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Old 2nd May 2011, 11:50 AM   #1
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Join Date: Mar 2004
Location: At the output stage
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Default F4 questions

Hi everyone,

I'm planning to switch from class D to class A and the F4 is my choice.

Quick q's:

- I'm using 86dB speakers and as per my measurements 30W is enough to reach levels above average but I have to ask: wouldn't the distortion reach inacceptable high levels when nearing max output on peaks?

- what preamp is most recommended? would a stepped transformer mate well with the F4?


Thanks in advance

Last edited by mr_push_pull; 2nd May 2011 at 11:55 AM.
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Old 2nd May 2011, 11:57 AM   #2
cjkpkg is offline cjkpkg  United States
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Join Date: Aug 2008
Location: Roswell GA
I just completed an F4 and have a few comments/pointers:
1. F4 has no voltage gain so you will need a robust preamp with sufficient gain >20 probably. I am running a JFETBOZ and an Aikido both putting out about 15 which will drive the F4 loudly but there could be a little more.
2. My speakers are about 89dB which does not seem to be an issue
3. You may want to consider an F5 which is a slightly cheaper and simpler build - it has a gain of about 15 on its front end and would be happy with a simple preamp.

So far I am really happy with it but I am now going to revisit my preamp strategy...either tweak the JFETBOZ or possibly different input tubes on the aikido.

The Pumpkin Preamp is very highly recommended here especially for the F4. Search this forum for it.
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Old 2nd May 2011, 12:04 PM   #3
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Thing is that someone has a ready built (not commercial but DIY mind you) on offer and the price is so attractive that I decided to buy it. Not to mention that it saves me all the hassle of sourcing the immense heatsinks, matched transistors etc.

I'm aware that with F4 the gain is 0dB but I want something that is the exact opposite of what I'm using now: brute force instead of complex circuitry, feedback and the like. I'm really not one of those religious guys scared of the mere ideea of feedback, I just want to experiment.
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