100hz buzz from 1 channel in a newly designed buffer amp

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Jessica_kh

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Joined
Oct 26, 2021
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1
Location
UK
Hello all,

My name is Jessica, and I am (almost) tearing my hair out with a design I have been working on It is a buffer amp using a 6SN7 tube in a boosted cathode follower arrangement, and nice it sounds too.
But I have 100hz buzz from 1 channel, the other is as quiet as a mouse. It is not loud but significantly heard from the listening position with volume set min (inputs shorted)

Any thoughts as to why one of 2 identical channels could do this? It is a two layer PCB design laid out symmetrically with dual grounds. I have tried pretty much everything so thinking it must be the layout, anyone else had this type of issue?

Thank you for all that read this

Jessica xx
 

gyraf

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Jun 4, 2004
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10,496
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Aarhus, Denmark
Hi Jessica,

Welcome to GroupDIY!

- one channel closer to the power supply transformer than the other? try moving the transformer around to see if this changes anything?

- you shouldn't have dual grounds - try connecting/unifying them? Also, one of the grounds perhaps being "dirty" by being in-between a power consumer it's supply ground?

- bad tube?

..a good picture and a schematic would help to help..

/Jakob E.
 

Bo Deadly

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Dec 22, 2015
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2,780
Location
New Jersey, USA
It's hard to say what the problem is but 100Hz is the second harmonic of power supply mains in the UK so the first thing to check would be sources of mains AC. In particular the heater wires are notorious for inducing hum. Describe how your heaters are powered and maybe also post a pic showing where the heater supply gets from the transformer to the tubes.
 

Diabolical Artificer

Active member
Joined
Aug 12, 2014
Messages
31
Typical cause of 100hz hum is incorrectly placed ground or a leaky de- coupling cap. A scope is your friend here, check your grids. You don't say whether your heater supply is DC or AC or whether the stage is decoupled from the main HT/B+ power supply, both are normal for high sensitivity circuits.

As mentioned a pic/schematic speaks volumes, Andy.
 
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