Very Small Hum

[kambo]

when u short input, arent you grounding your whole circuit ?
so, isnt it normal not to have any hum/noise, when input is shorted (assuming rest of your circuit grounded properly) ?
dont you think, you should have a min 150ohm resistor, and/or resistor/cap combination, connected to DI input for measurements?

 

[JohnRoberts]

I think a few other posts point this out - but some caps with an outer foil will have lower hum if installed with the outer foil attached to the side closer to ground.
If you flip the cap does the hum decrease?
There is a thread on the hoffman amp forum talking about this and testing caps for the best orientation (foil to low impedance side).
Orange drop caps are shown as having this effect and they need to be measured if I recall correctly - there is no indication from the pachage or label which lead is connected to the high impedance side.

That would be significant if it was a decoupling capacitor, but here, as it's a coupling cap, both armatures are at the same potential AC-wise, except at ultra-low frequencies.

I don't know the circuit but the issue is termination impedance on both ends of the capacitor. Of course this is only a concern at LF where the cap itself is NOT low impedance.

I have a hard time imagining any application except for balanced gain trims, where both ends of the cap see similar termination impedance.

Of course I am just speculating since I do not know the application being discussed, but back with my polystyrenes it did make a difference.

JR

It's the input cap, that comes from the input jack and goes to the grid. It's a 100nF. Whatever the direction, 100nF is >> than the stray capacitance, so the difference would be negligible. Indeed, with a shorted input, there would be a difference.

OK I think I get it now... It's an input to a guitar amp? So source impedance from guitar pickup is not exactly low Z while relatively lower than the grid and it's termination.

Thanks

JR

@ kambo, for noise measurements it should be terminated with equivalent source impedance that signal is coming from.

 

[kambo]

@ kambo, for noise measurements it should be terminated with equivalent source impedance that signal is coming from.

thats where i am getting confused... when u short input,  isnt your source impedance becoming zero ?
is there any audio equipment with zero impedance ?

 

[ruffrecords]

when u short input, arent you grounding your whole circuit ?
so, isnt it normal not to have any hum/noise, when input is shorted (assuming rest of your circuit grounded properly) ?
dont you think, you should have a min 150ohm resistor, and/or resistor/cap combination, connected to DI input for measurements?

When I short the DI, that is true and there is no hum

With the DI unplugged the mic transformer secondary is connected. The mic transformer primary is shorted for the test. So the impedance across the tube input is the complex impedance at the transformer secondary. The schematic is on the third page of this document:

https://drive.google.com/file/d/0B_n67A1hN3qtRzFaOUNpVTczUkk/view?usp=sharing

The capacitor we are talking about is C2.

Cheers

Ian

 

[ruffrecords]

@ kambo, for noise measurements it should be terminated with equivalent source impedance that signal is coming from.

thats where i am getting confused... when u short input,  isnt your source impedance becoming zero ?
is there any audio equipment with zero impedance ?

Not necessarily but that is a whole other conversation.

Cheers

Ian

 

[dmp]

Most tube designs omit that capacitor - either unbalanced guitar amps or mic preamps.
Why did you include it in the design?
Maybe replacing it with a jumper would give your design an improvement.

 

[ruffrecords]

Most tube designs omit that capacitor - either unbalanced guitar amps or mic preamps.
Why did you include it in the design?
Maybe replacing it with a jumper would give your design an improvement.

That is definitely my next test.

As to why I included it I can only plead old age. My other tube mic pre design has a first stage where the grid is about 90V above ground so the cap is required there for dc isolation. So when I drew this design it was sort of in the back of my head that I need an input dc isolation capacitor, but of course this design has the grid sitting at 0V as far as dc is concerned so it does not really need it.

Cheers

Ian

 

[JohnRoberts]

@ kambo, for noise measurements it should be terminated with equivalent source impedance that signal is coming from.

thats where i am getting confused... when u short input,  isnt your source impedance becoming zero ?
is there any audio equipment with zero impedance ?

I am agreeing with you... to measure noise for a guitar amp input, termination should be equivalent to a guitar pickup,,, (probably several K with some inductance in series too).

Yes, op amp outputs could deliver very low source impedance (m Ohms)  but best practice involves adding non-zero build out resistors, so in typical use between 50 ohms and 2k actual. 

JR

 

[abbey road d enfer]

@ kambo, for noise measurements it should be terminated with equivalent source impedance that signal is coming from.

thats where i am getting confused... when u short input,  isnt your source impedance becoming zero ? 

yes, and that is not  a normal operating condition of the circuit.

is there any audio equipment with zero impedance ?

No, but indeed there are sources with very low impedance. For example, moving-coil phone cartridges (about 2 ohms). The notion of impedance is relative. For a tube or FET stage, noise is almost independant of the source impedance (except for externally induced hum/buzz), so 10 kohm can be considered as low impedance.
For a bipolar stage, the optimum source impedance will typically be between 1 -10kohm. So 150 ohms (typical of many microphones) can be considered low impedance.

 

[ruffrecords]

I just tried removing the unnecessary cap and replacing it with a wire link.  A lot less susceptible to the presence of my finger but the level of background hum is unchanged.

As a kind of last resort, I decided to try changing the orientation of the mains toroiod. To my surprise the hum completely disappeared when I rotated it through 90 degrees. I had previously abandoned this Hammond enclosure when testing the version 1 PCB because I thought the top and bottom were made of steel and the H field was being conducted via the enclosure to the input transformer. I later discovered the enclosure is in fact 100% aluminium which is why I decided to use it for the V2 PCB tests. So it appears the culprit was H field all along; not conducted via the enclosure but directly through the air. The mic input transformer is a Sowter so its mu metal screening should be good but clearly not good enough in this instance. I now have a hum free tube mic pre with a shorted input EIN of -123dBu - I am happy with that.

Here is a picture of the competed unit with its top off. The mains tproid was originally on the floor of the enclosure which probably means it was in the same plane as the core of the mic transformer. Now it is mounted on the side wall there is no hum.

https://drive.google.com/file/d/0B_n67A1hN3qtaktyQUdtZGRrX28/view?usp=sharing

Cheers

ian

 

[Deepdark]

Congrats! By the way, why did you ground input xlr pin 1 to the jain bolt?

 

[ruffrecords]

Congrats! By the way, why did you ground input xlr pin 1 to the jain bolt?

Originally it was a cheap plastic one so there was no shell to ground pin 1 to.  I changed it for a 'proper' metal shelled one but left on the connection. I should really take it off.

Cheers

Ian

 

[Deepdark]

Congrats! By the way, why did you ground input xlr pin 1 to the jain bolt?

Originally it was a cheap plastic one so there was no shell to ground pin 1 to.  I changed it for a 'proper' metal shelled one but left on the connection. I should really take it off.

Cheers

Ian

That explain why then haha. I was going to send you grounding 101 

 

[JohnRoberts]

Glad you found it, torroids are notorious for asymmetrical magnetic fields, from things like where windings start and stop, etc. So lower field in general but can vary depending on orientation.

JR .