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F3 pcb group buy...

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#182
Calculating R5

Hey all.

Got a question about calculating the value for R5.

According to the ZV9 manual, there are three ways to do it. Using a distortion analyser (no dice), testing the jfet as per the schematic (which I tried, but for some reason, I was getting voltages like 0.74 across the resistor, well outside the range in the manual. I'm assuming I did something wrong, dunno what though ...), and the third option was to set R3 so that Vds is 2.5V, with the output DC at 23V ...

How do you determine that value then ?
 
#183
Lordvader,

I can share a little about the source resistance and distortion.

http://www.diyaudio.com/forums/showthread.php?postid=1248736#post1248736

When I built the second amp the total source resistance of 0.5 ohms (2//2//1) produced a little more than 0.04 % distortion (1 watt and 8 ohm load). When I adjusted the total source resistance to 0.55 ohms the distortion went to about 0.015%. Yes thats right .05 ohms and most of us are using 5% resistors. My point is you can get pretty good distortion numbers by using what Nelson had presented in the Ver 9 articles.

Vgs at lowest distortion was 0.87V for one and 0.89V for the other.

BDP
 
#185
F3 cap voltages

50 volt for electros is fine, although sometimes wider range at 63v (okay).
The old stock Elna Cerafines are quite good, otherwise look to Pana FC, Nichicon Muse KZ (maybe FG), Rubicon B/Gates (maybe ZL), etc.
For C8 it is better to go for a higher voltage propylene cap and hence spend some money on this one. C2 can be 35v, maybe even 25v.
Suggest higher current rating ripple caps (physically bigger, too) in the power supply, rather than the compact size ones (for same uF) and avoid wirewound R in the C-R-C.
Good quality connection wire is essential.

The amp is not dead silent, but still quiet enough.

Will maybe need to upgrade the ICs and perhaps set your speakers up again - extraordinary performance, this amp - well worth extra care, attention, & cost.
 
#188
F3 caps

Peter has designed a number of options into the pcb, including leaving the C1 off the board and so can use the vacant space for other options - if the pcb is raised higher above the heatsink, you can add components below the board also. such as trimming resistors, bipass caps (if desired), connecting wires, etc.

Many people find that "bipassing" good electros detracts from the sound - simpler the better - often true - continuing technical discussion on another thread, worth a visit.

The 1817s are okay.

My C1 (at present) is a large Rifa 15,000/40v [50mm dia x 75mm high] mounted off-board and has a big, fat companion 4.7uF ICW Clarity Cap added which complements the C10 BlackGate (a bit surprising, this!) plus the 0.1uF Styrene (now sub for C8) - the sound of this "construction site" is a little "too-top-end" until the caps settle down.
.
Obviously, a simpler &/or better assortment of caps will result in a different sound for you - this selection is just an example of going a bit over-the-top - it's good to push the limits!
 
#196
Sorry to bump this, but a quick question about the AC current gain.

I've managed to confuse myself by looking at the ZV2 and ZV9 articles while tired (as well as this thread http://www.diyaudio.com/forums/showthread.php?s=&threadid=107958), so I'd like someone to make it all better.

Now, the AC gain is altered by varying R19 (in the ZV9). The article states that as it currently appears, the gain is set to 50%. However, using the formula in the afore mentioned link, to have an AC gain of 50%, R19 needs to be of the same value as R13 (which is the way the equivalent resistors are setup in ZV2).

So ... on the F3 board, if I go by the ZV9 article, I can adjust P2 so that it plus, R18 is about 3x the value of R14, OR, I can omit P2, and have R14 and R18 be equal (ala ZV2).

What do I do ? I'm kinda at my wits end 🙂

(actually, going by the formula, i get an A value of 1.66 for the ZV9, which sounds like a 66% gain, not 50% ....)

(but then, if I put equivalent R values in the formula (so that B = 1), I get a = 0.5, which sounds like attenuation ...)

(and then I get unity gain if B=0.5, so one resistor is twice the value of the other ...)

AAAAAAARRRGH !!!
(sorry if this is all over the place, kinda doing two things at once atm ...)
 
#199
Quick question ...

While reviewing my board, i noticed that I'd purchased the wrong resistor value for R8 (the schematic wants 2ohms, I've got 1ohm there). This puts the overall value of R6|R7|R8 at 0.33ohms rather than 0.4 ohms. The difference is tiny, but would it have an impact overall, especially when it comes to current gain ?
 
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