[AMRadio] Class AB and B audio XFMRS


Jim candela jcandela at prodigy.net
Mon Mar 13 22:38:44 EST 2006


-----Original Message-----
From: John Coleman ARS WA5BXO
Sent: Sunday, March 12, 2006 10:17 PM

>Very interest Jim:
>On magnetic bucking:
>I guess just a few turns of some heavy wire would suffice and
>the bucking current could be many amps to compensate for the fewer
>turns.  But the DC CCSource would need to be fee of HUM or any
>variations else it would modulate the secondary with a step up.  Perhaps
>more turns in the bucking coil would be better. It is an interesting
>concept though.

Keep in mind that the normal audio will modulate the CCS as well, and if
it's CC capability is not perfect, the mere presence of the CCS will
"colorize" the audio.


>On LOW freq distortion in the output XFMR:
>I found it interesting, how the cross over distortion reappeared
>(in my PP KT88s CLASS AB1) amp as the Frequency of the driving signal
>when down (talking below 20 CPS).  It shows the need to limit the low
>end of the pass band in the preamps to a value that will coincide with
>the pass band ability of the output XFMR.  Accidentally hitting the
>grids with too low of a frequency while trying to pass high frequencies
>at the same time caused not only distortion in the lows but it modulated
>the higher frequencies with the distortion.  And it really sounds bad!
>Limiting the low frequency pass band in the preamps really cleaned up
>the audio.  You might ask what low frequencies we are talking about.
>It's the 4 - 10 CPS stuff that comes from turn table rumble.  On class A
>XFRMerless solid state audio the speakers just moved with the rumble
>(yes, a total waste of power and could damage the speakers).  But with
>the tube equipment it created quite a stink with intermodulation right
>in the output XFMR.  I am sure that some of the bad stuff we hear some
>times on the air is due to incomplete modifications where the preamps
>and drivers of the audio equipment have been modified to pass 20 to 30
>CPS but the modulation XFMR circuitry can not pass below 100 with out
>intermodulation occurring in the XFMR due to core saturation and/or
>cross over distortion.

I once took a Sherwood S8000 hi-fi tube amplifier and changed the coupling
capacitors to the 78768 output tubes to a value about 3X stock. It was going
from .05uf to .15uf. This was in a feedback looped amplifier that was flat
down to 20 hz or so stock at rated power of 30 watts RMS. After changing
those capacitors, I noticed one big difference. Whenever I tuned through FM
broadcast stations my speakers would "thump" real hard. In fact if the
volume was up, the speakers would bottom out very easily. Enhancing the low
frequency response the way I did it was a real bad idea! After 20 years I
took out the capacitors, and went back to stock.


>John, WA5BXO

Jim WD5JKO





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