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Old 21st May 2012, 11:49 AM   #1
Doose is offline Doose  Australia
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Default Buzz/Hum in Matrix amplifier

Hello,
I am having trouble removing a hum/buzz from an amp.
The hum's volume is the same as quiet music.
I built the amp a couple of years ago, gave up fixing the hum, and have taken it up again.

Its a Matrix amplifier, based off circuits from:
Steve's Tube Pages MATRIX

The problem only happens in the left channel.

With the left or right input connected, no hum.
Left AND right input connected, hum appears.
Moved ground leads of inputs to different locations, hum remains.

Tested feedback from grid of left signal triode, to inverting triode, hum was halved.

Removed mains ac lead while signal is playing (until B+ drops), hum disapears.
Tested signal with laptop running from battery, hum disapears.

Any help would is greatly appreciated.


(The photo has the left input bypassing the inverting triode,
I was just testing to see if the heater wires were affecting it.)
Attached Images
File Type: jpg Matrix.jpg (166.6 KB, 119 views)
File Type: png Matrix Circuit.png (43.3 KB, 108 views)
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Old 21st May 2012, 12:33 PM   #2
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Perhaps you could start with separating your high mains ripple power supply circuitry - and bring the 0V from the last filter cap over to the star ground point.

Also perhaps move your star ground point to the last filter cap of the B+ supply, and separate the ground returns from the output valves, from those of the input valves, and don't ground the input sockets at the socket.
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Old 21st May 2012, 01:04 PM   #3
DF96 is offline DF96  England
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Yes, ground arrangements look a bit random at present. Difficult to follow the layout when almost every wire is orange (I exaggerate, but not much!).
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Old 22nd May 2012, 11:09 AM   #4
Doose is offline Doose  Australia
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Thanks for the ideas.

I moved the psu ground to the last filter cap.
Changed the valves ground to star ground (it was originally at the psu).
Input sockets are grounded at the signal valves.

The hum/buzz is still as loud as before.
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Old 22nd May 2012, 11:21 AM   #5
DF96 is offline DF96  England
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Perhaps there is a hum loop set up via the left and right channel grounds, so you need both connected to establish the loop.
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Old 22nd May 2012, 12:22 PM   #6
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Your hum loop appears to include the mains earth, based on your initial comments. How many 0V points do you have that go to the chassis? Does the mains protective earth go to the chassis by itself?

Have you tried other 'nice hi-fi style' signal sources that are mains powered - using the same 'matrix of connection combinations.
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Old 22nd May 2012, 03:49 PM   #7
Doose is offline Doose  Australia
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The chassis is connect to earth only at one point with a bolt.
I used that bolt as the center of the star.

The only 0v running through the chassis, is the shield of the inputs.
Input cable is a twisted pair with a seperate outer shield.
Had previously tested with the shield disconnected, but the buzz was still there.

My only mains powered audio source, with earth, is a PC.
My Laptop and Turntable don't have a direct connection to earth.
It's only with the PC connected that the problem appears.
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Old 22nd May 2012, 04:03 PM   #8
DF96 is offline DF96  England
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So the PC is injecting hum into the ground loop created when plugging in both channels. Remember that PC PSU designers don't think about audio.

Generally not a good idea to combine the signal star point with the safety chassis ground, as you can lose control of where your currents go.
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Old 22nd May 2012, 10:49 PM   #9
MelB is offline MelB  Canada
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You have three amplifying stages on the left channel and two on the right channel and the right and left channels use the same output transformer? That's an interesting design?? Looks like it's designed to humm?? Where did you get the schematic?
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Old 23rd May 2012, 02:41 AM   #10
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You indicated that when the laptop was battery powered, then you didn't get hum when just one channel was connected, but you got hum when both channels are connected.

Was that hum the same type/sound as any other time you've noticed hum?

Do you have separate channel outputs from the laptop, or just a 3.5mm jack with a common ground?

Have you taken each channel from the laptop jack using a separate twisted pair over to the input sockets on the amp chassis?

I think you indicated the amp input sockets were isolated from chassis, and the incoming channel 0V was terminated at star point for each channel?

Have you tried a simple shielded stereo cable from the laptop, using just the normal shield as the 0V, and then splitting the channel wires out of the cable when near to the valve inputs?
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