[TenTec] Heil headphones

[Bob McGraw - K4TAX]

Do not confuse source impedance with load impedance.  Typically the load 
specified  Z is 5 to 10 times the source impedance.  Thus a 3 ohm source 
impedance is adequate to drive a 30 ohm load impedance.  To correctly drive 
a 600 ohm load the source Z should be about 60 ohms otherwise inadequate 
voltage will be developed should a lower source Z be available.  At the same 
time with a 60 ohm source, connecting a 8 to 16 ohm load would likely cause 
excessive current and could likely damage to the output circuit.

Also as discussed, another factor has to do with sensitivity of the 
headphones vs. sound pressure level produced.  Some simply require more 
power to produce the required SPL.

In my case currently with the Omni VII  the headphone suggested load is 8 to 
16 ohms but higher Z phones can be used without damage to the transceiver 
but audio output to the phones  may be reduced.

All in all, I believe it is a hardware issue.  A small transformer might be 
though to work but frequency response will suffer on both ends and still the 
circuit may not have adequate power to drive phones with low sensitivity.


73
Bob K4TAX



----- Original Message ----- 
From: "pfizenmayer" <pfizenmayer at qwest.net>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
Sent: Tuesday, January 15, 2008 4:57 PM
Subject: Re: [TenTec] Heil headphones


> Hi Vin - I had issues with CW headphone sidetone level using Sony MDR V6
> phones which are 65 ohm phones. So I MEASURED the source impedance of the
> headphone amp IC in the OII. It is slightly above 3 ohms (not 32 ohms as 
> the
> manual says - at least in early OII before the headphone RF filter was
> added). It is capable of just at 1 volt RMS open circuit before clipping.
>
> Now when TT began offering the RF filter on the OII headphone it added 
> about
> 24 ohms series resistance in both sides so you are up to about 27 ohms
> source impedance - making the manual about right when the filter is
> installed.
>
> I played with several small audio transformers trying to get it so I could
> get a little more sidetone level but nothing was very effective . I was 
> not
> able to find (at a reasonable price) small milliwatt level audio
> transformers that would go down to 100 cycles or so and transform from 27 
> to
> 65 ohms -  Closest I found were a couple 16 to 100 ohm and the primary
> inductance was too low as well as the turns ratio not really any help 
> either
> so it ended up about an even trade , no gain , no loss.
>
> I get exactly the same issue you have - with headphone levels set so they
> are comfortable with the Sony MDR V6 - then unplugging the phones will 
> blast
> you out of the room. I know this will happen so I just reduce the AF gain
> before I unplug - I hardly ever use the speaker anyhow. But you could add
> some series resistance in series with the speaker as a quick and dirty 
> fix -
> especially if using an external speaker.
>
> There are at least two issues with phones , impedance matching is one  ,
> but the other is sensitivity, or how much sound pressure is generated for 
> a
> given input power or voltage across the phones. The SONY only produces 106
> db/ mw . My sidetone was about 50 mv open circuit -and about 30 mv across
> the phone.
>
> I suspect the Heils , even if they are 200 ohm phones , are significantly
> more sensitive. Everybody I have talked to says their sidetone level is 
> just
> fine. I have not asked Heil for their specs yet as the Sony MDR V6 is so
> comfortable. I asked Heil at Dayton but he did not know off hand, and I
> never followed up with an email.
>
> I suggest you send a note to ditsnbits about the differential audio 
> levels -
> I suspect they may address it in the next OII firmware if enough guys
> comment about it.
>
> 73 de Hank K7HP
>
>
>
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