[ARC5] Hum in BC-453-B

[Dennis Monticelli]

The problem I have observed regarding hum when using modern headphones has
to do with their good bass response and high sensitivity.   The high
sensitivity causes you to turn down the volume pot.  However, the hum
voltage on the B+ does not go down so the audio stage after the pot sees a
reduction of signal to hum ratio.  Couple that with the bass response and
you've got a problem.  One easy trick (besides or in addition to reducing
the ripple further) is to put a series resistor in the headphone line so it
causes you to turn up the volume pot.  I did that with a re-cap'd Drake
R-4B and the low level hum I was hearing was no longer objectionable.

Dennis AE6C

On Wed, Jan 21, 2015 at 2:58 AM, Leslie Smith <vk2bcu at operamail.com> wrote:

> Ken & list readers:
>
> I took some readings from my power supply using a DMM and "CRO"
> I'm still chasing the hum I hear in cheap modern headphones.
>
> Readings below were taken with inexpensive DMM.
> T-2 is the (nominal) 240:24V transformer.
> "To 180 ohm" is the output from the 1/2 wave doubler.
> The 180 ohm resistor 'feeds' x5 zeners, in series, each 12V/1000mW
> B+ reg is the other side of the 180 ohm, the regulated supply.
> I(low) is the measured current drain to the BC-453-B
>
>
> Output T-2    To 180 ohm    B+ (reg.)  I (low)   I2 (high)
>  Volts AC      Volts      Volts         mA       mA
>    25.2         65.7       59.2        16.3       20.6   <-- reading #1
>    24.8         65.2       59.6        16.4         nr    <-- reading #2
>
>                 o-o-o       o-o-o
>
> Next:       Readings with a Fluke 123 scope-meter.
> I measured the mains voltage - 239V AC RMS.
>
> In the table below:
> Ripple(R) - means ripple seen on the zener regulated B+ supply.  This is
> really
> "hash" that's floating around from various switching supplies.
> I see this on many things.  I have no idea what it is. (Really)
>
> Ripple(U) means the AC component of the 1/2 wave doubler - at the "top"
> of the 180 ohm resistor.  Min(pk) is the min DC voltage from the
> doubler.
> Max (pk) is the peak voltage from the doubler.
>
> In other words I have just under 5 volts to 'play with'
> across the 180 ohm resistor.  It seems I don't have significant ripple
> on the B+ line.
>
> Measured with Fluke 123 scope-meter.
> Mains.     Ripple(R)  Ripple(U)     Min (pk)    Max (pk)
>     V AC        mV          mV                  volts       volts
>      239         43         1390                 64.9        65.6
>
> Some specific points for you, Ken G.
> (1) Haven't disconnected the zener chain yet.
>  BTW, the card here is very close (earlier model) to the card I sent you
>  last year.
>
> (2) 470uF, you suggest - it's too small.
> OK I haven't found a 100V many uF cap yet to incr. the "muffs" in my
> circuit.
> Will do this tomorrow.
>
> (3)  In reply to your obs that there is very little hum picked up by (or
> before) the 12A6 grid.
>      I confirm this is so.  When I put the top of a screwdriver on G1 of
>      12A6 - I introduce hum
>      in a form that's missing until I do that.  Conclude:   no hum from
>      G1/12A6.
>
> (4)  You wrote, "At 68V, 470uF isn't enough filtering"
>      OK, I said I would get more "muffs" - but check the ripple voltages
>      (from CRO).
>       I'll put 470mF across the B+ supply.
>
> (5)  I measure the ripple at 1.39Volts, on the unregulated side -
>     but there is a discrepancy between this
>     and the peaks and trough readings.  1.39 vs 0.70V.
>
> (6)  I read your advice to 'get a dyno'
>      I want you to know we 'down under' folk are clean living types.
>      Get a dyno?  Sheeesh!  Next think I know you'll want me to trade
>      in my command set and buy one o' them new-fangled transistor
>      thingies!
>
> Les
>
>
> --
> http://www.fastmail.com - Same, same, but different...
>
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