F5Turbo Illustrated Build Guide

[stereonutty]

Yes, the hum (which is of course 50 Hz, my bad) goes away with only one input connected. But I also have only one ground connection from psu to amp pcb (per channel). Chassis ground and system ground are (well, should be) only connected through the ground lift.

 

[stereonutty]

I measured for continuity between the grounds of my inputs and outputs and the chassis, and after a couple of seconds there is, but at 9 Ohm so through the resistor of the ground lift I guess. There is another thing: every time I switch on the amp with sources connected and running, there is a 'shivering' going through the chassis and I hear mechanical hum from the (800 VA) transformer. Then after a little while I hear the hum through my speakers (and no more mechanical hum). Could this have to do with my problem?

 

[Sangram]

The shudder at initial power-on is probably magentisation current and is not related to electrical hum in most cases.

I see you've used fairly thick unshielded twisted pair for the input signal, I've not had much success with anything but screened pair with a grounded screen.

Do you have access to a portable source (not connected to mains)?

 

[stereonutty]

It is actually thin silver plated copper wire in a pvc sleeve. I connected my cell phone to the amp and played a song: the hum was still there (maybe only just a little bit less loud).

 

[stereonutty]

I might have found the cause of the problem: the right pcb gets the correct voltage (63 V between + and -), but the left almost nothing (a couple of mV), both with a not insignificant ac part that I'm not being able to measure with my dmm. I used solid core copper wire for the connections between the psu en the pcb's, and that has underwent quite some stress during the build. I suspect one or more of the wires to the left pcb is (almost completely) broken..

 

[Zen Mod]

moral of the story is ... ?

 

[stereonutty]

Not exactly sure what you mean...?

 

[Zen Mod]

...... never use solid core for power routing

- you can, if soldering points are fixed in space and time, but in our most often used arrangements, that's rare case

- just see difference in approach, between what's most often here in Papaland and tube amp made in point to point fashion

 

[stereonutty]

I was thinking that might be it, or else "measure your psu voltage before setting bias". I see the wisdom of this moral. Wires will be replaced with flexible ones. Any suggestion for a good quality make/brand?