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Old 11th August 2014, 03:39 AM   #261
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Thanks Fab.

The amp does not have any bottom and I left a space of 6mm between the heatsink and the Plexiglass. I'll run some temp test tomorrow. If it heats up I'll become creative with the press drill...LOL

Rgds,
Eric
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Old 22nd August 2014, 03:48 PM   #262
cwtim01 is offline cwtim01  Hong Kong
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I made one channel of the mini-F5 for testing, with component values according to the attached schematic. Except that I used a 19.5V notebook smps to power the circuit while testing, since I'm still waiting for the toroid to be delivered.

The amp seems to work, I fed it with a 2.8Vpp sine wave from my scope meter and it's amplified (in fact, it clipped but the generator output from my cheap scope meter is fixed at 2.8Vpp so cannot lower it.)

However, I have an issue. I noticed that Vgs of Q4 is only 0.89V and Vgs of Q3 is only -0.53V. That correspond to around 140mA of bias, too low actually. So I want to know whether I should increase R3 and R5 to get higher Vgs, or this is simply due to the use of the 19.5V supply, and will be fixed when a 24V supply is used (I would like to have around 750mA of bias current).

Thanks and regards.
Attached Images
File Type: jpg Mini_F5a.jpg (630.0 KB, 137 views)
File Type: jpg Mini_F5b.jpg (634.2 KB, 127 views)
File Type: jpg 1K_out.jpg (278.0 KB, 123 views)
File Type: jpg 20K_out.jpg (259.7 KB, 112 views)
File Type: jpg Schematic.jpg (58.9 KB, 124 views)
File Type: jpg Vgs_Id.JPG (184.7 KB, 74 views)
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Old 22nd August 2014, 04:51 PM   #263
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Nice looking PCB.

In my amp R3, R5 are 1K1 (with a 24V supply) but that is because my NOS LatMOS seems to be lazy.

Before changing R3 and R5 and would wait to use your 24Vdc supply.

I think your low bias is mostly due to a low supply. Do you own a small bench test 24V supply ?

Rgds,
Eric
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Old 23rd August 2014, 12:20 AM   #264
cwtim01 is offline cwtim01  Hong Kong
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Thanks, I don't have a bench top power supply yet, but this is certainly on my to-buy-list for the future.
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Old 23rd August 2014, 02:54 AM   #265
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This is the one I'm using presently. Maybe one day I will try a regular power supply, just maybe. I do not fear SMPS.

120W DC24V 5A Switching Power Supply ?Suitable for TK2050 Amp | eBay

Rgds,
Eric
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Old 24th August 2014, 03:55 PM   #266
cwtim01 is offline cwtim01  Hong Kong
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Default It sings

It'll take another 2 weeks for the 200VA dual 18V toroid I ordered to be delivered, but I took an undersized 35VA dual 9V from a linear regulator to give the 24V dc I needed for testing.

At 23.8V Vcc, I got a bias of 500mA for each MOSFET, so I increased R3/5 from 680R to 820R. However, it was too much to ask for the tiny 35A toroid and Vcc dropped to 22V, anyway I got a bias of around 600mA with that.

Next I connected a 5R 20W resistor to the output as dummy load, and I saw oscillation from the scope meter when test signal was present. I removed the 4.7uF output bypass capacitor and the oscillation got even worse. It turned out oscillation was due to the nest of crocodile lead wiring mingled altogether, but was resolved by separating the input and output wires with proper distance.

Finally connected the test channel to a cheap Denon speaker, and it sings !! Source is just a Samsung smartphone's audio out, but even with a single channel the sound is quite nice, it has a taste of sweetness resembling that of tube amps
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Old 24th August 2014, 04:23 PM   #267
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Great news !

I really enjoy this amp, if I build a second one I will modified the PCB for R3/R5 : fix 500 ohm in series with a 1K multi-turn pot.

BR,
Eric

Last edited by e_fortier; 24th August 2014 at 04:25 PM.
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