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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