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Old 20th September 2021, 07:12 AM   #231
xrk971 is offline xrk971  United States
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Balanced output (BTL) LuFo coupled with a common core dual inductor.

Like this:
Click the image to open in full size.
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Old 20th September 2021, 06:45 PM   #232
xrk971 is offline xrk971  United States
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This is going to need a DC servo circuit to keep output offset near 0mV. I am going to see if I can repurpose the DC servo circuit from the Xmas amp that Jhofland Dibya designed.
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Old 20th September 2021, 10:57 PM   #233
xrk971 is offline xrk971  United States
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Both channels of LuFo work! Uneventful firing up for first sound on each channel separately. Then initial tests if balanced SuSyLu shows that bias is stable and output DC offset changes within 20mV through warmup cycle. Balanced common choke appears to be working.

-9f0a948e-9688-4154-9f4f-6db7dd247041-jpeg

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Old 21st September 2021, 01:06 AM   #234
poseidonsvoice is offline poseidonsvoice  United States
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Here are more updated photos on what my ALPHA 20 board build looks like:

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

I’ll complete the next one hopefully this week.

Best,
Anand.
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Old 21st September 2021, 01:42 AM   #235
xrk971 is offline xrk971  United States
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Nice work, Anand!

That’s a fancy cap!
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Old 21st September 2021, 06:47 AM   #236
wiseoldtech is online now wiseoldtech  United States
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I've already done numerous amps, both tube (EL84) and impressive SS. (120 to 200w)

Among custom designing some radio gear and other electronic stuff.


So, for a big change, I decided to delve into a small amp for a refreshing change.
Really small, like a couple of watts, maybe 10.


Taken from a trashed reel-to-reel (VM) that had wing speakers, I salvaged the poor little amp board.
Stereo, all class A, 6 transistors (3 per ch) cascaded single-ended driving a pair of output transformers.
Initial testing surprised me, using the test speakers on my bench, so I'm giving it some respectable mods and we'll see how it goes.
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Old 21st September 2021, 10:17 AM   #237
suresh is offline suresh  India
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Anand / XRk ,
IXTK90P20P P channel mosfet has 12nf Ciss. is the gate drive sufficient ? how many milliamps current is required to drive such high gate capacitance.
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Old 21st September 2021, 10:32 AM   #238
xrk971 is offline xrk971  United States
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Looks like Anand is building the 4 ohm variant of the Alpha:
Click the image to open in full size.

The IXYS p channel is driven by the VAS, a KSC1845, which works fine. Hugh has often used the combination of a KSC1845 and p channel IXYS with large gate capacitance as a CFP and there is never any issue of gate drive current. Perhaps you can ask Hugh in the Click the image to open in full size. for the Alpha?
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Old 21st September 2021, 02:27 PM   #239
poseidonsvoice is offline poseidonsvoice  United States
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Yes, what X said.

Best,
Anand.
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Old 22nd September 2021, 06:27 PM   #240
xrk971 is offline xrk971  United States
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On the bench tonight is the SuSyLu max power stress test:
SuSyLu Where Are You?-susylu-max-power-setup-jpg

It worked fine and can run all day at max power. It's Class A so the amp actually runs cooler. Thermal efficiency is 100w/168w or 60%. So at max power the heatsinks only dissipate 68w vs 168w. The opposite of what happens on a Class AB amp.

SuSyLu is verified to put out close to 90Vpp into a 10ohm dummy load:
SuSyLu Where Are You?-susylu-max-power-dmm-vrms-jpg

This is the o-scope trace - yellow is output after a signal divider - actual value is 89.7Vpp. Input signal is 4.64Vpp for overall gain of 25.6dB.
SuSyLu Where Are You?-susylu-max-power-scope-jpg
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