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Just another lateral FET amp
Just another lateral FET amp
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Old 5th September 2019, 09:44 PM   #141
maton00 is offline maton00  Mexico
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May be none but you seem eager to know, also you say you have seen everything and that also know the same, why do you think you have those hum problems?
If you need to add more capacitance in the rails, it certainly has to be because de PSSR, dont you think?, also you may have problems with grounding,



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Old 5th September 2019, 10:11 PM   #142
voltwide is offline voltwide  Ireland
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In my posting #133 you find the details and the background information. As the modification of the filter caps killed the hum with input shorted I consider this specific problem solved. There are loads of other sources of hum disturbance possible depending on grounding, and sometimes I observe strange effects when I modify the wiring of the test setup. So this mod just fixed ONE source of possible hum disturbances - others may follow...
Always keep in mind that I measure very small disturbances in the uV-range here.
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Old 5th September 2019, 10:35 PM   #143
voltwide is offline voltwide  Ireland
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Let us play around with the numbers.
starting with 1W/8Ohm corresponds to 2,8Vrms=9dBV
thus a hum level of -115dBV is -124dB below 1W
A high efficient pa speaker box with 100dB/1W sensity at 1m distance driven with that voltage outputs
100dB-124dB = -24dB hum sound level at 1m distance.
This is a sound level 24dB below any audibility - ignoring the fact that hearing sensitivity drops significantly at 50Hz.
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Old 8th September 2019, 07:14 AM   #144
voltwide is offline voltwide  Ireland
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Meanwhile the new pcbs were in my postbox. Besides re-routing the driver circuitry I had added some decoupling MLCCs and two LEDs in series with the clipping clamp diodes. First these increase max gate drive voltage 2~3V above the power rails reducing the voltage drop of the power MOSFETs.
Second they flash when clipping occurs - a practical free gift.
A first check took place - and surprise, surprise - it oscillates at 200MHz. The new design had not changed anything to that. So I investigated more time to locate the culprit. To do this I monitored the output with my oscilloscope and used an isolated test prod to probe the circuit traces. Most sensible was the neg aux supply track, here the result changed dramatically over a length of some cm. So I assumed this track to be the inductor of the resonant tank I was looking for. To dampen this it is good practice to break the loop by inserting some ferrite bead. Doing so with a 0603 bead (600Ohm @100MHz) tamed that oscillation. I added a 2n2 MLCC blocking cap behind that bead and did the same on the pos side. I conclude there has been some need of a RF decoupling filter between the driver circuitry and the gate driving emitter followers.
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Last edited by voltwide; 8th September 2019 at 07:27 AM.
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Old 8th September 2019, 09:29 AM   #145
voltwide is offline voltwide  Ireland
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A friend of mine provided me with a taylored u-shaped aluminium sheet serving as subshell of the housing. Beneath I mounted two handles made of stainless steel.
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Last edited by voltwide; 8th September 2019 at 09:32 AM.
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Old 8th September 2019, 09:30 AM   #146
voltwide is offline voltwide  Ireland
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some pictures
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File Type: jpg IMG_20190908_103501827.jpg (85.4 KB, 239 views)
File Type: jpg IMG_20190908_103529532.jpg (125.8 KB, 241 views)
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Old 8th September 2019, 07:49 PM   #147
voltwide is offline voltwide  Ireland
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THD vs output was measured at 1kHz with 10R/5R load.
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File Type: png LatFETamp_2019_09_08_THD_1kHz_10R_2.png (35.7 KB, 209 views)
File Type: png LatFETamp_2019_09_08_THD_1kHz_05R.png (35.7 KB, 199 views)
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Old 8th September 2019, 07:52 PM   #148
voltwide is offline voltwide  Ireland
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The small increase of THD when load drops from 10R to 5R indicates that output current related distortion of the amp is not the dominant contributor to total distortion.
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Old 20th September 2019, 10:03 PM   #149
voltwide is offline voltwide  Ireland
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The last days I had focused on the frequency range above 100kHz. Recovering from clipping did not look so nice with some ringing.
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Old 20th September 2019, 10:15 PM   #150
voltwide is offline voltwide  Ireland
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The main reason obviously was overdrive recovery of the hi-speed/low voltage BJT (BC850C) driving the VAS cascode (DZT5551). Besides some fine-tuning of compensating networks (a never ending story) I replaced the BC850C by a low signal NMOS 2N7002 hoping to improve recovery from overdrive. This was a 1:1 replacement without any adjustment of DC biasing: Output DC offset measured 5mV after this patch. All in all there is no reason not to put such NMOS into this very position.
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Last edited by voltwide; 20th September 2019 at 10:25 PM.
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