[TenTec] Heil headphones

[pfizenmayer]

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