WTB: help fixing a humming F5 in MA

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#21
Have a look at the http:\\Pass Diy site.

Under PROJECTS there is a very informative article about GROUND LOOPS.

I'm not familiar with the F5 - I've built the Aleph 4.

Are you sure you are driving it properly ?

The Aleph 4 has GAIN - 24dBs of it. A standard 2V RMS signal from a CD player can fully drive it - Hence my B1 pre.
 
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#22
Ok, here are some observations I have made.
I only made one bypass RCA jumper and I plugged the other side into an old cd player.
The jumper side has mVDC at the output but the cd player side has ~4vdc. This is swapable. Then I noticed that if you power up with no RCA attached the vdc shoots up to ~20. Also the LEDs go out with no RCA. So that must be the power supply rails straight to the output.
 
#27
I have a squeezbox feeding a ARC pre with 18db of gain that I have driven a F4 with. Also my current amp is a Zv9 that I never go past 11 O'clock. My speakers are 96db. 11 O'clock is very loud. With the F5 i can turn the pre all the way up and it's like 830 with the Zv9.
 
#39
Zen Mod said:
"F5 yadayada building thread "
Click to expand...

(All joking aside, here is the relevant yadayada...)


Biasing an F5:

dial both pots to 0 ohms ( check with ohmmeter )

place one voltmeter across PSU caps ( best between + and - of PSU) to observe max voltage of PSU

place one voltmeter at output - to observe offset

place one voltmeter across one source resistors of output mosfets ; it doesn't matter which one .

for test - slowly dial up Variac ( presuming that you have one , as man with many skills) up to full mains voltage , observing voltage at PSU ....... thinking about max cap voltage ( 25V as in FW ? ) , because with 0 Iq PSU is unloaded and voltage is maxed

if nothing is smelling - leave Variac at full mains ;

what's important - Iq must be very low , offset is irrelevant in this moment .

now turn one pot one turn ( assuming that you have multiturns )

then turn other pot one turn

observe Iq and offset

proceed one then second pot , again just one turn

observe Iq and offset

again one turn + one turn

now you are probably in range when you can see which pot is pulling offset in right direction - to 0 .

proceed iteratively with pots , while you set - say - 75% of desired Iq and zero offset

now - put lid on box and let it cook for a while - until yo get thermal equilibrium on heatsinks

it's best to use wire/clips to leave those voltmeters in place ;

open the lid , up bias to - say - 90% of desired one ,while maintaining offset

put lid on , let it cook

check ;

if all is OK - move voltmeters for Iq and offset to other channel and repeat procedure

use it few days at 90% of desired bias , then check and set to 100%

remember - temp. equilibrium with lid on is important
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