F3 vs. ZV9 - the differences...

[HVfanatic]

I got Peter Daniels ZV9 boards and collected parts according to F3...

Well, I'm stupid.

To understand why there is a difference, please answer me some questions:

- Why do we have 22,1K in the cap multiplier in ZV9 and 47,5K in F3 (Papa had a box of 47,5K resistors?)
- F3 has additional a RC Filter (R21, C6). Is this a big improvement?
- Why do we have emittor resistors different in the Aleph current source - better distortion specs?
- Is there a reason to make the current source in the F3 adjustable?
- In ZV9 we use 20pF for shorting the feedback resistor. In F3 its 33pF. Why???

Sorry for my stupid questions.

Regs, Dirk

 

[Nelson Pass]

Most of the differences are arbitrary and/or the result of
historical accident. The pot on the current source allows
precision trimming of the output gain figure, which I set at
12.5 dB.

 

[Bigun]

I've never built any of these, but I like to throw in my 2c sometimes...

The capacitor across the feedback resistor is something that is usually 'tuned' by the constructor once the amp is finished. By looking at the output on a good oscilloscope with a square wave input test signal you look for ringing (small oscillations) wherever the signal has a sudden transition from low-to-high and high-to-low. These capacitors are usually helpful in damping out that ringing which is felt to be of benefit. I believe Nelson has documented his recommendation for DIY - use fine insulated wires twisted together for about 1pF /cm and adjust to taste...

RC filters on inputs (I didn't check but I assume this is where R21 and C6 are) are there to prevent radio frequency interference from getting into the amplifier via the input cables. These signals are purported to reduce sound quality because they can be 'detected' by semiconductors in the amplifier to produce signal in the audio frequency range (and some people believe a capacitor across the feedback resistor can help rf get into the amplifier via the speaker cables acting as antenna). I've never used an rf filter, but I don't live close to any radio stations either.

 

[Dzius]

I built ZV9 but I've got question about output capacitors. Does that make significant difference in sound to have 3 caps 15000uF//220uf//1uF, instead of just two 10000uF//10uF ?

 

[Vix]

Does that make significant difference in sound to have 3 caps 15000uF//220uf//1uF, instead of just two 10000uF//10uF ?

Probably you know that capacitors aren't Nelson's biggest concern, so just enjoy and don't worry about it. As long as they are of decent quality, there shouldn't be any appreciable difference.

 

[Dzius]

I read "The Zen Variations - Part 2, The Penultimate Zen’s Current Source" and found description of how adjust the Aleph current source. Can we use that procedure modificated for First Watt F3 as follow ? :

"The percentage of current contributed by Q3 is a function of the values
of R9 and R18,P2 along with the other values of the current source, and
in this circuit are set for best sounding performance, which is close to
50%. As a practical matter, we would adjust P2 to vary this value.

To achieve this 50%, you would have to adjust P2 so that
the AC current through R6,7,8 is one half the current going through the R9.
In the case of the values shown, that means that when the amplifier is driving a load,
the AC voltage across R6,7,8 is 43% of the voltage across the R9.
You can use an AC voltmeter at 60 Hz to check this figure if you need to confirm proper operation of the circuit. Decreasing P2 increases the percentage, and
vice versa. (F3 R6,7,8=0.4 OHM R9=0.47 OHM
Zen2 R1= 0.33 OHM R13,14=0.235 OHM) "

Or just better to trim gain figure to 12.5 dB ?

Pawel

 

[Nelson Pass]

This approach will work with any "Aleph" current source.

The trim to 12.5 dB is simply for consistency in commercial
product. You can set the values anywhere it makes you
happy.

 

[vdi_nenna]

Based on a discussion elsewhere regarding "Favorite Pass amps", I may now want to try the F3. I do have home-made boards for the Zen 9 and most of the parts (have the LUV Jfets). Is there any sound quality difference between F3 and Z9?

Most of the differences are arbitrary and/or the result of
historical accident.

I interpret this to mean they are very simular design and sound the same.

Also, Zv9 is 20w at 10% distortion, correct?

Thanks,

Vince

 

[flg]

To be honest with you, the article states a current value from the current source and a different I value from the self bias that I thought disagreed with the math of what R's were in the thing? I'ld have to go back and read it again to be more specific