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Thread: Audio Stereo valve pre amp NFB

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    Audio Stereo valve pre amp NFB

    I'm just about to complete a build for someone. And he wants a 2 - 3 way NFB switch on the front of his pre amp. The switch has been fitted.
    Now he owns a power amp that he built without NFB and he fitted a stereo jack connector on the back of the power amp to connect to the pre amp when built.
    http://diyaudioprojects.com/Tubes/Un...-Preamplifier/
    The owner is disabled and now unable to complete the project and posted the item too me.. So I don't have his power amp.
    All this is a bit annoying but that's life.
    I need some advise on the values of resistors and caps I need to get the NFB.
    I've been structed to use the above circuit. And all parts were provided. Using 12bh7 as the main tube. On the sheet I have there is an update on using them.

    I did ask to have the power amp but due to is health he wants to avoid sending me that

    I have built the amp and at present all is well

    So if anyone can help I would be thankful
    A

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    Supporting Member The Dude's Avatar
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    Maybe I'm not understanding your question/description, but NFB is a power amp feature. I'm not sure what you're getting at here. You can't build an NFB circuit without the power amp? Or, I'm not understanding your question,............ at all.

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    OK the owner built his own power amp and decided that it sounds better without nfb. He said he tried various resistor and cap arrangements. But settled for no negative feedback. He fitted a steroe jack on the back of the power amp . So when he got round to building a new pre amp he could try adding nfb to the pre amp. Since then he has been unable to complete the pre amp. Due to ill health.

    So he has sent it to me. To complete. I did say I will need his power amp. But does not wants to send it... As as he is not local to me at all I can't collect

    So I have to try to get nfb circuit in the pre amp. He has fitted prior to me getting the chassis a stereo rotary switch.. I have made the pre amp and all is working OK. But this NFB circuit is the last bit.
    I hope this clears it up
    A

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    So you just have a single preamp stage. The logical thing would be to make the "optional" cathode bypass (33µ) cap switchable as this changes local feedback.
    Of course this will noticeably change gain.

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    I was going to do that anyway. As an option not sure on what values of resistors to use as yet

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    Quote Originally Posted by Arbutt View Post
    I was going to do that anyway. As an option not sure on what values of resistors to use as yet
    If you search the forum for "switchable cathode bypass cap" you will find some advice.

    E.g.: https://music-electronics-forum.com/...ode+bypass+cap

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    Many thanks helnholtz. I'll do that later

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    The problem is that without knowing the input sensitivity of the power amp, its not clear how much minimum preamp gain is required.

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    I understand what you are saying. Well a bit anyway ;>). I'll do my best to get it right without the power amp.

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    Quote Originally Posted by Arbutt View Post
    I understand what you are saying. Well a bit anyway ;>). I'll do my best to get it right without the power amp.
    If not clear, just ask.

    Remembering that the power amp has no NFB, chances are good that preamp gain will be sufficient even without employing the cathode bypass cap.

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    Supporting Member Jazz P Bass's Avatar
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    It was never stated exactly What the customer expects out of all this.

    Especially seeing as negative feedback is a real nasty word to Audoifiles.

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