TC-TB3 (Transistorized Tone Board)

[Gurpreet Turna]

Hi DIYAUDIO Friends , I make transistorized tone control (TC-TB3 Namec), It's working but l have little issue. When
I put this preamp on a surface little hum sound though speaker🔊, but when I lift preamp in air (air gap from surface) hum sound goes so far back. I used this preamp or tone control with AX11 and many other amp result same.
How can I solve this issue.
PCB layout and ready PCB pic uploaded.

 

[PRR]

Put it in a box. If it works OK just in the air, a plastic box (take-out salad tray) may be enough.

Usually we need a metal all-around box for totally hum-free operation. Your room must have very low electric noise.

 

[Mark Tillotson]

The ground loop around the outside of the pcb is a big magnetic loop antenna. Cut it somewhere perhaps. As far from the signal path as possible.

 

[sgrossklass]

Sounds like electrostatic coupling - the power supply's idea of ground potential may not coincide very well with the surface the board is sitting on, and the rather high-impedance circuitry is coupling to the latter - the board is probably best placed in a grounded metal enclosure or on a metal plate at least. The aforementioned ground loop all around the board (oldschool - I think this went out of fashion quickly after 1977) is not exactly good either, it could make the thing quite prone to hum pickup if placed near mains transformers.

Somehow I suspect that a line level tone control using just two transistors may not be the last word in lack of distortion either. With it being single channel and all, I almost assume it is intended for PA or instrument use?

 

[Gurpreet Turna]

The ground loop around the outside of the pcb is a big magnetic loop antenna. Cut it somewhere perhaps. As far from the signal path as possible.

I cut the ground track from two place, little improvement. So I remove ground track on PCB in Right Top side area,now it's working nicely. I post the new PCB layout with little change below.

Somehow I suspect that a line level tone control using just two transistors may not be the last word in lack of distortion either. With it being single channel and all, I almost assume it is intended for PA or instrument use?

Yes I use this circuit for PA amp.For me Best thing of this circuit sub control on board. I am using Apex PP8 discrete preamp for my home theatre. It's awesome
Regards