On 11/23/2014 08:31 AM, Bill Guyger via FlexRadio wrote:
A couple of separate answers:
- Capacitors in series do roll off the lows, but if you pick a cap whose reactance at one octave 
below the lowest frequency that you wish to be "flat" is equal to the reactance (though 
you can probably get away with simply using the DC Resistance of the headphones for all practical 
purposes...) of the phones, the frequency response will be 3 dB down at that frequency but will be 
back to flat at one octave above the "corner" frequency. So if you assume a lowest 
desirable frequency of 300 Hz. for communications purpose pick a corner frequency of 150 Hz.
- And back in the day (1920's - 1930's) the voice coil of the speaker was used 
for the filter choke, and if you look at old radio schematics, say in the ARRL 
Handbook, you will see the headphones in series with the B+ for the amplifier 
tube. But that went the way of the Dodo when transformers became more readily 
available.
Most high end amplifiers either use a transformer, or in the case of transistor 
amps have complimentary output devices with B+ and B- supplies so (in theory) 
there is very little DC at the common point the speaker is connected to. If you 
do loose one side of the power supply the full tilt boogie voltage of the other 
supply will be applied to your speaker and you have a good chance of blowing 
the voice coil.I have literally seen a speaker cabinet catch fire and burn 
because the amp went into DC. High quality amps have protection circuits to 
prevent that, but the Certain Vagrant (Cerwin Vega) amp that caused the fire 
did not because Vega was really cheaply designed.
Bill AD5OL
        From: Rob Keijzer <pa3...@amsat.org>
  To: Bill Guyger <bguy...@yahoo.com>
Cc: flexradio <flexradio@flex-radio.biz>; Mike Sonn <fatfend...@sbcglobal.net>
  Sent: Sunday, November 23, 2014 7:34 AM
  Subject: Re: [Flexradio] Flex 5000A w/earbuds
There is indeed dc on the 5kA phones output.
When I plug in a headphone I hear a loud pop (almost to the
level it makes me think it'll be blown).

I asked once why on earth this is needed, and got the answer that
blocking this DC (by transformers or capacitors) would also block
the lowest audio frequencies.


I simply don't use the headphones output.
BTW: I never heard that high end audio required our equipment to run  DC 
through our voice coils.

Rob, PA3CNT

2014-11-23 5:06 GMT+01:00 Bill Guyger via FlexRadio <flexradio@flex-radio.biz>:

Forgot to add grab a voltmeter and check for DC

Bill

Sent from my iPhone

On Nov 22, 2014, at 10:04 PM, Bill Guyger via FlexRadio 
<flexradio@flex-radio.biz> wrote:

Heating in headphones and speakers is usually caused by D.C. coming out of the 
amp. I underline usually maybe something I'm not considering though.....

Bill AD5OL

Sent from my iPhone

On Nov 22, 2014, at 8:27 PM, Mike Sonn <fatfend...@sbcglobal.net> wrote:

Anyone else use earbuds? I've used a set for years with my Flex 3000. I
now have upgraded to a FLEX 5K and when I stuck my earbuds in for the
first time, they were very warm.  I thought I had them leaning against
something warm in the shack, but after sitting on the desk for 10
minutes, they are still warm.  My ear canals won't get cold, but
clearly, there's something strange going on here. Any ideas?
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In the old days the loudspeaker used a "field coil" instead of a
permanent magnet.  The B+ supply had a capacitor input,
then the field coil, then the rest of the radio.  A hum bucking
winding was sometimes used.

I don't know of any radios that had B+ on the headphone terminals.
A bit shocking I'd think.

--
     Chuck Forsberg WA7KGX   c...@omen.com   www.omen.com
Developer of Industrial ZMODEM(Tm) for Embedded Applications
  Omen Technology Inc      "The High Reliability Software"
10255 NW Old Cornelius Pass Portland OR 97231   503-614-0430


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