[ARC5] PS Hum in BC-453-B (solved!)

[Roy Morgan]

On Jan 30, 2015, at 5:00 AM, Bill Cromwell <wrcromwell at gmail.com> wrote:

> Hi Brian,
> 
> In private discussion Roy already said he isn't sure there is a shield.

I have learned a bit more about Cat 5 type cable:  It does not have a shield normally.  It turns out that each of the usually four twisted pairs are twisted at different turns per meter (the “pitch”) in order to reduce crosstalk.  Cat 5E cable is the same but with improved characteristics.  Cat 6 cable is further improved.  Wikipedia says:
“...Compared with Cat 5 and Cat 5e, Cat 6 features more stringent specifications for crosstalk and system noise. The cable standard provides performance of up to 250 MHz…"

> ... Maybe Roy saw some of the shielded variety along the way. 

No, I don’t remember that.  It seems that I was just making an unfounded assumption about there being a shield.

> I have observed that AC heater wiring can be run as 'twisted pairs' and dressed near the metal chassis to help against hum radiation.

And in some hifi amplifiers (tube type) you find a variable resistor across the filament winding with it’s wiper grounded.  This is to balance out the hum in the amplifier as a whole.  The hum comes from stray wiring capacitive and inductive coupling and heater to cathode “leakage” or capacitance.  We can’t do this in the command receivers as you have noted, but injecting a tiny bit of AC filament voltage of one phase or another into the audio stages of the receiver may balance out the hum.

> ... I will try twisting the DC wiring I am already using. I'll probably get to that this weekend. I'm retired ya know so I don't have time to do it right now. I'll put the caps across the battery terminals, too, and perhaps extend the earth to those as well.

Indeed.  Retirement brings so many obligations and important ways to use our time.

I’m not sure what the effective AC internal resistance of low voltage batteries is, but I’d guess it is very, very low. At power line frequencies, it may well be the same as the DC internal resistance.  I think that experimenting with various ground and non-grounds will make an improvement for you.  It’s possible that you’ll discover that the hum is not coming from the battery filament supply at all, but from some other source.

I’ve read some articles about hum and noise in microphone and other audio cable situations in professional sound systems.  They are dealing with microvolts of unwanted signal in some cases.  The causes of unwanted hum and RFI are sometimes hard to discover and cure.

Roy


Roy Morgan
RoyMorgan at alum.mit.edu
K1LKY Since 1958