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Old 28th July 2008, 01:27 PM   #11
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P.S. Does anyone know what the secondary PSU is about? It is a 9V tranny in the grey case.

The arcticle I've given a link to talks about it but is in French - Pages 10 and 11.
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Old 28th July 2008, 02:20 PM   #12
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I will take a look in the original article when I am home tonight what the secundary psu is for.

The sound is hard to discribe. I always feel the need to laugh loudly when I read reviews descibing the sound as "with a lot of air" of "very black". I was able to compare it head-to-head to a NAD S300 amp and the lfa50 (in combination with the elektor The Pre-Amp) sounded better. In comparison to the Marantz PM7001 the Marantz gets blown away. The lfa50 sound natural is al I can say.

The lfa50 is able to deliver quite a lot of power (current). The impedance of my Scanspeak Reference Plus dips to 2 ohm at certain (low) freqs and this is no problem with this amp. It keeps good control on the speakers.

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Old 28th July 2008, 05:15 PM   #13
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The 2x 9V transformer is in series with the main transformer, to raise the rail voltage to +/- 70V, the front end of the amp feeds from +/- 60V regulated rails.
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Old 28th July 2008, 10:14 PM   #14
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I have just looked in the schematics of the amp. The secundary psu is to power the voltage-amp (pcb in the middle of the triple stack).

If the voltage on the rails is 29v, then you the lfa50. otherwise the voltage would have been 56v (lfa150).

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Old 28th July 2008, 10:14 PM   #15
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Quote:
Originally posted by jacco vermeulen
The 2x 9V transformer is in series with the main transformer, to raise the rail voltage to +/- 70V, the front end of the amp feeds from +/- 60V regulated rails.
This is for the lfa150. For the lfa50 the voltage is raised to 44v.

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Old 29th July 2008, 09:10 AM   #16
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Slip of the fingertip, my bad.

The heatsinks used are either Fisher SK56 or Seifert KL146 types.
A single 12"x4"x1.6" size is good for 0.45C/W : a pair of those heatsinks have a combined thermal factor of 0.225C/W.
The one who assembled these LFA50s set them up to fully bias the amps to 50W in 8 Ohm, not 25.
(= 200mV across the 0.22 Ohm emitter resistors instead of the Elektor prescribed 138mV.)
Biased to 50W should still keep the temperature of the heatsinks below 50C in a room with 25C temperature.)

The difference in output devices between the LFA50 and LFA150 only had to do with logistics, the Toshiba ringemitters used in the LFA150 became unavailable around 1990.
Your 828MKII has both Sanken as Toshiba MahJong bricks : 6 Sanken output devices and a pair of Toshiba drivers (2SA1094/2SC2564 IIRC)

BTW : the LFA50 and LFA150 have a large open loop bandwidth, >10KHz, an important factor for them to sound rather good.
Both amps also have a Sziklai output stage, also known as compound output, aka complementary feedback pair output stage (CFP).
A Sziklai output has less distortion, can deliver more clean power, but is inherently less stable.
The Threshold power amps have/had compound output stages, but the Stasis operation and bulk of those amps make them rock solid.
The LFA50 is not that stable, it swings if you use it with tricky electrostats.
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Old 29th July 2008, 09:50 AM   #17
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Haven't had any trouble with my "not-so-easy-to-drive" speakers.

The design runs quite hot, especially during hot summer days. I have build both mono-amps in a single casing from monacor. I have never measures the temp but I am nog able to hold my hand on the heatsinks for more that 2-3 seconds. But it doesn't seem to affect the relaibility.

Anyway nat.d.berg, I hope you have the same fun with this amp as I have got during the last 12 years. Let me hear your comments!

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Old 10th November 2011, 12:36 AM   #18
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I'm ashamed to say that they'e sat on a shelf since 2008..

Plugged them in for the first time err a couple of months ago! Unfortunately one is silent so in need of some repair.. It's on my list but lower down than a nice big beefy 80's Bartolomeo Aloia D100 Referenza that burned out some kind of bias resistor on first turn on.
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