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 Post subject: nfb more like pfb
PostPosted: 28 Jan 2018, 02:07 
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Joined: 27 Nov 2016, 07:15
Posts: 11
Location: Holland
Hi,
I built a little el84 pp integrated amplifier. I try to apply some global negative feedback. But when i do so, it seems more like positive feedback. The feedback goes, as in all schematics I have looked at from the 16 Ohm OT tap to the kathode of the first stage. I don't get it.. Is my transformer output in phase with my first stage cathode?
For instance if I apply feedback through a 100K resistor I get more amplification. What am i doing wrong?
thanks,
Daniel.


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 Post subject: Re: nfb more like pfb
PostPosted: 28 Jan 2018, 03:17 
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Joined: 02 Mar 2009, 12:41
Posts: 1039
Location: Vänersborg, Sweden
Sounds like the transformer is out of phase. Try swapping the O/P leads of the transformer to get the feedback signal 180 degrees out of phase. Normally the amp use to howl when getting the FB wrong.

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 Post subject: Re: nfb more like pfb
PostPosted: 28 Jan 2018, 04:27 
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Joined: 27 Nov 2016, 07:15
Posts: 11
Location: Holland
Soundbrigade wrote:
Sounds like the transformer is out of phase. Try swapping the O/P leads of the transformer to get the feedback signal 180 degrees out of phase. Normally the amp use to howl when getting the FB wrong.


That did the trick. Didn't even realise the possibility. Learned something again! thanks!

Other question. The thing has a loud pop when turning off. Any suggestions to reduce that?
And yes it howled like a mother, but with 100k resistors it seemed to just amplify.


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 Post subject: Re: nfb more like pfb
PostPosted: 28 Jan 2018, 09:00 
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Joined: 01 Jun 2013, 09:05
Posts: 601
Mains pop,

is probably back EMF from the power transformer primary.
Fit a MOV across the primary and or X class suppression capacitor.
You can suppress the switch with a contact suppressor, however you loose the isolation of the switch.

There is always more like inrush suppression (which I always fit) but thats the basics.
https://www.allaboutcircuits.com/techni ... apacitors/

On large current like Toroid transformers the inrush current can weld the power switch closed or blow the contacts open.
So its always a good idea to use inrush suppression.
The other thing that can give mains pop is not earthing the mains TX iron circuit.
All extraneous metal (that can be touched) should be at earth potential and no difference should appear between metalwork under fault conditions.
For interest:
http://www.fuseco.com.au/rfi_emc

Regards
M. Gregg

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 Post subject: Re: nfb more like pfb
PostPosted: 28 Jan 2018, 17:46 
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Joined: 08 Aug 2009, 03:11
Posts: 2208
Location: Chilliwack, BC
Danielk wrote:
Other question. The thing has a loud pop when turning off. Any suggestions to reduce that?


A 10nF line safety cap across the mains switch eliminates that.

Cheers!

*edit*
I see the other Gregg already said that, D'OH! :blush:

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