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DC Servo MOSFET Amplifier
DC Servo MOSFET Amplifier
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Old 7th February 2016, 10:16 AM   #591
jwilhelm is offline jwilhelm  Canada
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Is there DC on the output when the input is shorted? Does the hiss go away with the input shorted?
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Old 7th February 2016, 10:34 AM   #592
prasi is offline prasi  India
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Quote:
Originally Posted by jwilhelm View Post
Is there DC on the output when the input is shorted? Does the hiss go away with the input shorted?
DC offset with the i/p shorted is 0.6 mV. And yes, there is same amount of hiss even when i/p is shorted.
The hiss/hum I am talking about, to hear it, I need to be pretty close to the speaker ~10-15 cm.
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Old 7th February 2016, 10:43 AM   #593
jwilhelm is offline jwilhelm  Canada
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The thump is likely coming from your source. I have a cheap DA converter that does the same thing.

Hiss can be harder to locate. It could be oscillation. A scope reading of the noise helps.
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Old 7th February 2016, 11:23 AM   #594
prasi is offline prasi  India
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Quote:
Originally Posted by jwilhelm View Post
The thump is likely coming from your source. I have a cheap DA converter that does the same thing.

Hiss can be harder to locate. It could be oscillation. A scope reading of the noise helps.
Yes, you are right. The thump (during track change) gone when I changed the source (mobile). Funny thing, I had used the same source , for my previous builds w/o such a problem.
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Old 7th February 2016, 11:37 AM   #595
apexaudio is offline apexaudio  Serbia
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Quote:
Originally Posted by prasi View Post
Tested the amp. sound is nice, neutral (only test speakers now). I didnt have proper ceramic caps, used 22p instead of 10 p across the MPSA B-C. The blue ceramic caps used are 1kV and dont know if they are suitable.
three problems,
1. there is hiss when sound is muted. should I remove the 2r2 ground lift and connect i/p ground directly to psu?
2. during changeover from one music track to next track, there is a thump noise.
3. minor problem is that there is a start thump, which a speaker delay will eliminate.
reg
Prasi
Nice work... remove 2R2 and connect input ground directly to psu ground.
Regards
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Old 7th February 2016, 11:38 AM   #596
still4given is offline still4given  United States
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It still sounds like a grounding issue. Try running a wire from input ground to star ground and see if anything changes.
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Old 7th February 2016, 03:33 PM   #597
prasi is offline prasi  India
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Removing the 2r2 and connecting i/p gnd to psu gnd solved the problem. There is still a minor hum when ear is next to woofer, but not audible even from 10cm away.
connected to proper speakers, slight start-up thump, other than that no unwanted behavior. sound is very nice. bass is good

Once I get correct value NPO/C0G caps, I will change to the recommended values and install rest of the pairs and listen again.

bias that I set-up is 32mV across the emitter resistor pair, everything is cool.
even with only single pair, with 32V supply, its loud !, even at very low pot volume setting. I can only imagine what will happen at 3 pairs, 70V .

reg
Prasi

Last edited by prasi; 7th February 2016 at 03:40 PM. Reason: supply voltage comment
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Old 7th February 2016, 11:16 PM   #598
andrewlebon is offline andrewlebon  India
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Hello apexaudio
greetings F100 #30 schematic and FX100 #540 SCHEMATIC i see changes in
schematic values is #540 is it new modified schematic i want to use it with
2 pair AX14 my transformer 40 0 40 AC
warm regards
Andrew
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Old 9th February 2016, 11:57 AM   #599
apexaudio is offline apexaudio  Serbia
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Quote:
Originally Posted by prasi View Post
Removing the 2r2 and connecting i/p gnd to psu gnd solved the problem. There is still a minor hum when ear is next to woofer, but not audible even from 10cm away.
connected to proper speakers, slight start-up thump, other than that no unwanted behavior
Start-up thump is from DC servo and this behavior is ok.
Regards
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Old 9th February 2016, 12:44 PM   #600
prasi is offline prasi  India
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Quote:
Originally Posted by apexaudio View Post
Start-up thump is from DC servo and this behavior is ok.
Regards
Thanks Mr. Mile for letting me know.

As I said I will install rest of the pairs, correct caps and then listen again.
Should I increase the bias, as these are MOSFETS. Currently, I have it at 32 mV across the emitter resistor pair.
reg
Prasi
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