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OTL, Output Transformerless Amplifier User Group.

P.S.

"Each Basic PP Amp is formed from two equal basic SE Amp joined together ,and need at least double separate SE OPT Ra primary coil load for normal operation ( 2x SE Ra = PP Raa ) ."

It is true that a Class A PP circuit requires a primary 2 times the impedance that is needed for each tube.

In a Class A PP circuit both tubes are using the whole winding at the same time and the effect is that each output tube is seeing 1/2 Raa.

But in a Class B amplifier 4 times is needed. Each half of the primary winding is only 1/4 of the impedance of the whole.

This is a problem with Class A/B amplifiers. The load shifts as soon as one tube is no longer changing in an equal but opposite way.

As long as both tube are contributing in an equal way the load seen by each output tube is 1/2 Raa. When one tube cuts off (or reaches the non-linear "cutoff region") the load seen by the other tube is shifted to 1/4 Raa.

The same is true with a Class A/B OTL amplifier.

When one bank of tubes reach the non-linear cutoff region and stop contributing equally, the output impedance of the amplifier goes up and that would have the same negative effect as a load change in a transformer coupled amplifier.

In a Class B amplifier each tube always only uses one half of the primary winding and the two output tubes take turns. Each tube sees it's 1/4 of the total primary impedance and that doesn't shift.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"



Edits: 08/22/11

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