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F5 V3.0 Help with Hum Issue
F5 V3.0 Help with Hum Issue
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Old 18th February 2019, 09:26 PM   #1
TheNuge is offline TheNuge  Germany
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Default F5 V3.0 Help with Hum Issue

Hi!
I'm hoping to get a bit of advice / help - and yes, I did try searching for answers
I recently completed the build of two stereo F5s, using the V3.0 boards & kit, the universal PSU boards, Toroidy 400VA 2x18V transformers and Modushop housing.
Click the image to open in full size.

The amps sound great - I use them to power my co-axial horns - but do hum ever so slightly when input is connected. With no RCA cable connected they're dead quiet.


So, I gather I have a ground-loop issue, and I'm looking to fix it!


To implement a star ground, I'd create a central ground point off the ground on the PSU board, and hook up the speaker negative posts and the ground "supply" to the boards using equal-length and gauge wire. Right? This post then also gets connected to chassis/safety earth via a thermistor.
However, how do I go about connecting the shield/negative of the inputs? At the moment, I'm using regular shielded cable. Would one option be to use XLR-type cable, using the two inner conductors for hot and ground, and just connecting the shield at the board end?


My preamp also has XLR outputs (not true balanced I gather) - would it be of an advantage to use these over the RCA connections?


I'd obviously be grateful for any other advice too!

Cheers


Es
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Old 18th February 2019, 09:30 PM   #2
Zen Mod is offline Zen Mod  Serbia
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F5 V3.0 Help with Hum Issue
connect one fat wire between left and right pcb GND points , shortest possible

or lift one input gnd with 10R resistor - just insert it between screen and RCA gnd tab
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Old 19th February 2019, 09:46 PM   #3
TheNuge is offline TheNuge  Germany
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Quote:
Originally Posted by Zen Mod View Post
connect one fat wire between left and right pcb GND points , shortest possible

or lift one input gnd with 10R resistor - just insert it between screen and RCA gnd tab

Thanks!
Just to be sure I understand correctly:
Method 1:

Speaker grounds and PSU grounds are atteched to both boards as per standard setup. Additionally there's an extra (fat) wire connecting the two boards?


Method 2:
Speaker and PSU grounds connected as per standard setup, but the shield of one of the inputs is "lifted" with a 10R resistor?



Sorry if my questions are stupid - I'm not very electrially minded, but can "solder by numbers" so to speak


Cheers


Es
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Old 19th February 2019, 10:11 PM   #4
Zen Mod is offline Zen Mod  Serbia
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F5 V3.0 Help with Hum Issue
you got it right in both instances
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Old 20th February 2019, 02:16 AM   #5
astromo is offline astromo  Australia
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Side comment/question - how hot is the STM CL-60 thermistor likely to get during normal operation?


From the photo (top right), it looks like the thermistor is lying on top of the AC mains cabling (could just be skewed perspective). If it's a potential issue, it might be worth making sure that there's a good air gap.
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Old 20th February 2019, 09:47 AM   #6
Zen Mod is offline Zen Mod  Serbia
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F5 V3.0 Help with Hum Issue
you got it right ......... more than right

mains connected NTC get's hot enough ..... to disturb plastic housing around relay tabs on which ( I resume) NTC is mounted , let alone wires in vicinity of NTC body

NTC must be placed on massive enough pads ( connection block or pcb , whatever it is) to endure heat of NTC legs

remember - never try NTC temp with finger ....... especially when amp is connected with mains
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Old 20th February 2019, 10:08 AM   #7
mpmarino is offline mpmarino  United States
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What is this wire? Probably better to ground it outside of the toroid regardless.
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Old 20th February 2019, 10:11 AM   #8
Zen Mod is offline Zen Mod  Serbia
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F5 V3.0 Help with Hum Issue
!!!!!!!!!

exactly !!!!!!

that's short winding through toroid : case- bolt-wire to case

well , I'm wearing glassssssessss , no wonder that I didn't saw it

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Old 20th February 2019, 03:12 PM   #9
mpmarino is offline mpmarino  United States
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I have to see the I doctor next week for the first time - same boat as you ol' buddy.

I can't tell for sure where that wire is coming from, but it 'might' be a shorted turn


EDIT - Mr Nuge, nice job on the cleanliness of the internals!
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Last edited by mpmarino; 20th February 2019 at 03:19 PM.
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Old 22nd February 2019, 01:24 PM   #10
TheNuge is offline TheNuge  Germany
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Quote:
Originally Posted by astromo View Post
Side comment/question - how hot is the STM CL-60 thermistor likely to get during normal operation?


From the photo (top right), it looks like the thermistor is lying on top of the AC mains cabling (could just be skewed perspective). If it's a potential issue, it might be worth making sure that there's a good air gap.

Hi!
The thermistor up at the mains wiring is just the one for the ground - that shouldn't get hot!


I built-in a kind of soft-start using a thermistor and a relay, which is positioned between the PSU and the right amp board.


Es
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