[TenTec] 565 Low level audio on headphones

[Bob McGraw - K4TAX]

As Jim K9YC said earlier, it is normal or typical for designers to put some 
value of series R between the audio output device and the HP connectors. 
This will prevent shorting of the device should some inadvertent event take 
place like a shorted HP cord, bad connection or such.

Now with the series resistors then add the HP impedance to ground and one 
has a voltage divider.  The larger the series resistor values and the lower 
the HP impedance the less voltage across the headphones.  It's an Ohm Law 
thing.  The idea is to have a series R of such value that shorting the HP 
connection to ground will not cause destruction of the headphone amp device. 
In general, higher impedance headphones i.e. 30 to 65 ohms, should perform 
better, again depending on the actual sensitivity of the headphones than 
those headphones of lower impedance values.  Yet impedance is not related to 
headphone sensitivity, so two factors must be considered.  Thus headphones 
of lower impedance should by choice have a higher sensitivity while 
headphones of higher impedance can have lower sensitivity values.

I recently heard a station with quite good sounding audio and inquired as to 
his radio and mike being used.  He was using a headset with boom mike.  He 
provided me the information and I ordered the item.  It has the numbers 
SM-750M.V and the name Stereo Dynamic Headphones on the band.  Nowhere does 
it say "Made in xxxxx"  you fill in the blank.   The item was delivered by 
postal service with a total cost for the merchandise and shipping in the 
amount of $5.95.  Yep, complete to my door for a total of $5.95.   I find 
the headphones work very nicely and sound quite good and are moderately 
comfortable and even the in-line volume control works.  The boom mike has a 
nice wind screen and when connected to the Omni VII gets great audio 
reports.  Mechanically and structurally, I view the $5.95 purchase is a bit 
more than a piece of junk.  If it didn't work it would be total junk.  I 
would expect parts to fly around the room should I drop the headphones on 
the floor.  I've never seen the likes of molded plastic snapped together. 
This goes to show that one doesn't have to spend a hundred bucks or so for a 
name-brand headset that works.

73
Bob, K4TAX




----- Original Message ----- 
From: "Richards" <jruing at ameritech.net>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
Sent: Friday, March 15, 2013 9:10 PM
Subject: Re: [TenTec] 565 Low level audio on headphones


> Er...so, what DO the engineers at TT say about this?   Have you asked 
> them?
>
> I know loads of guys who swear by the CM500 on the Orion and Orion II
> and other TT rigs.  If you suffer low receiver volume, I suspect something 
> is fundamentally wrong with the radio.   Jim Brown is a big fan of the 
> CM500 - and he is serious about audio.  If it did not perform well, he 
> would have said so.
>
> I suspect there is something fundamentally wrong with the rig, and adding 
> a band aid resistor is not an appropriate remedy - it might cover for a 
> larger problem, but I doubt that is the best course.
>
> Trust your instincts...  discover the real problem.   When this happens to 
> me, I ask the local rabble for help in diagnosing it - and they have 
> always helped me find the real root problem, bless their pea-picking 
> hearts !
>
> Just my take.
>
> -------------------------  K8JHR  ----------------------------
>
>
>
> On 3/15/2013 6:00 PM, Paul K. wrote:
>
>> So, now I am considering looking at the resistor change.  Although, I am
>> still seeking additional advice and suggestions. I hate second guessing
>> the engineers at Ten Tec.
>
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