F5 Ground Loop - Need Ideas.. - diyAudio
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Old 23rd April 2013, 10:13 PM   #1
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Default F5 Ground Loop - Need Ideas..

Hello,
I believe I have a ground loop in my F5. It varies from day to day and ranges from almost totally inaudible to audible from 5-6 inches away from my hornspeaker (Cornscala). When only one RCA input cable is connected, the amp is dead silent (this is the same for both channels). When both are connected, I have a very slight hum/buzz. It just bothers me because I know how silent the amp is supposed to be.
I am about to do a couple of upgrades here in the next few days so I would like to fix the ground loop problem while I am at it. I connected a thick (15 ga) wire between the grounds of the two amp boards. This was advice given to me when I first built the amp.. This took care of about 50% of the hum..

Should my speaker ground wires be removed from the amp boards and soldered directly to star ground?

Also, should both the speaker binding posts and RCA input jacks be totally isolated from the chassis?

I am just trying to sort out what would be best for me, due to having dual-PSU with a single transformer.

Star Ground connections:


AMP GND=>PSU GND=>NTC Thermistor=>Star Ground<=NTC Thermistor<=PSU GND<=AMP GND
&
Earth Ground (From Mains)=>Star Ground<=Transformer Shield

I am biasing at .59VDC and have DC offset of extremely close to zero.

I drew out a diagram of my current wiring... I hope it is clear enough.

The F5 is the finest amplifier I have ever heard and if I get this ground loop fixed I will be extremely happy.

Please, any ideas will help. Thanks in advance!
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Old 23rd April 2013, 10:23 PM   #2
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Default More Photos

Here are some more photos of the F5.. I also included a blank diagram if you would like to save it and draw wires the clearly illustrate a solution. I am hoping this will be an easy fix.

(I squeezed a couple photos of my Hot-Rodded DCB1 that is nearly completed)...
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Old 23rd April 2013, 10:56 PM   #3
Zen Mod is offline Zen Mod  Serbia
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try just one NTC from either left or right PSU pcb to your "star" GND
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Old 23rd April 2013, 10:57 PM   #4
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Huh. I would either combine your PSU boards to form a single PSU, or drop one. the ground connecting the two output boards together is definite GL. For that many bridges, you really need 4 secondaries to really be of any advantage.
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Old 23rd April 2013, 10:57 PM   #5
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Listen to ZM, although it seems a bit overdone.
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Old 23rd April 2013, 10:59 PM   #6
Zen Mod is offline Zen Mod  Serbia
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that bridge is a cure , but evidently not strong enough

however , it's always better without need for cure
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Old 23rd April 2013, 11:01 PM   #7
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Really. Simplification seems like best route, but I yield to your experience, as I would have thought the bridge was part of problem. Shows what I know
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Old 23rd April 2013, 11:18 PM   #8
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you can try to switch green and blue on ONE of your sec.
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Old 23rd April 2013, 11:49 PM   #9
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Ok great, thanks for the advice. I will try using only one NTC, with both PSU boards connected. What do you think about moving the Speaker grounds from the amp boards and soldering directly to star ground (Speaker Ground=>Star Ground). Also, I have been looking at the various ways to connect the RCA input jacks. I am seeing some use a twisted series of 3 wires between RCA input and Amp board. Something about one of the wires is floating earth...?

The attached image shows a couple of ideas...? In bright green, I have traced the ground through the components and there seems to be what I would call a "loop". I remember reading about lifting one of the channels ground (from AMP GND=>PSU) and interconnecting the two amp boards ground. But by lifting that one channels ground, you have effectively removed the loop... In dark pink, I have connected speaker ground directly to Star Ground.. I don't see how this could hurt anything.. But, then again, I am far from a professional.

I don't have good DIY Audio vocabulary yet, sorry... I have been learning about electricity/circuits/diyaudio/speaker building/acoustics pretty much obsessively for the past 15 months and I have literally only scratched the surface.. I love it though..

I see there is some concern about my dual power supply with a single transformer and 4 bridge rectifiers.. Honestly, I was initially planning on going full dual mono with 2 toroids, etc. I went to order the second toroid from Antek and learned that their factory had burned down a few days earlier.

I got the idea for my configuration from the "Official" build guide for the diyAudio universal power supply. This configuration is used on a Honey-Badger amp.

From the build guide..
Quote:
As you can see in the picture on the previous page, a couple of PSU Board PCBs were used to make a “dual mono” power supply. The advantage of this is better channel separation and independence, as well as an extra power reserve for those loud music passages that are completely separate for each channel. The “dual mono” configuration proved to be a better choice for me, using a single PSU Board will work of course, but the additional benefits of using a dual mono configuration were just too great for me to pass up!
This amplifier quite literally sounds better than any other audio component I have ever heard. Sometimes I wonder how it would even be possible for my ears and brain to hear something that is more satisfactory than the F5. Thanks Mr. Pass!

Anyway, thanks for the help up to this point. I have a good place to start now.

Be warned, I am going to keep this thread alive until my problem is solved. Hopefully one day I will be able to return the favor to a newcomer.
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Old 24th April 2013, 12:11 AM   #10
6L6 is offline 6L6  United States
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Are the grounds of the input RCA jacks touching the chassis? Or are they using the insulated washers? The metal should not touch the chassis.

Do what Zen Mod suggests and attach the grounds of both PSU to one thermistor.
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