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Old 24th November 2009, 11:38 PM   #1
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Default Driver for Musical Fidelity 550K

So I might have a chance to get my hands on a pair of Musical Fidelity 550K superchargers.

Looking at the specifications, they feature a 50 Ohms input impedance. The specifications say:
Quote:
Input sensitivity for full power out into 8 ohms: 33V
UGH! Is this 33V input RMS or peak-peak?

Anyway, the folks at Stereophile measured the gain as 5.5dB (1.88V/V) with 54 Ohm input impedance with maximum output of 650W. This correlates to 74VRMS output into 8 Ohms. This means the input must have (74/1.88) = 39VRMS to drive the amplifier to full power. This is reasonably close to the "33V" stated in the amplifier's official description, especially since Stereophile measured 650W output instead of the rated 550W.

Now, (using factory specifications) 33VRMS across the 50 Ohm input impedance is (33V * SQRT(2)) = 47V peak voltage drive. This is equivalent to (47V^2 / 50) = 44 Watts peak, or 31 Watts RMS.

I would like to drive the 550K with an SET. For example, I can get an SE OPT from Electra-Print wound for a 50 Ohm secondary (for example, a 7k:50)

However, even with the higher 50 Ohm load impedance, it doesn't seem fair to ask a SET to drive 31WRMS to push the 550K to full power.

The obvious solution is to dismantle the 550K and swap the load for 600 Ohms. For purposes of this discussion, let's leave the 550K intact and unchanged.

Any suggestions to drive the 550K's 50 Ohm input?
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Old 25th November 2009, 08:54 AM   #2
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33V over 50K represents a current demand of just 0.0066 A which translates to a wattage of just 0.2W.
Any SET should be able to cope with that.

Shoog
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Old 25th November 2009, 07:04 PM   #3
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50 Ohms, not 50 kilo-Ohms.

33V RMS across 50 Ohms is 0.6A RMS, quite a bit for an SET.
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Old 25th November 2009, 07:25 PM   #4
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My apologies - thats just plain bizarre. I can guarantee that the input stages is all but ******* away all of that power.

Shoog
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Old 26th November 2009, 03:12 AM   #5
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The 550K is a "supercharger". It acts as an impedance buffer between your existing amplifier and the speaker. Click for some write-ups here (Stereophile) or Power-Booster Amplifiers.

In other words, it can take a 33 Watt amplifier and boost its output power to 550 Watts. My Magnapans would appreciate that kind of drive

Lots of people (including this forum) are exploring unity-gain amplifiers, "power buffers", and other topologies to get the SET sound on a wider variety of loudspeakers.

I built a SE unity-gain MOSFET buffer stage for my Aikido, published in AudioXpress. This took a small power signal (from the Aikido) and boosted it to about 12-18 Watts. The 550K just works on a bigger scale, boosting a 33 Watter to 550 Watts.

The 550K needs a "low" input impedance to make it compatible with a wide variety of source amplifiers. 50 Ohms is a bit high for tube amp compatibility, but it seems to work somewhat. Because I'm a DIYer, I'd get an OPT wound for 50 Ohms output, or I'd build/modify the booster for 600 Ohm input and use a custom 600 Ohm OPT.
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