Fairchild's SC amp explaination

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lolo-m

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I think this can be interresting for a lot of poeple here... The Fairchild is fascinating a lot of poeple, and there are a lot of nebulous points on it's basic operating principle...

It's also a way to check if I understood correctly ...

On the drawing down there :
1 - the SC amp input tranformer symetrises the signal
2 - each 12AX7 triode is mis-biased in the first stage so they amplify only the over modulation. The operating point of the triodes is set by a mono pot, and the dual pot (threshold) set the amount of signal we want to go into the SC amp.
3 - each side of the amp amplify it's part of the over modulated signal to get the voltage and power needed for charging the TC network.
4 - The SC-amp output transformer substracts the two signals ( signal A - signal B )
5 (not shown) - The diode bridge after the transformer rectify the wave into DC to charge the TC network

FairchildSC-ampprincipe.jpg

 

rotheu

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The SC-amp output transformer add the two signals ( signal A - signal B )

If I'm not wrong, two opposite in phase signals are still present at the secondary of the output transformer until they actually get rectified. Then the positive sides of the waves are grounded and negatives are combined in to negative CV and sent to the TC network.
Very good explanation with visuals, thanks a lot, Lolo! ;)
 

lolo-m

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rotheu said:
The SC-amp output transformer add the two signals ( signal A - signal B )
If I'm not wrong, two opposite in phase signals are still present at the secondary of the output transformer until they actually get rectified. Then the positive sides of the waves are grounded and negatives are combined in to negative CV and sent to the TC network.
Maybe my english isn't always good enough  :mad: ;D :mad:... It's hard to be a frenchie !!!
You're right, the SC-amp output transformer don't add but substract the two signals like shown on figure 4. Figure 4 actually show what signal is present at the secondary of the transfo... and the bridge ( meant diode bridge ) transform this wave into negative CV...

Edit first post thanks  ;)
 

DaveP

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Hi,
Lolo has made a good point, can anyone out there explain why Fairchild would choose to operate the control amp 12AX7 in class C?  I can see that you can adjust the DC threshold that way but you can also just use the AC threshold control as well to adjust control signal, just like all the other later vari-mu's .
I 'm gathering the parts to make one of these so I want to totally understand every aspect to give myself a halfway chance of success, I spent 6 hours going through the entire thread last night!
best
DaveP
 

lolo-m

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DaveP said:
12AX7 in class C? 
12ax7 is a high gain sharp cutoff triode. The DC threshold is actually the real threshold of the comp. This the set point of the 12AX7 to pass the signal. The AC threshold is much a "how much do you want to compress" pot. This is the frontplate knob.
I'm maybe wrong but someone here can tell us if I am ?

I don't have any answer on the 12AX7 choice rather than another sharp cutoff triode...

 

RedNoise

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Thanks Lolo-m !!! Very nice indeed...(I mean : it helps me to understand things!)
 
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