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How to build the F5
How to build the F5
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Old 20th March 2011, 05:15 PM   #431
6L6 is offline 6L6  United States
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How to build the F5
Russellc - Yep, I remember that thread. Thanks for linking it.

The next time I am working on the amp I will see what bias current I can get with the existing setup, and if I'm just out of adjust range, then I will look for some resistors to swap.
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Old 20th March 2011, 05:25 PM   #432
CanAm Man is offline CanAm Man  United States
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Hey, all......

Question for you, on the "intimate relationship" between the two pots on the Amp PCB. As I got close to the magic .59 volts (say, any value above .50 volts) I found tweaking one pot (and measuring the voltage across, say, R11) had effectively the SAME EFFECT on the voltage across the other resisitor (R12 in my case). Conversely, tweaking the other pot did the same. Prior to testing, I had assumed changing one pot would only change the voltage for its one associated resistor.

In on case, during final tuning, I found myself "chasing" a 0.012v difference between R11 and R12 readings, and by tweaking either pot I could increase or decrease the R11 and R12 readings, but they continued to show that 0.012v difference. Since I had a low output offset (like, 0.1 mV), I attributed the 0.012v bias difference to tolerances in R11 and R12 values, and declared victory.

Have you seen that "intimate relationship" between the two pots, in which tweaking one effects both bias setting almost identically?

P.S. This is probalby a minor point, since the amp is performing magnificently--but I sure mulled over it, while I was doing the bias settings!
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Old 20th March 2011, 05:35 PM   #433
Marra is offline Marra  United Kingdom
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How to build the F5
Yeah thats exactly what threw me to start with.I initially set one trimmer and then the other thinking I would have zero on the output; it came as a shock to find I'd got 100+ma! I was doing it with a DMM across both source resistors; once I was reminded to use one source resistor and the output the final setting up was a doddle.When I recheck things I'll have to check if there's any differance between settings across the sources. As somebody said this amp gets better with use.
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Old 20th March 2011, 06:09 PM   #434
Zen Mod is offline Zen Mod  Serbia
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How to build the F5
observing one source resistor and output offset is way to go ;

chasing symmetry elsewhere in amp is nonsense ;

all you have to do , after initial bias setting is to use THD meter and tweak one source resistor , looking for lowest THD , trying to neutralize differences in xconductance of input Jfets , same as output mosfets

if you are in mood for that (chasing symmetry) - EUVL gave few tips ( additional source resistor for one input Jfet , needed Ids for that etc. ) ........

do not forget - current is flowing through feedback loop , too ......... so that's where is "missing" current through one output source resistor , if you compare them ......
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Old 20th March 2011, 06:48 PM   #435
CanAm Man is offline CanAm Man  United States
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Good points, ZM..... In fact, after "chasing" that 0.012v difference in the R11/R12 bias voltages, I elected to go for the zero DC offset. Considering just the tolerance of the various amp components, going for perfect bias tracking is futile/unnecessary.

I borrow the test equipment later this week for THD and IMD testing...... But since the real test is my ears, I'm pretty content right now.

I consider my outcome to be a perfect "42".....

I'll continue to nag Antek about my power transformer, though....


Ken Mod
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Old 21st March 2011, 01:27 PM   #436
budwiser is offline budwiser  United States
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Any reason not to use these to mount the mosfet's.

MAX08G Aavid Thermalloy Heatsinks

Also , I need to purchase some mica anyway so was considering using aluminum oxide pads from mouser. Any thoughts?

4180G Aavid Thermalloy Thermal Interface Products
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Old 21st March 2011, 07:04 PM   #437
merlin2069er is offline merlin2069er  Canada
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What do you guys recommend for speaker binding posts / rca jacks?

I got these huge connex speaker binding posts, but it's not threaded all the way - and I'd need to use a washer or spacer (which stinks)...

So, I'm shopping for new pairs...
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Old 21st March 2011, 08:23 PM   #438
MagnumOpus is offline MagnumOpus  Norway
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Jim, what kind of speakers are you running/going to use with the F5?
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Old 21st March 2011, 09:06 PM   #439
CanAm Man is offline CanAm Man  United States
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Missing your posts in the thread for the last day or two (since we talked on the phone).

I hope that's a good sign--that you're busy being mesmerized by the sounds of your F5.

Is bias still an issue for you? Let me know if I can help......

Today I located some pretty nifty hardwood I hope to use for the sides of my amp. Will machine the top plate, probalby tomorrow. Only thing left are the ultra-bright LEDs for the VU meters, and the front panel. I've been playing my F5 "semi-naked" for the last few days. I'm humbled by the sound.


also known as Ken Mod, these days.....(!)
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Old 21st March 2011, 09:33 PM   #440
eyoung is offline eyoung  United States
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can we shorten it to KM

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