputs:4 pairs WBT binding posts (2 pairs per channel)
Operating Voltage:100V, 120V, 220V, 230V or 240V at 50 or 60 Hz
Dimensions:19 in. W x 9.5 in. H x 27 in. D
Weight:160 lbs / 75 kg

Power Supply Upgrade

This Magnus Audio MA-300 came in for a factory power supply upgrade. Magnus Audio has developed a new board which they fit to their current top of the range model and offers as retro-fit for some older models like the MA-300.

Any time you put yourself out there as willing and able to work on gear like this, you take on great challenges and risks. This upgrade, whilst technically straightforward, requires soldering and critically precise work. One mistake and this 12K USD amplifier ashes itself at power on. As you can imagine, note-taking, photography and quadruple-checking are utilised before any switches are flipped.

As always, I gave the amp a thorough pressure dusting, clean and of course, careful testing her once the work was complete.

Nuts and Bolts

Check out my video about the Magnus Audio MA-300 on YouTube:

Let’s dive in…

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Here’s the monster, with the new, factory-supplied power supply in the package sitting on top.
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Digging inside, we see an impeccably well laid-out amplifier, using premium parts throughout. Very nicely done.
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This toroidal transformer must be 1.5 kVA, or more.
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This is the factory original power supply module. It is old-school in design, using four large Mallory filter capacitors and no bypass caps.
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These capacitors are large, rated at 37,000uF a piece. This gives 74,000uF per channel, a decent amount for sure. The heatsinked devices at the bottom are the bridge rectifiers, one bridge per channel, very nicely heatsinked. Before removing anything, EVERYTHING must be photographed, marked and documented, to remove any possible risk of damage. There is zero margin for error inside a $12,000 USD amplifier.
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This is the new power supply from Magnus Audio. Note that it features a more modern design using more smaller capacitors in parallel. The new module actually has slightly greater total filter capacitance. of 80,000 uF per channel, and likely lower ESR as a benefit. What you can’t see here is the row of four WIMA bypass capacitors through the middle, further improving power supply noise rejection and high-frequency performance.
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The main filters are Elna For Audio, very nice. These heatsinks are also slightly larger. Everything here is done about as well as possible in consumer gear. Well done Magnus Audio.

Out With the Old…

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These two images clearly show the differences between the power supply modules. Note the WIMAs in the new supply.
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This shot shows just how much smaller the new power supply module is, at least in vertical terms. I love the old-school approach using massive capacitors, a-la Accuphase, Krell, Levinson et al. There’s no denying the merit to the more modern approach though, and it will probably sound better.
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Do you know how hard it was to hold this old module like this and photograph it, with one hand..?! Note that someone has had this out before and used a crappy Jamicon capacitor to replace a factory one.
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Bad move, whoever did this. I can never understand why people use crappy replacement parts in absolute premium gear like this. It’s like using no-name Chinese piston rings in your expensive Porsche engine rebuild – no sane person would do this.
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With the power supply module removed, the MA-300 looks empty. This provides a good opportunity to look around though. note that there is minimal wiring, a result of thoughtful design.

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Magnus Audio have used nothing but premium parts here – BC capacitors (the four blue electrolytic capacitors you see to the left) plus WIMA polypropylene capacitors to bypass them (the pink rectangles next to them). They also used Dale emitter resistors for each bipolar output device and socketed op-amps. There isn’t much, if anything, one could do to improve this.

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In With the New…

After carefully documenting everything, the new power supply is ready to go in.

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Plan view of the finished unit. I’m very happy with the results and the excellent serviceability of the Magnus Audio MA-300.

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The new module, soldered and screwed back into place. I always pay careful attention to cable and component dress and this, to me at least, looks like a neater installation than the original module. Remember though that the original had been out and messed with by someone…
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Quick shot of the back end, showing WBT speaker terminals and input connectors. No expense spared here either.

Results & Feedback

You can see the work I’ve done, but sometimes the best way to gain a sense of how things have changed is to hear directly from the owner:

Hi Mike

Got the amp set up last night and listened for a number of hours. It still runs cool. The sound at the start was different to the sound by the end of the session….better. This amp is definitely my keeper. It sounds absolutely wonderful. Since the update I’ve noticed more detail and greater timbre. The bass is tighter and more extended and generally cleaner than before. Transients are quicker and speakers seem to disappear. Very happy and again thanks for the great work.


Feedback like this says more than I can about my customer’s thoughts on this Magnus Audio MA-300 power supply upgrade. Thank you very much to her owner for this feedback.

In the very rare event that you’d like me to upgrade your Magnus Audio MA-300, or any other big power amplifier for that matter, don’t hesitate to get in touch.

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All cleaned up, upgraded and ready to go back home.

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6 thoughts on “Magnus Audio MA-300 Power Amplifier Upgrade”

  1. Too cool Mike!! I bet that thing just slams out the transient impact
    whilst still rendering fine detail & sweet tone of course :>)

  2. Hi! Awesome job! Other than the MUR3060WT in the rectifier circuit, do you remember the part number for the device of the negative rectifier?

  3. Hi. Thanks for your fast reply. I’m working on a few vintage amplifiers , and I’m try to upgrade the their power supplies with soft recovery diodes. Again thanks.

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