F2 amp - What difference to the original Zen amp?

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Mr. Pass, looking at the description and the basic schematic of your F2 First Watt amp, it looks to me exactly the same as your original Zen amp design, published in TAA February 1994 issue (which I have been using for a number of years).
Please excuse my ignorance, but why is the F2 amplifier a transconductance amplifier, and the original Zen isn't (original Zen had no NFB either)?
Or put it the other way around, what is different about the F2 compared to the original Zen design?


We will wait for the answer from the Master

In the mean while, as far as Nelson has described the F2 circuit is the same topology of the original Zen but with almost no feed back and a 25V single rail, good for 5W class A intended to be used with a high efficiency Full range drivers.

You may have a look to the thread “ Zen w/o FB some place in the second page of the forum.


Let me add that the original Zen does have feedback through the biasing resistor. In the F2 this resistor seems to have a high value.

I will post a revised schematic of my interpretation.
Re F2 comp to original Zen

Tony, thank you for your comments and pointing out the already ongoing thread about F2/Original Zen - I have completely overlooked it!
Had I seen it before I would not have asked my question. In the thread you pointed out to me Mr. Pass suffiecently comments on the similarity of the two amp designs and I understand through the thread what would make the original Zen a transconductance amp in the fashion of the Pass F2.
It was not my intention to try to squeeze out of Mr. Pass more information about his F2 amp than he wishes to divulge at the time being. I have sufficient information now to try making a transconductance amp out of the original Zen design.

Regards everyone, Andrej
The one and only
Joined 2001
Paid Member
The biggest difference is that the feedback is for the DC
operating point only, making it a current source. This is
accomplished by using very high values for the bias network
resistors. The gain is set by Source resistance in the
single gain transistor. And the bias is higher.
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