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    Question about Open loop gain for VTA-70

    peterh
    peterh

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    Post by peterh on Tue Mar 24, 2020 12:12 pm

    Hi,
    I have been tinkering with the feedback ratio in a VTA-70
    ( low gain model with CCS and 12au7, EL34 finals)

    By examining distortion spectra i found that 2k as R7 R8 seems
    reasonable ( down from 7.5k)
    In order to calculate the NFB i need to know the open loop gain.
    Yes, i forgot to disconnect the feedback and measure when it
    was on the bench. Anyone that has the value ?


    Last edited by peterh on Tue Mar 24, 2020 2:18 pm; edited 2 times in total (Reason for editing : spelling)
    peterh
    peterh

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    Post by peterh on Wed Mar 25, 2020 8:24 am

    My measurment on a VTA-70 shows Open loop gain to be 48
    ( input 500mV out 24V on the 16 ohm tap when feedback disconnected)
    Bob Latino
    Bob Latino
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    Post by Bob Latino on Wed Mar 25, 2020 3:22 pm

    peterh wrote:Hi,

    By examining distortion spectra i found that 2k as R7 R8 seems
    reasonable ( down from 7.5k)
    I

    Hi Peter,

    Over the years Roy and I have played with R7 and R8 > the two feedback resistors on the VTA ST-70 and ST-120 CCS driver board. These are 7.5 K resistors. I don't recommend altering these two resistors at all. If you go down below about 3K for R7 and R8, the amp will sound a little "harder" and less tubelike. It may now even measure slightly better with respect to harmonic distortion BUT > you will change the character of the sound of the amp. How about we go above 10K (which will give less feedback)? Above 10K (to my ears) the bass will get a little "boomy" with less speaker control. 7.5K is a good compromise which gives good speaker control and good sound.

    Bob
    peterh
    peterh

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    Post by peterh on Wed Mar 25, 2020 4:24 pm

    Bob Latino wrote:
    peterh wrote:Hi,

    By examining distortion spectra i found that 2k as R7 R8 seems
    reasonable ( down from 7.5k)
    I

    Hi Peter,

    Over the years Roy and I have played with R7 and R8 > the two feedback resistors on the VTA ST-70 and ST-120 CCS driver board. These are 7.5 K resistors. I don't recommend altering these two resistors at all. If you go down below about 3K for R7 and R8, the amp will sound a little "harder" and less tubelike. It may now even measure slightly better with respect to harmonic distortion BUT > you will change the character of the sound of the amp. How about we go above 10K (which will give less feedback)? Above 10K (to my ears) the bass will get a little "boomy" with less speaker control. 7.5K is a good compromise which gives good speaker control and good sound.

    Bob

    I actually found by trying the 2k is an excellent choice. Low distortion ( half of stock)
    According to my calculation it will give me a NFB of 10dB and is very forgiving
    of tubes used.
    At 1w i get 0.06% THD, 0.22@10w, JJ EL34 and JJ ECC82 used.
    The feedback cap seems reasonable, square wave tests looks fine with current cap.

    It could be my speakers or my ears, but i find a good amp to sound even better.
    Spectrum analyses supports that improvement, again, other speakers indicate otherwise.
    ( i only use UL )

    tubes4hifi
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    Post by tubes4hifi on Thu Mar 26, 2020 7:40 pm

    hey Peter, it's been about 20 years since I measured the actual amount of NFB, but at that time it was 13db, so I suspect you're using at least 20db (which is actually about the same as the original 60s ST70.
    Make that same measurement you did on the outputs and let me know what you get. 48db open loop sounds about right, it was about 40db with my original design 30 years ago.
    peterh
    peterh

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    Post by peterh on Fri Mar 27, 2020 1:58 am

    tubes4hifi wrote:hey Peter, it's been about 20 years since I measured the actual amount of NFB, but at that time it was 13db, so I suspect you're using at least 20db (which is actually about the same as the original 60s ST70.
    Make that same measurement you did on the outputs and let me know what you get.   48db open loop sounds about right, it was about 40db with my original design 30 years ago.
    Open loop was measured to 48 @500mV input  ( out 24Volt) not 48dB !!
    with 2k feedback resistor 1.75V is needed to get 24Volt on 16ohm outlet. According to
    my calculation this is 10.3dB
    peterh
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    Post by peterh on Sun Apr 12, 2020 6:35 am

    I'm more and more convinced that i like this level of NFB ( 10 dB). I keep rediscovering records
    in my possession that i never thought of had such good sonics.
    Bass notes are less wooly, highs clean & airy.

    As for health issues, we seem to have passed the top of covid-19 in sweden, myself and my
    families have escaped unharmed, none has needed the (free) swedish healthcare system. But
    economy is still highly affected, no Volvo's out of the factory, no SKF ballbearings rolling out.

    avatar
    audiobill

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    Post by audiobill on Sun Apr 12, 2020 3:35 pm

    PeterH:

    I don't understand your point in mentioning it, but unless your doctors, nurses, and administrators are working unpaid, and wealthy benefactors are donating hospitals and all else, your health care by no means is "free".  Think about it, you're too smart to state this.

    "The health care system in Sweden is financed primarily through taxes levied by county councils and municipalities. A total of 21 councils are in charge with primary and hospital care within the country."
    peterh
    peterh

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    Post by peterh on Sun Apr 12, 2020 3:46 pm

    audiobill wrote:PeterH:

    I don't understand your point in mentioning it, but unless your doctors, nurses, and administrators are working unpaid, and wealthy benefactors are donating hospitals and all else, your health care by no means is "free".  Think about it, you're too smart to state this.
    It's free in any sensible meaning since it does not cost me when needed and
    the sky is the limit if i need it. Free when needed, but of course not gratis as it's included.
    It is payed by the taxes we pay, which is an very efficient way of collecting money,
    no in-betweens, no layers and no profits made by insurance companies.
    Yes we pay a large portion of out income to taxes . But we get back, starting with
    scool childcare university hospitals and pension, not necessarily in that order.

    The comment was intended to express the relief that this horrible pandemi seems to
    have it's peak. I hope this will happen in every readers country too.


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