A.D. Audio amplifier, «Sforzando» model

A.D. Audio amplifier, Sforzando model

This appliance is totally made with tubes, except the rectifier (4X BY127) and the 2 expander diodes (OA81). Tubeset : 4X 7189, 1X EMM801, 5X ECC83, 6X ECC82.

This amplifier is really excellent. Even with my laboratory loudspeakers, the sound is really nice. The expander circuit is a little bit particular. And also, the mode selector has a filter position, sending low frequencies on one channel, and high frequencies on the other. It's a simple passive filter and I don't find that very good... But this is just an accessory and it don't change anything to the high quality of this appliance, a wonderful «Made in Belgium» part!

Sadly, i don't have a lot of information about this mnufacturer. He was called André Dumortier and his brand was A.D. Audio. I've just learn he has work with mister Frank à at the Radio-Union era. This has to be confirmed! If you have more information, don't hesitate to contact me. I will put it here, for all the people wondering about the designer of this wonderful appliances. I'm also searching other appliances from mister Dumortier! If you have, please contact me!

I found this one in June 2008. A part of the tubes was replaced already, but not all. Inside, some capacitors were changed but not all... and it was even missing one: de-coupling for the output tubes cathodes! It was working anyway...

To repare it, i've started to draw the schematic. Download it here. It's a 470ko pdf file. Attention, this is not the manufacturer schematic but my drawing! Maybe some small differences can be found between models. This document is free of copyright and can be distributed and copied, but in any way it can't be placed on commercial websites!

A few pics before and during the works. Clic on small pics to enlarge.

	The amplifier with the cover. The 7189 power tubes are placed at rear of frame. The two boxes near the 7189 are the power transformer (left side) and outputs transformers (right side).
	Opened. On right side we can see the big smoothing coil.
	View of the power tubes wiring. Overheated! The wires are not naturally brown, the insulation is burned. I've totally re-wired this part of the amp. For the security I've also replace the 4 «mustard» coupling capacitors. If they went out, the output tubes will quickly be defective.
	Wiring : output tubes, smoothing caps, input and output connectors. We can see small black capacitors from the former repair. I've change it by better ones (Evox Rifa). I've also replace the electrolytics cathode capacitors, and of course placed the missing capacitor! Thanks to Yves for the information.
	Wiring: rectifier, tubes. I've change the rectifier wiring because the bakelite support was broken. And the only one change regarding the original wiring : i've place the fuse protection before the recitifier and not after like it was. This way, if the rectifier has a problem (and diode goes to short cut) the power transformer won't burn out !
	Wiring ; pre-amp, expander and input stage. The yellow «block» capacitors are also replaced. Some of them were defective and there was a sound difference between the 2 channels.

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